Good gauge for Jumper cables?

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#1, the vast majority of alternators are temperature-compensated and cut back on output voltage as they heat up. Ohm's law tells us that lowering the output voltage of the alternator reduces the amount of current drawn out of it.

#2, the regulator in most alternators, certainly anything you'd find in a car, is NOT a linear regulator.

#3, the alternator is often bench-tested in a way that makes it produce and records it's maximum output, which often exceeds it's rated output since the test is conducted at room temperature. This does not seem to damage them.

#4, the battery on the donor vehicle will supply a significant part of the current needed to start the engine.

#5, most alternators cannot put out anywhere near their rated output at idle.

#6, diodes can be operated at higher currents for short periods of time without damage. The limitation is thermal heating of the diode. This is why heatsinking a diode allows it to handle more current.

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Maximum (average) forward current = IF(AV), the maximum average amount of current the diode is able to conduct in forward bias mode. This is fundamentally a thermal limitation: how much heat can the PN junction handle, given that dissipation power is equal to current (I) multiplied by voltage (V or E) and forward voltage is dependent upon both current and junction temperature. Ideally, this figure would be infinite.

Maximum (peak or surge) forward current = IFSM or if(surge), the maximum peak amount of current the diode is able to conduct in forward bias mode. Again, this rating is limited by the diode junction's thermal capacity, and is usually much higher than the average current rating due to thermal inertia (the fact that it takes a finite amount of time for the diode to reach maximum temperature for a given current). Ideally, this figure would be infinite.

Maximum total dissipation = PD, the amount of power (in watts) allowable for the diode to dissipate, given the dissipation (P=IE) of diode current multiplied by diode voltage drop, and also the dissipation (P=I2R) of diode current squared multiplied by bulk resistance. Fundamentally limited by the diode's thermal capacity (ability to tolerate high temperatures).

Operating junction temperature = TJ, the maximum allowable temperature for the diode's PN junction, usually given in degrees Celsius (oC). Heat is the “Achilles' heel” of semiconductor devices: they must be kept cool to function properly and give long service life.
 
The only way I can see the alternator output diodes being damaged is IF
The donor engine is being revved AND
The donor battery somehow got disconnected AND
These conditions persist for some length of time, enough to overheat the diodes.
 
Voltage spikes can also damage a diode, but I find it difficult to see how you might generate a voltage spike high enough to do that while jumpstarting a car. The PIV on these diodes is probably several hundred volts.

I should note that some instructions for jump-starting recommend turning the blower to high on the donor car, presumably to soak up any voltage spikes that you might generate by hooking up the jumper cables.
 
brian1703, regarding your:

#1, the vast majority of alternators are temperature-compensated and cut back on output voltage as they heat up. Ohm's law tells us that lowering the output voltage of the alternator reduces the amount of current drawn out of it.

Are they spun up while supplying a dead battery, and then while that is going on connected to a starter motor and hit with the surge in load that occurs while the starter engages? In other words are they put into a situation that the regulator is driving the armature hard and then hit with the surge in load while the armature is being driven hard.

The regulator has NO sense resistor connected in series with the output of the alternator. It does not have any means to get feedback to use as an input to cause it to limit the driving of the armature. And with the armature being driven hard, it is possible that the IFSM will be exceeded when the load of a starter motor engaging is placed on the output.

#2, the regulator in most alternators, certainly anything you'd find in a car, is NOT a linear regulator.

OK, if it is not a linear (and I am being sarcastic) then are you saying it is a switcher.

The regulator output transistor is fed by a comparator that compares the alternator output voltage to the voltage that it should be at. The voltage that it should be at is determined by the ambient temperature. Now the armature of the alternator is driven by the output transistor of the regulator. The mechanical energy of rotation is used to excite the Three Phase Delta connected field windings. The Three Phase Delta connected field windings supply the 6 diode Three Phase Bridge Rectifier that puts out the current for the output of the Alternator.

While this is not you common linear regulator running from a filter capacitor like normal application of a LM723 or a 7812 or a 7805, it is a linear regulator. It is a form of linear regulation. It certainly is not a switcher.

#3, the alternator is often bench-tested in a way that makes it produce and records it's maximum output, which often exceeds it's rated output since the test is conducted at room temperature. This does not seem to damage them.

This in not duplication of the condition that occur when connected to a dead battery and then having a starter engaged.

#4, the battery on the donor vehicle will supply a significant part of the current needed to start the engine.

The battery on the donor vehicle usually has just started the donor. Connecting the donor to a dead battery loads it down significantly already.

#5, most alternators cannot put out anywhere near their rated output at idle.

Normally yes this would be correct, but we are not talking about normal. Normally when a starter is pulling its maximum peak load the engine is turning slower than idle. What we are talking about here is placing the surge load of a starter that can pull a good battery down to as low a 6 volts for a fraction of a second. Now normally the alternator is turning much less than idle. If the alternator is at idle, it can produce too much current for a fraction of a second and that is all it takes to damage an output diode. Interesting enough when this happens, usually only one of the 6 output diodes is damaged.

#6, diodes can be operated at higher currents for short periods of time without damage. The limitation is thermal heating of the diode. This is why heat sinking a diode allows it to handle more current.

The very brief times the surge current lasts is too short to allow heat to leave the diode case and go to the heat sink. This is all about a very short time very high peak current exceeding the instantaneous peak surge current of the diode.

If you run the good vehicle while you are cranking the bad vehicles engine you are rolling the dice about damaging the alternator of the good vehicle.

To quote berniedd " If the other guys have not experienced any damage to their charging system while running the engine and crank-assisting another vehicle, I'd say they are very lucky indeed."

I have seen several times that a output diode of an alternator has gone leaky after someone used the vehicle as a donor to jump start another vehicle with the donor engine running. It is rare, but what makes it bad is that the leakage that it causes is not that great of a load. It takes about two days of not using you vehicle for this slight constant load to discharge you battery enough for you vehicle not to start. Often the donor vehicle is used for several weeks and the problem is never noticed until it is allowed to sit for two days unused. Then it won't start. Many people do not make the connection because of this delay.



dljefino regarding your:

There are kits to make DC welders out of old alts and they seem to survive that!

I know there is a way to do this, I have never looked it up, but I would think that one of the key things in doing this would be to remove the diodes and run directly across one leg of the D winding.




I am not making this up. I have seen alternators that were damaged this way. If you run your engine while jump-starting another vehicle you can damage you alternator. And these days even a rebuilt ain't cheap. Not to mention the inconvenience of having you vehicle not start. And the time involved. It ain't worth the risk. If the bad vehicle will not turn good enough with you good vehicles engine off, then get all of the keys to the bad vehicle in you position. Connect the two and run the good vehicle for ten minutes under normal conditions to charge the bad vehicles battery. If it is real cool out run the good vehicle for 20 minutes for this charging. Then turn off the good vehicle, leave them connected and start the bad vehicle. It is time consuming, but it works and there is no chance of damaging the alternator in the good vehicle.
 
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#1, an alternator charging a dead battery heats up pretty quickly...and cuts back on the voltage as it does.

#2, the regulator controls the field coil with a pulse width modulated voltage. It's not analog, and therefore not linear. Yes, I would call it a switching regulator, because that's exactly what it does, altering the duty cycle of the voltage applied to the field coil between 0% and 100% as it switches the field coil on and off. I do believe this is the mode of operation used to regulate the voltage in a switch-mode power supply....

Note also that the mechanical regulators in the days of yore work exactly the same way, modulating the field coil by turning it off and on.

There may be a regulator out there that uses an analog voltage to control the field coil(Was it one of your designs?), but that would be a poor design.


#3, why not?

#4, I have done the math on this and I will do it again. Are you ready? If it took 150 amps to start the donor vehicle and it took 3 seconds to do so, at a modest 15-amp charging rate (30 amps is probably more typical in a modern vehicle with a larger alternator) it will take about 30 seconds to replace the charge in the battery.

I've also measured the charging current consumed by a battery after starting the car. The charge current tapers off to a few amps within a minute of starting.

How long does it take to connect the jumper cables? Unless you work exceptionally quickly, by the time you get them connected, whatever the donor vehicle took out of the battery to start itself has already been replaced.

#5, the alternator cannot put out more than it's rated current at idle. It's not physically possible, even with the field coil duty cycle maxed. Also note that it takes a certain amount of time for the voltage regulator to ramp up the field coil duty cycle in response to lowered system voltage, which could be as long a second or two. And then when that happens, the engine has to respond to the additional load as well--which means it needs to open the idle air control valve more to maintain the idle. That does not happen instantaneously.

#6, the diode has thermal mass. Thermal mass is what allows you to quickly pass your finger through the flame of a candle and not get burned. Likewise, a short burst of current is not going to burn the diode.
 
To elaborate on #2, all of the regulators I've looked at use PWM to control the field coil duty cycle. That's Ford, GM, and Chrysler, as well as the Bosch alternator/regulator used in my Saab. I expect that it's an industry standard. Many newer cars use the field coil duty cycle as an input to the PCM for better idle control, as well as to up the idle speed if the load is high for several minutes or more.
 
Since it only seems to be one diode going out, it is probably much more likely that the diode was already on its way to being bad and just happened to fail around the same time when the jump occurred. Or some idiot smacked the clamps together to make sure they had a good connection...

I don't know all *that* much about electronics, but I do know that electricity, like water, takes the path of least resistance. If you drop a big load on a running car, I have to think that the battery is going to take up the slack. Not the alternator. Because if it did, every time you tried to jump a car with the donor idling, the alternator would instantly seize and squeal the belt...
 
Well, actually, electricity takes all paths in an amount inversely proportional to their resistance. But your point is taken.

Quote:
Because if it did, every time you tried to jump a car with the donor idling, the alternator would instantly seize and squeal the belt...


Either that, or stall the engine. It doesn't take much to stall an engine at idle, as anyone who has ever driven a stickshift knows well.
 
The older GM delco one wire internal regulator alternators use a simple 3 transistor one Zener regulator and it is Linear. Many of them are still in use today. I do not know if/when they switched to a PWM drive of the armature field. With the old GM three transistor units if the vehicles voltage requires more current from the alternator output to maintain the desired voltage, the drive current to the armature field winding slip rings will increase to the required amount and then stay/regulate at that required amount. This is Linear regulation. The mechanical energy being put into the rest of the production and the stator field windings and three phase diodes are simply part of the output loop along with the gain they supply.

I do not recall ever seeing a neutral from a Y on any stator field winding. All the ones I looked at were Delta. However the field can be either Y or Delta. But almost all use a 6 diode 3 phase full wave bridge. Many also used an additional low power 3 diode circuit. That additional low power 3 diode circuit might not be as popular today.

I do remember seeing some relay regulators when I was a kid. Even saw one or two that still worked when I worked on cars. I know how they worked, but never had to work on one.

The Chrysler units were also Linear regulators. The regulator was external from the alternator and mounted on the wheel well. They may now use a PWM field drive.

A little research shows that now days Motorola makes the L585,L9407, L9444VB, L9484, CS3341, CS3361, MC33099, IRVR101, and L9473.

Most of these use PWM drive of the armature field.

The MC33099 and be run either PWM or Linear.

The IRVR101 is only Linear.

The L9407F includes circuitry to shut down the output drive to the armature field winding during start and for a short time there-after. This implies the desire to protect the alternator diodes from the abuse of high current that would be placed on them if the alternator were to drive the armature field and try to regulate while the load of the starter is applied, and also for the brief time after the battery had its voltage pulled low by that load. Why else would they include this circuit?

Most regulate to within 0.1 Volts to 0.2 Volts. This would mean that if the voltage were to go low by more than that amount the drive to the armature field would go to maximum.

Most notable, NONE of these regulators has any means of sensing the output current of the alternator. They do talk of output short circuit protection but this is all about the current to the armature field.

Finally, if you want to run you engine while you jump-start a vehicle, well it is your vehicle so go ahead and risk it.

But you won't ever see me do that. I have seen the worst case results more than once and don't need to learn that lesson the hard way by having it happen to a vehicle I own.
 
Very interesting. Ive started 2 or 3 vehicles with my Vic while running. Last one on the Vic was 6 months ago or so. Worst was probably the 7.3L Powerstroke I jumped with a 1500 Ram. That was Christmas Eve. I let you know if that fails within the next month or so.
I know that disconnecting the battery while the engine is running is bad, as the battery buffers the remnant AC voltage.
It takes some time for them to adjust the current output. Plus they dont put out maximum amperage at idle. I know on my Vic with a dead cold (50F) alternator a 70A (Not counting electrical systems necessary for the engine to run) load will make it sit on the battery with a system voltage of approximately 12.6V. This was with a cold idle 800-900RPM. I imagine heat soaked with a hot idle it would be lower, maybe 40-50A would be pushing it.
 
It would be interesting if someone used a fast storage scope and current transformers with the wire from the alternator output on the good vehicle, and the wire to the starter on the bad vehicle to record the real peak current that occurs during the first few milliseconds when a starter is initially engaged during a jump start with the good vehicles engine running. That would tell the real story about how much peak current we are talking about. These very brief peaks in current are very likely many times more than the average amounts normally talked about, and probably very close to and sometimes exceeds the maximum a diode can handle.
 
Sounds like more equipment than I have. :p It would be interesting to know how fast the alternator is reacting and what happens with the current output.
 
Originally Posted By: JimPghPA
This implies the desire to protect the alternator diodes from the abuse of high current that would be placed on them if the alternator were to drive the armature field and try to regulate while the load of the starter is applied, and also for the brief time after the battery had its voltage pulled low by that load. Why else would they include this circuit?


To ease the load on the starter. Why have the alternator loading down the starter? This circuit could mean the difference between an engine that fails to start and one that does.
 
Originally Posted By: JimPghPA
That would tell the real story about how much peak current we are talking about. These very brief peaks in current are very likely many times more than the average amounts normally talked about, and probably very close to and sometimes exceeds the maximum a diode can handle.


Here's why I think that isn't a problem...

Several years ago, I had a car that wouldn't start, so I push-started it. The battery was not completely dead, but it would not turn the starter.

After it was push-started I took it out on the highway in an attempt to charge the battery a bit. While I was driving it on the highway, it started to rain, so I turned on the headlights. As soon as I turned the headlights on, the engine stumbled, then recovered.

Even at highway speeds, the 130 amp alternator couldn't handle the startup surge of the highlights. Neither, apparently, could the battery. So the system voltage dropped way down, causing the engine to stumble. It recovered, but it seems clear to me that this particular alternator (Ford 3G) is designed so that the battery is used to handle peaks until the regulator can react.

The battery was not taking a charge when all of this was going on.
 
FWIW, I remember reading how a some poster on a different forum swore off jump starts after experiencing alternator failures after jump starts. Not every time but often enough that there was a pattern.

All I know is that the load of recharging a battery and starting an engine is far higher than typical. I'll take Jim's advice.
 
Originally Posted By: brianl703

To ease the load on the starter. Why have the alternator loading down the starter? This circuit could mean the difference between an engine that fails to start and one that does.


I looked at the wiring diagrams for my '97 Crown Vic. The alternator is only powered up through the I terminal when the ignition switch is in the RUN position. It is not powered up in START, and therefore the alternator produces no current when the key is in START. The wiring diagram says that regulator is activated when voltage is applied to the I terminal. No voltage at the I terminal, no output from the alternator.

I believe most cars have similar wiring that prevents the alternator from receiving power to energize it in the start position.

The regulator you mention, which contains circuitry to do the same thing, may be designed for equipment that does not have a typical ignition switch, which has a 12V output only energized in RUN (pushbutton start switch, perhaps).
 
brianl703>an alternator charging a dead battery heats up pretty quickly...and cuts back on the voltage as it does.

The heat isn't what causes that. Alternators are cooled very well. What causes that is that the regulator is designed to have a voltage current curve as a natural result of the circuit design. The regulator puts out more as the voltage drops. If it weren't so then the alternator regulator system would be unstable. A properly designed regulator won't put more than 12.0v across the field which limits the power output of the alternator.

That doesn't matter since alternators from the early years are designed to be full fielded. During full field there is no current limit. No other condition can output more. The output increases as the input increases. I'm sure an alternator could be made to fail spectacularly by full fielding into a capacitor that can't dissipate the power. With a battery and starter in the mix there is a practical limit to the voltage output which limits current output. I know people who jump a lot that place full field switches in a discrete location. As they chat with a nickel in the throttle for high idle they casually drop the field switch to charge then start the dead vehicle. They don't lose alternators and they have way more current capability than a regulated alternator. 18v kicks over the starter really well.

brianl703>coil with a pulse width modulated voltage

PWM regulators came into being when the ECM had to control the field. ECM produce varying pulse width much easier than a varying voltage. Through the years with PWM, varying voltage, and on-off mechanical regulators we didn't lose alternators from running engine while jump starting. Interesting to note that the varying voltage regulators are the only ones that don't full field their alternators.

>There may be a regulator out there that uses an analog voltage to control the field coil(Was it one of your designs?)

They all did up to the late 80s, after mechanical regulators and before PWM regulators. The voltage current curve is why they were just as good as PWM designs.

JimPghPA>All the ones I looked at were Delta. However the field can be either Y or Delta

I've only seen delta stator windings, 3 or 4 phase. The phase count doesn't matter since all fields rectify to the same place. More phases allows more output power without having to buy bigger diodes. The field coils are all DC one phase. Count the brushes. 2 brushes === one phase. More than two brushes I've never seen so I can't imagine what they would do. Besides you probably wouldn't want whatever it is that a 3 phase rotor produces.

JimPghPA>The older GM delco one wire internal regulator alternators use a simple 3 transistor one Zener regulator and it is Linear

Are you sure it wasn't two transistors and a linear IC that resembled the circuit of this Internation Rectifier IVR101 Voltage Regulator. Go look at the equivalent circuit for an Linear Regulator. If they could do it in 3 parts they would, but they can't. It is unlikely that early electronic regulators used 3 transistors because the design would probably have been less reliable than the mechanical regulators which were working fine so there was no need to press a poor design onto the market.

I'll do some math to help you out with that degree.

A starter takes 200 amps at 9 volts for a total power of 1800 watts. An alternator produces 105 amps at 12 volts for a total of 1260 watts. Subtract 30 amps for donor vehicle operation and we still have 900 watts. If I can get another 75 amps at 12 volts from the donor and dead batteries then I haven't exceeded the alternator rating. If the starter draws the donor system voltage below 12 then the alternator can no longer put out it's maximum current at any RPM. There are few jumper cables that can provide the contact area needed to carry this current without an arc fault unless the donor battery is going dead. We *always* run the donor engine to prevent that possibility. One dead car is bad. Two dead cars is really bad. The alternator raises the voltage which reduces the starter current requirement.

An instantaneous current (which doesn't exist) through one of the phases burning out one diode is an area where nature takes care of itself otherwise we'd be burning things out every time we turn on an inductive load. In a 3 phase system only one phase at a time could be at zero output current so any current draw is always coming off at least two phases so the load is shared by two diode sets and most of the time all 3. An alternator running at 60 amps over 3 diode pairs is producing 20 amps per winding and diode pair. Each diode pair is passing 240 watts. So long as the field current stays the same the watts output stays the same. Drop the system with a large load to 9 volts and the current could increase to 240/9 = 26 amps if the regulator is supplying the same field current. The field current would also drop as the voltage drops until the regulator decided to increase it which it would.

Such details become uninteresting when looking at Sanken alternator diode ratings. My 105 amp alternator could not use diodes less than 105/3 35 amp diodes and probably uses the 45 amp diodes. Even with the 35 amp diodes the IFSM (Current Forward Surge Max, peak forward surge current) is 350 amps. That means that a single diode too small for my wimpy alternator can take more than what the entire starter takes if only there was some way for the alternator to produce and supply it through a single diode pair without any help from the batteries.

Point me to some failed diodes on brand new cars or with brand new diode arrays and I'll be impressed. I'm not impressed when a diode array in a used or rebuilt alternator that has spent many years at 150*F loses the weakest one when subjected to the largest draw an alternator will ever see--that is until all your children hit the window up button at the same time your ABS resets. An alternator that fails during a jump start needed to be serviced.

The only way I know to burn out diodes is to hook the battery up backwards. The diode pair that is the least resistance will get upwards of 1000 amps. Your alternator will be your cheapest repair.
 
The reason that the alternator cuts back on the voltage is it heats up is that it's temperature compensated. At least all the ones I've ever seen are. This is done because lead-acid batteries need higher charging voltage at lower temperatures. Now, what the temperature of the regulator (where the temperature sensor is) and the temperature of the battery really have to do with one another I'm not sure (since the alternator can be cooking and the battery still relatively cold), but it apparently works well enough that way.

As far as PWM vs. linear control goes, the advantage to PWM is decreased power dissipation in the transistors--so Ford, at least, went to PWM long before they ever had a PCM controlling the field current. (I remember reading somewhere, or seeing in a wiring diagram, that the regulator on the 3G alternator, which they started using in the early 90s, uses a PWM drive to the field coil).
 
severach: Re your : " A starter takes 200 amps at 9 volts for a total power of 1800 watts."

Who said 1800 Watts is enough to crank a modern vehicles engine?

Starter motors are unique in that if they were built to be continuous run dc motors they would weigh as much as the ICM they are used to start. Starter motors are designed to over heat. By the very nature of the application they must be able to produce at least as much as the energy that the ICM looses to friction at an idle when cold. This is often much more than 1800 Watts. Starter motors are designed to overheat during a normal start. If ran for more than a minute or so they may be damaged.

The number of Amps someone probably measured with a meter, is never going to tell the truth about what is going on here.

What we are talking about here is the very brief peak current. People make the mistake of thinking that what they measure with a meter represents peak current. A meter will totally miss any peak current. They only show average. They are limited by the response time. Even most digital meters with a peak sample and hold have limited response time.

It is the very short time, peak current that we are talking about here that is possibly damaging a diode in an alternator. There are very few digital meters that will measure this. The best way to see it is with a good storage scope.

One of the previous post wrote something about if the load was that great the belt would be squealing, And there was a comment about stalling the engine. These references are thinking in terms of seconds and tens of seconds. The peak current(s) we are talking about are in terms of milliseconds (1/1000s of a second) and until it is measured we do not know, it could even be less than a millisecond. These peaks are measured in milliseconds, sometimes in microseconds. The rotational mass inertia of the armature at an idle might be all that is required to carry the alternator through these very short time intervals. This is not the kind of time that would necessarily result in a notable change in the rpm of the donor vehicle.

We could argue the fine points regarding this until we are blue in the face, or our finger tire of typing. Arguing is not adding the required scientific measurements that will show the true story.

I also realize that disabling the field drive to the alternator during starting might lower the mechanical load on the starter, probably not by much, we are not talking about high rpm's during starting. It also lowers the load on the battery by not using the current to the field. This is another fine point we could go on an on with. PWM vs Linear, might have some effect on the peak current the diodes see, who knows, until that is also measured with a large enough sample group to statically find out?.

Regarding the simple transistor regulators, some used three transistors, some used four. Most were well enough designed to include a forward biased diode in series with the zener so the total voltage reference was stable for a wide range of temperature. By the way, the drive to the armature field was not a regulated voltage. The drive to the field is simply a controlled current, the over all wiring of these linear regulators senses the final voltage produced by the alternator and regulates the drive current to the armature field to maintain the desired output voltage from the alternator. There is a slight filtering to allow for a normal, slight ripple, from the 3 phase full wave bridge. These regulators do not try to regulate out that ripple. It is enough to filter the sense voltage and regulate that.

While it is true that a regulator that regulates the voltage to the armature field would probably be a bad design, I never said that any of my designs regulated in that way. None of them did that. You jumped to that conclusion. Mine sensed the output voltage of the alternator and did the required filtering of the ripple, and regulated to keep the output voltage of the alternator at the desired voltage. And they work just fine thank you.

As for the Y stator field reference, I was even starting to believe that some draftsman preferred drawing Y instead of Delta because I never saw a Y. Then recently one site showed a picture of the Neutral lead for the Y so I guess they do or at least did exist. I never saw a 4 phase but it would be notable, I am sure one could design with as many phases as one wished, same goes for AC power, I have heard of 4 or more phases but never saw any.

As to a means to measure the peak current with a storage scope. A current transformer could be used, but some might question the response, and the derivative effect. One sure true method would be to solder two sense wires, one to each end of the power + cable from the battery to the starter. Then calibrate that cable as a shunt. The same could be done with the wire from the alternator, but one could only do one channel at time because the scope would have to float the ground and that ground could only be tied to one of those shunts. Also note that there would be an initial current draw when the solenoid of the starter was engaged and the real peak would occur when the contact of the solenoid first energized the starter armature. Therefore it would be necessary to adjust the trip of the scope, or use a time delay, to ketch the real peak. If one had hall effect probes that had the correct range and response, they would enable the simultaneous looking at the output current of the alternator and the current to the starter.

Only by thinking about the peak current in the millisecond range and actually measuring what these peak currents are will we ever truly put this question to rest.

A great place to do this would be at some junk yard, and the best thing to measure would be recently brought in vehicles. A recently brought in bad battery or a discharged battery of the correct size could be put into the vehicle to simulate a bad vehicle. This way you would be dealing with a good range of old engines and old starters with a low or bad battery.

Also, a good running junk vehicle could be used as the good vehicle. That way no one would have an alternator damaged on their personal vehicle.



I think brian1703 is correct in:

As far as PWM vs. linear control goes, the advantage to PWM is decreased power dissipation in the transistors--so Ford, at least, went to PWM long before they ever had a PCM controlling the field current. (I remember reading somewhere, or seeing in a wiring diagram, that the regulator on the 3G alternator, which they started using in the early 90s, uses a PWM drive to the field coil).

It makes sense electrically.



prax wrote:

FWIW, I remember reading how a some poster on a different forum swore off jump starts after experiencing alternator failures after jump starts. Not every time but often enough that there was a pattern.

I am not the only one to figure this relationship of cause and effect / jump-start with the engine running have a diode go leaky and discharge the battery the first time you do not use the vehicle for several days. And I've seen it several times.




I will be the first to admit that if you run the engine the bad vehicle will crank better. I saw that done many times before enough friends vehicles had their alternators damaged that we stopped doing it that way.

I just tried to pass on the warning so people know about it. Who knows today with PWM driving the armature field may be it is no longer an issue. Until someone does real test in enough numbers to be statically sure, do you really want to take the chance? I don't, and I will not.
 
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