Odd GM (CS130) alternator issue

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Apr 8, 2012
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What would cause an alternator to become a *load* on the electrical system at idle?

Some backstory:
When I bought my Lumina at the end of 2019, I also purchased a 220A alternator for it. On a cold start, and at or above 1500 RPM it appears to work fine. Once everything warms up, at idle I see my voltage drop below battery voltage. I've seen it drop as low as 10V, and unplugging the terminal from the field winding gives me batter voltage as expected, the alternator is becoming an inductive load at idle when hot.

Voltages measured as follows:
  • ALT + to case
  • ALT + to BATT -
  • ALT + to engine block
  • ALT + to body
  • BATT + to BATT -
  • As reported by PCM (ignition voltage)
  • As reported by my OBD-II scanner (voltage at DLC)
  • As reported by my stereo
All of these voltages are within 0.2V of each other.

I sent it back once and it was returned to me with a new pulley and a suggestion to replace my belt and tensiopner (both of which were brand new). I didn't bother with this, as a slipping belt would result in reduced voltage no lower than battery voltage -- and because the parts were new, and the stock alternator did not (and to this day does not) exhibit this issue. I got them to agree to an RMA but they wanted me to pay $50 in shipping on the *chance* that they'd accept it for a refund, they refused to guarantee a refund on receipt. So, I threw the stock alt back in and boxed up the 220A to come back to at a later date.

That later date was yesterday.

Suspecting perhaps a failing voltage regulator, I swapped to one from the stock alternator before driving home from the dealership I work at. Same result, after about 15 miles of driving I had 0.2V less than battery voltage at idle, 14.2V above 1500 RPM. Pulling the field terminal gives battery voltage.

Both when I originally installed it and yesterday, I put a scope across the terminals to check for presence of AC current (one or more failed diodes) and found a clean DC signal with only a couple milivolts of ripple, probably one of the cleanest outputs I've seen from an alternator. It's not bad diodes or a failed winding. Given that failed diodes and windings don't magically fix themselves under load, I expected to see what I measured, but it's always best to verify.

It's not a bad voltage regulator, as I've got the stock alternator back in now with no issues, with the voltage regulator that was originally on the 220A unit; both voltage regulators are now known to be good.

At this point, I'm planning to take it to a local rebuilder and have them work out and fix whatever is wrong with the thing, but I figured I'd check here to see if anyone's familiar with GM CS130 alternators and might have a simple fix. My in-depth training is MOPAR-centric and I've already looked over all of the generic stuff.

All I can think is that it's the LED I've got in the idiot light, but I would expect that to be a problem for the stock CS130 as well, if that were the issue. I'm willing to further investigate this as a potential cause, as I do know the alternator is expecting a certain amount of current to pass through it and into the field winding. I started down this road, but everything I'm reading on the subject is conflicting as to whether an LED would be passing too much or too little current, thus whether I would need to add a resistor in series or in parallel. I guess I could drop a 194 bulb back in there to test, but I'd really rather not have to take the dash apart twice if that doesn't help. I guess this is another question I would need a GM guy to answer.

Also potentially of note, the VOLTS light does illuminate when the engine is not running and when I run the engine with the belt removed, it is functioning as intended. It does *not* come on when the output of this alternator drops below battery voltage.

Any help I can get is appreciated.
 
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The bulb is part of the circuit, I believe a missing or burned out bulb will result in the alternator not charging. The LED might be your issue, worth a shot - the price is right! Why it's only impacting the one - 🤷‍♂️
 

TuBon_gRips

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The bulb is part of the circuit, I believe a missing or burned out bulb will result in the alternator not charging. The LED might be your issue, worth a shot - the price is right! Why it's only impacting the one - 🤷‍♂️

Thank you for the reply. In the meantime, I ruled this out, actually, and gathered some more useful information.

I have a few spare GM 4 pin connectors, one of which happened to be keyed correctly. Using one of these connectors, I made a pigtail with a 194 bulb soldered inline and connected one end to the battery positive.

I swapped the 220A alternator back in (I've done it enough times I've got it down to a 15min job by now) and idling it until it dropped to 12.0V. Then, I unplugged the 4 pin connector (in which only the L terminal is populated) and plugged in my pigtail, thereby replacing the LED with the 194 bulb intended for the application. Same result.

So, issue is internal to the alternator. This time around, though, since I was in my driveway and not my noisy shop, I was able to notice a whine with the connector plugged in, which goes away when the connector is unplugged (field winding not powered, alternator unloaded). That does seem to indicate a shorted winding or diode, but measuring AC output of each winding to ground (alternator body) shows identical voltage on each and measuring DC output of each diode shows the same. Plus, all goes good at around 1500 RPM, which shouldn't happen with a short, but might happen with a cracked solder joint -- but that wouldn't whine, if my understanding is correct. At least, that's the case on that I work on daily. To my "finely tuned" ear, the whine appears to be about a 33% duty cycle and coincides with engine speed, which does seem to point to a winding... it's just not behaving like I would expect.

And I can't condemn the voltage regulator, as it's the known good regulator from the stock alternator. I know this thing isn't killing VMRs, as the one that was originally on it is now on the stock alternator and working without issue.

In honesty, this is really the first time I've tried to dig into *why* an alternator was bad. Usually I'd just replace the dang thing and be done with it, but this isn't an OEM equivalent, as stated in my original post.

At any rate, I'll be reinstalling the stock unit once things cool down and probably calling that rebuilder tomorrow. I clearly have an internal fault in a winding somehwere and neither my garage nor my shop are equipped to re-wind the dang thing.
 

TuBon_gRips

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Also of note, AC/ripple on the DC output is measuring between 40 and 90mV currently. According to service data, 300mV is allowable. I believe this rules out the diode stack.

I also just looked up the wiring diagram for this vehicle (which I should have done from the start and saved myself a lot of trouble). The bulb is, in fact, not a part of the circuit as I (and dishdude) believed. The bulb is controlled purely by the PCM and the alternator is sent a PWM signal on the L terminal to control its output. This is further borne out by the ability to illuminate the VOLTS lamp via OBD-II, another fact which should have popped into my head sooner, as I have performed that very action in the past.

1629682971750.jpg

1629683023014.jpg

1629683378518.jpg
 
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I have a few customers into competition audio, A single CS130 can't put out 220amps for very long & likely can't put out 150amps for very long either.

A CS144 is a much better alternator with a much bigger Stator, And you will have much better luck having a High Output 200 or 250 Amp Stator for one of these actually outputting close to that.....Though 200amps is about the sweet spot when used with TWO Rectifier Bridges.

*When the rectifier heats up the diodes won't pass as much amperage through them, therefore, the amperage output drops, Double rectifiers means you have twice the amperage carrying capacity and double the cooling surface area to keep the diodes cool.

One of my customers has a 2001 Tahoe that will literally shake the ground, Had my Alternator guy build TWO 200amp CS144's with Dual Rectifiers......That's 4 Rectifiers & 24 Diodes.
 

TuBon_gRips

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I have a few customers into competition audio, A single CS130 can't put out 220amps for very long & likely can't put out 150amps for very long either.

A CS144 is a much better alternator with a much bigger Stator, And you will have much better luck having a High Output 200 or 250 Amp Stator for one of these actually outputting close to that.....Though 200amps is about the sweet spot when used with TWO Rectifier Bridges.

*When the rectifier heats up the diodes won't pass as much amperage through them, therefore, the amperage output drops, Double rectifiers means you have twice the amperage carrying capacity and double the cooling surface area to keep the diodes cool.

One of my customers has a 2001 Tahoe that will literally shake the ground, Had my Alternator guy build TWO 200amp CS144's with Dual Rectifiers......That's 4 Rectifiers & 24 Diodes.

I absolutely get what you're saying and I would be right there with you if not for the fact that the stock alternator doesn't exhibit this issue with the current load and I haven't added what will eventually be my extra loads, so they can't be a contributing factor at this point.

Certainly good information and the reminder is absolutely appreciated. The loads I intend to run will be short term use, a 400 watt inverter (35A draw at full load, fused at 40) and a jump box which will charge at 10A max. Truth be told, I could run this off of the stock alternator if not for the modest sound system (35A total by fuse ratings) I have installed. I would just hook up the charger for the jump box and call it good, except that I've needed (and not had available) the inverter a number of times over the past year and a half, and haven't needed the jump box once.

Interestingly, GM released a TSB stating that the behavior I am seeing with this high output alternator is typica (to a lesser degree)l, even of an unmodified unit -- minus the heat factor and the fact that I see a voltage increase (rather than decrease) by disconnecting the control wire or (as I tested after my last post) commanding the alternator off when it is acting up. As described in the TSB, an affected alternator should exhibit the behavior even from a cold start and that is just not the case here.

I'm not sure a CS144 is an option for my application, unless I'm missing something. The 144's I've seen have all had this style of mount:
1629694829221.png


That's a 130D (left, and actually what I have) and a 144 (right), note the wide mounting lugs. The 130D comes in a few different cases, the one above, one with a small ear as the upper mount and a lateral screw as the lower mount:
1629694980405.png


And one with a small ear as the upper mount, and a pair of smaller ears as the lower mount, which is what my Lumina calls for:
1629695093008.png


If it turns out nothing can be done at reasonable cost to make this CS130D function as expected, a CS144 would be an option, if they're available in a case that mounts like the 3rd photo. I have looked, briefly, to no avail. I'd ask my parts guys but, if I'm being honest, they have enough trouble getting me the correct parts for our own brands.

For now, the stock alternator is back in and I'm giving a local rebuilder a call tomorrow.
 
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I've not been inside a cs130.

Can you essily test for continuity between the 3 stator leads?


I suspect a shorted stator.

Often higher amp rated alternators in same case size, sacrifice low rpm output, and the lower rated one can output more amperage in low rpm around town granny style driving.

A dc amp clamp is enlightening, as voltage alone can be misleading
 

TuBon_gRips

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I've not been inside a cs130.

Can you essily test for continuity between the 3 stator leads?


I suspect a shorted stator.

Often higher amp rated alternators in same case size, sacrifice low rpm output, and the lower rated one can output more amperage in low rpm around town granny style driving.

A dc amp clamp is enlightening, as voltage alone can be misleading
The diodes are on the back, outside the case. The stator windings poke through the case to connect. Measurements are taken from there. Everything checks out when cold, can't really properly check in vehicle due to location (and the presence of a plastic cover which prevents everything shorting out against the power steering lines that right right across it). There's a notch cut in the cover to allow access to a pair of test points, one connected to the diode outputs and the other connected to the case, but those test points are not present on the diode stack on the upgraded alternator. As such, I can only test it at temperatures where I can safely handle it, which means I can only test it outside the temperature range where it exhibits this condition. Thus why I am taking it to a rebuilder who will have the equipment to test and certify it for me. If they say it checks out, I'll probably just run it as-is; I have a hunch it's not gonna go that way, given that its output is what I expect until it's been run for 15-20min.

I do realize that higher amp alternators can sacrifice low RPM output and I would assume that's what I was seeing if I didn't start with 14.5 at a cold start, even after letting the idle come down to 750. I would expect 13.3 or thereabouts when heat-soaked, I would accept battery voltage. 10 volts is a problem, that's the alternator becoming a load, not sacrificing output. Disconnecting the control wire results in a voltage *increase*; that's not right.
 
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I have a 'lifetime warranty' 50/120 amp chrysler externally regulated alternator.
Used that warranty many a time since 2004.

The most recent rebuild failed a while back, and the warranty provider magically claimed to have no record of me every having bought an alternator from them.

I moved on, purchasing New, shelving the old, and when I later tinkered with it, I found the brushes well out of alignment with slip rings.

In the course of adding some spit and polish to the parallel AC stator to diode straps, and attempting to increase thermal conductivity of rectifiers to casing, the three stator output leads were tweaked.

When i reassembled it and spun it, I heard interference.

I found one stator lead had become loose and was touching the rotor, and it had done this before my ownership of it, and the rebuilders simply glued it back.

I prepped it and JB welded it and it has been working better than ever for a year.

I just tested the 'new' er alternator to its limits with a healthy well depleted TPPL AGM with a new tinned arine 2awg circuit between it and battery.

Max field current sent to rotor, that I noticed, from external voltage regulator, was 5.4 amps at just below battery voltage, at idle and ~ 75 amps output. Higher rpms spiked voltage briefly, then both voltage and amperage fell while output amperage was over 100.

Interested to see what your rebuilders find wrong with it.

What does your battery voltage fall to when cranking engine?

I wonder what max field current for a cs 130 is at battery voltage.

My engine requires 8.2 amps to run ignition and fuel pump at idle and 12.2 amps at 2K rpm, though this does not include the field current sent to rotor.

i can see more modern vehicles requiring significantly more and battery voltage falling faster lower when teh alternator is not functioning properly, and a weak battery dropping to 10v making it seem there is a much larger load than actually present on it.

So if battery is dropping below 9 while cranking the engine, I would consider it weak and if below 8 then extremely weak and due for replacement.

When it falls to the ~ 7.75 volt range, is when the dreaded click click happens.

A fast sampling voltmeter on the battery terminals during engine cranking is a great load test, comparable to itself, but do note engine and battery temperatures when comparing at extremes.

Do consider your battery, or the cable terminations at battery or alternator, could possibly be a variable in your issues, but especially if you ever saw cauliflower or brocolli forming at battery post clamps in the past.
 

TuBon_gRips

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Messages
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I have a 'lifetime warranty' 50/120 amp chrysler externally regulated alternator.
Used that warranty many a time since 2004.

The most recent rebuild failed a while back, and the warranty provider magically claimed to have no record of me every having bought an alternator from them.

I moved on, purchasing New, shelving the old, and when I later tinkered with it, I found the brushes well out of alignment with slip rings.

In the course of adding some spit and polish to the parallel AC stator to diode straps, and attempting to increase thermal conductivity of rectifiers to casing, the three stator output leads were tweaked.

When i reassembled it and spun it, I heard interference.

I found one stator lead had become loose and was touching the rotor, and it had done this before my ownership of it, and the rebuilders simply glued it back.

I prepped it and JB welded it and it has been working better than ever for a year.

I just tested the 'new' er alternator to its limits with a healthy well depleted TPPL AGM with a new tinned arine 2awg circuit between it and battery.

Max field current sent to rotor, that I noticed, from external voltage regulator, was 5.4 amps at just below battery voltage, at idle and ~ 75 amps output. Higher rpms spiked voltage briefly, then both voltage and amperage fell while output amperage was over 100.

Interested to see what your rebuilders find wrong with it.

What does your battery voltage fall to when cranking engine?

I wonder what max field current for a cs 130 is at battery voltage.

My engine requires 8.2 amps to run ignition and fuel pump at idle and 12.2 amps at 2K rpm, though this does not include the field current sent to rotor.

i can see more modern vehicles requiring significantly more and battery voltage falling faster lower when teh alternator is not functioning properly, and a weak battery dropping to 10v making it seem there is a much larger load than actually present on it.

So if battery is dropping below 9 while cranking the engine, I would consider it weak and if below 8 then extremely weak and due for replacement.

When it falls to the ~ 7.75 volt range, is when the dreaded click click happens.

A fast sampling voltmeter on the battery terminals during engine cranking is a great load test, comparable to itself, but do note engine and battery temperatures when comparing at extremes.

Do consider your battery, or the cable terminations at battery or alternator, could possibly be a variable in your issues, but especially if you ever saw cauliflower or brocolli forming at battery post clamps in the past.
Why use a fast sampling voltmeter when I have a Midtronics charging system tester at the shop. I am, after all, a dealership mechanic. At last test, I was above 11 volts during cranking. I was honestly surprised by that, as it's the no-name battery that was in the car when I bought it, but that's what I've got.

You seem to be implying that 10V is battery voltage with the alternator cutting out. That implies, to me, that you missed the multiple times where I mentioned that disconnecting the control wire from the alternator (e.g. shutting the alternator down) results in a *return* to battery voltage, anywhere between 12.0 and 12.3 volts depending on battery state of charge (e.g. how long I let the failed alternator draw from the electrical system before disconnecting it).

That said, the rebuilder I took it to today did not have a tester capable of running it at 750 RPM. It's fine above 1500 and his tester ran it at 2000, so no issues were revealed other than the stator only being capable of 150A (based on visual inspection), rather than the 220A it was sold as being capable of. His tester was only capable of testing up to 110A and it put that out without dropping below 14 volts.

The lady at the counter, who was very knowledgeable, referred me to another rebuilder who she knows has better equipment for testing idle performance. I may visit them next week, or I may simply scrap this thing and buy from one of the two local suppliers she referred me to. I'd have had them rebuild it with a new stator but she told me the best they can get for a CS130D from their suppliers is only capable of 160A.

I did learn, however, that a CS144 will in no way mount up to my engine without replacing the bracket. I would have to have something custom fabricated, which is really more than I'm caring to put into it at this point. Both the rebuilder and the lady at the counter did confirm what 35 years of electronics experience already told me, though: While I *should* upgrade my power wire as a matter of course when installing a higher output alternator, not doing so won't have any ill effect until I try to draw more current than the stock alternator was capable of providing in the first place. In other words, until I actually hook up the additional loads, it's not necessary; and I already knew that and have 2ga power and grounds already made and ready to go in. Working space at the back of the alt is tight, I would rather not have to stick my hand back there with a fatter wire multiple times to keep swapping the alternator until I get one that works, thus why I am waiting to install it.

Eventually, this car is getting gutted and rebuilt. I might consider putting that effort into it then, but I'm not quite there yet. When I get there, I know where I can get a CS144 capable of 400A if I wanted it. I may decide it's easier to put a red top in the trunk and run my excess loads from that until then, isolated from the alternator when in use.
 
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