Battery discharging every 48 hours.

[Mustang_Cougar]

About two weeks ago the battery (the alternator, I pulled that and replaced it w/a Borg Warner; two days later, battery is again dead.

At first I thought I had a parasitic drain somewhere, but before pulling fuses to go that route, I decided to check the voltage at the battery while the car is running (duh - something I should have done far earlier). Anyway, after charging the battery it reads ~12.43v; w/the car running, it reads ~12.33v. Now I'm no wizard w/electrical (and pretty amateur w/a multimeter), but shouldn't the battery be showing north of 14v when the car is running? And if that's the case, it would appear that the car is not charging the battery under load.

Any tips on where I should start looking? I'm worried that I have two separate issues: The charging system isn't working and I have a parasitic draw. If I can't flush this out soon, I'm going to have to take it to a garage...and I wince at the cost of doing an electrical diagnostic.

Thanks for any help.

M_C

 

[Quest]

check for parasitic current draw.

Q.

 

[Donald]

First you need to use an AC charger to charge the battery fully, with one battery cable disconnected and let it sit for a day and measure the voltage, then another day latter. Forget the voltage right after its charged its a surface charge and you need to wait 1 day to get an accurate reading. The voltage should not drop at all over a few days with the battery cable removed.

Then reattach the cable at night, so you can see any spark (or hear). Except for a clock maybe, should be almost nothing.

Then to your charging. Before you measure the voltage kick up the RPM to 2000 for a few seconds. Then measure the voltage at 1200 RPM.

Curious question, did they or you do a retrofit for the dangerous drop-in gas tank that Lee I. came up with.

 

[brianl703]

Is your alternator warning light working?

 

[HerrStig]

Trunk light stuck on? Put your meter on the higest amp scale and measure the current flow between the negative terminal and ground with everything off. You should see zero , (working your way down to the lowest amp scale) unless your gas gauge is one of those on all the time.

 

[friendly_jacek]

Originally Posted By: Mustang_Cougar
Anyway, after charging the battery it reads ~12.43v; w/the car running, it reads ~12.33v. Now I'm no wizard w/electrical (and pretty amateur w/a multimeter), but shouldn't the battery be showing north of 14v when the car is running? And if that's the case, it would appear that the car is not charging the battery under load.


Your conclusions are correct, it should be about 14V with car running.

 

[oggb]

Get someone to get in and give the car some gas and check the voltage. If it's below 13.5 you have a problem with the alternator/voltage regulator/etc.

 

[Mustang_Cougar]

Thanks for the suggestions, Guys.

Dan: I've done everything you've suggested, no luck. Also: Yes, it still has the unbanded, tank-on-the-bumper...albeit, a new one. As the car is rarely driven, I've sort of accepted the "Pinto risk."

As far as parasitic draws, this year Mustang has very few creature comforts and idiot lights. No trunk light, no dome light, no clock, gas gauge only works w/ignition "run," no alternator light. I've eliminated floor board and glove box lighting, along w/radio (OEM-style speaker).

M_C

 

[Garak]

Originally Posted By: brianl703
Is your alternator warning light working?


Yep, check this. On two separate Fords I've had the alternator go south with no warning from the dead warning light, which I promptly replaced with voltmeters in both vehicles.

 

[Eddie]

tHE ALTERNATOR SHOULD BE CHARGING THE BATTERY AT ~ 14.2-14.4 VOLTS AT ~ 1,500 RPM. [censored] caps lock. Start checking for bad cables and especially battery grounds before spending any more dollars.

 

[TFB1]

Originally Posted By: Eddie
tHE ALTERNATOR SHOULD BE CHARGING THE BATTERY AT ~ 14.2-14.4 VOLTS AT ~ 1,500 RPM. [censored] caps lock. Start checking for bad cables and especially battery grounds before spending any more dollars.


Agreed... Unless battery was almost totally dead, running at 2K RPMs voltage should be over 14v, if not alt isn't charging... Basically the running voltage being less than batt voltage, pretty much verifies that...

If the battery cables look to be in good shape and the engine cranks strongly, they aren't your problem... My guess is maybe a bad ground on the regulator, or maybe the alt excite circuit through the warning light is open...

 

[djb]

Originally Posted By: Mustang_Cougar

As far as parasitic draws, this year Mustang has very few creature comforts and idiot lights. No ... no alternator light.


The alternator light isn't a comfort/convenience light. It's an essential part of the alternator circuit. It powers the rotor magnetic field during start-up. Once alternator is generating power on its own, it self-powers the rotor coil resulting in no current flowing through the light.

It is cleverly used as a warning light, but unlike other warning lights the alternator will not work if it's burnt out. Starting in the 1980s it became common to have a resistor in parallel with the lamp to a no-charge condition just because the lamp burned out.

 

[Dave Sherman]

Originally Posted By: djb
Originally Posted By: Mustang_Cougar

As far as parasitic draws, this year Mustang has very few creature comforts and idiot lights. No ... no alternator light.


The alternator light isn't a comfort/convenience light. It's an essential part of the alternator circuit. It powers the rotor magnetic field during start-up. Once alternator is generating power on its own, it self-powers the rotor coil resulting in no current flowing through the light.

It is cleverly used as a warning light, but unlike other warning lights the alternator will not work if it's burnt out. Starting in the 1980s it became common to have a resistor in parallel with the lamp to a no-charge condition just because the lamp burned out.



X2, older alternators depended on the warning light, for the reason djb describes. The alternator in my MG is not too different from other alternators, and it won't work without the warning light to get the regulator circuit started.

I'd look for anything resembling a "GEN" or "ALT" warning in the dash. Alternators of that vintage also tended to make the warning light glow and fade out as the engine was coasting to a stop after turning the key off. At least, my MG does and a friend's '72 Chevelle did.

 

[Warstud]

Hood lights, trunk lights, headlights or glove box lights that do not turn off when the door is closed will drain the battery. Relay switches that are stuck in the "on" position can also cause a battery to drain. Does it have an alarm? You may also want to invest in a battery tender.

 

[friendly_jacek]

Interesting info re: burned out lamp, learned something new.

 

[TFB1]

In '67 Ford was already using a alt shunt resistor, but I dunno about '66...

 

[meep]

wow--- the old relay-based regulator. Now THAT brings back some memories!!! amazed that those mechanical devices lasted as long as they did, while vibrating those contacts all day long.

agreed--- charging system inop.

while running, test for voltage at field terminals on alt (white to ground). it will vary between 2-12.... at idle, especially w headlamps on, field voltage will approach 9+ in a vehicle like that with a small alt. it might actually be at a full 12 until it gets some rpms.

 

[Bamaro]

Sounds to me like an internal short

 

[Donald]

The 1964 1/2 Mustang had a generator and they went to an alternator in 1965. I do not remember my 289 Mustang having an external voltage regulator, but it was awhile ago.

 

[Donald]

I am a fan of local rebuilders. Bring in the suspect alternator and voltage regulator and see what he has to say. They typically can do these kind of things with their eyes closed. And they do not often get to work on vintage ones like these.