Alternator & Battery question on 87 GN

Scott A.

Member
Joined
May 24, 2001
Had my first flatbed ride home with the GN last night (not bad after 12 years of ownership). The car died at a stoplight and wouldn't crank back over, like a dead battery. The volt light didn't come on before either.

Got it home late, come home today and the battery is reading 12 volts as-is, but won't start the car. Sat the charger on it for about an hour and got it to start. I then put my Fluke clamp-on ammeter on the lead out of the alternator (with the charger disconnected and engine running) and was reading about 36-40A, with 14.4 volts at the battery. The battery is about (8) months old (Interstate battery). Think I'll take it in to have it tested, but does the 36-40A sound low at idle for a low charge battery? It was slowly decreased current until I turned the car off.

Any suggestions?
 
When you take your battery in to have it tested,you should also have your alternator load tested to see if its still good.It sounds like an alternator problem to me because the longer you run the car,the lower the amps are getting.The heat must be affecting the alternator as it runs.In the 12 years you owned the car did you ever have the alternator replaced or checked?Even if your battery was bad the car should of never turned off since the car should be able to run without a battery.
 
I've never changed the alternator since I've owned the vehicle, but do periodically check it with a voltmeter. I'm puzzled why the idiot light didn't come on, it fires up on the bulb test (key on engine off). I've had alternators die on other vehicles and the light clued me in before I got too far down the road.

Guess another thing I should double check is for good solid grounds. I replaced the battery cables about (10) years ago and, again, never had a problem until now.
 
recheck battery

Your battery may not be taking/holding a full charge. 12v is often not enough to start a car. Fully charged should = at least 12.6v
:)
 
Took the battery to a battery/alternator shop, looks like the battery is good. Shop said it sounds like I lost a diode on the alternator, <40A is not probably not enough to run the car when the battery dies. Gonna take it over tomorrow to confirm, rebuild runs $65. Never had one fail like this before (where it still reads 14.4v).
 
Battery & Alternator tested good

Looks like I'll continue troubleshooting....both tested good.
 
Check the starter. Might be a bad solenoid. Try tapping it with a hammer while someone is cranking the car over with the ign. key.
 
There is a good possibility that the alternator bulb circuit in the dash is intermittent. The connection is nothing more then some Mylar foil stuff.

If it goes open the alternator stops charging and the bulb will not light up. This is a common issue with these cars.

Caspers has a field fix kit to alleviate the problem.

Note that the decreasing amperage you observed while running the car is normal. As the battery charges it draws less current.

RemoveBeforeFlight
 
RmvBfrFlght,

Thank you! That explains what I think happened. I just ordered a kit from Caspers. Friday night while driving, my left turn signal light randomly came on (without me touching it), and I bumped the dash to see if it would go off (it did), guess the bump may have opened up the alternator bulb circuit opened up then (or intermittently). I've known about the dash problems on my car for awhile, guess its time to fix that. Never imagined that the charging control circuit would not function as a result of a dash problem!?!

Wonder how many people bought new alternators & or batteries when this happened to them?
 
I have had a similar problem and threw everything I could think of at my last GN. It turned out to be my ignition switch internally shorting. It is located on top of the steering column. This may not be the exact problem you are having but it does sound similar.
 
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