87 SIII VDP with 82K on the clock. Alternator light is staying ON after the engine is running. I've read the service manual and searched here and jag-lovers for possible causes. First thing I did was test for voltage at the warning light terminal on the back of the alternator (what a GOOD time I had trying to get my hand behind the alternator to attach a test lead...). I tested twice with the engine running (at idle) and the voltage was 18 - 19 volts. Based on my understanding of the operation of the warning light circuit, that would cause the light to stay on since there is a voltage difference between the ignition side of the warning light (12 volts) and the alternator side of the warning light.

I was surprised to see voltage greater than 12-14 volts. I suspect the 18 - 19 volts is indicative of a bad diode pack?

Would you all agree that the alternator should be replaced?

By the way, I did check voltage at the battery while the car is running and I get about 13.8 volts. That makes me think the alternator is charging properly. It just appears to be a problem with the warning light circuit.

Thanks

 

I'm a bit flummoxed by the 18v at the exciter wire but the mulling-it-over part of my brain is already gone to bed for the evening.

The more common symptom of a diode failure is the light staying on after the key is turned off...but I reckon that isn't the only failiure mode.

Yes, I'd replace the alternator...or have it repaired locally.

Removal is a little tricky. I think I posted some hints and tips a few months ago. If a search doesn't yield anything shout back

Cheers
DD

 

Thanks Doug.

I was perplexed by the 18 -19 volt reading. I suppose it's possible for an alternator to put out that kind of voltage?

I have read a couple threads on removal. Seems the key is removal of the anti sway bar and cooling system plumbing in the vicinity. Looks like a fun job.....

Final question: What brand alternator should I buy? I normally get parts from Autohausaz. They've got no less than 6 Bosch alternators listed? What about NAPA?

Thanks again.

 

My experience with rebuilt *anything* has been very iffy the last few years. I, and others I know, have had multiple failures of rebuilt units regardless of brand name.

What I've been doing, and what I suggest you do, is take the alternator (or starter) to a local rebuilder. Most any town of medium size or larger has one. If Longview Washington has one, I'm sure Ft. Worth does. Local business, local reputation to protect, actual people to talk to. I like when the guy who is actually doing the work talks to me.

I've had MUCH better results taking this route.....nary a single problem....even the the cost is usually higher ($120-$160 versus $75-$100 for a mail order/internet alternator, for example).

Cheers
DD

 

Thanks Doug. I will go the local rebuilder route....

 

I finally got the alternator out of the car. Holy cow, what a job. Should be real fun re-installing.

I've been spoiled working on BMWs. This Jag doesn't go quietly.... Even flushing the brake fluid was difficult. Getting to the rear caliper bleed screws was not easy, much less getting enough leverage to crack 'em open.

I should get the alternator to a local rebuilder next week....

Thanks.

 

The alternator can be removed in less than a half hour with a lift and air tools. I just displace the lower trans cooler tube a little and pull the 9/16 stabilizer link nuts to rotate the bar out of the way. The alt falls out of the bottom.

I had a customer call me to recharge the A/C after he replaced the alt and I asked why he removed the compressor. He told me the alt would not fit out the bottom.
I lifted his car in the air and removed the 2 cooler tube 3/8 bolts and the stabilizer bar link nuts in 5 minutes. I showed him the huge opening in the area for the alt to 'fall out'.
I told him I usually charge about an hour labor to R&R the alt.
He was visibly upset about the 4 hours he spent in his driveway wasting his time and Freon pulling the air pump and compressor.

Also there is no need to disconnect the battery. Just CAREFULLY remove the 5/16 BS nut from the bulkhead power post to isolate the alt wires from the system.
You can also check the voltage/amperage at the same connection so you don't need to crawl under the car or reach to the back of the alt.

bob gauff

 

I installed the rebuilt alternator and all is working well. No more alt warning light! Also replaced the anti-sway bar bushings, which I'm sure were the original bushings.

For those in the Fort Worth, TX area, I used Alternator Service, Inc. It's a small shop just north of the city. Chris (the owner, I think) is a nice guy. They diagnosed the problem on the spot and had the alternator fixed the next day. Cost was $75 plus tax. Good deal as compared to buying a rebuilt unit from a parts source.

Their address:
Alternator Service, Inc.
3600 N Main St
Fort Worth, TX 76106
817-626-2990