alternator problem?

[terryb]

My dad's 1988 Carrera died on the freeway the other day and a Shell service station tested the battery which turned out to have a bad cell. I got a replacement and put it in there on the side of the freeway and the car started right up and drove fine. Problem solved, right?

The next day my dad drove approximately 50 miles on the brand new battery and the same exact thing happened--dead battery--and was put on a flatbed to his home.

So today we started running some tests:

- No alternator warning lamp with the key in the On position
- Took the temp/oil gauge out and tested voltage at the alternator lamp bulb socket:
At rest: 8.73V
With key in On position: -3.21V

- Battery:
At rest: 12.09V (after being on a 6A charger for ~5 hrs
At idle: 11.85V
At 2500 RPM: 11.85V

Are there other tests to run to diagnose? Or is the only likely culprit the alternator?

Thanks for any guidance.

[terryb]

Ooops, forgot to mention that we tested continuity through the alternator warning bulb itself and it passes current, so it doesn't seem like it's shorted out.

[dshepp806]

Sounds like the alt/reg to me.....is it original? If so, it's time!!

Best,

Doyle

[tdatk]

You didn't mention that you checked the fan belt....right? It sounds like you are pretty through but I had to throw that out there.

[terryb]

dshepp806, I believe the alternator is original but the car only has ~60k miles on it. I'm also leaning toward the alternator/voltage regulator but I wanted to see if there are other diagnostic checks that might point to another (easier to remove and replace!) culprit. We pulled the alternator on my car and I'd like to avoid doing it on my dad's car unnecessarily.

tdatk, yes, we checked the belt and it's still on there. And thank you for not assuming I know what I'm doing. Please feel free to throw out other "obvious" checks to make, I can use all the help I can get!

We cleaned the ground connection at the intake manifold, which looked fine. We also inspected the ground connection at the battery and it also looks good. I couldn't get under the car enough to see the tranny ground strap very well.

We also got a replacement bulb to try in the alternator warning lamp and it didn't light up with the key On either. The thing that seems weird to me is with the key in the On position, the voltage goes negative at the alternator warning bulb socket.

[stevej37]

Voltage too low when running...alternator due to be replaced...especially if it's the original one.

[wswilburn]

The alternator light is actually in the circuit that powers the alternator. If it is burned out, it will not charge. It should come on with key in 'On' position before starting. Try changing the bulb.

[terryb]

wswilburn. Thanks for the suggestion. We did try a replacement bulb and it didn't light up either with the key in the On position.

I'm curious, does anyone know what specifically would cause the voltage to the alternator warning lamp to go negative (-3.27V) when the key is in the On position? Burned out diodes? Bad voltage regulator?

It's looking like an alternator removal is in my future.

Thanks,

[wswilburn]

Terry,

Looking at the diagram in Bentley, there are a few possibilities. Here is what I would do.

Basically, if the alternator light is good, it either is not getting voltage from the battery, or no ground through the alternator.

1. With ignition on, check voltage on RED/BLK wire at alternator light. Should be 12V with respect to ground. If no voltage:
- check fuse 8.
- check voltage at fuse 8

2. Check resistance from BLU/RED wire at alternator light to ground. Not sure what this should be, but would guess a few ohms. Open is definitely a problem. If open:
- Check resistance from alternator terminal where BLU wire connects (D+) to ground. If open, alternator is bad.
- Check resistance between terminals where BLU/RED and BLU wires connect to anti-theft control unit. Should not be open. If so, could be loose connection or problem with anti-theft (make sure it is turned off).

[rick-l]

Originally Posted by terryb

dshepp806, I believe the alternator is original but the car only has ~60k miles on it.

At 55,000 miles the brushes in my 88 were worn to the point of intermittent contact.

A new regulator comes with new brushes and will fix it but on my car the slip rings had 1/8th inch grooves dug in them and I had problems again after a year that a rebuilt took care of.

[terryb]

wswillburn, thank you for spending time on your Sunday evening looking through Bentley electrical diagrams! We can do those tests and report back.

rick-l, wow, 55k miles, thanks for the heads up. Sounds like if it's gotta come out it's best to get it rebuilt.

[jlex]

I haven't changed out the alternator in my '85... has only about 50,000 miles on it though.
Has always puzzled me why Porsche didn't put a gauge on the dash showing the charge. In order to keep an eye on it, I bought one of those digital gauges that plugs into the cigarette lighter so I don't get stranded. Works pretty well.

http://www.cetsolar.com/voltmeter.htm

regards,
jlex.

[TroyGT]

If you don't see anything obvious, then just pull the alternator and have it checked out. I was recently having charging issues and pulled mine out and had it tested... cost me $10 bucks to find out that the alternator was working perfectly on the bench. Turns out that I had reconnected mine incorrectly after previously dropping the engine.

-Troy

[dshepp806]

Alt test....

DS

[47silver]

+ for the alternator

also after going through 2 bad rebuilds, a third one of the wrong size, i recommend that you get the one in the car rebuilt. It will cost +200 or so and take longer than an exchange but the rebuilts are inferior to the originals due to the diodes that are used. The orignals use pressed in diodes for the main diodes...rebuilts use loose diodes that suffer from the heat... At least this is what the alternator rebuild guy told me.

[dshepp806]

it would be cool to get some specs for the "various" diode packs used........

Best,

Doyle

[terryb]

We pulled the alternator yesterday and externally it looks fine but after removing the voltage regulator we noticed a whole bunch of what looks like brass sawdust/shavings clumped up inside. Also, the windings were black, not the nice copper color we were expecting. That doesn't seem good.

It's at an alternator/starter shop getting tested. Will hopefully hear back tomorrow and post the diagnosis.

Thank you all for the kind assistance!

[terryb]

Okay, to (hopefully) close out this thread--alternator repair shop reported that it was severely shot and rebuilding would be more than half the cost of a new one. So my dad ordered a new one that's apparently an exact replacement from an OEM supplier.