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Old 11-29-2020, 12:04 PM   #1
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Join Date: Jul 2016
Location: MA/NH
Default idiot lights inop after battery swap

Hello all,
The other day I swapped my group 47 AGM battery for a mini pc680 battery. I also replaced all of the high current battery wires and grounds. First time I go to start the car with new battery, the idiot lights all shine in key position 2 as normal. Engine cranks and starts normal. A couple minutes later I re-start the car and none of the idiot lights light up in key position 2. (except the service and check engine) The speedo is also jumpy when cranking the car. Car starts just the same though.

The alternator is definitely charging. 100 amp denso recently refreshed. does not need a rev to self-excite. I get 12 volts at the exciter wire in key position 2, and grounding this to the block allows the lights to shine as they should.

Right now i'm about to clean up the low current engine grounds and verify all the new alt/starter/ground wires I installed the other day.

Anything else I should check to try to get the dash lights to work as expected? Thanks!!
1991 244 LH2.4 M46 352k miles

Last edited by esmth; 11-29-2020 at 12:09 PM..
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Old 12-02-2020, 03:04 PM   #2
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Join Date: Jun 2002
Location: S NJ, a suburb of Phila.

It is probably time to check or replace some fuses. The ground on the alternator can also cause the excitor circuit to not function so I think you are on the right path.
1982 242 turbo. 338k miles. MVP coilovers and 3" exhaust. Flowed 405 with a V15. Cossie turbine housing with upgraded compressor housing. 90+, IPD remote oil filter. Some other goodness, too. Been lots of fun over 25 years. Restored in 2k. Now ready for a 2nd restoration.

1993 245 Classic, 435k miles, enem V15. IPD bars and chassis braces. Simons sport exhaust from Scandix. sbabbs ezk chip. Been a good road warrior. Genuine Volvo rebuilt leaky M47.
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Old 12-02-2020, 05:52 PM   #3
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Originally Posted by esmth View Post
... I get 12 volts at the exciter wire in key position 2, and grounding this to the block allows the lights to shine as they should. ...
How are you reading the 12V at the exciter wire? Are you pulling it off of the alternator and sticking your meter probe in the end of it? Or are you measuring it in-circuit, backprobing the connector?

I suspect the former. If grounding it gives you the lights back, and with it reconnected, the warning lights fail lamp test but remain out when the alternator is charging, you have a problem at the alternator. The ground wire seems a likely suspect as Dave mentions.

Backprobing the exciter wire should yield about a volt and a half key on engine stalled, and 13.6 or more with engine running.
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Old 03-22-2021, 08:51 AM   #4
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Location: MA/NH

Late reply, but thank you both for your input.

honestly I forgot how I had probed it, probably as you predicted. as the alternator itself charged I just let it be for a bit. Yesterday I decided to dig more and it turns out the regulator on the alternator was bad. It 'worked' as in it regulated the alternator output voltage correctly, but it would not sink current from the cluster through the exciter circuit to light up the cluster lights. I bought a replacement regulator from maniac electric motors and all seems well. The cluster lights work and I get 14.2v cold and 13.8v warm, just like the old regulator but it now sinks the current correctly. very odd!
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Old 03-22-2021, 02:16 PM   #5
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Location: Portland IN

If you forgot to connect the battery idiot light wire to the alternator most of idiot lights wont work. There are at least 4 of the idiot lights on my 88 765 connected to that circuit.
Trying to understand stupid people is like trying to pick up a turd by the clean end.
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