home register FAQ memberlist calendar

Go Back   Turbobricks Forums > Mechanical > maintenance & nonperformance

Thread Tools Display Modes
Old 11-29-2020, 12:04 PM   #1
Board Member
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 341k miles

Last edited by esmth; 11-29-2020 at 12:09 PM..
esmth is offline   Reply With Quote
Old 12-02-2020, 03:04 PM   #2
Happy playing the blues
dl242gt's Avatar
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, 430k 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.
dl242gt is offline   Reply With Quote
Old 12-02-2020, 05:52 PM   #3
Board Member
cleanflametrap's Avatar
Join Date: Sep 2009
Location: near baltimore

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.
cleanflametrap is offline   Reply With Quote

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

All times are GMT -4. The time now is 10:35 PM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.