This is on a 1990 legacy.

 

Lately I've been having a problem I think is a bad ground. The AT temp, brake, and stop light "idiot lights" will stutter on and off while driving (all three will light simultaneously).

 

At the same time, I will feel a slight stutter in engine power, and the blower fan will slow down for a moment.

 

So far, I've cleaned and checked the ground at the battery and done the same for the wire that goes out to the chassis (by the headlights). I've cleaned the ground wire that goes to the transmission, and actually relocated it from a plate by the starter so that it connects directly to the transmission. No help.

 

Given that, I suspect that one of the grounds under the dash is messed up. But I don't know where they are. Any ideas?

Load test the charging system. A dying altenator can cuase everything you just described.

 

 

nipper

Even if the battery shows no signs of being undercharged?

Even if the battery shows no signs of being undercharged?

 

battery is only 1/2 of the charging system. It stores energy. the altenator runs the car. If the alt is not keeping up with the load from time to time, the battery is part of the load.

 

Also you cant tell much with a volt meter, hence the load test.

 

nipper

Nipper's right. The conditions you described are just what a dying alternator looks like.

Even if the battery shows no signs of being undercharged?

If the problem is intermittent and the electrical load isn't high, the battery may still get a sufficient charge.

 

As already mentioned, a dying alternator can cause the symptom you're experiencing. However, so can loose/poor connections, and they don't have to be grounds.

 

If you can attach a voltmeter that can be read (safely!) while driving, see what it says when the problem occurs.

Yes a lot of the indicator lights get their ground through the alternator field circuit. Definitely check out the alternator and things. If you are getting this problem intermittently, the alternator may be getting ready for retirement.

 

Here's some copy&paste from EndWrench.com articles:

Erratic Warning Light Operation

When diagnosing a vehicle with erratic operation (flashing, dim illumination, etc.) of the handbrake/brake fluid warning lights, check the charging system and the charge light system.

 

The charge light system (which receives a ground through the alternator field when the alternator is not charging) is used to check the bulbs of the handbrake/brake fluid warning lights (and some other warning lights on select vehicles). End Wrench® http://www.endwrench.com

 

Electrical Problems? Check The Alternator

If a customer complains of a speedometer needle jumping, dash warning light(s) on/glowing dimly, or an engine driveability problem, etc., check the alternator. One of the simplest ways to see if the alternator is causing the problem is to unplug it.

 

The vehicle may run fine with the alternator out of the picture. If the problem is gone with the alternator disconnected, then quite possibly you have found the source of the problem. Check the alternator/charging system more closely to determine the exact cause.

 

The battery is only used to start the vehicle and to back up the charging system as necessary. When the vehicle is running, it’s the alternator’s job to supply electrical energy. If the alternator isn’t capable of supplying the correct voltage/amps (too much or too little) problems may result. Many shops overlook this simple test. End Wrench® http://www.endwrench.com