My dashboard dims and the turn indicators glow

Question:

I have a 1971 Pontiac Lemans Sport convertible with an electronic gremlin that is driving me bonkers.

The car has a peculiar problem with the dashboard illumination where once I turn the headlights on and drive with them on for a few minutes my dashboard lights dim a little bit and the turn indicator lights and high beam light indicator on the dashboard come on.

Once this occurs, I step on the high beam foot switch to turn on the high beams and this returns my dashboard lighting to its normal stage shutting off the left and right turn signal indicators and high beam indicator. But the “fix” lasts only for a few minutes and then the cycle returns.

The car was never modified and it still has the original AM radio and factory electrical components and accessories (switches, knobs, levers, etc.). Where or what should be the first step in the repair?

Answer:

My best guess is that the odd behavior your Pontiac is exhibiting stems from current back-feeding into the dash lamp harness. The most common cause for this strange phenomenon is inadequate grounding of the electrical circuit.

As a quick, “down and dirty” test, run a heavy gauge wire directly from the negative battery post to the instrument housing. It is likely that one or more important ground connections in your car are corroded, loose or broken entirely.

Give the entire electrical system a comprehensive examination, starting with the battery cables and moving throughout the whole car. Look for corrosion, loose connections, and other problems at every ground point and harness connection.

Another possible factor contributing to or causing your car’s symptoms could be a failing alternator. One or more bad diodes in the alternator may be to blame.

If you don’t find any ground problems and auxiliary grounds run directly from the negative battery post to the dash, body and chassis don’t fix the issue then you should have the alternator tested.