The negative side causes a voltage drop and all the electrical consumers look for another path to ground, and one path is through your gauge cluster. Porsche has a technical service bulletin on this but it only pertains to the 968. Believe me, it happens to all 944’s from 85/2 up.

To check it out you have to do a voltage drop test on the negative ground cable after the car has been running for some time with all electrical consumers on (a/c, rear defogger, lights, radio, etc.), then do the voltage drop test. We have had problem cars and done the test on a cold cable and it passes with flying colors, it needs to be hot!

A voltage drop test is not hard to do, just complicated to explain, and a carbon pile type tester should be used. Find a shop with electrical knowledge to perform the test.

The cure is to replace the negative ground cable. Porsche is the only one to sell the complete cable, it also comes with the positive side cable. Trust me, you should replace both and not skimp on this one, more electrical damage can occur if not repaired properly, plus the Porsche cable is of very high quality.

The tech bulletin also says to replace the printed circuit board behind the gauge cluster, this is not a cheap part, and a used one probably had a bad ground cable. Try the repair without replacing the board, we have had success with just the battery cables, and the boards were fine.