I have a 1988 740 Turbo. WE've had it for about nine months without any major issues. Last week the voltage meter started going crazy, so we took a look and saw that the positive battery post was severely corroded, and the battery was 6 years old. We replaced the battery, and my mechanic replaced the positive post. This helped, but the gauge was still erratic, just not dipping as low as before (which was well into the red, occasionally killing the engine). He found a short and said he fixed it, and for about half a day the gauge was relatively steady, but now the problem is back worse than ever. Now the voltage is dipping so low on idle that the engine is periodically revving up to 1.5 - 2k rpm to compenstate for the low voltage, and I can feel brake pedals popping up or down a little bit on low dips (I'm assuming this is the anti-lock or power brakes kicking in and out with the voltage). The alternator and battery have been tested and are good. Does testing the alternator include testing the voltage regulator? Or are they separate? Is this a short, or should we be looking at something else? Any advice or direction would be good at this point.

Thanks

 

Is your engine harness in good shape?

 

I'm not sure how to check that, so I'm not sure how to answer your question. Thanksfully, I was able to figure out the problem. The bolt securing the ground off the battery had come out, so the ground was sliding around as the car moved, causing all sorts of voltage craziness. All I needed to do was secure it with a new bolt. Its not a perfect fit, but it'll work until I find out exactly what size I need.

Thanks for the interest though.