I have a '91 W250 5.9 (see signature) that is giving me great headaches because of electrical issues that have gotten out of control. It originally started with the speedo, temp, and pressure gauges giving faulty readings. I read here this could be caused by the speed sensor in the pumpkin which I replaced. No good same deal. I also read it could be related to air in brake lines which seemed like a good place to start since I had a leaking master. So I replaced the master bled the brakes and still same deal. I then replaced the voltage meter, same thing. Over the course of the couple months I have been working on this the electrical problems seem to get worse. Radio barely works because it seems to cut in and out. Headlights don't work in cold weather. They seem to cycle on and off, which to some degree I understand is normal, however mine will virtually go out. It seems that I have a major short somewhere but for the life of me I can't find it (although diagonsing electrical problems is not my strong suit). Has anyone experienced this kind of problem and could point me in the right direction. Even a brief tutorial in how to diagonsis an electrical problem like this would be great. One other thing I feel is important is that these problems don't present themselves when the truck is sitting still (like at a red light) or when I put the rear wheels up on jack stands and run the drivetrain.

 

I would start off making sure I had a really good, hard ground to engine and chassie. Run grounds from Batt. to chassie and engine and engine to chassie.
Clark

 

How is/are the battery(ies)?

Checking the battery connections is a good idea (pos and neg).

 

It has a new battery and I have checked and rechecked all the grounds from the battery.