I'm having alittle problem with my 91 D-150. The truck runs fine intill it warms up. When you give it gas most of all the gauges start to go crazy, but when you let off it goes back to normal. The light will dim, the temp. gauge, volt gauge, tack thats power is from the battery, and the transmission will not shift into 4th gear intill I let off the gas. I had a volt meter on the battery and it was at 13 volts the whole time. I had a Jet Performance Chip (what a joke) on the ECU about 3 month ago and it was doing the same thing because it would not fit all the way into the ECU. So I sent it back and now its starting to do it again. I'm lost

 

Maybe a bad wire in the bundle??

 

I was thinking a ground, but i think that would make the truck run like crap to. It doesn't run like crap, the truck runs fine it's just the elec. system when I'm giving it gas. I don't know right now. About at the point of giving up on the truck and get rid of it.

 

Sounds like power problems.
Make sure all ends of the battery cables are squeeky clean and tight.
If the charginf system is working correctly then you should have 14+ volts at the battery with the engine running. Might have a weak alternator?
Pull the error codes off the computer and post any codes here on the board.
Is the belt tight and in good shape on the alternator?
Have the battery "load tested".

 

Find the problem! I was a ground. Most have a ground from the block to the firewall or something like that. No not my truck. It has a ground from the block to the battery and had a small *** wire, like the smallest speaker wire you can find going from the battery to the body. The wire was melted. Find that this weekend. Replaced it with a another battery wire. Works awsome now. Thanks