Help Needed electrical issues

[John Johnson 69]

I have a 1999 7.3L with several electrical problems. The problems are no speedometer, no turn signals, when the RPMs increase the voltage from the ALT increase up about 18V, if the MAP sensor is plugged in and the truck is in gear the RPMs only increase to 900 if you unplug it the truck revs. normal. We have changed the flasher, the insturment cluster, the main computer, the ALT three times, the diesel preheater, the MAP sensor, and went the wiring harness as good as possible without removing the hole thing

 

[JLDickmon]

that harness has a screw shot through it somewhere..
only thing I've ever seen cause anything remotely resembling your grocery list of problems..

 

[bigrigr]

Could also be a bad ground connection some where, check the batt cables at the posts, and at the other end also.(ie frame, engine block, firewall, etc) Also check for the usual ground connections from the cab to the frame.(ie ground straps) Your wiring diagram should show you where they are supposed to be. Good luck, you'll need it!

 

[silverF250]

Couldn't it also be the gem module? Fairly common on a 99.

 

[bigrigr]

Do tell! Give us the rundown on the gem module, this is news to me.

 

[silverF250]

It's proven there are a lot of 99's that had leaky windshields the water goes down and gets into the gem module and short circuits it out. I have a buddy who instead of replacing his actually took it apart cleaned it up and used di-electric grease and put it back together and he hasn't had a problem with it yet. Do some searches on other forums it's fairly common.