Need some help. My 96 3500 seems to have a common ground issue. The tach, CC, AC, and alternator all go dead at the same time. Then after a few hours they all come back on at the same time. I thought it was ECM, swapped it out and still no go. I have traced every wiring harness looking for bad connections or worn spots. Nothing. If i ground the alternator and hook up a hot wire it will work but it charges full blast with no regulator on it. I do a lot of night driving so I need it to charge a little. Every once in a while everything will come on and work fine then the next time i park for a few hours nothing works again. Any ideas. Twenty years and 333K miles and this is the first time it has me stumped.

 

Guarantee you it is your Engine Speed sensor circuit.

Just for grins, on the front of your engine,m somewhere around the water pump area is a connector that hooks the speed sensor to the harness.

Take it apart and clean that connection with contact cleaner.

Mine got oil soaked and would intermittently pull the same stunt.

Anyway, without the Tach signal your CC,AC, Charging, and O/D ( auto trucks) go away because the PCM doesn't know the truck is running.

 

Thanks Patdaly. I'll give that a try this evening. 'll let you know how it turns out. I took that off because it looks bad but it cleaned up and i put it back on. If the speed sensor is out then that could be the whole thing. I'll give it a try. Thanks

 

All the wires look good but It took them off and cleaned everything up and tested the power. It tested for 5v in to the sensor but the out signal was very erratic. I had to order a new sensor. It should get here today and I'll put it on tonight.

 

Make sure you properly gap it.

Specifications are .049/.051 gap, but a folded matchbook cover works.

This info is from the 97 FSM ( I don't have a 96 ) but should be the same in case you have to start diagnosing further in.

 

Yep that did it. I put the new sensor on and set the gap and now everything works like a new one. Thanks for the tip. I was about to go crazy checking wires and testing things. Should be good for another 300K.

 

Glad you got her going!