Electrical problem???

[thecos2002]

I have been chasing an electrical gremlin for a while now... no overdrive, speedometer, odometer, cruise control, blinking o/d light and check engine light on.

Have isolated the problem (I think) to having 12 volts from the speed sensor to the GEM when there should be none. I then cut the grey/black wire (from the speed sensor) at the GEM connector with no drop of voltage but if I remove the entire connector from the GEM the voltage goes off.

This appears to indictate a problem with a shorted or crossed wire, at least I think, or could there be something else I am missing?

Anybody have any expertise in the aforementioned portion of their wiring harnesses of their Ex...? My truck is an 1996 XLT with 167,666 miles and holding.

 

[dogfriend]

This post might help you out: Dead Link Removed

The links show the wiring schematics for 95 Control trac, the 96 should be the same as far as I know.

The driveshaft speed sensor is a Hall effect type of sensor. It detects the change in magnetic field caused by the movement of the exciter ring (toothed ring) inside the t-case. In order for a Hall effect device to work, you must apply voltage across it. No current flows (or very little) when the magnetic field is strong, small current flows (with corresponding voltage drop) when the magnetic field is weak. The GEM uses the change in voltage (average voltage?) to determine driveshaft speed.

So to sum up, it may be normal to detect voltage across the speed sensor (if I have this correct)

 

[TPLYNCH]

An electrical gremlin that effects numerous systems is 99% a bad GEM.

 

[thecos2002]

Problem identified

For any future searches related to speedometer and cruise control not working along with the o/d light flashing and check engine light on. The code was VSS (vehicle speed sensor) not working. The wiring for the VSS uses a grey/black wire to send out the signal.

Traced that wire through the truck and found the short in the overhead display. Even though the VSS signal was not used, the wiring still was run for future uses. I snipped the shorted grey/black wire at the display pad and the problems were gone.

 

[tenikiwon]

Good work!!!