GEM module 97 4.0 SOHC

[glfredrick]

I'm having problems that seem to be GEM module related on the 97. This isn't the first time I've had GEM troubles, and I'm wondering if anyone else has had these issues, and what it takes to fix them.

Symptoms...

4x4 sometimes engages when it isn't needed, especially when backing up and turning sharply (like backing out of a parking spot on dry blacktop). Once engaged, the 4x4 will stay locked in until we shut off the truck, making it drive hard, steer hard, and generally suck.

Dash lights do not function when the headlights are on. All the various warning lights (like 4x4 -- speed control -- OD off, etc) go away when the headlight switch is turned on. They are still lit (turn off the headlights and they are right there) but never show when the lights are on.

Failure to allow a computer scan. When I try doing a code read, I get a malfunction code and the scan stops. I'm using top end scanner tools from Snap On with all the latest updates (Modis, Color Graphing Scanner, Solus) and also cheapy simple OBD II code readers all with the same issues.

In the past we had Ford re-flash, and then replace the GEM, which helped drivability for a short time, but the problems quickly returned. Is something killing the GEM or do I have another issue at hand?

 

[Opera House]

Sounds like a bad instrument cluster ground. It only starts working again because someone moves the wires around. Solve the simplest problem first. Get the dash lights to work and your other problems will go away.

 

[glfredrick]

Just so you know... All the actual "lights" in the dashboard light up (instrument panel lights controlled by the dimmer) but the warning lights go away when the other lights come on.

Still sound like a ground problem?

If so, I'll tie into the dashboard over the weekend.

 

[Opera House]

Others have reported these dashboard ground problems. Turning on the headlights is usually associated with bad grounding between the engine and the vehicle body. You can get a significant electrical potential between the engine and body with a high current draw like headlights. Do you see the voltmeter change much. This can also happen in the dash. The dash illumination would use the same ground point as the GEM. The extra current of the dash lights would float the GEM above ground from resistance in the ground path. I design these dedicated microprocessors and this is always a potential problem when the wiring systems get larger than a couple feet. You'll have to try both locations.

 

[glfredrick]

Thanks...

I'll get to tearing into things to see what gives.

Now to find that pesky dashboard ground... (or the several of them that I know exist)

I'll be spending some time surfing the factory manual.