GEM module

I was told by the local mechanic that me "GEM module" was ou on my t-case... what the hell is this and what does it do?

The GEM module in not “on” the transfer case but it does control your auto 4wd as well as other things. It is located behind the radio in the dash next to the interment cluster. The annoying chime you hear when keys are in the ignition come from it. Calling it a computer is a stretch, it is more of a programmable dumb logic box.

As far as its duties concerning 4wd, it monitors the axle speed (from Hall Effect speed sensors) and engages the transfer case when it detects the rear wheels spinning. It also controls the hi/low shift motor I believe.

If you have to buy one new, I would take it to your guy who told you that and have him replace it. Try and stay there while he changes it so you make sure he doesn't change anything else. That way, you know if the GEM fixes it, and if it doesn't cure your problem, he eats the new, very expensive, GEM module and not you...

I'd be very surprised if it was a bad GEM causing the problem. It has no moving parts... it's just a circuit board with three large connectors on it, and it's securely tucked up inside the dash, bolted to the instrument panel. I would seek a second opinion before investing the money in a new one (not to mention the parameters need to be pulled off the old one and programmed into the new one).

Maybe we should begin at the beginning? What's your truck doing, and when is it doing it?

I trust the guy, he is also the local preacher... He said that he is going to replace it and see if it fixes the problem first before we have to pay for the part..


While I was mobalized with my national guard unit, she said that she was driving it down the road and it just quit pulling (at 75mph on the interstate).. they pulled over and had it towed to her friends house.. when I got home I went and got it and drove it directly to my house. It seem to me like it is not going into OD. and it will ocasionally quit pulling.. when it does it acts like the T-case is in neutral, and you can hear the park pin clicking when you try to place it in park. when you put it in neutreal, and then low range, and back in drive it will catch. But then put it back in neutreal and auto, you can take off again. My first thought was that the T-case motor was bad, so i pulled it and checked it. It seemed to be working fine. after putting it back on I checked it in high and low range, and you can feel a deffinate difference so I do beleive that the T-case is going in and out of low range. We took it down to the transmission shop, he hookeed his computer up to it, and drove it and what not and said that the 4x4 switch in the dash was bad. So I bought and replaced that. No dice samething. So I took it to where it is currently at (he also works a a ford dealership) and he had a "better" computer.. and said that it traced all the circuits and the only two possibilities that came back were the GEM module, or the Tran's comp... I don't know much about all this crap, but he said that he didn't think it was the trans comp, because when he drove it with his computer hooked up, it showed that it shifted through everything....

So now what oh holy ones?

If I was a bettin' man, I would bet you've got a transfer case that's suffering from the infamous shift rail bore issue. The fact that it was grinding when you would shift to park tells me that the transmission output shaft is spinning (since that's where the park pawl is) but that power is not getting through the transfer case. There are several possible causes... the transfer case shift motor can stop in the 'neutral never-never-land', but shift motor failures on second-gen Explorers are notoriously uncommon. The shift rail bore tends to wear out, and then the shift rail fails to hold the gears in position. Generally this manifests itself as a buzzing or grinding noise on coast-down, say at freeway speeds when you let off the gas, and goes away when you step on the gas again. It eventually results in the gears slipping into neutral, and sometimes staying there. Shifting to low range and back into high sometimes moves the gears far enough to engage the forward direction, and the forward-pressure actually holds the gears in place (due to the helical cut of the gears). But shift to reverse, or coast-down again, and the problem will often re-surface.

The other possibility that comes to mind is an issue with either the shift rail itself, or the range fork not holding the gears in place. Omega Engineering makes some redesigned components to repair those issues.

The only way to know for sure is going to be to open yours up and see what's going on in there. There are several write-ups on them... Unfortunately, for your 97, to swap in a used one, you need one from a 97 specifically. A 95-96 44-05 transfer case won't have the proper front output shaft, and the 98-01 won't have the speed sensor you need in the rear half of the case. There are several members here that have built hybridized cases (back half from a 97, front half and guts from both a 97 and 98-01), but it's not exactly a fun thing to do.

That's my best guess at this point... sorry to be the bearer of bad news....

thank you for your reply, My only issue with that theory is that when it is in gear and pulling, it does not want to go past 35 MPH (at around 4500 rpm) and I can get it all the way up to 55, but it just keeps racing the motor, and if I stop and put it in low range, it won't go above 30 MPH or so...

Also when I am driving it the OD light flashes..

any other ideas?

If the gem module does not work then I will be pulling the T-case off..
can't be no worse then rebuilding an NP205 right? LMAO

wombat said:

Also when I am driving it the OD light flashes..

any other ideas?

Click to expand...

Hold the phone... You left out the part about the blinking overdrive light... That makes a world of difference. It's possible that you have two concurrent issues: One with the transmission and one with the transfer case, but this one's a little tough to tell by the symptoms alone. The place to start is with a scan of the codes to find out why the OD light is flashing. That could be the root cause of all the issues.

At any rate, it's certainly NOT the GEM causing the blinking OD light. Of that I'm quite sure.

thanks for your insight,
But we got the truck back today, and it does seem the the t-case is the issue, so I will now be looking for another.
Hopefully once I replace that it will fix it all..

Apparently it welded itself into 4wd... go figure.