Replacement GEM module 1997 AWD 4.0L SOHC?

[pcrussell50]

If you don't have 12V on the brown wire when switched to 4High mode, check to see whether there is voltage on the TOD yellow (or is it yellow/brown?) wire. If there is, the TOD is probably blown (assuming no wire or connector problems).

If there is not, the GEM is not outputting this control voltage to the TOD (again assuming no wire or connector problems).

OK, I did this...

I have 12v on the brown wire at the TOD when I select 4 High (no voltage when I leave it in AUTO)
I do not have 12v on the yellow wire at the TOD in any switch position

Suspecting the GEM?

-Peter

 

[J_C]

^ Then I am not understanding how the TOD gets energized, unless it's a normally closed (solid state) relay but it's a bit beside the point.

With 12V on the brown wire, you should have the 4WD clutch engaged in the transfer case. I would measure for 12V on the brown wire at the transfer case and inspect both that plug and the mating TC connector. If you have 12V going in, either the clutch is damaged or something mechanical is wrong in the transfer case.

While I don't know what the resistance value should be, you should also be measuring some resistance between the brown wire and chassis ground, that resistance being across the clutch coil windings.

This topic is a bit aged now and I want to make sure I am still up to speed:

1. You are certain, through testing, that the front wheel drive is not engaging?
2. You still suspect the GEM but the code it set is related to airbags not 4WD?

I don't know much about the airbag system, would presume a bad connection (or deployed bag?) would set a code. As for the rest of testing the 4WD, if the transfer case isn't engaging the front but it's getting 12V on the brown wire, it would seem that the shop put a non-working TC on the vehicle.

To be clear, you do not need the GEM at all to engage the front wheel drive. Well you do, to energize it from the dash switch, but its purpose is to be able to energize it in Auto mode intermittently using feedback from the speed sensors. Electrically it is not needed for full time 4High mode and if there were no Auto mode they (Ford) could have just wired straight from the switch to the transfer case.

You could cut the brown wire at the TOD and wire it straight from the battery to the transfer case and 4WD front wheel engagement should be working if there's no problem with the wire, connector, or inside the transfer case.

All it needs is a sufficient amount of 12V current on the brown wire. Some wire or connector damage could limit current too much, but I'd as soon suspect that the shop got confused about the code and thought their work was done when it wasn't.

 

[pcrussell50]

OK, I think I get it:

The GEM is only responsible for cycling power to the clutches of the TC based on need (front to rear driveshaft speed differences), when in AUTO mode. If the GEM has failed, this won't happen in AUTO mode.

If you select 4 HIGH, the GEM should be bypassed altogether and power should go through the brown wire, to the TC and 4WD should be engaged full time until it is deselected by going back to AUTO on the switch.

So right now, since I'm getting 12V on the brown wire at the TOD relay when I select 4 HIGH, our main suspect is power not getting to the TC due to a wiring fault, or a problem with the TC itself. Sound right?

-Peter

 

[CDW6212R]

The A4WD system is made of many components, and the transfer case is the largest and toughest to fix. The shift motor is a weak link, and typically it's just brushes or minor to rebuild them.

The control system is made up of three modules, the GEM, TOD, and 4WD module. I think the TOD and 4WD modules might be common for many many Ford 4WD models. The GEM for 98-01 Explorers is one of two parts, the 4WD version, and the 2WD or AWD version. I think the GEM for 95-097's might be just two in number also, but the front differential disconnect system I'm not sure if it complicates that at all. So you may be able to locate a GEM just as easily as the 98-01's, find a 4WD truck, not the AWD/2WD versions. A lot of things changed for 1998, you can't use a later GEM in the 97 models.

The GEM is to the right of the radio, the TOD is right of that behind the airbag, and the 4WD module is to the right of the steering column area, behind and left of the AC controls.

I agree with J C, identify the signal going to the TC by the brown wire. If power reaches the TC then the problem is inside the TC, the clutch most likely. If the power isn't getting there, than it's the control circuits or wires not providing the signal.

 

[pcrussell50]

12v is leaving the TOD, headed for the TC when I select 4 HIGH, and it goes back to zero as expected when I select back to AUTO. so as far as I can tell and as you guys have said, it's either not making it to the TC, or there is a fault in the clutch coil or shift control module. Question: Are the shift control module and magnetic clutch coil physically located inside the TC?

-Peter

 

[J_C]

With 12V on the brown wire, it can't be a Shift Control Module problem. That module only activates 4Auto and 4Low, is not needed for 4High with 12V already on the brown wire due to the dash switch set to 4High. However AFAIK it is located behind the glovebox or in the passenger footwell side panel.

The magnetic clutch coil is in the transfer case. That, the electrical connection to it, or a problem with the mechanical movement/engagement of it would be my best guess.

You can try to rule out the electrical connection to it by having 4High turned off and measure for a resistance between the brown wire and chassis ground. There should be one, the resistance of the coil itself, unless it has a bad connection or the coil wire is damaged.

 

[CDW6212R]

You should be able to apply power to the brown wire which engages the magnetic clutch, to test the functioning of that clutch. I don't know what the power would do affecting upstream, you could disconnect the brown wire from the control output source etc. People who have installed a switch in the brown wire to disable the front wheels, could do that more easily to diagnose a clutch problem.

 

[pcrussell50]

Power goes to the brown wire at the TOD relay under the dash as it should when I select 4 HIGH. I suppose the next step is to probe the brown wire as it enters the transfer case to make sure the power is getting all the way down there. Is the place where the brown wire enters the transfer case easily accessible from underneath?

--Peter

 

[CDW6212R]

Yes, there is a connector under the floor near the TC. That is part of a short harness which plugs in above and below the floor, it's about a foot or so long if it needed to be changed.

 

[pcrussell50]

sorry to be a pain but I am out of town right now, and I'm trying to visualize... is it under the driver or passenger front seat?

 

[CDW6212R]

Driver's seat, here's a picture showing the two large holes that two wiring harnesses go through. It's under the back part of the seat. One harness goes to the frame, and the one you want is optional for 4WD trucks, it goes to the transfer case. This is my fuzzy memory, I swapped all this into my 99 back in 2006.

 

[Dave98XLT]

You should be able to use a volt meter to measure the resistance to ground on the brown wire at the relay to see if the coil is connected and has continuity. I think the resistance reading should be around 100 Ohms from memory.

 

[Dave98XLT]

Can anybody confirm what the resistance should be on the brown wire if measured at the relay plug with the relay disconnected?

 

[J_C]

^ I'd guess single-digit, somewhere between 0 and 8 ohms, due to ohms law at 8 ohms it'd be 1.8A on a ~ 14.4V system, would only be 26W so probably lower.

 

[Dave98XLT]

With the torque on demand relay unplugged I just measured it at infinite resistance, so I guess I have a problem underneath I need to investigate.

 

[pcrussell50]

You should be able to use a volt meter to measure the resistance to ground on the brown wire at the relay to see if the coil is connected and has continuity. I think the resistance reading should be around 100 Ohms from memory.

is this a test of electrical continuity between the magnetic clutch coil and the brown wire that energizes it?

in my case, if I select 4WD with the switch, I get 12v on the brown wire, but no actual 4WD action out of the vehicle. sooo... if the brown wire is not electrically connected to the coil, or if the coil has an open inside the transfer case, I should get no continuity, or high (infinite) resistance. does that sound right?

-Peter

 

[J_C]

The magnetic clutch coil, is literally a coil of wire, acting as an electromagnet when energized.

When not energized, that coil will measure a resistance between the power input (brown wire) and its own connection to ground.

Yes if you get no continuity and infinite resistance with the brown wire disconnected from the TOD, then the circuit is not completing through the clutch coil whether it be due to the brown wire damaged somewhere along its run, connector problem, clutch coil itself, or clutch coil grounding problem.

 

[Wolfgang G.]

The magnetic clutch coil (brown wire) should have a resistance of 2,6 Ohm with an open 16pin connector near the shift motor.

 

[pcrussell50]

Bringing this back from the dead, since it's ongoing...

-1997 Explorer XLT 4.0L SOHC Automatic 4WD-

Sooo... the problem is... getting no drive to the front.
1) there is 12v on the brown wire all the way to the transfer case when switched from AUTO to 4WD HI and 4WD LO
2) for whatever it's worth, the low range works just like it should (except no drive to the front wheels)
3) BUT two different mechanics have verified that when up on jacks, both axles drive when you select 4WD
It's just that...
4) on pavement if you select any 4WD mode, and make a tight turn, there is NO (detectable) scrubbing
5) on snow, if you pull the front up against one of those parking strips to stop you from crossing forward into the landscaping, and select any mode of 4WD, (including AUTO) there is NO (detectable) drive to the front axle... the rears just spin and spin helplessly. Nothing to the front, either side.

So what I'm thinking is that the clutch discs in the transfer case are burned/worn*. They are capable of some very slight drive of the front axle when there is no load, such as when up on jacks, but not when there is weight on the axle as in real life.

IF this is the case, I've seen that there are companies out there that sell rebuilt transfer cases. Is there one that is well known that you guys trust that you can recommend?

-Peter



*or damaged. Years ago, the car locked in 4WD one day. I think a speed sensor failed and reported that the rear was slipping so drive was sent to the front all the time. It was undriveable because of the scrubbing in turns. And some HACK repair shop I sent it to tried and tried and kept coming back scrubbing in turns, locked in 4WD. But they stood by their work and kept taking it back and trying to fix it. Then they finally "fixed" it. I think what they ended up doing was somehow destroying the transfer clutches that sent torque to the front. It felt a little "different", but I never cared because we lived in California (and Nevada) and never needed the 4WD. It was when we started spending winters in Denver that we noticed it.

 

[CDW6212R]

That does sound more like the TC is bad. Have you been checking the TC fluid, replacing it as needed? If the fluid was ever black or dark, that is an indicator of clutch wear.

The TC shift motor is also a weak link, I'd verify that is good before being serious about a TC rebuild or swap. Those motors are rebuildable, used to be about $75 new, last time I looked ran $75 used. I'd rebuild one next time if I need it.