99 & up 7.3L Power Stroke Engine and DrivetrainDiscussion of the 99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.
I called a tech to make sure I was looking at the right one. I also asked about the 4WD functions. He confirmed that the t-case motor only engages/disengages 4L. Moving between 2WD and 4WD is engaged inside the t-case and is controlled directly by the GEM. He said this is why you don't feel anything when engaging 4H. Almost like a clutch. However 4L is a mechanical engagement triggered by the motor and therefore the clunk when going to 4L.
That guy is WRONG. The motor (or single shift lever on the manual t-case) controls all the functions. The manual shifter is all the way forward for 2L, middle is 4H, and all the way back is 4L, (with neutral in between 4H and 4L), all done with the same lever. The motor works the same way.
The position switch controls the motor operation thru the GEM, by activating or de-activating the relays. The switch tells the GEM to engage the relay to start the motor and the switch tells the GEM when it's in the position selected and opens the relay to stop the motor.
And the switch does use different resistances to talk to the GEM.
I wouldn't change out the GEM till you confirm that all your wiring matches the diagram. It's possible that you didn't get everything reconnected right, or still have an open circuit somewhere.
So the shift motor has three positions? Looking at the diagram I don't see how that happens. On page 1 I see the L2H Relay and the H2L Relay both go to the transfer case shift motor. I don't see the t-case shift motor anywhere else on the diagram. The only wires going to the shift motor are the Orange and Yellow and they come from the L2H and H2L relays. Is there more inside that I can't see or that aren't on the diagram?
On page 3 it shows the switch going to the GEM, which in turn goes to the t-case contact plates A, B, C, D, and VCC. I don't know what those are. It also has leads to the brake pedal position and trans range sensor so you can only go in 4L with the brake depressed and trans in neutral.
Am I missing something? I will be testing wires tonight after work. Not sure I can get a GEM right now anyway. Every place I looked is out of stock.
The shift motor doesn't really have "positions". It just runs forward or backward. The position switch tells the GEM when to turn the motor off. Say you're in 2H, and flip the switch to 4H - The GEM starts the motor going toward LOW position, and the position switch tells the GEM to stop the motor as it gets to 4H. Same thing going in the opposite direction.
A, B, C, & D are the position switch outputs to the GEM, The VSS (Vehicle Speed Sensor) input is for going to 4L, telling the GEM that the vehicle is stopped, and it's OK to move to/from 4L. Similarly, the brake and tranny shift on position inputs restrict the motor from moving unless in neutral and brake is on, as you already figured out.
So the relays aren't just for H2L and L2H. They move the motor along all three positions. One relay moves the motor one direction, the other relay moves the motor in the other direction. The GEM reads the outputs from the transfer case and stops the motor in the correct position based on the switch position.
So I'm still left with confirming integrity of the wires, relays, and fuses. If those are good then the GEM is the only thing left. Correct? If so I might be up a creek. The factory is out on those until March and all of the places online are saying out of stock until then. Not really keen on getting one from a junkyard.
Yes, if you can confirm that everything is wired correctly and you check the actual function of the position switch (they can go bad also), then that pretty much narrows it down to the GEM.
I don't think that a wrecking yard GEM would be a giant risk, but that's up to you. You do have to get one that matches your year and trim level, and they need to be programmed to exactly match your truck. So I'd exahust all other possibilities first.
Well I've tested as best I could. The wires I tested have continuity. I hope I tested all the necessary ones. So I searched again for a GEM. The Ford factory parts are non-existant until March. I ended up ordering an A1Cardone unit. 30 day money back guarantee so I can return it if it's not the problem. If that doesn't do it I guess I have some more diagnostics, though I don't know what else I can check.
As an aside, while pulling the 7 pin connector from the t-case I broke the retaining clip. So I have the plug held tight with wire. Anyone know of source where I can purchase those types of plugs to patch in?
Also, the GEM matches my trim. ESOF, power windows, no keyless. Am I going to need it programmed by the dealer anyway?
I believe it's the GEM that takes care of speedo calibration, but I'll check that for sure later. You'll at least need to get that checked. Were you able to check the condition of the position switch? To do that, you'd have to remove it from the t-case and run it thru all positions while checking continuity thru the switch contacts at the terminals.
Here's the stuff the GEM needs to be programmed for (from the Ford service manual):
Remote Keyless Entry (RKE)
One Touch Down
Brake System Configuration
Continuity between pins as you move the switch. I think pin 4 ground on the diagram I posted would be common for your meter, and the other lead to each of the lettered terminals (1-D, 3-B, 6-C, and 5-A). Not sure about the VCC contact. I thought that was VSS (Vehicle speed sensor) previously. I'll have to dig a bit deeper and will post what I find out.
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