I have a 1999 Explorer that was heavily modified. Replaced both axles with 1978 f250 axles and welded some 50" tractor tires to 15"wheels to create my log puller/crawler.
My issue is the 4x4 low doesn't engage. The factory setup was the auto/high/low and was able to do the modification for manually locking in 4 high no sweat, but when I am pulling 500lb logs up a 20 degree incline, she struggles a little. Anyone know if there is a single wire I can hit feed a switch into the transfer case to put it in 4 low?

 

So you can see why she is called 'The Creature'

 

The original 4WD mode switch used resistors to select the transfer case setting via the Generic Electronic Module (GEM) and Electronic Shift Control Module. There's also a Torque on Demand (TOD) relay that activates the magnetic clutch coil in the transfer case. So there's not a single wire to the shift motor that will allow you to easily just throw a toggle switch, but if you still have everything except the mode switch it wouldn't be too hard to bypass that.

Do you still have the GEM installed along with the mode switch and the transfer case motor still doesn't let you change the setting? How much of the original wiring is intact?

-Rod

 

Original switch was bypassed and a toggle installed for the 4 high. The differentials obviously don't have the sensors in them any longer. I am not sure where the GEM would be as much of the wiring was destroyed when the previous owner cut away the body panels. If memory serves, that gem is behind the stereo correct?
Still seems I should be able to provide 12v somewhere and ground somewhere else and it fool the internal relays. Knowing the precise wiring if the harness would be the best way to figure.
Thanks for your reply, btw.

 

I don't know the wire colors, but the electric motor on the transfer case chooses HI or LO... you should be able to put 12 volts as mentioned to "the wire" and go into LO gear.. You just need to know what wire.

 

You're correct, the GEM is installed behind the center of the instrument panel.

Enabling 4L is not quite as easy as installing a toggle switch because of the functionality of the shift control module. I've pasted in the wiring diagram for the transfer case, GEM, TOD relay, and shift control module. The speed sensors that would have been in the original axles are inputs to the GEM.



-Rod

 

Looks like OG and VE wires are the key. So what color is OG? Orange? VE? Violet? Very grEen?
The GEM still send 12v to a wire that leads to the transfer case motor to engage into LOW.
My only concern is if this needs to be a constant 12v, or is it a 12v only while engaging, like a window motor if you will. Applies 12v until it's in place then no more power.

 

OG = Orange
VE is actually YE = Yellow

With regards to power for engaging Low, this is from the factory service manual:

When shifting into or out of LOW range, the GEM requires that the vehicle speed is less than 5 km/h (3 mph), the brake is applied, and the transmission is in NEUTRAL.
Feature inputs:

• Throttle position information.
• 4WD mode switch.
• Contact plate position inputs A, B, C, D.
• Vehicle speed signal (transmitted from ABS module).
• Brake ON/OFF switch input (battery voltage when brake is depressed, open circuit when not activated).
• Digital transmission range sensor (ground when transmission is in NEUTRAL, open circuit otherwise).
• Start/clutch depressed input (manual transmission only).

Feature outputs:

• 4x4 shift motor relay outputs.
• 4LOW cluster indicators (ground when activated, open circuit when deactivated).
• Solid state clutch relay (pulse width modulated signal; grounded when relay is on, battery potential when relay is off).

And for a bit more detail of the solid state clutch relay:

The solid state clutch relay is used to activate the A4WD clutch within the transfer case.
System Function
Hall effect sensor monitors input and output shaft speed.
In the A4WD (C-Trac) system, the GEM varies the torque sent to the front driveline by controlling the transfer case clutch. At rest and under cruising conditions, the GEM activates the transfer case clutch a minimum duty cycle (percentage of time the clutch is turned on), which allows for the slight difference between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. Under any of the following conditions, the GEM will increase the duty cycle in order to prevent or control slip:

• Slip is detected (by using the Hall Effect Sensors)
• Heavy acceleration (throttle position).

So it sounds to me like, for locked 4WD Low operation, you'd want the clutch completely locked which would be a constant 12V source once in 4WD Low.


-Rod

 

You need two power sources. As shorod mentioned, you need the 12V full-time to the clutch to get you 4WD. For the shift motor to get from 4H to 4L, from what I can gather from the schematic (you'll need page 34-3), power is applied to the motor only until it's in position. There are a number of "plates" in the transfer case motor (inside the blue plastic housing) that provide position feedback to the GEM. Ford didn't provide the recipe to the secret sauce, so I don't know for certain what logic is in the GEM, but those plates are definitely inputs. As to which should be open and which closed for 4L, I don't know. That was one of the things I was going to figure out when I got around to troubleshooting my 4L problem. I built a breakout using the TC motor connectors so I can monitor each wire, but, so far, life hasn't stopped happening just because I don't have 4L and I just keep waiting for the time to investigate.

Anyway, if there are no speed sensors, there's no way to keep the GEM happy so you'll have to bypass it, unless the GEM assumes a default value of 0 mph for an open circuit (I doubt it).

I've played with a transfer case in the junkyard and was able to move the transfer case shaft to 4L with my fingers. You could probably remove the TC motor and put it in 4L manually (pun intended), but I don't know if it will pop out once there's a load on the transfer case. The TC motor has a worm gear setup so it will hold the shaft position.

 

From what I've gathered from the diagram above and the text description, the key signals to the electronic shift control module from the GEM will be GY (Gray) and BN/PK (Brown/Pink tracer), both of which it sounds like could be momentary since Mikeman confirmed the shift motor uses a worm gear setup. You'd also need battery power on a toggle switch to the magnetic clutch (BN - Brown wire) as this is what engages the 4WD via the TOD relay when slip is detected.

You could momentarily provide ground to the BN/PK wire to shift from High to Low (H2L input), then momentarily provide ground to the GY wire to shift from Low to High (L2H input). The duration of that momentary press would need to be determined to fully engage the gear in the transfer case. This appears like it should bypass the GEM.

You'd also need to ground the BK = Black wire from the Electronic Shift Control module and provide power to the DG/LG = Dark Green/Light Green wire and ignition switched power to the LG/OG = Light Green/Orange wires so that the H2L and L2H relays work correctly.

I think the above connections should successfully bypass the GEM and TOD relays, but I need to caveat that I have not tried this.

-Rod