Four wheel drive works after computer reset, then nothing.

[Tacsat117]

If I pull the battery connection then replace it the four wheel drive works when I turn the switch. After a short time the four wheel drive stops responding to the switch and the computer begins to throw codes related to emission. Do you guys think I can replace the vehicle's brain? Could it be loose connections? Once the four wheel drive goes out it won't shift until I reset the computer.

What do I need to do to fix this?

 

[Tacsat117]

I was really hoping someone would have some insite on this situation. I hate to pay a ton of money and still come up with the wrong fix.

 

[Canadian Expy]

Never heard of this problem before ...have you had any water leaking into the Expy?

 

[Exia]

Well for one what are codes, and we can help you from there

 

[Exia]

But since I am in a " Class in session " mood

Here is a description of how your system works. Most likely the shift motor, GEM or switch. See highlighted.

Edit added another highlight

The Automatic Four-Wheel Drive (A4WD) system is an electronic shift 4x4 system that allows the operator to choose between three different 4x4 modes as well as Two-Wheel Drive (2WD) . The operator can switch between 2WD, A4WD, and 4WD HIGH modes at any speed. To engage or disengage LOW range, the vehicle speed must be less than 5 kph, the brake depressed, and the transmission must be in NEUTRAL.

In A4WD, the Generic Electronic Module (GEM) varies the torque split between front and rear drivelines by controlling the transfer case clutch. The transfer case clutch allows for slight speed differences between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. When the rear wheels are overpowered, the GEM detects this slip condition, and the duty cycle to the transfer case clutch is increased until the speed difference between driveshafts is reduced.

The 4WD mode switch is a rotary switch which allows the vehicle operator to choose between 2WD, A4WD, 4WD HIGH, and LOW range positions.

The Vehicle Speed Sensor (VSS) , mounted on the rear of the transfer case, informs the GEM of vehicle speed.

The Hall effect sensors, mounted to the transfer case, are used to detect the speed of the front and rear driveshafts.

The electric shift motor, mounted externally at the rear of the transfer case, drives a rotary cam which moves the mode fork and range fork within the transfer case to select between the 2H/A4WD, 4WD HIGH, and LOW range positions.

The shift motor sense plate, an integral part of the electric shift motor, informs the GEM to the position of the transfer case.
The Transmission Range (TR) sensor, located on the left side of the transmission, informs the GEM of the transmission gear position.

The Expedition/Navigator is equipped with 2WD and 4WD vacuum solenoids attached to the upper RH side of the engine compartment dash panel, they alternately route vacuum to the vacuum motor which engages/disengages the center axle disconnect collar in the front axle assembly.

The 4x4 shift motor relay is a module containing two relays which, under the control of the GEM, shift the transfer case shift motor between the 2WD and the three 4WD modes.

The solid state clutch relay is used to activate the A4WD clutch within the transfer case

 

[Tacsat117]

Thank you all for the response, sorry it's taken so long to get back to you.

There has of been any water leaking into the vehicle since I've had it and I don't see any evidence of water damage.

The codes are:
p0401
P0174
P0171
From previous posts I've learned that these are emission/vacume related issues. The thing that trips me out is that the four wheel drive engages just fine after I pull the battery cable then replace it, but then stops working after a day or two. In my monkey brain that seems to indicate that the hard ware is good to go, but there is an electrical/software/computer issue.

Right?

 

[toms89]

But since I am in a " Class in session " mood

Here is a description of how your system works. Most likely the shift motor, GEM or switch. See highlighted.

Edit added another highlight

The Automatic Four-Wheel Drive (A4WD) system is an electronic shift 4x4 system that allows the operator to choose between three different 4x4 modes as well as Two-Wheel Drive (2WD) . The operator can switch between 2WD, A4WD, and 4WD HIGH modes at any speed. To engage or disengage LOW range, the vehicle speed must be less than 5 kph, the brake depressed, and the transmission must be in NEUTRAL.

In A4WD, the Generic Electronic Module (GEM) varies the torque split between front and rear drivelines by controlling the transfer case clutch. The transfer case clutch allows for slight speed differences between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. When the rear wheels are overpowered, the GEM detects this slip condition, and the duty cycle to the transfer case clutch is increased until the speed difference between driveshafts is reduced.

The 4WD mode switch is a rotary switch which allows the vehicle operator to choose between 2WD, A4WD, 4WD HIGH, and LOW range positions.

The Vehicle Speed Sensor (VSS) , mounted on the rear of the transfer case, informs the GEM of vehicle speed.

The Hall effect sensors, mounted to the transfer case, are used to detect the speed of the front and rear driveshafts.

The electric shift motor, mounted externally at the rear of the transfer case, drives a rotary cam which moves the mode fork and range fork within the transfer case to select between the 2H/A4WD, 4WD HIGH, and LOW range positions.

The shift motor sense plate, an integral part of the electric shift motor, informs the GEM to the position of the transfer case.
The Transmission Range (TR) sensor, located on the left side of the transmission, informs the GEM of the transmission gear position.

The Expedition/Navigator is equipped with 2WD and 4WD vacuum solenoids attached to the upper RH side of the engine compartment dash panel, they alternately route vacuum to the vacuum motor which engages/disengages the center axle disconnect collar in the front axle assembly.

The 4x4 shift motor relay is a module containing two relays which, under the control of the GEM, shift the transfer case shift motor between the 2WD and the three 4WD modes.
The solid state clutch relay is used to activate the A4WD clutch within the transfer case

Just want to point out this is not accurate for all models or at least the 2000 expys. Not sure what year expy the op has. There were a few years there between the use of the center axle disconnect and the IWE's where there was no 2wd selection. The 2000 expy actually has no center axle disconnect or IWE's. Not sure exactly which model years this applies.

Curious if the years were mentioned in the article?? Maybe you could post the link or let us know the source.




Tacsat117..

Its not clear on the exact sequence of events or your situation but the 4wd issues and codes may be unrelated. When you disconnect the battery and reconnect you are reseting the ecu. Soon after if there is an issue the ecu will recognize and set the codes.

P0174 & P0174 are banks 1 & 2 lean.

Likely have vacuum leak.

P0401 is insufficient egr flow. This has to do with dfpe readings. Could be bad hoses leading to the dfpe sensor from the egr tubing or there is too much carbon buildup in the egr system.

There may also be an issue with the 4wd that is no recognized until shortly after the ecu is reset or its just a matter of time after the battery is reset.

Think Canadian Expy was considering gem module issues as they are somewhat common to the expys and it would link the various issues. When I had water in my gem/fuse block the floor was completely dry!! The floor will dry out the gem/fuse block does not.

I am betting you have several seperate issues that are in reality related to you resetting the battery and not the 4wd.

 

[toms89]

Found thread you posted last month. Believe all advice given is sound....did you ever check your vacuum lines??

When you reset ecu you clear the codes and if you have faults its just a matter of time before the ecu spits out codes and may or may not go into limp home mode. If you keep getting identical codes I would take it at face value. Sounds like you purchased someone elses problems. They may have reset it hoping in your test drive it did not surface?!?

http://www.expeditionforum.com/f42/bought-yesterday-threw-codes-today-17203/

 

[Exia]

Just want to point out this is not accurate for all models or at least the 2000 expys. Not sure what year expy the op has. There were a few years there between the use of the center axle disconnect and the IWE's where there was no 2wd selection. The 2000 expy actually has no center axle disconnect or IWE's. Not sure exactly which model years this applies.

Curious if the years were mentioned in the article?? Maybe you could post the link or let us know the source.




Tacsat117..

Its not clear on the exact sequence of events or your situation but the 4wd issues and codes may be unrelated. When you disconnect the battery and reconnect you are reseting the ecu. Soon after if there is an issue the ecu will recognize and set the codes.

P0174 & P0174 are banks 1 & 2 lean.

Likely have vacuum leak.

P0401 is insufficient egr flow. This has to do with dfpe readings. Could be bad hoses leading to the dfpe sensor from the egr tubing or there is too much carbon buildup in the egr system.

There may also be an issue with the 4wd that is no recognized until shortly after the ecu is reset or its just a matter of time after the battery is reset.

Think Canadian Expy was considering gem module issues as they are somewhat common to the expys and it would link the various issues. When I had water in my gem/fuse block the floor was completely dry!! The floor will dry out the gem/fuse block does not.

I am betting you have several seperate issues that are in reality related to you resetting the battery and not the 4wd.

I can even remember what I searched for, I know it was about the issue, ill search and find the link, Also I think that was for 97 - 2002

 

[Jay87LX]

Sounds like possible vacuum issues