Greetings

I bought this 96 4WD off of eBay last year, it was used by the USPS for the postal inspector, and I have put 25,000 trouble free miles on "Catfish". I'm reading the forum and hear references to the 4WD light and I realize that I've never seen the 4WD light on my car. I check the owners manuel and sure enough, it's suppose to have one, right beside the steering column. I looked under the drivers seat and found that 4WD module was unpluged. Plugged it in and voilà, the light works. I took it for a spin and everything seems normal.

So, why would the module have been disconnected? Would driving like that damage anything? Should I be looking for any problem to crop up with the module connected? Am I driving on borrowed time?

I had believed that the 4WD was working properly, but have not had a simple and reliable way to test it.

Any enlightenment appreciated?

 

I have a '91 AWD and can help a little. If the van drives ok
and doesn't do strange things when turning a corner it is
probably ok. Drive it for awhile and watch for a flashing AWD
light on the dash. I know 3 flashes means there is trouble with
the front speed sensor and/or wiring and 4 flashes means trouble
with the rear speed sensor and/or wiring. I am refering to the
speed sensors mounted on the transfer case. There are other
codes that that will flash the AWD light but don't have them handy.
The only way I can tell my AWD is working is when I am driving on
ice or snow. I am sure if was something wrong with the transfer case
or associated drive components it would be obvious in a short time.
I think that with the computer disconnected the clutches in the transfer
case are disabled. Hopefully one of the Aerostar wizards here will
offer up some more info.

 

Let me pass on my learning experience.

The AWD was disconected when I bought mine also. I plugged it in and everything was good, or so I thought. The previous owner had unplugged it when they replaced the front tires with sizes different than the rear. There are sensors that determine if the front and rear drive shafts rotate at different RPMs, an indication of wheel slip. When this happens it will engage the electromagnetic clutch in the transfer case and lock the system into 4x4. All was good until my first trip on the high speed expressway. When I exceeded 70 MPH there was enough of a difference in axle rotation speed to engage the system. Scarey! You could feel the whole frame torquing as the van tried to force the front and rear wheels turn at the same speed. I suspect this is what led to or at least accelerated the front half shaft replacement 6 months later.

The good people on this forum gave me a lot of knowledge on this system and how it works and the van has been trouble free since.

 

Let me first say, that regardless of the status of the control module, the van is always in AWD. Inside the transfer case, there is a center differential that 100% of the drive torque always goes through. This differential then splits the torque 1/3 to the front and 2/3's to the rear. It is doing this always, and has no way to do anything else.

The module under the seat controls a clutch, which is connected to the center differential. When the wheel speed sensors detect a difference in speed between the front and rear that is above a programmed threshold, it assumes that the wheels are spinning due to lack of traction. It then signals the clutch to lock the center differential to get the vehicle unstuck or stabilized and then disengages the clutch again. There is torque sent to both axles, however, regardless of whether or not this is working due the center diff's splitting action.

The problem with running different tire sizes front to rear is two fold - if the clutch unit is working, it constantly cycles the clutch on and off trying to correct a wheelspin condition that it only thinks is occurring. This leads to the shuttering pbrunner saw as well as wears the clutch out faster. It can also directly contribute to fatiguing other driveline components as well. However, as he also observed, if the tire sizes are only off by small percentage, then you have to reach a higher speed before the wheelspeed difference rises above the control module's threshold.

Even if the control module is unplugged, however, the center differential is always subjected to the wheelspeed difference induced by the different tires. That means that it is differentiating 100% of the time while the vehicle is going down the road, rather then the 15-20% of the time, when the vehicle is cornering, which it is designed for. So, doing this can potentially significantly shorten the center differential's lifespan.

So it's pretty important that the tires be the same size all around. They should also ideally be set with a pressure that gives the front and rear tires approximately the same rolling radius, even if there's more weight on one end then the other to squish the tire down.

Anyway, with that out of the way, to answer the original question: no, running with the module unplugged will not hurt the drivetrain. It just doesn't take full advantage of its capabilities. It may have been unplugged, as it was on pbrunner's van, because the former owner was having issues with it or was running mismatched tires. If the tires are now all the same size and the van doesn't do anything objectionable, then it probably is OK. You might try to back the rear end only a dirt or gravel surface or something else slippery and have a buddy watch the wheels to see if they just spin a lot when you gas it. They should spin initially and then slow down to same speed as the fronts if the clutch is operating properly...

 

I think everything's OK, I've had it out on the freeway once, it feels normal.

I understand about the tire size, this van came with a matched set, including spare, of Goodyear Regatta P215 70R14's. I have them rotated and balanced every 10,000 miles and run 35psi. They are wearing well, with plenty of tread left.

Everything I've seen suggests that the USPS kept it well maintained, I was just puzzled about the disconnected module.

As soon as it snows in Alabama, I'll check out the 4WD.