afanboy

There's a set of seemingly unrelated symptoms on our cars. I was chasing them separately until I realized they all have the same cause.

The symptoms I had, in the order they started, each starting about a month after the previous one:

1. Keyless entry system started randomly locking and unlocking when car was sitting. Then it stopped working entirely. When I tried to program in new remotes, it never entered programming mode. Unable to connect to keyless entry system using Techstream.

2. Interval wipers and slow-speed wipers stopped working. Spray button only sprayed, didn't wipe. Wipers only worked in high speed position. Switch checked out fine. Replaced wiper motor, which didn't fix the problem.

3. One-touch moonroof stopped working. Holding the switch down worked as normal.

4. Temperature readouts switched to celsius.

All of these are caused by a failing MPX, multiplexing body computer. It sits under the steering column, but don't confuse it with the tilt/telescope computer. Also don't confuse it with the four outstations it has, one for each door. You need to replace the main MPX under the steering column, not any of the outstations.

The multiplexing body computer part# for 1999-2000 US cars is 89221-50110. (Other markets have different part numbers, because of celsius vs fahrenheit, possibly also because of remote signal differences?)

Replacement requires digging under the dash then unplugging the old one and replugging the new one. It does NOT require any programming or syncing and does NOT affect your immobilizer. However, you will need to reprogram your remotes and your seat memory.

Yamae

It seems to me that the voltage regulator in the MPX body computer is causing a high frequency oscillation.
Anyway thanks for the post, afanboy. This would be useful for some people who suffer similar problems.

Regarding the oscillation, I encounter it time to time at my local shops. A spectrum analyser is quite useful to find out the root cause of the problem. Just 3 days ago, I used it to fix the repetition of unlock and lock problem caused by the oscillating voltage regulator on a JDM car Subaru Pleo. Replacing only one capacitor completely stopped the oscillation and the strange lock/unlock problem was gone.

YODAONE

Does anyone have images of MPX body computer circuit board components?

Scraape

I am having issues with my door locks in my 99. Wondering if this could be the issue.

My car will go into programming mode but the remote won't program for some reason (maybe wrong remote FCC ID, or maybe something wrong with actuators?)

If I manually push the lock button , then close the door , I hear the locks cycle a second time after I have already pushed the lock button.

Also if i try to push the unlock button on the inside to unlock all the doors, some of the doors don't unlock.

It seems unlikely that all my door lock actuators would go bad, any suggestions ? Could it be this MPX thing?

Yamae

I'd rather think it's the lock/unlock actuators issue.
I have experienced your type of problem many times at local shops. The quickest way to fix is to replace the actuator.