How to test rear tailgate multiplex ecu 89222-35010

[1950884]

Does anybody know how to test the multiplex ecu mounted on the tailgate? It controls rear wiper, rear defrost, rear power window. None of those function in my vehicle. The tech manual says you can connect a scanner to look at the ecu’s inputs and outputs but I don’t have that tool.

 

[1950884]

The factory repair manual has a section titled MP where the multiplex troubleshoot procedure exist. You don’t have to have a scanner to test.

I did learn that the rear door courtesy switch is part of the rear door latch/locking mechanism.

 

[1950884]

Found an issue. The back door ecu harness should have 10-12 volts on the blue and blue-red wire at all times, irrespective of key position. My blue wire has 0.2-0 volts. Ecu cannot function due to this. This blue wire travels through the back door BD1 rubber to a connector in the roof, then finally transitions to blue-orange where it receives power from the 30amp fuse in drivers junction box. I have power across that 30A fuse, so the issue lies between that fuse and the back door ecu. That’s a lot of territory to cover. I guess the first step is to remove the headliner and test for volts at the connector in the roof.

 

[ArthurKotb]

Are you able to use a jumper to the connection to give it 12 vdc to confirm that's the problem? If so, I would just find a way to run a new wire from the back to the front. I ran a Sirius antenna wire along the drivers bottom door opening, under the carpet. It would be easier than taking down that headliner I would think

This blue wire travels through the back door BD1 rubber to a connector in the roof

Have you checked the voltage on the blue wire close to or at the connector? Maybe the wire is pinched/broken inside the rubber tube

 

[MikeinNH67]

Found an issue. The back door ecu harness should have 10-12 volts on the blue and blue-red wire at all times, irrespective of key position. My blue wire has 0.2-0 volts. Ecu cannot function due to this. This blue wire travels through the back door BD1 rubber to a connector in the roof, then finally transitions to blue-orange where it receives power from the 30amp fuse in drivers junction box. I have power across that 30A fuse, so the issue lies between that fuse and the back door ecu. That’s a lot of territory to cover. I guess the first step is to remove the headliner and test for volts at the connector in the roof.

It's not uncommon for wires to break where they bend between the chassis and the hatch. If you pull the rubber boot out of the chassis side and dig for that wire, look for a soft spot, that's generally the break. You can also test for 12v close to the chassis using a pin to dig into the wire with little harm.

 

[1950884]

Another update. Upon searching the drivers kick panel I found a partially unplugged automate am1 security or alarm system. I thought this might be the cause of my issues so I uninstalled it. Whoever installed it didn’t follow mobile electronic professional certification protocol. They removed insulation from existing wires, wrapped the new wire around, and covered with electrical tape. After 2 hrs of uninstalling I learned it was not the source of my problem. I did learn that the wire I was searching for exits rear of the drivers j/b and travels down the driver and passenger threshold. It was easy to pop off the threshold and reveal two looms of wire. The wire is blue with orange stripe but all I can find is blue with red stripe. Hm..

I thought about installing a new wire, but I’m curious how the oem wire failed and having been MECP certified want to ‘do it right.’ It would also save me time if I found the wire did fail at the hatch because then I could run a lot less new wire.

I did inspect the rubber boot on the hatch side and didn’t find any frayed or broken wires. It was terribly difficult to get that rubber boot back in so I did not removed the rubber boot on the chassis side, but now you have me wondering.

 

[1950884]

I replaced the blue wire from the rubber boot in the hatch to the back door ecu. The result was that the exterior switch to open the door works now! Also, the rear window will go up and down. However, the rear defrost nor the rear wiper turn on. It worked this way for several days and then quite for a few hours and then started working again. Hmm.

Does anybody know how the ecu knows when the rear window is down? I cannot find the location of this sensor but know that it exist because the defrost and rear wiper sections say the rear window has to be up for them to function.