Rear wiper has a mind of it's own!

[Jerseyrider]

First of all, this forum has been extremely helpful in diagnosing and fixing some common issues so thank you. Now a new issue popped up. Recently, I've noticed a clicking or buzzing noise coming from the rear hatch area. Then the rear wiper will do a single wipe and park itself and at random intervals repeat the process. If I turn the wiper on to any of the positions, the clicking/buzzing stops and the wiper works normally. If I shut it off, it parks itself and then it begins to randomly wipe/park/repeat. Also, it seems that all other hatch function operate correctly, such as the rear defogger, license plate lights and the locking mechanism.

A little background is necessary here. My '02 Trailblazer LTZ has 245K miles and there is a rust hole in the lower passenger side of the hatch. I'm wondering if there is a corrosion issue with a plug or maybe a shorted wire. Any suggestions? I'm going to take the interior panel off and check the wires and connections. I'll post what I find afterwards.

As I'm writing this, one of my co workers comes into my office laughing and tells me that he was walking past my truck when he heard a noise and then a thump and he observed the rear wiper just doing it's thing. I just went outside and opened and shut the hatch hoping to temporarily get it to stop and not kill my battery. I can't wait to fix this issue.

 

[IllogicTC]

You can pull the RR Wiper Circuit Breaker (15 amp breaker) in the rear fuse block to stop it while you diagnose and repair. I'm not sure exactly which breaker that would be.

It sounds to me like the rear washer assembly itself is flaking out. It receives a speed signal from the LGM, and controls itself forward and backward, and to and from the rest position. It receives its own power independently through the breaker mentioned above, with the LGM just telling it what it should be doing. I would also check the wiring from the LGM to the wiper module, as the method by which speed instructions are passed is interesting...

The wiper module puts battery voltage out on the speed circuit. The LGM then uses PWM to put the circuit to ground. The rate at which this pulse occurs tells the wiper motor what to do. An intermittent failure condition of the speed wire could be making it go bananas.

 

[Jerseyrider]

Thanks. I'll post whatever I discover when I'm done.

 

[hdodd1]

Had the same problem. New wiper motor corrected the issue.