atomic_fungus (atomic_fungus) wrote,

#5603: Well, that certainly makes sense

So, this evening I decided I'd have a gander at the Jeep's rear wiper motor.

When we bought the Jeep in 2007, it didn't work. In September of 2010 I replaced the rear wiper motor, and it worked for a little while; it feels like about 10 minutes but it must've been more like 10 months. Then, one day, in the middle of wiping, it just stopped.

I watched a video where a guy bought a used Cherokee for $300 and spent a total of $1,800 massaging it into a daily driver. One of the things he talked about was how his mechanic fixed the stuck rear wiper, saying "it just needed some grease".

Well, WTF, I can do that!

This evening I had a gander. Pulled off the interior trim and pulled the wiper motor, popped the cover off the gearbox and verified that it was all good inside. Reasoning that the output shaft was the likely culprit, I pulled it out and--sure enough!--it was rusty.

Got the rust off with some 800 grit; did the same for the bushing, lubricated with fresh grease. Reinstalled shaft, packed grease around top of bushing to keep water out, put on seal, completed reassembly, reinstalled into truck and tested.

I could hear the relay clicking, but the motor wasn't turning.

Pulled it out again. Reasoning that the motor was at fault, I tried supplying power to the motor itself, bypassing the relay etc. Nothing, not so much as a quiver. Got out the test light, tried seeing if power was flowing through motor; power was not flowing.

"Is the motor open?" I wondered aloud. Well, to find out, have to take the motor off, so I did that. Turned out that the motor was not self-contained, but that worked in my favor anyway, and I found that the motor shaft itself turned freely, meaning it wasn't seized. Well--pulled the gearbox cover back off, pulled the gears, and yanked the motor armature. No obvious issues with the commutator.

Next, check the brushes--ah, this is my problem. Plastic had melted around one of them, keeping it from contacting the commutator. All I need to do is pull the brush out, clear away that crap, and presto.

...pulled brush spring, tried to push it out, and it would not move. The brush on the other side wouldn't move, either, even though it looked okay. Looked at commutator again, saw a blob of silvery stuff on one of the contacts, right in the middle, not shorting anything...but that blob could only have come from one source.

I put it all back together and put it back in the truck, knowing--now--that I need a new rear wiper motor.

Here's what happened: water got down into the thing and rusted the shaft. That made it harder to turn. Eventually, it stopped turning, right in the middle of a stroke, and with the motor stalled the motor drew its maximum current. The brushes overheated and melted the plastic around them. Thermal expansion pushed them back into their bores while they were still getting power; when the power was disconnected (when I got to where I was going and shut the engine off) they cooled, and in shrinking they broke contact with the commutator.

DC motors are--or should be--designed not to catch fire if they remain stalled for a long period of time; this one was obviously no exception. But the fact that the brush holder melted is unforgivable, and now I'm having to replace a $60 assembly because a $0.15 part died. Well, the $0.15 part died because the guy who rebuilt the thing didn't pack it with grease the right way; and if I get another one of these things I'm taking that end seal off and making sure that end's packed with grease, to keep water out.

What a pain in the butt.

* * *

Speaking of pains in the butt--

Mrs. Fungus and I went to bed a bit after 11:30 tonight, both of us pretty tired. Well, there's some thunderstorms on the way, and the weather radio started bleeping with the first severe thunderstorm warning just as we were drifting off to sleep. Then it did it again just as I had managed to fall asleep.

So I got up and told Mrs. Fungus I'd stay awake and keep the thing from yodeling her awake, at least until this line of storms passes. Looks as if I'll get to go to bed about 1:30 or 2 AM. That's okay. She's going to work tomorrow, and has a long day ahead of her; the most I've got to do is to go pick up some dry cleaning for her and wash the dishes. That's about it.

And then it'll be Friday.

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