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Wiper weirdness


I have an issue with my '99 4-door Blazer that I hope someone here can help me with…

Some months ago my rear window wiper began acting odd. The problems started with the wiper failing to make a complete pass across the window. At first I could get it to work by turning it off, and then back on, but as time progressed, this no longer helped. After the wiper stopped functioning completely, I disassembled the rear liftgate and removed the whole wiper motor/gearbox assembly.

When I took it apart, I found the shaft and metal driving gears all rusty and gunked up. I took the gearbox apart and cleaned up and lubed all the metal parts. Everything appears to be mechanically sound. I even tested the motor, and tweaked the metal sector contacts under the large crank gear. I know everything in the motor assembly works now.

The problems with this unit start when I turn the wiper motor on (via the dash switch). For whatever reason, sometimes the wiper motor/gearbox assembly only clicks and runs for about 2 seconds and then shuts off. Other times it fails to turn on at all. And yet other times it does nothing but chatter the relays in the black plastic control box. Now I am no slouch at troubleshooting electrical issues (low-voltage tech by trade), and I have checked all the voltages and grounds on the incoming wiring harness that comes from the main switch. According to the service manual I have all of the proper voltages on the plug, yet the wiper assembly refuses to behave normally.

Since the first bout of diagnostics, I have replaced the relay box (twice) and the entire motor/gearbox assembly. The POS still refuses to work. Examining the electrical wiring diagrams has led me nowhere. What else is there to check? No fuses are blown, and I can find no relays related to this circuit.

Has anyone else run into this kind of issue? Is there something I failed to check? Does the main gear in the wiper gearbox need indexing? HELP!


PackRat, I Really Think It’s Great That You Have Gotten Into This So Much. I Do The Same Thing. I Like To Service “No User Serviceable Parts Inside” Things.

It very well could need indexing. Are ther any little timing marks on gears or anything?

However, if you really want to just get it going, here’s what I’d do. My local “take-it-off-yourself” salvage yard has 10 year vehicles sitting for just a short time before they crush them. They’d charge me a small fee if I went in and extricated a motor from another Blazer. Do you have such a place near you?

If you still really had to fix yours (so that you are not defeated), you’d at least have another one to look at.

Good Luck.

Thanks for the reply,

Yes, I’ve gone that route. Got 2 different wiper motor/gearbox combos from a local yard. I could see no physical differences between my original one and the new/used ones. Just a year older. From what I can tell, the crank arm that attaches to the sectored gear can only go one way. If I rotate the gear 180 degrees out, the crank doesn’t reach. I’ve also cleaned off all the grease and found nothing that I could identify as timing or indexing marks. Applying 12v directly to the motor connector’s pins results in some real nice back and forth wiper shaft movement. This is not happening when the control module is in the circuit.

Anyway, seeing as how 3 different assemblies have all shown the same symptoms, I’ve gotta think that there is something wierd in the vehicle’s wiring. Aside from the fuse, the on/off switch, and the rear window open switch, what else is there to this circuit. Is there another relay somewhere not shown on the wiring diagram? I have been working under the impression that all of the wiper delay functions happen inside the black plastic box on the wiper control module. Is this incorrect?

Can anyone tell me what voltages I should be reading on the various pins of the connector that plugs into the wiper control module when the control switch is in the various OFF, SLOW, and FAST positions?