The Stovebolt Site Forums The Shop Area The Electrical Bay 1956 GMC Factory Electric Wiper Motor

I need a little help here with the proper way to bench test my wiper motor. Quite a few years ago, I pulled the original electric motor that was in my truck, and replaced it with a Newport motor. Problem is I can't for the life of me remember if I bought the motor or if it was in a box of parts that came with the truck! Well, the truck has rarely been driven in the past few years as it was stored up at the mountain cabin and only used to haul firewood. So the Newport motor has about 2 hours of cumulative run time, and that's probably a stretch. It seems to be locked up. I would like to test it, to be sure, before I call them, but I would also like to test my original motor and see if it's bad. I honestly don't remember if I replaced it just thinking 'might as well' or what... In any case. What is the proper way to test these single wire motors? The replacement Newport motor wires in the same way as the original. It grounds to the under side of the dash, and the hot wire goes to the top terminal on the ignition ballast resistor. Thanks for any insight on this.

I would think an ohm meter test on your installed motor would prove it is not an open circuit. Pull fuse and clip one lead to motor side of fuse holder and disconnect battery for safety. Other lead to ground. Read meter, if infinity then make sure wiper switch is on. If everything is normal then you will read the resistance of motor which is a low resistance.
Just use a battery with jumpers wires to bench test the spare one.

Thanks! I got them both tested. The Newport motor when bench tested tries to run, but fails in high speed, and just growls in low speed. Thankfully, the original motor works just fine! I will clean and lube the OE motor and reinstall it with cleaned up grounds. I may send the Newport motor back and have them look at it. I spoke with them on the phone and they said they could check it out. I looked through some of my old notes and I believe now that the motor did come with my 56 Carryall that I've since sold, but it was new in the box at that time. For anyone who might be looking for the same answer I was about bench testing, the way to test the original motor was to hook the positive side of the battery with a jumper to the brown power wire, and to use a jumper from the negative terminal of the battery to just touch the bracket that grounds the motor to the underside of the dash with the switch in in any of the three positions. The Newport motor can be tested by clipping a jumper from positive to either the yellow or white wire terminal ends coming out of the motor, these are the high and low speed positions, and then touching the negative terminal jumper to the threaded brass shaft coming out of the transmission.

I'm somewhat confused. Your Newport is two speed, your orginial is one speed. What is your control switch?

I'm not sure what you're confused about. In fact the factory 55-57 GMC electric wiper motor is two speed as well. I have confirmed that this motor is correct and original to the truck using the GMC master parts book. The factory switch control is a cable, just like the vacuum motor would have used. The switch itself is onboard the wiper motor. The Newport motor uses an electric switch.

I just got the factory motor reinstalled in the truck. What a nightmare of contortion that requires... A few years away from being under that dash and I forget. The motor runs strong in both speeds, but for some reason it's not going through the complete return to park. It's parking at 90 degrees straight up rather than horizontal along the bottom edge of the windshield.

Does anyone have a service manual for these wiper motors that might shed light on what I can do to fix this issue?

So it was an easy fix. At first I was really frustrated because I figured all I needed to do was just clock the wiper arms where I wanted them to be. Then When I turned the motor on it wanted to drive the arms into the cowl! So I pulled them and observed where the arms were wanting to park, which was straight up. Then it occurred to me I had the transmission arms on wrong pivots. Switched them and all is now well!

What confused me was the orginial motor has one brown wire and the other has two hot wires. That made me believe one was two speed, the other one speed.
I was also unaware you were using two different switches.

I gotcha. Well thanks for your help anyway. Electrical is my weakest area on these old trucks so sometimes I ask what seem to be rudimentary questions. Just trying to learn as I go!