Kevin S
Junior Member
- Joined
- Jul 19, 2017
- Posts
- 12
- Reaction score
- 19
- Location
- Japan
- First Name
- Kevin
- Truck Year
- 1979
- Truck Model
- GMC C2500 High Sierra, Colonial Yellow
- Engine Size
- BBC 454, manual SM465
Squarebody Chronicles: Windshield Washer Pump fix
Replacing a broken washer pump motor in an old 78-? full-size Chevy/GMC platform? Read on.
Project scope:
Added a "universal washer motor" to bypass the broken factory washer pump system. (LMC 36-4081) This system comes with a bit of hose and two fittings to tap into the rubber wiper hoses, the washer pump with two screws and two wires (red+ and black-) and two clamp-on wire taps.
GMC full-size trucks did not have factory wiper delay systems until 1978 onwards. My 1979 GMC C2500 3/4-ton pickup has the wiper delay system. There are three components: wiper switch (LMC part 36-0671) in the dash, connects to a delay & park sensor module behind dash, then the wiper motor/washer pump assembly on the drivers side firewall. Good idea to use Di-electric grease on all contacts and perhaps a bit of contact cleaner spray on disassembly, these are crusty trucks at 41 years old. I have heard sourcing the factory delay module part is tough (LMC staff). Only the pump was bad on mine. Why do I know the part number for that switch? I replaced that part first which didn't solve the problem... sooo
$25 but its shiny...
The hardest part was finding the correct wires to splice into on the factory system. Key to understanding was that GM is wierd and the wiper system from Acc/Ign ON is always "hot", and activating the wash button takes it to ground to complete the circuit. Current flow path is (+)Battery->keyON->motor->switch->ground(-). Trying to wire it before understanding this led to the pump emptying the bottle once activated, etc. Same concept for wiper motor (switch goes to ground to complete the circuit). Every wire is hot back there unless a switch is pushed...
I used a troubleshooting light from inside the cab's always-hot cigar lighter + terminal to find which wire coming out of the switch went to ground when the wash function was pushed, then an Ohm meter to identify continuity to the correct wire on the back of the wiper/washer motor assembly going to ground. That wiper motor has 8 wires. On the lower section, four wires for Hi-Lo wiper motor speed, middle two-wire connector for the washer pump motor (white, red), top two-wire connector is the "park" sensor so it knows the wipers are at bottom stroke and can leave them there.
Turns out the middle two-wire connector, with the red wire closer to drivers fender, is the one to splice the new pump motor's black ground wire into to complete the circuit. Red +positive wire for the pump goes onto white wire next to it (always hot wire). When using the trouble light with key-ON, both wires were hot; push the wash button the red wire goes to ground completing the circuit and activating the washer pump. Easy peasy once you wrap your head around the "switch goes to ground" concept.
Key article that helped me understand washer delay GM going-to-ground idea:
http://www.rowand.net/Shop/Tech/DelayWipers.htm
Replacing a broken washer pump motor in an old 78-? full-size Chevy/GMC platform? Read on.
Project scope:
Added a "universal washer motor" to bypass the broken factory washer pump system. (LMC 36-4081) This system comes with a bit of hose and two fittings to tap into the rubber wiper hoses, the washer pump with two screws and two wires (red+ and black-) and two clamp-on wire taps.
GMC full-size trucks did not have factory wiper delay systems until 1978 onwards. My 1979 GMC C2500 3/4-ton pickup has the wiper delay system. There are three components: wiper switch (LMC part 36-0671) in the dash, connects to a delay & park sensor module behind dash, then the wiper motor/washer pump assembly on the drivers side firewall. Good idea to use Di-electric grease on all contacts and perhaps a bit of contact cleaner spray on disassembly, these are crusty trucks at 41 years old. I have heard sourcing the factory delay module part is tough (LMC staff). Only the pump was bad on mine. Why do I know the part number for that switch? I replaced that part first which didn't solve the problem... sooo
$25 but its shiny...The hardest part was finding the correct wires to splice into on the factory system. Key to understanding was that GM is wierd and the wiper system from Acc/Ign ON is always "hot", and activating the wash button takes it to ground to complete the circuit. Current flow path is (+)Battery->keyON->motor->switch->ground(-). Trying to wire it before understanding this led to the pump emptying the bottle once activated, etc. Same concept for wiper motor (switch goes to ground to complete the circuit). Every wire is hot back there unless a switch is pushed...
I used a troubleshooting light from inside the cab's always-hot cigar lighter + terminal to find which wire coming out of the switch went to ground when the wash function was pushed, then an Ohm meter to identify continuity to the correct wire on the back of the wiper/washer motor assembly going to ground. That wiper motor has 8 wires. On the lower section, four wires for Hi-Lo wiper motor speed, middle two-wire connector for the washer pump motor (white, red), top two-wire connector is the "park" sensor so it knows the wipers are at bottom stroke and can leave them there.
Turns out the middle two-wire connector, with the red wire closer to drivers fender, is the one to splice the new pump motor's black ground wire into to complete the circuit. Red +positive wire for the pump goes onto white wire next to it (always hot wire). When using the trouble light with key-ON, both wires were hot; push the wash button the red wire goes to ground completing the circuit and activating the washer pump. Easy peasy once you wrap your head around the "switch goes to ground" concept.
Key article that helped me understand washer delay GM going-to-ground idea:
http://www.rowand.net/Shop/Tech/DelayWipers.htm
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