Electric Radiator Fun Gremlin Wrangling

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Jul 19, 2018
Kodiak, AK
I’ve been chasing my tail troubleshooting a temperamental radiator fan and have reached the ends of my meager electrical skills.

My ‘78 FJ40 started overheating the other day, and low and behold the radiator’s electric fan wasn’t running despite the coolant temp getting up over 200 degrees. Prior to this episode, the fan had been running well without significant issue.

The previous owner installed a full Kwik Wire kit, along with a single speed radiator fan and oversized aluminum radiator.




I checked the usual suspect, namely the fan fuse, and it’s continuity was intact, then jumped the fan motor directly from the battery, which got it running up to speed without issue. When I pulled the relay, I noticed the previous owner had not connected the relay as the relay’s diagram instructed; he had connected the wire sending power to the fan to the terminal from the power source (87), connected power from the battery to the terminal for the fan (30), relay ground wire to the relay ground terminal (85), and the switch wire to the switch terminal (86). I assume this is just to reverse polarity of the motor and get it spinning the way he wanted.


Jumping the relay (85 to 86), it clicked close and showed proper continuity across the power portion (30 to 87).

When I start the truck and let it warm up, the gauge indicates a marginally accurate temperature (thermal gun at the upper coolant housing says 190, gauge says 210), but the temperature continues to rise without the fan ever coming on. I tried dropping in a new relay, wired the “correct way” as indicated on the relay diagram, with no change in symptoms.

I’m about all out of Schlitz; do any of you fine people know which gremlin I should start chasing?
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I think I got it; temp sensing at the gauge doesn’t pull from where I’d thought. Fairly certain this switch has gone bad- does anyone recognize this switch? TSMs and Haines don’t say it exists...


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