Radiator Fan Thermo Switch Testing
Posted: Sat Dec 13, 2025 7:16 pm
I'll do a more complete testing write-up in due course here, but thought I'd show off my makeshift test rig. I pretty much had confirmed my switch was bad based on symptoms, but wanted to see it for my own eyes while testing. I didn't want to submerge the whole switch in boiling water. Probably fine to do, but I don't really think it's designed for that, so came up with this. I tried a heat gun, but that took forever to heat the sensor up enough -- long enough to make you think neither trigger worked.
I took an old scrap of copper and drilled a couple of holes in it -- one for the switch and one for a temp probe -- and bent it to sit in a small paint can. I filled up the can with water until the level was right at my little fixture and then put the whole thing on a hot plate to heat up the water. Took a while, but sure enough the low-side trigger didn't work, but the high-side (two outer pins) did. You might need to add antifreeze to increase the boiling point if you have a 102C switch -- I always use the 85/93 version. That was consistent with what I was seeing in the car, where the fans would stay off until the car got near the upper white hash, then drop all the way to the lower white hash when the fans ran fast for a while. Just thought the little test fixture was worth sharing.
The only interesting thing I learned is that there is only one OFF temp -- around 82C on my 85/93 switch. So once the high temp side is triggered, the fans stay on high until the temp comes all the way back under the low-temp trigger point. As the temp comes down, they never switch back to low speed -- once the high speed is triggered, the switch/fans stay on high until the temp is low enough to turn them off completely.
I took an old scrap of copper and drilled a couple of holes in it -- one for the switch and one for a temp probe -- and bent it to sit in a small paint can. I filled up the can with water until the level was right at my little fixture and then put the whole thing on a hot plate to heat up the water. Took a while, but sure enough the low-side trigger didn't work, but the high-side (two outer pins) did. You might need to add antifreeze to increase the boiling point if you have a 102C switch -- I always use the 85/93 version. That was consistent with what I was seeing in the car, where the fans would stay off until the car got near the upper white hash, then drop all the way to the lower white hash when the fans ran fast for a while. Just thought the little test fixture was worth sharing.
The only interesting thing I learned is that there is only one OFF temp -- around 82C on my 85/93 switch. So once the high temp side is triggered, the fans stay on high until the temp comes all the way back under the low-temp trigger point. As the temp comes down, they never switch back to low speed -- once the high speed is triggered, the switch/fans stay on high until the temp is low enough to turn them off completely.
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