what causes cooling fan relay failure

[moore_rb]

Heat and vibration-

If the fan is drawing amperage near the relay's upper limits, then the relay is going to get hot while the fan is running. Check the gauge of the feed wires to the fan, and then check to make sure the 12v source, and the ground wire to the relay are both larger gauge wires than the fan feeds. make sure the ground wire is connected too a good, clean chassis ground spot with a solid/unlimited electrical path back to the battery negative...

Inside the relay are a bunch of solder connections- Heat and vibration can cause these solder joints to crack and result in open circuits. When this happens the relay stops working.

I always buy 2 of every relay for my fans and fuel pumps- when one fails, I swap in the spare, and I pop the failed one open, hit the solder joints with a soldering iron, then test it to see if it works. If it does, I pop the cover back on it and put it back with the spare parts... if it still doesn't work after re-soldering, then I toss it and order a new spare.

wash, rinse repeat