Electric fan cutting out.
 

Been chasing an overheat issue. Finally determined that my 2-speed electric fan was cutting out on high after about 15 minutes of driving. Replaced both high and low speed relays and the switch. Test drive yesterday with same results; temp going up rapidly at the last 2 traffic lights before getting home. Shut off the engine and the fan was not running, it is direct wired so it can be run without the ignition on. Turn off the fan switch and then back on and the fan comes on. Can the fan motor be intermittent , and why cutting out after a certain period of time? Any other ideas?

When you have it on, make sure you have power and ground at the fan connection. Check with the fan plugged in so you test under load. I'd look for a poor ground or high resistance connection to power first. The fan could be bad, but power/ground need to be checked first.

Try a full factory shroud back or radiator
Electric fans need fan shroud back side of radiator draw air like a vacuum.
Try it report back..

Does your fan have a thermal breaker on it? If so that may be going bad.

Thanks guys for the info. Will test some of these ideas first of the week. One other thing I had forgotten about is the 40A circuit breaker where the power comes in and then goes out to the 2 relays. Is there a way to test that breaker or just replace it and see what happens?

40a is a bit small for a cooling fan. I'd replace it with a 60A fuse or a 60a breaker minimum. If you go with a fuse, buy an extra to keep as a spare in the glovebox. If your current breaker is auto resetting after it trips, would explain the intermittent nature of the issue. Breakers trip from thermal/heat load, so make sure it's mounted away from heat sources as well. Breakers do wear out, look up the specs on your fan and see what the full load amp draw is. Whatever the rating, size at least 125% of that number. Fans draw a higher current when starting, far less to continue running once moving.

Full load amp draw is 22 on high. I know they draw more on startup but not sure how much. Anyway, thinking about going with a fuse instead and then carry a spare as you suggest.

Inrush current on startup is usually 2-3x of running current, so a 60a fuse should be about right.

Got the new 60A fuse link installed and took a test drive. Overheating problem solved. The fan ran on high as it should; no cutting out as before. The old circuit breaker was indeed the problem. Not sure why it took me so long to figure that out. Thanks for the help from all of you who weighed in.