My radiator fan turned on today at what sounded like regular speed and then slowed down and sped back up. It would cycle fast and slow several times and then shut off. Every time my fan came back on it would cycle up and down until it shut off. It was as if the fan had a rheostat and it was being turned up and down very slowly.

Any ideas where to start?

 

I don't know the model at all but a rheostat is a variable resistor so I would suspect varying resistances. Check the temp sender first, some are two wire so bridge the wires together and see if the fan now runs steady. If it does, it looks like the sender is faulty. Hondas are now two wire but work t'other way round so unplugging from the sender makes the fan run. Some senders are single wire, so earth it. If the fan is still running unsteadily with sender by-passed, check the fan by wiring it up directly from a battery, if it then runs OK you have a wiring fault. This ignores the fact that many makers wire fans through the ECU, but an ECU fault ends up expensive. Old Honda 350s had the fan wired through the CDI (why) and these used to fail, making the fan run all the time the ignition was on, most owners let it run rather than buying a new CDI, as the ignition side of the CDI continued to work OK.

 

Thank you for your help. My concern was that because the fan was "oscillating" that it could not be the fan switch or the thermal switch and it had to be either the fan itself or the ECU. I will do as you have suggested.

 

I just realized mine does not have an ECU/M as mine is not EFI. Looks like it could be the thermal switch, fan switch, radiator or the breaker, I think. Could the speedo also be a possibility?

Machine is an hour away at my farm so I plan on heading there tomorrow to take a look at it. First thing I plan to do is unplug the thermal switch on the engine. I believe this will cause the fan to run continuously with the key on.

 

It turned out to be the thermal circuit breaker located next to the battery.

 

Glad you found it, though not sure what a "thermal circuit breaker" is. Do you mean fuse?

 

It's a breaker that resets itself. In my case it's a 10amp breaker that trips when it exceeds 10amps and after it "cools" down it resets. I don't believe I was actually exceeding 10amps but because the breaker was faulty it was limiting current to the fan circuit which caused my fan to change speeds.

 

There is no function for the fan speed to oscillate. If it is then there is a fault in the ECU. The temp sensor has no bearing on fan speed. There can actually be a problem with the fan itself.
It turns on the heard up and begins to slow down. That can be a symptom of the fan motor going bad. Normally this would either blow the fuse or trip the circuit breaker for the fan as the current draw would exceed the fuse or breaker rating.

 

I thought the same thing but so far after replacing the breaker my fan is working fine.