Cool Cat fan behavior - Typical?

I just installed a Cool Cat fan on my S1, with the Cool Cat switch, and a relay.

If I start the car from cold, the switch triggers the fan at 185F, just as designed. Then the fan runs for a couple of mins as it pulls the temp down, and switches off…perfect!

However, from that point onward, the fan cycles on and off about every 15-20 seconds. It almost seems as if the switch can’t reset itself to the 185F once warmed up. The net result is that the fan is running almost continuously around town. The fan doing a great job of keeping the temp under control, but I expected longer periods between cycles.

Have other people had similar experience?


Ask the source!


I figure Mike would see this, and chime in, but given the number of board members with this set-up, I figured it would also be good to get a number of data points.

I have a S1 with a Cool Cat. It runs of what I suspect is the original Otter switch, no relay. Works perfectly. Paul.

On second thought, I think it does have a relay.

Can you describe what “works perfectly” means?

How long are the cycles once the car is up to temp?

I suppose you could bypass the relay and observe

I use CoolCat fans and the thermo switch Mike supplies with the aluminum radiator in my S2. The fans cycle normally, e.g. they may come on at a stop light then go off once I am moving again, or come on in traffic and go off once I am on the open road.

The switch is a modern style and may differ slightly from the one currently offered on the website (both stand-alone and with a fitting to match the otter switch mount).

I’ve been through half a dozen different switch elements over the years, but they all are supposed to work the same way. If you drop me a message offline, I’ll send you a new switch element.

I installed the Cool Cat fans and otter switch on my S2 over 2,000 miles ago and they work perfectly. They come on ~185° and off ~ 175° using the stock radiator. They’re way more powerful than the stock windscreen wiper motors.


Yes. It cycles on and off but there several minutes between the on an off set points. The temperature moves a few degrees celsius between each point. It sounds like your set points are too close. Paul.

Thanks Mike,

So, you think this is a characteristic of this particular switch?

I don’t know. The easiest way to find out is to try a different switch.

Ok, I have another switch on-hand, so I’ll give that a try, and see if it changes the behavior.


I would guess normal. When the water temp cools in the radiator the fan shuts off but the convection effect is still working, cooler water in the radiator falls and pushes hot water from the engine block up. The switch sees it and turns on the fan again. A few cycles might occur.

Convection cooling, like a Ford Model T’s thermosyphon cooling. They didn’t have water pumps.

Mine does that. I’ll pull into the garage with the fans not running and shut down the engine and while I’m unloading groceries the fans will kick on. Mine are set up to do that on a variable time relay I’ve set for a maximum of about 6 minutes.

I believe if anything, it has to do with the LOCATION of the otter switch.

My friend has an S1 and the otter switch on his car is mounted in the header tank.

This makes no sense to me. If the thermostat is doing its job the otter switch should see very little temp variations.

Most modern cars have their fan switches mounted on the cold side of the radiator. That does make sense to me.

I’m referring to having the engine running. Sitting at a traffic light, the fan is cycling on-and-off every 20 seconds or so.

I understand that. My response was in reference to Paul’s post.

For your rapidly cycling switch I’d agree with those who suspect the switch.

Just to close the loop on this, I changed out the switch for another one I had on the shelf…same brand, I believe.

The cycles are now upwards of 1.5-2 mins long, which is more in line with what I would have expected them to be…night and day.

Thanks for the feedback