An observation about my cooling fans that don't cycle on and off

On the BSOL noticed that fans on other cars would cycle on and off, I had forgotten about this as my car used to do the same thing.
I have the CoolCat otter replacement and CoolCat dual fans on my seriesII.
Don’t recall the temp rating of CoolCat switch but it must be lower than the 190 degree thermostat that I am using. Using the 190 degree stat because I found that in cooler weather (50 deg.) engine would not get up to operating temp. Engine runs in the middle of the normal range most of the time and never goes past the “L” of normal.
My guess is that under speed the fans cycle? Maybe I should install an indicator light so I will know. Have used an IR to confirm temps.


The CoolCat switch is on at 185, so with a 190 thermostat you will run the fans all the time. You can replace the switch element with one from a mid-90’s Honda and it will give you 195 on. But the 190 thermostat is a mistake. It won’t get the car warm any faster than a properly operating 180.

I know that cold water in a hot block is bad. But why not get the garden hose and spray the radiator to cool it off and see if the fan cycles on and off as you cool the radiator and let it warm up again.
…just a thought.

Was this on parked cars with their engines off? I don’t know how else you would notice.

If so, the stock fans only cycle when the ignition is on but with a simple change can be made to continue to cycle after engine shut-down (that is how mine is set up).

Not everyone agrees this is a good idea but I prefer the fans to be doing some cool-down, especially at short gas stops in the midst of a long hard run (or while waiting for flagmen and pilot cars).

The cycling of the fans were on an idling engine. I used the 190 deg. stat not to warm up engine faster but to get it to run in the normal range. With the cooler stat engine ran too cold. Years ago drove from Indiana to california in January. Gauge needle never got into the normal range until I got to warmer temps in AZ and CA.


First time I’ve heard an E-Type owner complain about an underheating engine :laughing: One for the record books…

Not that I don’t believe you Glenn, it’s just a very unusual complaint for the cars.

My after market fans cycle after the engine is off also. I’ve also added a safety switch which bypass the otter switch and can turn on the fans at anytime.

My 70 etype is an underheater too. In the fall i disconnect one fan, sometimes i put a piece of cardboard in front of the radiator. Always have to pull the choke out at a red light for it to idle right.

I did find that raising the two carb needles up into the piston gives it a little more gas and now it runs great.

My fans will cycle on and off as needed.

I do not understand the purpose in our cars for the fan to cycle after the engine is off. No water is circulating …so what is the purpose? There are a few modern cars that have an additional electric pump that will circulate the water as well as cycle the fan on and off for a period of time.
In my 997 Porsche (water cooled) the fans do cycle on/off without additional water circulation …but its purpose is to cool down the engine compartment to protect the electronics from high heat after the engine is shut off. I do not think that applies to our cars and only increases the risks of a dead battery should one of those fans decide to stay on.


IIRC Model Ts didn’t have a water pump. Water circulated by thermosiphon action; cooler water coming out of the radiator into the block pushing hot water out into the radiator through the upper radiator hose where it’s cooled and goes back into the block. It works because warm water, like warm air rises compared to cooler water. The same thing will happen in an XK engine as long as the thermostat isn’t completely closed . And the fans are blowing hot stagnant air out of the engine bay.

Is it necessary? Probably not, but a lot of owners like the bit of extra cool down.

Ditto for older John Deere tractors. I have a 1947 and a 1950 and nether one has a water pump or thermostat. I still work both and no o’heating problems. JD in the time felt that keeping it simple increased reliability, even to the the extent that more than 2 cylinders was unnecessary (but they were BIG cylinders.)


This is interesting input by ever. I know that there are a variety of thermal switches but but their switching temps seem not to be mentioned. So how do you get a mid 90’s honda switch element for 195 C on !?

Here’s the Wells-Airtex part list. The thread size is 16mm-1.5. Find a switch in the range you like, and check the photo to see if the terminations would work. Then buy it by part number at Rockauto.

Well, Abe, there is ONE reason for allowing the fans to spin after the ignition, a reason that does not come immediately to mind. My car is one of several on this list that suffers from a paint blister directly above the exhaust manifold. I am convinced that allowing the fans to spin for 30 seconds or a minute after the ignition is turned off would reduce underbonnet temperatures enough to eliminate this expensive phenomenon. I intend to do some experiments with a remote thermometer and will report back.

Maybe the issue is not the engine heat…but the primer used, else everyone would be having the same paint problem. Wonder if we can blame the epa on that one. How I miss leaded oil based paint.

My cat is not running again yet but i plan to replace the fan relay and otter switch with FET drive and temperature measurement. I did not look at the cabling from AC radiator fan relay and just have a basic circuit based on BTS50015S and STM8 yet but softstart and outside temperature dependent run should not be a problem then. I had an issue with the fan relay back when the jag was on the road but could force the fans on by the A/C when necessary if i remember correctly.

I will post circuits / PCBs / firmware when/if this all is done.

I have my fans on a manual switch and have always wondered at what speed do the fans no longer add cooling and should be shut off? I usually just turn them on in city traffic.
Len Wheeler

Tweety, on the hottest of days, with the CoolCat fan and the massive NASCAR radiator, would barely reach 100C, in traffic, and in anything under 15C, ambient, I had to use a winterfront to keep the engine at 70C.

Never a complaint!!

Had same problem… It turned out engine wasn’t, hot enough to trigger the temp sensor. ONCE I tuned the engine, got the exhaust manifolds to register 300 degree and the fans started working again, IN short your engine may be running too rich.

I have been in the situation that my fans do not cycle off except at idle. (Cool Cat sensor) My temp gauge is pegged most of the time but using the Cool Cat temp indicators, the car is not running nearly that hot, and this has been verified with an infrared temp measurement.
I have never had my system boil over and even after a hot run releasing the coolant cap does not produce boiling.
i could have used a bit more heat from the heating system on the BSOL, but it was adequate.