TR Register Forum

Electric Fan thermostatic switch
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Author:  NevilleT [ Mon Oct 26, 2015 6:32 pm ]
Post subject:  Electric Fan thermostatic switch

I put an electric fan on my TR about a decade ago and just used a toggle switch to turn it on. Worked fine 99% of the time but occasionally I would glance at the temp gauge and see it in no mans land when I forgot to turn it on. I decided to put in an automatic switch before the alzheimers took over completely. That was a few years back.

First was a electronic unit from Davis Craig. Probe through the radiator baffles. It worked fine for about a year then just stopped working. Full credit to Davis Craig. When I rang them they said that unit had been withdrawn due to some faults and replaced it at no cost with a capillary unit. The probe goes inside the radiator hose.

That lasted a year. When I pulled the capillary out, the end was gone. Contacted DC again and once again they replaced it with a new unit. This one was their new electronic unit with probe through the radiator baffles.

That was about a year ago, and now it has failed. It has a digital screen which displays the temp of the probe, and where you can set the turn on point. Screen is completely blank. There is power to the unit but nothing happening.

I did not go back to DC on this one. Too much trouble. I found a unit on Ebay which has a temp probe, 2m of wire to a control unit, and a relay. You can set turn on and turn off temps and it displays current temp. All for the princely sum of under $12 delivered. Will it work? Don't know but there was a write up on an auto website that said it did the job. Only drawback is that the unit is a little larger than I would like, and not sure where I will mount it. Maybe under the dash. Size is 8.5x7.5x3.5cm Web link is

How long will it last? Not sure but I certainly will not be out of pocket. I would be interested to hear from other people who may have had experiences with Davis Craig or other temp switches.

Author:  Rusty [ Mon Oct 26, 2015 11:51 pm ]
Post subject:  Re: Electric Fan thermostatic switch

Hi Neville, I am sure some people have had success with thermos switches but I gave up after a couple of types. Reverted to a toggle switch with an led light in it. Pretty easy to see if its on and I tend to switch it on after travelling for a short time and just avoid any overheating chances. R

Author:  fingoon rob [ Tue Oct 27, 2015 12:04 pm ]
Post subject:  Re: Electric Fan thermostatic switch

I have an old fashioned controller with relay and capillary tube. The bulb is fitted into a themo-pocket, made from a piece of small bore copper tube set at an angle into a piece of hose dia copper tube. this means the bulb is not in direct contact with the coolant and has lasted about 30 years so far. Switches tend to have a large hysteresis or dead band which is often 15 dec F, so it switches on reasonably accurately but does not switch off until the temp has dropped that much, whereas a controller should be good for about 3 or 4 degrees if you can find a reliable one. Sometimes old fashioned can be better....

Author:  rob noonan [ Tue Oct 27, 2015 7:14 pm ]
Post subject:  Re: Electric Fan thermostatic switch

Hi Neville, Just a little suggestion. Check the coolant Ph with a ph tester, obviously it should be neutral, about 6.5. If not you'll have an electrolysis / corrosion problem and the more reactive metal like small tubes and thermo housings will get eaten away. If ph is high put a bit of vinegar in or if low put some bicarb in. Tap water is usually high, if you've use that it could be the problem? Cheers, Rob.

Author:  NevilleT [ Mon Dec 07, 2015 3:46 pm ]
Post subject:  Re: Electric Fan thermostatic switch

Just to finish off this post, I have had my Chinese thermostat in the car for a few weeks and it is working perfectly. Attached is a photo. I had to put it in a case, but found a suitable box in Jaycar. It was plastic so I painted it with wrinkle finish and it looks good. It has a display of the temperature of the probe which is wedged in the baffles. Push another button and you can set the cut in temp.

If you look at the picture, you will see I have the biggest horn in the club. Came off a boat and sounds a bit like a B double bearing down on you.

File comment: Thermostatic Switch
Thermostat.jpg [ 186.73 KiB | Viewed 4942 times ]

Author:  fingoon rob [ Sun Dec 13, 2015 6:30 pm ]
Post subject:  Re: Electric Fan thermostatic switch

This brings me to a recent thought. Do we measure the temperature at the inlet or outlet of the radiator?
I have always believed in puting the fan sensor in the radiator inlet, or near the thermostat. Howevr aftr my fan failed and caused some research I have hanged my mind.
Now the engine normally inceases the coolant temp and the radiator reduces it so my newlogic is that it is better to control the fan based on the performance of the radiator rather than the engine! After all the fan is required only when the airflow through the radiator is insufficient to provide the neccessary drop in temp. There is no need for the fan to run if the radiator is providing enough temp drop.
If one uses the radiator inlet to trigger the fan then it will have a much higher duty cycle than necessary and probably be the cause of premature fan failure.
If the fan is only triggered when needed, the duty cycle will fall dramatically and thus the fan will last longer?

The other issue might be that fans normally produce about 1000cfm per 10 amp current draw so taking into account the inrush current or spike of 2 or 3 times that and the car idling, will the generator/alternator and wiring etc be up to the task?

Using a huge fan is not then such a good idea! Well that is the current thinking.. no punn intended..

Author:  aladdin [ Mon Dec 14, 2015 1:45 pm ]
Post subject:  Re: Electric Fan thermostatic switch

The temp leaving the engine is the one that tells you the most about you coolant system. It tells you the average temp of the coolant exiting the engine. The temp exiting the radiator is the actual temp after being processed by the radiator/fan. You can get an engine pipe from Moss that has a thermo switch incorporated in it but I would not personally use it.

I will not enter the discussion on switch life. I do not like the capillary type that must be fitted in the radiator hose and use an electronic probe the sits in a fitting in the top radiator hose. I have fitted the type that sits in the radiator core to other members cars without any problems to date.

Location of the fan has been a topic of discussion for a long time. Behind the radiator is the best location, especially for the open road. It offer the least restriction for the air flow but greatly restricts the choice of fans due to space availability.

My fan is behind the radiator and is only an 12" air conditioning fan drawing about 6 amps. I do have a Volvo radiator, and electric DC water pump fitted though.

See my article on cooling in the Technical section under pre 2006 articles.

Author:  paulcos [ Mon Dec 14, 2015 3:19 pm ]
Post subject:  Re: Electric Fan thermostatic switch

Revington TR in the UK supply a stainless bottom radiator pipe with a welded thread to accept a thermostatic switch. They also have a range of switches that have different on/off temperatures. I have fitted this and use a relay with an override switch circuit so I can leave it on during hot weather. Works great.

Author:  fingoon rob [ Mon Dec 14, 2015 3:57 pm ]
Post subject:  Re: Electric Fan thermostatic switch

An interesting debate indeed, which has raged for a long time.
I am particularly interested in what the fans function really is and how often and for how long it is required to operate. If the thermostat is correctly sized and operating throught it's full span, one could assume that the tep exiting the engine will be say 82deg, unless the thermostat has opened fully and the cooler fluid entering the engine is not cool enough. This then creates a situation where the radiator is not good enough or does not have enough air flowing through it due to lack of progress. Enter the fan which compensates for speeds of less than say 30kph.

Having the fan triggered from engine outlet works just fine because the fan runs most of the time, even when travelling faster than the fan velocity. I know that because my system worked well. It just wore out the fan.

My current thinking is reducing the amount of time the fan runs. Down to when the engine inlet temp is insufficiently low to deal with the temp rise and the thermostat can no longer maintain the set point.

I gather many modern cars. Even Holdens monitor engine inlet rather than outlet temp to control the fan?

I also doubt the need in a 100hp engine for more than a 10 amp fan of 12" or14".

Perhaps there is no correct answer, otherwise the debate would not have interested so many people for so long.

As always I am keen to get input.

Author:  TR1954 [ Mon Dec 14, 2015 5:51 pm ]
Post subject:  Re: Electric Fan thermostatic switch

I have had an electric fan behind the radiator of my TR2 since I bought the car 40 years ago. The car has a standard TR radiator (no crank handle hole now), water pump etc.

The fan died about 5 years ago at an All British Day so I replaced it with a similar one. I've always had a switch under the dash as I don't trust automatic ones and am too lazy to fit one anyway.

I'm used to scanning the instruments, the aviator in me, so don't have a problem using a manual switch. I just turn the fan on when the temp gets to 185 if I am held up for more than 30 seconds or so, it usually runs about 170. I leave it on in stop start traffic on hot days until I get a bit of a clear run and then give it a rest until the next hold up. After 40 years it is almost a sixth sense, you just turn it on instinctively.

Manual switches aren't for everyone but it works for me.

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