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MG MGB Technical - Fan Thermal Interrupter Replacement?

The thermal interrupter for the fans in my 1980 MGG died recently. Is there a recommended replacement available? It's jumpered at the moment...

Thanks!
Scott In Ohio
Scott Rodriguez

Hi Scott.

Do you mean the thermostat with two wires going into it that goes into the top tank of the rad on the passenger side (LHD cars) ?.

If so, they are readily availible, but some ppl have had quality problems and believe that it is best to use the thermostat to operate a relay rather than switching the feed to the fans directly.

Don
Don

Scott. There is no "recommended replacement", merely some options available to you. When the system was first used, it had a habit of leaking or blowing out. On the 1980 model, the problem was solved by using a slightly different style of thermocouple (an electric switch which is turned on by a rise in heat) with a wire retainer. I have not used one of these myself, but have never heard of problems with the 1980 style switch. So, your first option is to purchase a new, replacement thermocouple for your radiator. The Moss part numbers are:

Switch, thermostatic fan, 542- 213 @$43.95
Bush, sealing, 542-203 @$3.35
Clip, switch retaining, 542-206 @$1.25

Any number of dealers, such as the Little British Car Company can order the parts for you at a slight savings over the Moss Catalog price.

Your second option is to have a patch soldered over the hole in the radiator and use a different form of thermatic switch to activate the fans. I had the radiators for my cars cleaned out, the hole for the switch patched over and installed the system marketed by Bob Ford at Brit-Tek. I have used this on a 77B and 79B for over five years with no problems noted.

The use of a relay to supply power, using the switch and the existing wires to turn the relay on and off, is a seperate item. You can continue to operate without it, as the factory designed the system, or, you can add a relay to the system. Proponents of adding a relay point out that it will provide better current flow than the old wires and will put less strain on the system. It is probably worth considering and I may use that system in the car I am currently bringing on line. However, many of us have not needed it over the years. Thus, your decision because it is your car.

Les
Les Bengtson

It was the cheaper switch that people have had problems with, including me. I think it was for 77-78? I went through at least 3 before I switched to the later, more expensive switch and have not had any problems. The cheaper ones would last from as little as a couple of months to a year. I have had the more expensive one for around 4 years now with no problems. I would just buy the new switch,seal,and retainer.

Jayme
77MGB
Jayme

I did almost as Les did and used a different switch and plugged up the hole in the radiator. A pic is on here: http://www.asciimation.co.nz/pics/page9.html

I did is a bit differently though. The new switch has a bulb on a copper capillary that fits into the top radiator hose. It is adjustable so I can change the point the fan switches on at.

To plug the hole in the radiator I machined up plug in the shape of a fake switch in steel. That pushes into one of the normal rubber grommet things. In the back of the plug is a tapped 1/4 hole. I made a U shaped bracket that fits over the top of the tank and a bolt goes through that into the back of the plug. No way that thing is exploding out (like it did when I first started the engine)!

Overkill probably but when you have access to a lathe little jobs like this are fun to do!

Simon Jansen

Simon. Yes, lathes are great fun. Last week I was making the Bob M. heater control valve bases from scratch using the lathe and mill. There was a time when people having a lathe and a mill around the house was not uncommon. Hard to find people today who even know what they are.

By the way, the kit from Brit-Tek also has the adjustable feature allowing you to dial in exactly when you want it to come on. If one has an IR thermometer, the system can be set to come on about 20 deg (F) above the thermostat rating (i.e. when the thermostat is fully open and there is maximum water flow). This seems to work well in our summers.

Les
Les Bengtson

I will have to get myself a lathe some day soon. At the moment I do a night course at the local school which gives me access to machine tools. Unfortunately the government is changing how they fund the courses so in a year or two there won't be anymore night classes.
Simon Jansen


What is involved in changing to the later style switch? My fans aren't currently working due to the earlier style switch going bad.

Thanks,
Dave
David Plantz

You need the switch,grommet, and clip for the late cars. You will also need to change from the molded plug to female spade connectors on the harness. This didn't bother me because I had already cut the harness to allow the fans to run with the ignition on, after the switch kept breaking. I was living in Missouri at the time and I needed the fans. It's funny, now that I live in the northwest, my fans hardly ever kick on anymore.

Jayme
77MGB
Jayme

This thread was discussed between 11/04/2006 and 13/04/2006

MG MGB Technical index

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