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MG MGB Technical - Electric Cooling Fan Problem

While timing my 1977 MGB I had an overheating issue. The fans seemingly did not come on to cool the radiator. I have tested the fans by pressing the two leads together that connect to the fan switch in the radiator. At first the fans responded but further testing of the leads give me no response when they are pressed together. I see no evidence of a fan relay switch. The green lead and the white/brown lead attach to a two prong (I am going to say circuit breaker) as that is how Moss Motors lists the part. I have one on order however all fuses seem to be ok and if I test the leads with a volt meter they are getting power but do not respond when pressed together. I will be replacing the thermostat (no fault) and the fan switch but am perplexed as to why the fans will not come on when the leads are tested. I am afraid that when I have everything installed and topped off I will get the same overheating problem. Any ideas on how to proceed or where to look or am I just missing something?
JCH Hibbard

Some thermostat switches fail without warning, so you should take it out of the rad and dip it into boiling water.
With two leads connected and the tester set to Ohm you should read '0.00' within some seconds or, if there is a 'beep' funktion selected, it should give alarm. Let the water cool down for some 15 minutes and it should stop beeping or read the max. resition for the range selected.
If it works this way, the sender unit is O.K. otherwise a replacement is needed.
Before you start this testing procdure, you should check the ground connection at your fans and all of the connectors used in this curcit as sometimes corrosion is found there, leading to endless problems in relyability there. There are further very helpfull information given on Paul Hunt's page, have a look there too!

Hope this helps

Ralph
Ralph

JCH
I have a '77 "B" also and had the same trouble some time ago. I ordered a new unit(the one that fits into the radiator) and have no more problems. Like Ralph mentioned, those units can die just about anytime.
Good luck,
Don
DonaldElliott

Thanks Ralph and Don.Ordered a new fan switch to replace the exisiting one in the radiator. I think it may be someting else though as the fans should come on when the leads at the end of the wires are pressed together and they do not. I still don't see a relay so I am hoping it is the circuit breaker I ordered from Moss. The hot water idea was brilliant never thought of that and the fans did not respond. So it got to be the switch or the breaker or a relay somewhere that I am not seeing.
John Hibbard

John. The thermocouple that fits into the radiator blows out on a regular basis. This was a known problem and the design of the mechanism was changed in 1980 to correct the problem. If you use the original system, some mechanism to keep it from being expelled is a good idea. Most of us have the hole patched over, then, use an after market fan controller. Brit-Tek is the model I use on my cars.

Use a volt meter, or a test light, to see if you are getting current flow on the wires going to the thermocouple. One should be hot when the ignition switch is in the run position. The other is the wire going to the fans and will show no current present unless the thermocouple switches power to it. If you have power present, you have a problem down stream of the thermocouple as power should flow when the two wires are jumpered. If you do not have power on one of the wires, you have an up stream problem.

If you have power at the input to the thermocouple, jumper the wires together and check for power at the fan motors. You also need to clean the connectors and check for a firm engagement. They are not well sealed and the prongs can oxidize over the years, keeping the fans from working.

You can pull the fan motors and hot wire them to the battery to test the operation of the motors. The bodies of the current fan motors are .025" undersized (as compared to the original motors) and are not held securely in the brackets, even when fully tightened. If using a new motor, you need to measure it and the old motor, then, shim the mounting bracket to hold the motor properly. If not, the motor will fly backwards and the fan will be damaged.

Removal of the old fan blades from an old motor may not be possible. The set screw becomes corroded in place and may have to be drilled out, an interesting process at best. Then, the hole has to be tapped to the next larger size and a new set screw installed. Great fun.

Les
Les Bengtson

The 'circuit breaker' between the white/brown and green wires replaced the earlier in-line fuse, it is not the thermostatic switch in the radiator, which goes between the green and black/green wires.

There is no *fan* relay (although there is an ignition relay that feeds the fans), so if the fans do *not* run when the green and black/green wires are making good contact with each other, then the problem is *not* with the thermostatic switch (or not entirely at any rate as you could have more than one problem). If neither fan runs it could be the 12v supply to the thermostatic switch i.e. the inline fuse or circuit breaker, or the 2-way connector in the black/green feeding the fans. If you connect a volt-meter to the green with the green and black/green wires disconnected ignition on you should see 12v. Now connect the green and black/green. If the fans don't run, and the voltage has dropped right down, it is the 12v supply that is the problem i.e. the in-line fuse or the circuit breaker. Do the same test on both sides of that. If the voltage stays up on the white/brown side but drops on the green, it is the circuit-breaker/fuse. If not then something else back towards the ignition relay. If the voltage stays up on the green and black/green when they are connected together but the fans don't run, it is almost certainly the 2-way connector near the fans, which is is a vert bad location as far as water ingress and corrosion is concerned.
Paul Hunt 2

At this point I found some connector corrosion that I cleaned up and now when I jumper the wires going to the thermocouple the fans run which they did not before. So power to the fans is seemingly good. However, when I connect the new (or old) thermocouple switch and test the fans by putting the thermocouple in high temp water nothing happens. The fans do not run. I tested them this way before chancing the engine overhating again after I got everything back together and then having to start over. So, power to the fans is good, jumper the leads and the fans run, test the thermocouple with hot water new or old unit and nothing, the fans do not come on. What do I do?
JCH Hibbard

JC. See my earlier post about doing away with the factory thermocouple and replacing it with an adjustable unit from Brit-Tek. I have used this system on my daughter's 77B and my two 79Bs. It works well.

In November of 1979, I purchased a new MGB in Bitburg, Germany. Owned it for five and one half years--until the second child arrived. During that period, when factory parts were still available, the thermocouple went out twice. (So did the turn signal switch.) The thermocouple is a poor design and does not hold up well. The modern reproductions do not seem to hold up as well as the original. And, the current gaskets do not hold the piece as securely as the original gasket did. (I have seen two thermocouples blown out of the gasket. One while I was driving my daughter's car and one when a fellow club member had it happen to him, when starting up, after a tech session I had hosted.)

Have a radiator shop patch over the hole in the radiator header tank, replace the factory thermocouple with a Brit-Tek fan controller, and do away with a number of potential problems.

Les
Les Bengtson

Thanks Les, can you fill me in on the installation? Do the existing fan motors get removed etc?
JCH Hibbard

JC. Everything stays the same except that the hole for the existing thermocouple is patched over by a radiator shop. While this is being done, the connector is clipped off of the existing wiring and two wires are soldered to the existing wires, running over to the driver's side of the engine compartment. The variable rheostat is fastened, by two self tapping screws, to the inner fender near the radiator. A probe is fastened to the upper, rear of the radiator and the rheostat is used to determine when the fan motors are activated. Rather easy process. One of our units has been in use for eight years and another for seven years. No problems noted and much longer life than the factory style part.

Les
Les Bengtson

JC

You might like to try a simple 'over-ride' switch to supply power to the 'output' side of the cooling fan thermo-switch. I, personally, would always use a relay in the circuit to ensure that maximum power was available for your fan(s). Also, make sure you have a good earth (ground).

FWIW

Peter
Peter Hills

Thank you Peter Les Don PAul and Ralph. Fans are back up and running. I finally found a faulty connection with some corrosion that I missed previously. I believe I will take Les' recommendation and switch over to the Brit-Tex Thermostat sensor. Thank you for the pictures by the way.
JCH Hibbard

This thread was discussed between 31/10/2007 and 08/11/2007

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