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MG Car Repair/radiator fans


QUESTION: Barrie. I recently asked a question about my rad. fans. I left out some facts. The car is 79 MGB USA. My fans do not work. The resister for the distributor was bad.  The new part that I ordered was substituted with a 5 prong relay.  When I tried to wire it, my gas gauge, tack and temperature gauge did not work.  I am not sure if I have the right part or if it needs to be rewired a certain way.  Could you help me.

Replacement Relays
Replacement Relays  
ANSWER: Hi Don.  Thank you for giving me the additional information that I needed.

It would appear that the original relays are no longer available, so modern units are being substituted.

I have attached a document from Moss Motors that explains everything.    

You only have 4 wires and the relay has 5 terminals.  They must be connected as follows:

86 = White
85 = Black
30 = Brown
87 = White with a Brown stripe.

Your relay has a fifth terminal marked 87a.  Do not connect anything to it.


[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: Barrie. The reason I'm confused is that there are only 2 wires available from the old resistor.One is white with a brown stripe,the other is solid green. I wired the new relay from the w/b 30 to the 37a green and lost my tach etc. Where do I find the white,black and brown wires that you refer to. Thank you Don


I am having trouble understanding your problem.  If the resistive wire was faulty, why did you buy a new relay?  And what has that got to do with the radiator fans?

The ignition is supplied via a white wire from the unfused side of fuse #3.  It changes to a pink resistive wire inside the loom.  It then changes colour again to white with a light green stripe before exiting from the loom.

The tacho and temperature gauge are both supplied with power via green wires from fuse #3.  I suggest you check this fuse.  

Keep the dialogue going.  We will sort this out eventually.


---------- FOLLOW-UP ----------

QUESTION: Barrie. I thought my fans did not work because of a blown fuse.I replaced the fuse. When I jumped the hot side of the ressitor to rad. thermo. plug my fans ran. Ergo bad resistor, I thought. I have since repeated the process, same result.I now know that it was just a set of circumstances. I have no power to my rad. thermo. switch. I checked the continuity and the thermo switch is open. I checked all my fuses with a continuity tester and they are o. k. I found one inline fuse with a grey wire that had no power. Fuse is o.k. Where do I start looking for the open thermo. wire. Thanks Don

Hi Don
1) The only resistor that I am aware of is the resistive wire between the coil and the ignition switch.  

2) I cannot find an inline fuse with a grey wire anywhere in your wiring loom.  Fuse holders have 2 wires.  What is the colour of the other wire?

Have you checked the inline fuse holder for the fans?  

One side of the inline fuse holder goes to the non-fused side of fuse #3 (White/Brown).

The other side of the fuse holder goes to the thermostatic switch in the radiator header tank (Black/Green).

All the items that have stopped working are powered via the FUSED side of fuse #3.

The power to fuse #3 comes via another (White/Brown) wire from the ignition relay.  Perhaps the relay is failing or there is a bad connection at the fusebox.  Please note that both the wires to fuse #3 are the same colour, so it is easy to get them mixed up.


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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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