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MG MGB Technical - Cooling fans

Looking for a little help with the twin electric cooling fans on my '79 B Roadster, which have suddenly stopped activating. I replaced the sensor/switch that inserts into the radiator, but that didn't solve it. I have a Haynes manual, which is telling me nothing. Can anybody point me in the next likely troubleshooting direction.

Thanks for any suggestions. PTG
P.T. Grater

PT. The first thing to do is check for voltage on the wires going to the thermatic switch which fits into the radiator. (By the way, these switches, especially the modern "replacements", tend to blow out of the radiator while you are driving along. Jolly good fun! Unless it damages something when it does, you are driving on the interstate highway system, or you are in Arizona where the air temperature is 110 deg F+). If you have power to one of the lines (do not remember what the color code is and the wiring diagram is out in the garage) you need to check for power at the fans themselves. If you do not have power on one of the lines, you need to trace the power back in the system until you find power, then examine where the break may be.

It is possible that the problem is both of the old fan motors are dead. They do this after 30+ years. Be aware that the outer diameter of the fan motors that Moss Motors Ltd sells is slightly less than that of the factory fans by about .025". Not a lot, but sufficient that the factory mounting brackets will not hold the new motors securely. Thus, when the fans come on, pushing air towards the radiator, the fans (with their unsecured motors) fly forwards like a Curtiss Pusher-type airplane. More great fun.

Then, we can talk about the difficulty of removing the set screws that secure the fan blades to the motors and how some of the Moss supplied fan blades do not have the set screws included with them.

But, the first step is to trace the flow of electricity to the fan motors, then to their grounds. When you have done this, and know if you have current flow to the fan motors, post again and we can tell you how to check out the fan motors (jumper wire them to the battery) and what would be needed if you have to replace your existing fan motors with the currently available fan motors and fan blades.

Les Bengtson

Ignition relay forward of fuse box feeds the fusebox through a White/browN wire. This has two other WN wires coming off the common terminal, unfused at this point. One of these goes the coil/dist feed, the other to an inline fuse just below the fusebox, from which a fat Green wire goes to the thermoswitch, then as Black/Green to the fan motors. Then there are Black grounds from the fans to body. All the connectors on these are suspect.

The switches are crap, operate at random temperatures, and fall out as Les said. I glue them in; others use tie wraps or make retainers.

The motors seize up, but usually not both at once; if one seizes it will blow the fuse. Some motors can be lubricated easily before or after they seize, some with more difficulty and knowledge.

I lubricate them before they die and I fit relays to operate them, with manual override switches for when the automatic one fails as it will. Install a fused dedicated power feed to the relay from the solenoid. A good quality aftermarket thermoswitch is very worthwhile, and many of these are adjustable for temp and have the relay and fuse built in.

While you are in the area, replace the IGN relay with a new one; it is easily available at any good parts store for about $5-10 - it (and the wire ends that feed it) will be well on the way to failure after having been overloaded by the fan feeds, and when it dies you stop.

And buy a Factory/Bentley shop manual or Owner's Handbook, or look up the wiring diagram.

FR Millmore

Les and FR,
Thanks very much for your expertise, and prompt responses. I will get underway with the exploration and see how it goes. Thanks again.
P.T. Grater

"replace the IGN relay with a new one; it ... will be well on the way to failure after having been overloaded by the fan feeds"

Ironic, since the relay was provided to take the load off the ignition switch and its wiring. UK cars at least had a batch failure where the relays would stick *on*, with obvious results, and because of that the ignition, heated rear window, indicators and heater fan were moved back to the ignition switch, leaving the fuel pump (amongst others) on the relay.

The V8 always had everything including heated rear window and twin cooling fans powered not only off the ignition switch but from the one and only green-circuit fuse, with no apparent ill-effects. Makes you wonder why they bothered with the relay at all.
PaulH Solihull

I have just removed the fan motors from my GTV8, the front bearing on both motors needs replacing, they were seized on the shafts and have been spinning in the cage for who knows how long! With mostly country driving, the fans are very rarely needed.

Does anyone know where I can source some new bearings in the UK? I should probably replace the brushes while I've got them apart, although they are not too bad.

David JM

If the bearings were spinning in the casing won't that have damaged the casing and make new bearings (even if they are available) a loose fit? I've never seen a parts breakdown for the fans, I replaced the motor complete when one of mine failed. By measuring the old bearing you may well be able to get a generic item rather than one specifically for this application. Annoyingly I had to replace the blades as well as I couldn't get them off the shaft, and it was a bit of a challenge getting the two together out.

I had a similar problem on a 'powered broom' (leaf blower/vac!) and opted to change the bearing as they *were* listed as a spare part. Unbeknown to me at the time there were four slightly different motors and I ended up with the wrong bush - too small for the spindle. By rolling it round a twist-drill I gradually 'reamed' it out to fit and all was well for some time, until that bearing started spinning in the casing and damaged it. At that point I opted for a new motor!
PaulH Solihull

PT bearings are available in most big cities at 'bearing stores'[for lack of a better name]. As far as the case wear, I've used tin foil, beer can tin, and epoxy, alone or in combination to repair the material loss and I've had great luck. If your truly want the original look go for it you can make it work.

Thanks Ric, I am now starting to track some down.

BTW Paul, one of my fans was stuck on the shaft, I found soaking with some penetrating oil then using a ball joint splitter did the trick. The bearing is held in place by a spring clip and it seems to have worn against the spring, rather than wearing the alloy casing. I reckon that I should be in luck and as Ric suggests, just use a bit of packing.

If it all works I will be at Kop Hill next weekend.

Thank you all, David
David JM

I could never get the grub-screw out, drilling it out accurately at home not possible.
PaulH Solihull

Paul, they were difficult. I ended up filing a screwdriver to get a good fit. One came out easily, the other is damaged, but the thread in the fan is okay. The grub screws are readily available, well so I believe!
David JM

I'll maybe have another go sometime, so I have a spare blade and motor (once the bearing is fixed). It was the slot in the grub-screw that vanished, hence drilling seemed the only way. If still no good I will have to weigh up which is worth more - the motor with the blades cut off, or the blades with the motor cut off!
PaulH Solihull

This thread was discussed between 01/09/2010 and 20/09/2010

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