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MG MGB Technical - Can't get electric fan to work...

Hi all,
I've been fighting with the rad fan not kicking in on my '80 this past two weeks. I've tested all the circuitry. The fan runs when I short the plug across where it plugs in to the temp switch. So I just replaced the switch with a new one. The thermostat is only a few months old. The top of the rad feels hot when I touch it, but maybe not hot enough? I also replaced the temp gauge sender this past winter so I'm sure it's reading properly.
So any ideas or should I be trying another thermostat?

If you are sure that the circuitry to the switch is functioning, then there are only two things that could be preventing the fans from comming on.

1. The temperature switch is not working at all or it is switching at a very high temperature.

2. You have a pocket of air trapped at the top of your radiator. The switch must be immersed in the flowing coolant.

Here is the best way to fill your system with coolant to purge it of any air pockets:

Fill the coolant expansion tank to two thirds full. Replace the radiator cap.

Disconnect the upper radiator hose from the thermostat outlet.

Loosen the hose connection at the radiator to rotate the hose 90 degrees so that the open end is pointing straight up. (make sure you re-tighten the clamp at the radiator before proceeding.)

Now fill the system through the hose until you see coolant steadily dribbling out of the thermostat housing. (do not let the level of fluid in the hose go below the top of the engine.)

Loosen the clamp at the radiator and quickly rotate the hose and re-connect it to the thermostat outlet. (try to keep as much fluid in the hose as possible when doing this step.)

S Rechter

Ron. S is correct that the system must be full of coolant to work correctly. Beyond that, we have little to guide us in helping you.

What is the cylinder head temperature when your engine is running? What is the outside air temperature that you are driving in? What conditions are you driving under? How far are you going? All of these, as a whole, need to be considered. Your fans, and their circuit seem to be in proper working order.

Thus, the only question is whether the thermocouple is switching on the fans when the engine temperature is sufficiently elevated to warrant them running. You have provided us with no information on which make any form of diagnosis. Here in Arizona, it is just beginning to get sufficiently warm that my fans will kick on--after an extended trip at highway speeds, then, turning onto surface streets.

You need some facts--to tell you what is actually happening and, if there is a problem, for us to be able to help you decide what might be happening.

Les Bengtson

We need to know where your temperature gauge gets to while the fan still isn't running.
When mine works, it kicks in just past halfway on the gauge.
This happens as Les suggests, when slowing down after a fast run, and when stuck in traffic.

I have had it fail twice now. Once coming on too early, so it pretty much ran as soon as the engine reached running temperature, and then blind refusal to come on at all.
New radiator switches seem to provide a temporary cure :-(

In light of this I have a dash switch for the second fan, (mine is a single fan model, but I fitted the second fan) and switch it on if the gauge reaches 2/3 to 3/4. To facilitate this, I moved the temp gauge to where I can see it, and stuck the fuel gauge where it is hidden by the steering wheel.
Martin Layton

Thanks for the replies.
It's only around +10-15 deg celsius here these days(mid 50'sF) so weather isn't a factor. I let the engine idle while watching the temp gauge and before the fan would cut in just above 1/2 on the gauge. Now I shut the engine down when it gets near red, still no fan. Again, I tried connecting the two ends on the connector to the sender at the top of the rad and then the fan works, so either the new sender is faulty, or more likely I have insufficient coolant in the top of the rad. I also replaced the gauge sender recently, plus the voltage stabilizer so I believe the gauge is reading properly.
So I will try filling the rad from the top hose and see if this helps.

Ron, If you can feel water in the top hose, there should be water in the top of the radiator.

I fill mine slowly through the bung hole atop the thermostat, (your thermostat does have a jiggle pin, or a pinhole to vent air?), leaving the cap on the tank. When it is full to the threads I replace the bung, half fill the expansion tank, then with the tank cap off, squeeze the top hose flat a few times, this pumps most of the air out through the expansion tank, which then needs topping up again.

BTW, if your header tank stays pressurised when cold, or blows out water through the overflow, the head gasket is leaking, and air will continually collect in the top of the radiator.

The replacement senders are not particularly good, you could have a duff one. (Don't forget to wire it in place if you don't have the very late type with a retainer clip.)

BTW When the fan is on, the top of the radiator is scalding hot, (as it should be, it turns on at 90 deg)if it isn't, there is no water in it..

Martin Layton

Thanks Martin,
I tried refilling the system yesterday as much as I could get in and then ran the car for a few minutes with the overflow cap off, then let it sit for a couple of hours in the hopes air would bleed off. Later I refitted the cap, started the car and same problem. Fan wouldn't kick in so the new sender must be faulty. I'll return it this week for another and try that.

Well the fan works! I tested both fan switches in boiling water and they both work. So I stuck the old one back in then ran the car. This time I just let the thing run until the gauge was reading just before red and then the fan kicked in! Ran for a bit then kicked out, then kicked in again. So it was probably working all along, but the gauge reading has changed. The fan used to kick in at a lower gauge reading. I wonder if the voltage stabilizer for the gauges has gone wonky again? Wacky, but at least it works!

Ron. Having had several of the thermocouples go bad--three times in 5 1/2 years of ownership on a new 79, and having seen several of the original style blow out of the radiator, I have the hole for the thermocouple covered with a patch and use an aftermarket fan controller available from Bob Ford at Brit-Tek. It has a probe which is attached to the rear of the radiator, just below the header tank. It also has a variable rheostat which allows you to control at what temperature the fans come on. Using an IR thermometer, it is easy to dial in at what temperature the fans come on.

I have used this system on my daughter's 77B (nine years in operation), my 79LE (seven years of operation) and just installed on one the 79 roadster I just put back on the road.

It is an easy conversion and has held up much better than the factory system.

Les Bengtson

I did the conversion described by Les seven years ago.

You can get one of these aftermarket fan controllers from any auto parts store.

I used a 7/8 inch hole plug for several years to plug up the fan switch hole in the radiator until I finally had a welded patch installed.

You can also get the hole plug from the auto parts store. You install it in the hole then tighten it with a 1/2 wrench so that it expands and seals.
Steve Rechter

This thread was discussed between 28/04/2007 and 30/04/2007

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