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MG MGB Technical - Electric cooling fans, opposite directions

I just bought a nice '77 MGB along with three older non-running B's. Those will probably become one good car with lots of leftover pieces someday.

The drivable car has the two yellow plastic cooling fans in front of the radiator. The pitch or slant of both fans is the same. The car was running hotter than it should on my 20 mile trip home. The temp gauge was almost to the red "HOT" mark. I'm not sure if it's an original gauge. I've never had a '77 before. It does not say Smiths or Jaeger on it.

I popped the bonnet to discover that the electric fans were TURNING in opposite directions. One was drawing cool air in, and the other was pushing hot air from the engine compartment out the front. I feel that this could easily account for the high temperature. There is plenty of coolant. SHOULD the front mounted fans both be pushing cool air INTO the radiator, or are they really designed that way? The fan motors appear to be original identical equipment as does the wiring. They are plugged into the connectors provided, and no wire splices are visible. If they should both be bringing air IN, as I suspect, can the errant fan be reversed?
Chuck Asbury

Well, off hand I would say that unless the previous owner had some cunning cooling scheme up his sleeve, the two fans should be pushing cold air into the radiator. The one which appears to be pulling warm air out can easliy be reversed simply by swapping the wires over. However, strange as it may seem, a "pusher" fan with reversed poliarity doesn't become a "puller". It continues to push air but at a much reduced rate. The only way that it could really be pulling hot air out of the engine bay would be if it is really a puller and therefore really part of the previous owner's cunning plan.

A final word is that the cars didn't have twin fans fitted as standard - not in UK anyway - and if you have an overheating problem then it is unlikely to be caused by one of your two fans performing at less than 100%. In fact a decently maintained engine and cooling system should be able to drive along without any cooling help from any fans at all.
Marc

At least some US cars had 2 electric fans as standard. For all I know all of the electric fan equiped US cars had 2. There will be someone along in a minute who will know for sure.

I am not sure that Marc is correct. I recently fitted an electric fan on my MGB. It was turning the wrong way so I reversed the wires and it ran the correct way. This was not an OE fan but a more modern one.

Getting both fans turning the correct way should make a big difference.

David
David Witham

David - what part is it do you think that I am not correct about?
Incidentally I understand from my local mg specialists in North London - who have been servicing and repairing MGBs for 20 years, that they find a very high incidence of fans wired the wrong way round and they have come to the opinion that many of them were wired like that in the factory.
Marc

Yes - it may be about the reversed direction of the fan still pushing that I am wrong about. I think that what I was referring to is that if the fan is fitted back to front, it still pushes rather than pulls as one might expect. Of course, that's not relevant here.

However, I still think that an engine which needs its cooling fans on all of the time has some other problem. Fans only should become useful at idle or in slow moving traffic.
Marc

One thing I have learnt is not to trust the temp gauge untill you have tested it.

Mine has always read between zero and half. This was one reaason for converting to an electric fan. I thought the car was running cold!

To set the kenlow thermostat I took the radiator cap off and waited for the water to start to boil slowly in the top of the radiator. I set the stat to click on at that point.

When I looked at the gauge in the car the needle was not much more than a quarter of the way up the gauge.

So, Chuck, once you have sorted the fan wiring so that they both turn the correct way you should try and get a double check on the gauge reading.

David Witham

Chuck-
North American Market Rubber Bumper cars had two pusher fans in front of the radiator. The fan drawing air out through the radiator becomes counter-productive once the car gets moving. The forward motion of the car builds air pressure in front of the radiator and the fan then works against it. The two pressures then cancel each other out at lower speeds, and at higher speeds the air pressure in front of the radiator may be enough to overcome that of the fan somewhat, but flow through the radiator matrix is still reduced. I'd correct the fan problem and flush and descale the components of the entire system, including the engine, radiator, and heater core, to remove the 20+ years accumulation of muck, rust, and mineral deposits which act as insulators that keep heat from being dispelled by the cooling system. Use a quailty engine flush and leave the heater valve completely open to make sure that it gets into the heater matrix. You'll be surprised at how much cooler the engine will run in the summer and how much warmer the heater is in the winter. Make sure that the system is refilled with a mixture of a good ten year antifreeze and distilled water. Why distilled water? Because it won't coat the interior of your cooling system with mineral scale. Why the more expensive ten-year antifreeze? Because it has special additives that will extend the life of your water pump and because you don't really want to do all this all over again next year, do you? Refilling the system so that there will be a reduced likelihood of air pockets is easy once you know how: First, fill the radiator and block by pouring the coolant in through the thermostat housing and refit its outlet cover, then disconnect the heater hose where it connects to the forward part of the pipe that runs along the top of the rocker cover. Insert a small funnel into the hose. Holding the hose above the height of the heater box, pour in the coolant until it flows out of the pipe from the rocker box, then remove funnel and reconnect the hose to the pipe. This will minimize the amount of air in the system. If your car is equipped with an overflow tank, fill it 2/3 full and check it when the engine cools off
Steve S.

Don't know about later cars, but it's possible on early cars to put the fan blades on backwards and have the right directional rotation and the wrong effect. Most fans I've seen can be reversed with polarity. You need to find out which direction the motors are supposed to turn and then go from there. Someone with a later car can help a lot more here and I am sure someone will. Steve's point is very valid. After you get moving the fans don't do anything and will not help if the car is overheating at speed. Also, I'd change the thermostat while your flushing the system. It's easy and inexpensive to do.
John A

John,

The idea that you can mount fans backwards is something of an urban myth. The pitch of the blades do not change if the fan is flipped 180 degrees.

To prove it, draw a rectangular box on a piece of paper, representing the hub of the fan. Now draw some diagonal lines within the box, representing pitched fan blades seen on edge. Spin the sheet around 180 degrees - the pitch is still oriented properly.

That's not to say that the fans are reversible on our cars - hub offsets and other geometry also play a part - but you can't get a fan to blow backwards by reversing its orientation.

Hope this helps.
Joe Cook

This backwards fanblade answer was actually published in Safety Fast (MGCC magazine) as an official answer to a reader's overheating problem. Makes you wonder doesn't it?

To set the record straight;
- An electric fan that pushes with one polarity will pull with the opposite polarity, although efficiency may be reduced.
- Any fan will blow in the same direction which ever way round the bladewheel is fitted (i.e. which side of the centre of the wheel is touching the motor/pulley).
- Some fans are available with reversible *pitch*. Changing the pitch of the blades changes the fan from a pusher to a puller (or vice versa) without changing the direction of rotation.
- Pullers don't last as long as pushers (they get hotter due to being surrounded by hotter air).

hth :o)
--
Olly
Oliver Stephenson

Thanks for all of your replies. I will check into changing the wires around in a day or two and let you know how it works. Tomorrow I will begin trailering my three newly purchased non-running Bs home. It's an 80 mile round trip. That task will probably last through the weekend as I have to work sometime too.
Chuck Asbury

My '77 came with two fans. The '79 LE I bought this year, has only one.
On my '77, one of them quit, turned way too hard. I installed an after market fan, that is about 14" across. I have a fresh engine, with a 180 degree thermostat, and even under the hardest runs, just reaches ' normal' When sitting at a light in a small small town coming off the highway, it would get about 3/4 of the way up the guage. Then the fan turns on and the temperature drops to normal within seconds. I'm very pleased with this.
As far as I'm conerned, I don't care if I ever see another belt driven fan.
Safety Fast
Dwight
Dwight McCullough

This thread was discussed between 24/09/2003 and 25/09/2003

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