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MG Midget and Sprite Technical - Heater Fan puzzle

Two heater fans, both look identical, but they spin in different directions.
Which is the right one for a 74 midget?


Mark '74 Midget

Whichever one works best and spins in the correct direction to get the most air in the car.

Test connect and run off a battery, in the heater box if you can or just bear in mind which direct the blown air needs to go in, if you swap the wires round on the fan motor you may find it's more efficient one way than the other because of the hamster wheel.

See this "John Twist of University Motors" video to give more details. - https://www.youtube.com/watch?v=2gCN3hbtVgQ
Nigel Atkins

The one that spins in the "right" direction to produce the most air will be the loudest. So I choose for the "wrong" direction.
Flip
Flip Brühl

If you have the fresh air valve/flap set correctly (fully open at all times, even through winter, unless drawing in choking air) and the trunking connected fully at the front of the car then the fan isn't needed above about 20-25mph.

Operating instructions are in the Driver's Handbook as below, crossed through text is because I have a separate electric switch as like many the original doesn't work.

Second photo below is of fresh air trunking fitted fully at front (not just lying nearby as many are).






Nigel Atkins

Nigel

You’re misquoting the good book.
Dave O'Neill 2

David,
I live at the top of a hill, it's very windy, the good book couldn't have known that.
Nigel Atkins

Mark,

The fan on my 1275 ‘72 midget is the same as the one on the right in your photo.
I wonder if the one on the left is for a 1500 which has the heater box the other way round?

Simon
S Holt

As an aside, the motor should always spin in the same direction regardless of polarity, just like a starter or wiper motor.
Bill Bretherton

MGBs are Timmy-two-ways.
Nigel Atkins

Nigel, are you sure about that?
Bill Bretherton

Many thanks for the replies! I like the idea that one is 1500 version. Since both work, I'll install the one on the right.

I'll also have a look at reinstating the trunking, something I've been meaning to do for the last, err, 20 odd years.

m
Mark '74 Midget

Good to have a choice. The ram effect is rather limited without the trunking. 😀
Nigel Atkins

Bill,
I was going on my memory, plus I think this was discussed recently, but to be sure I checked the JT vid, at 1:35. - https://www.youtube.com/watch?v=2gCN3hbtVgQ

And I have an "upgraded" motor which is the one from the Bs I think as it seemed to work better with the wires reversed.
Nigel Atkins

Nigel

Thanks for the link and, yes, he does say the B motor has a permanent magnet field so it is a "Timmy-two-ways" as you put it. I'm surprised.
Bill Bretherton

For the heater fan to work the blades need to spin so that the curved ends of the blades 'hook' the air in the centre and displace it to the outside, so rotational direction is important

In the case of the wiper motor I don't think it matters which way it rotates. The wipers still wipe back and forth. Or maybe forth and back. 😁
GuyW

Bill,
you will have to join our 'improve your memory' group, we've been not meeting for two years as none of us can remember when it is or where.

I had forgotten your post but see the 'Polarity' thread from April on here and your post of 19 April 2020. 😉
Nigel Atkins

What Guy said. There is usually a direction arrow somewhere......
Oggers

Whether the blades curve forward, backward or radial depends on a number of factors, some information here https://en.wikipedia.org/wiki/Centrifugal_fan .
David Billington

Mark,

Have you seen the photo in Steve Mauer’s post “ Need Help With Last Piece of Ignition Puzzle”?
Part of the heater fan is visible, it looks the same as the fan on the left in your photo and the car is a 1500.

Simon
S Holt

Wow good spot. Thank you!.
Mines a 1275, so I've got to use the one on the right.
Mark
Mark '74 Midget

This thread was discussed between 25/09/2021 and 18/10/2021

MG Midget and Sprite Technical index

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