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MG MGB Technical - Heater motor fan speed ?

Hi all hoping some one can clarify something for me.

im lookign to install a 2 speed heater motor in my 69 B GT as its a daily user, but despite there beign a few heaters and motors for sale most of the sellers dont know if there 1 or 2 speed ? can some one tell me fro mwhat year 2 speed heater motors were fitted as this will be something at least i can go on.

many thanks

J
J P Phillby

All the motors installed in the B over the years were "one-speed". The difference between early and later years came in the form of a step-down resistor that was mounted inside the blower box.

When the fan is on "Hi" the resistor is bypassed. To obtain a lower speed, the current is passed through the step-down resistor before being sent to the fan motor. This, obviously, necessitates a 3-position switch (off/low/high).

Buying a new fan motor isn't sufficient. You need the heater box engine-compartment plate with the integral resistor also. This is the faceplate the motor is mounted to. I *think* your 69 will handle the plate from a later heater, but it's worth checking.

Technically, this upgrade isn't difficult. You need the step-down resistor, a new switch and some new wiring and that's about it. The tricky part is getting the switch to fit nicely into your dash and ensuring that your '69 heater box will accept the later model face plate.

I'm doing something a bit more radical in mine: I'm adding a 4-position rotary switch (off/low/med/hi) in place of the standard toggle, with the aid of Mr. Dremel. To control fan speed I'm plugging a GM fan resistor into the heater box. Additionally, I'm ***trying*** to fit the blower motor from a Fiero into the box. This fits, barely, but I have yet to put the pieces together - and sometimes that danged ol' reality thing gets in the way of beautiful theory.

This is being done in conjunction with an improved temperature control valve, like those shown on Bob M.'s site. Lotsa work, maybe not much gain, but fun....

R.
Rick Stevens

I dont know the exact year, "I think about 1975" of introduction of two speed motor but whilst the motors are the same the two speed has a resistor on the outside of the case where the motor is fastened on. Hope this helps. jim34
j soutar

Rick: You brought up a good question. What fan motors(from other vehicles)do fit in the B Box? Other than a faster motor, can you fit a different squirrel cage fan to increase the air flow?
BEC Cunha

The GT V8 allways was fitted with a two speed heater motor but I do not know whether it was switched with an inline resitor or had different terminals.

On later Austin limosines radiator vents were fittetd that have the same dimensions of the housing but using a longer shaft for the large prop and giving more torque/speed. These can be modified to fit the B's heaterbox.

In my cars I fitted a relay very close to the heatermotor (using one of the three screws the motor is fixed with) and run a thick wire from the starter motor (20 amps fuse in line!) to the relay, using the existing wires just for the coil of the relay. This way you do not lose @ 1.5 Volts via the small diam wires and the switch at the dash, helping to increas the rpm of the fanmotor.

Ralph
Ralph

J-
I have an 80 GT and the heater function was quite poor. I fitted an uprated heater marix and the air is quite a bit warmer than with the standard matrix. You can also purchase an uprated fan assembly as well, however this will require you to enlarge the hole a bit on the heaterbox cover, but as I understand it, you don't need to change the electric motor...just replace the old fan blade with this one.

I believe the ducting on the 69 is different than on the RB models, but I think it would be worthwhile to ensure all the ducting is in the right place and nothing has slipped out and leaking your valuable ducted hot air.

HTH, Matt
M Ruch

JP,
I have a '77 "B", which I purchased new. The fan motor is controled by two speed switch. The system works very well, despite being 30 years old.
Don
Donald E. Elliott

Donald brings up the salient point here: Properly maintained, the B's heater is NOT underpowered. I'm modifying mine just for the challenge, but it's not a necessity. On the other hand the multi-speed fan is a nice touch, particularly when the navigator is having "power surges". I've learned that the placebo effect is really, really, really important sometimes.

R.
Rick Stevens

Surely the two-speed fan is 'normal' and slower, rather than 'normal' and faster?

Personally, the heater in my B has always been really warm...no complaints.
Dave O'Neill

That's right: Normal and slow. The two-speed aspect is aided by the resistor mounted inside the heater box. All it does is increase resistance to the circuit, thereby reducing current - making the fan turn slower.

R.
Rick Stevens

V8 GT only ever had a *single* speed heater blower, the 2-speed arrangement only arrived for the 77 model year which was just after V8 production had ended.

The resistor usually has to be mounted in a forced air-flow to dissipate the heat and stop it burning out, although I believe Metros had it mounted in the open on the firewall in the engine compartment.

The fan was always a squirrel cage type. Uprated motors/fans and matrixes are available but I have heard of more than one person be disappoinred with the results. If you *do* find it is *much* better than the original then probably there was something wrong with the original system rather than it being inherently a poor design, whilst not as powerful as modern cars they really aren't that bad.

I reckon there are ten defects that can reduce the effectiveness of the heater system, and you could have any number of them.

Both my roadster and V8 burn our feet, and the screens will clear of mist (not droplets of water) in seconds. In good fettle it should be capable of dealing with most circumstances except sitting in traffic jams in cold winter rain, but then cracking a 1/4-light a fraction takes care of that.

Paul Hunt 2

Paul this may be one of your ten defects but it was new to me.

Most of us are aware of flow restriction from the valve not opening fully but I do not recall mention of flow restriction from rust in the steel pipe along the top of the rocker cover.

I recently removed the steel pipe on my car because I wanted to paint it. I found that the ends, which would normally be covered by the hoses, were badly rusted. When I looked in the end of the pipe I could see quite alot of restriction due to rust. May be time for a new pipe!

On the early cars the pipe was copper and ran along above the inlet manifold. With the '66 roadster I used to drive daily in the early '80s the demist would clear the rear window of the hood or hard top from the top down once it had done the front.
David Witham

By contemporary standards the blower cage and motor are pretty small, and improvements in both fan and motor efficiency over the years means that an appropriate blower from a new car which will fit the housing would be capable of moving much more air. As the air coming out of a properly working heater is pretty hot, moving more air could be seen as a good thing, distributing that heat around the car rather than concentrating it in the footwells.

If it weren't for the fact that I already fitted an oversized blower and housing to mine I would have already found a suitable blower and motor. All it takes is a walk through the junkyard, measurements in hand. But maybe someone would like to take this on? I have a very large 3 speed blower and the low setting may put out more air than the stock blower. I never use the high setting because it moves so much air that the breeze feels cool. The matrix only has enough capacity to incrementally warm that much air, so although it is getting warmer in the car my feet don't feel it. A perfect example of too much being more than enough. It would work very well with AC though so for now I'll keep it. Btw, it has a resistor set which is mounted through a hole in the housing. Very common on older GM's and other cars of the early 70's, these should work fine with any 1 wire blower.

Jim
Jim Blackwood

@ Paul,

sorry, my information of the two speed motor for the V8 was wrong... red this in one of the differnt books and did not check whether it was right...

@ all,

there had been two diffent motors during the production of the B. BL spare parts list for MG B up to 1976 quotes:
Earlier cars were fitted with 17H 2288 while 37H7973 replaced this early motor (G-HN5 G-HD5-303704 ON, G-D2D1-101 ON)

BTW, the part numbers are no special MG numbers and so there should be other sources for spares too.

Ralph
Ralph

I think the early motor was the same as the MGA and had a 'shunt wound' stator whereas the later motor used a permanent magnet stator. The effect of this is that the early motor always turns in the correct direction no matter the polarity, whereas the later motor is polarity sensitive. You might be tempted to think "Well if the motor is turning backwards I'd know because it would be sucking air from the car and pumping it out of the intake" but you would be wrong. Because the fan is a 'squirrel cage' it always pushes air in the correct direction, but the correct direction of rotation is a lot more efficient at moving air than when in reverse. If my V8 had been my first MGB I'd probably have been complaining about the heater just as much as everyone else. But comparing it with my roadster there was a huge difference - whereas the roadster shifted loads (relatively) of air and made little noise the V8 shifted hardly any but making a tinny whine whilst doing so. Reversing the connections at the motor of the V8 made a huge difference.

The early motor seems to have two black leads whereas the later is colour coded - normally the green/yellow being for the positive connection and the black for the negative, on negative ground cars 12v and ground respectively. But even if you think it is connected correctly try reversing it, you may just be surprised.
Paul Hunt 2

Rick, I've done the conversion using a GM 3 speed fan and resistor pack in my midget. It really does move a lot more air, not only because of the higher speed, but also the fan assembly is larger and fills the heater box very well so there's not much air 'wrap around' the impeller. If I can answer any questions about the conversion drop me a line.
Bill Young

Bill, thanks for the offer. I may well take you up on it! I'm glad to hear you say that about the squirrel cage (impeller) because I wasn't too sure if there'd be some sort of "volume discount" on the amount of air it pushed around. Right now my plans don't have me enlarging the engine compartment air intake - did you do that?

R.
Rick Stevens

As a matter of fact I didn't, in fact it got smaller because of clearance problems so I reduced it to a 3" diameter hose. The only mods to the original heater box was some trimming on the motor side opening, a hole cut to mount the resistor pack, and brazing a tube onto the box for cooling air to the motor. The B heater box is a little different from the Midget, but the GM motor should work about the same.
Bill Young

Bill

What GM motor did you use? I have converted the heater box on my C to a Fiero blower assembly, including the larger diameter squirrel cage. Available cheap on eBay. Not installed yet. What switch and resistor pack did you use? Where does the resistor pack mount?

Thanks

Larry
Larry Hallanger

I used the blower motor and squirrel cage from an '87 Camaro which I parted out for my motor and trans in my Midget. I believe that all these GM units from around the same period are identical. I mounted the resistor pack on the front side of the box, only because of clearance problems with the battery if mounted on the rear. It will mount on either side, just as long as it's down stream of the blower and upstream of the heater matrix and the wire resistance elements are exposed to the air flow from the blower. I used the standard GM high speed blower relay for high speed, the resistor really only supports 3 speeds, the fourth "max" speed should be through the relay. The wiring was with the original GM switch and done to match the circuit in the GM manual.
I'll check my files for a photo and post it on my Moss Pub album if I can find one.
Bill Young

Note to all: It was thanks to Larry Hallenger that I undertook this conversion. He was able to provide me with MANY tips and ideas, and deserves the kudos for this.

R.
Rick Stevens

Now - the resistor unit I'm using (likely the same as Bill?) is part GM#14093106. It's designed to be mounted on the outside of the blower box, with the resistors inside a (new) opening in the box, as Bill indicates.

For switch wiring, check out the following link from Vintage Air. Go to page 7 of 9 in the pdf file. When reading it, ignore the part of the schematic showing the A/C thermostat (connector "C" on the fan speed switch). Everything else is connected as-is. http://www.vintageair.com/DownloadsSection/Universal%20Series%20Evaporators/Vacuum%20Series/05000-Series%20C%20(Inet).pdf

The switch itself is being fabricated from a new 3-speed (4 position) switch, a couple of sacrificial B fan switches and found bits of hard plastic. The main tool here is just a dremel.

R.
Rick Stevens

For a neat appearance I like to use the commonly available lighted rocker switches that are (very)roughly 1/2 x 1". These can be stacked 3 high in the stock switch cutout. The connections are a little tricky due to the cross brace behind the dash but I've found some slip-on connectors that lead the wire out to the side that work well. This gives you a separate switch for each fan speed so I use red at the top, yellow in the middle, and either green or blue at the bottom. Then at a glance you always know your fan speed. The stack is slightly shorter than the opening and I've dealt with that 3 different ways: epoxying 3 switch housings together with a couple of pennies between as spacers, simply inserting them in the opening and pushing them together so that any extra space is at top and bottom but only apparent if one looks closely, and finally by creating a molded silicone rubber 9 terminal connector socket that the switches plug into which holds them together nicely. There ends up being something like 1/16 to 1/8" of play up and down which isn't hard to hide. I use the same switches at the headlight position, and I use relays. It's a system that has worked well for me.

Jim
Jim Blackwood

That's actually a pretty neat solution, Jim. Mine uses the stock bezel and lighted "FAN" label, but with a rotary switch. Doesn't look bad at all, but the rotary switch obviously is different than the others.

R.
Rick Stevens

Jim

Is there any problem with having more than one switch (speed) energized at one time? I would think not as you apparently would be enegerizing the blower motor through two or three parallel connections with different resistor values in them.

Rick

I don't deserve any credit other than reading about the conversion on the E Type BBS. A note on assembly of the blower in the MG housing. If you allow for a little adjustment in the positon of the motor you can adjust the gap between the squirrel cage and the point of closest "approach". When I did mine I bolted the motor up to the side plate and adjusted it for the desired position, and then tacked the nuts in place on the inside of the housing.

Larry
69 C still in restoration
Larry Hallanger

Larry,
That doesn't seem to cause any trouble but you have to design your circuit with the understanding that these switches connect the load to ground when they are turned off, and basically what that means is that three relays are needed as well to isolate the switches from the load lest you ground out through the resistor network. I tried it with 2 relays for med and hi and that didn't work out well. The combination of low switch off and high switch on tended to blow the fuse. But with three relays it's fine and the switches will last. Theoretically there might be a slight difference in motor speed depending on whether the lower speed switches are on or not but in practice it doesn't seem to make any difference. If you didn't care about the lights you could dispense with the relays.

Jim
Jim Blackwood

Something seems to be putting an awful lot of uneccessary complication in here. Surely all you need is a resistance in series with the motor to give a speed less than its max. As such it doesn't matter whether power is applied to the resistor when on a full-speed setting, the resistance will have 12v both sides and so be carrying no current.

Where you may get something different to expected is where you have two or more separate resistors and apply power to two or more of them at *less* than the full speed setting. Then the resistances will be in parallel and you will end up with a speed faster than you might expect. This shouldn't happen if you have a resistor assembly with, say, low, medium and full tappings as again the low setting will be bypassed by the medium setting, both being bypassed by the full setting.
Paul Hunt 2

This thread was discussed between 14/01/2007 and 19/01/2007

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