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Heater fan wiring

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Heater fan wiring
#1
  This topic is about my 1975 MG MGB
Soapy Avatar
Soapy Ken McGuire
Livonia, MI, USA   USA
I have 2 wires coming out of my heater fan motor. One is G/Y and the other is Slate with a very faint yellow tracer. The wiring diagrams show the G/Y on the plus side, getting power from the switch, but the negative side should be black. Because the switch wiring from Advance Auto-wire shows a 2 speed fan, I am wondering if I actually have a 2 speed motor with ground going through the motor housing and through the heater box to ground. Is that possible, or do I really have a single speed fan and the gray wire is really the ground.



My mechanic told me, "I couldn't repair your brakes, so I made your horn louder." (Steven Wright)

The doctor must have put my pacemaker in wrong. Every time my wife kisses me, the garage door goes up.

“There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” Will Rogers

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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Looking at the 78 schematic, the first with a 2-speed, it sounds like that is what you have. Get your meter out and see if there is a path from the leads to the case.

It should have a pigtail, maybe from a screw on the case for ground. I would not rely on the case to the box to the chassis.



Cogito ergo sum periculoso

course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
On a late model ('79) with the two speed fan, the motor has two terminals. To one side is connected a green/brown wire (+) and to the terminal in the opposite side is connected a black wire (ground). The voltage on the green/brown wire is either 12v or a reduced voltage for low speed operation (I haven't measured to see what that voltage is, but, most likely, it's about half).

With the blower switch set to high, 12v is routed via a green wire into the heater box where the green/brown wire and the green wire are joined (hence 12v to the motor).

With the blower switch in the low position, 12v is routed to the heater box via a green/yellow wire. There it connects to a wire-wound resistor and the opposite end of the resistor connects to the green/brown and green wires. Passing the current through this resistor reduces the voltage supplied to the green/brown wire (motor + terminal).

So, to answer your questions; a) No, the motor does not ground through its case and b) If you have a two speed fan, it will have a wire wound resistor inside the heater box. It is located just inside the mounting hole for the motor at about the 1 O'clock position (see attached photo).

The wire colors you have were probably changed by a PO. Possibly, they rewired the blower to make it a dual speed unit.

Having not looked at/tested an earlier fan motor, I can't say definitively that it is the same as a later dual speed fan motor, but, I would suspect it is the same, given the propensity of BL to reuse parts and minimize design changes.

This pic is looking through the fan motor mounting hole into the heater box and shows the wire wound resistor with green/yellow wire on one end and the green and green/brown wires on the other end.

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course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
Here are a couple more pics of the motor and the wiring that connects from the switch to the heater box resistor and the motor.

FWIW, I just measured the current draw of my blower motor at 12v (from a car battery) at roughly 5 amps.


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Soapy Avatar
Soapy Ken McGuire
Livonia, MI, USA   USA
Thanks, that was all very interesting, but only marginally helpful. My MGB is, as shown, a 1975 model. There are only 2 wires and no wires go into the heater enclosure, so there is not likely to be an internal resistor. The confusing thing is the color of what I presume is the motor ground, coming out of the motor. A photo of the motor is attached.



My mechanic told me, "I couldn't repair your brakes, so I made your horn louder." (Steven Wright)

The doctor must have put my pacemaker in wrong. Every time my wife kisses me, the garage door goes up.

“There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” Will Rogers


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Fan_motor.jpg

HoosierMGB Avatar
HoosierMGB Gold Member Steve Koller
South Bend, IN, USA   USA
1976 MG MGB
Here is a shot of my 76 heater - as far as I know it's never been monkeyed with so it should be stock. Looks like the same wiring you have - I can't see any faint tracer color on mine.

Hope that helps



"Make no little plans; they have no magic to stir men's blood"
Daniel H. Burnham, Architect

1976 MGB - pretty much stock but with a few sundry deviations


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Ahmed Avatar
Ahmed Silver Member Ahmed EL Abasiry
Calgary, AB, Canada   CAN
1958 MG MGA
1972 MG MGB
1979 MG MGB
Without hijacking the thread, I just would like to know what is this cable showing in both photos. Thanks



"... the motor car, after woman, is the most fragile and capricious thing on earth." - London Daily Mail 1908


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Fan_motor (1).jpg

HoosierMGB Avatar
HoosierMGB Gold Member Steve Koller
South Bend, IN, USA   USA
1976 MG MGB
It's a service interval counter - it was only installed on a couple years of MGBs (75 and 76 NA cars?). Driven by the speedometer cable. Supposed to "remind" you to get service done, I think especially the EGR (?) someone while chime in with more knowledge on it.



"Make no little plans; they have no magic to stir men's blood"
Daniel H. Burnham, Architect

1976 MGB - pretty much stock but with a few sundry deviations



Edited 1 time(s). Last edit at 2017-07-01 10:56 AM by HoosierMGB.


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IMG_2839.JPG

Ahmed Avatar
Ahmed Silver Member Ahmed EL Abasiry
Calgary, AB, Canada   CAN
1958 MG MGA
1972 MG MGB
1979 MG MGB
Thank you..



"... the motor car, after woman, is the most fragile and capricious thing on earth." - London Daily Mail 1908

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Get out your Ohm meter and see if either of the wires has lo-ish resistance to the case. If both open, then it is a singe speed. Which wire goes to + or - depends on which direction the fan will rotate.

Does your switch have multi-positions?



Cogito ergo sum periculoso

Soapy Avatar
Soapy Ken McGuire
Livonia, MI, USA   USA
The switch only has 2 positions: on and off. However, I have been unable to trust anything to do with wiring on this car. That's why I am re-wiring it from stem to stern.

Neither wire has any connectivity with the metal nuts on the motor case. There is nearly no resistance between the two wires. I\ connected them to the car battery to see if anything happened, and the fan motor powered right up. Thanks for the test information.



My mechanic told me, "I couldn't repair your brakes, so I made your horn louder." (Steven Wright)

The doctor must have put my pacemaker in wrong. Every time my wife kisses me, the garage door goes up.

“There are three kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves.” Will Rogers



Edited 1 time(s). Last edit at 2017-07-02 06:50 AM by Soapy.

HoosierMGB Avatar
HoosierMGB Gold Member Steve Koller
South Bend, IN, USA   USA
1976 MG MGB
My switch didn't work either - so I took it out and it wasn't connected. As a master of fact, the switch was missing all of its innards. So, bought another from VB, plugged it in and...voila...works perfectly.



"Make no little plans; they have no magic to stir men's blood"
Daniel H. Burnham, Architect

1976 MGB - pretty much stock but with a few sundry deviations

grahamts Avatar
grahamts Graham Prosser
Chelmsford, Essex, UK   GBR
Probably the easiest way to sort out which is which is to use an ohmeter. One connection to a good earth on the fan motor and the other to one of the wires. One should show virtually zero ohms which will be the earth connection and the other should be appreciably higher which will be the motor connection.
Graham.

jborkman Avatar
jborkman Jim Borkman
Clarence, USA   USA
I believe its the tachometer cable, connects to the transmission.

Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
Let me approach it in another way: these two wires seen entering the motor housing pass through two grommets in opposite positions. Since the heater motor is a DC-motor with permanent magnets, these opposite wires in my opinion lead to the two carbon brushes pushing against the collector of the motor. Putting 12Volts across these two wires either way will get the DC motor running, but in only one of the two possibilties the fan inside will move correctly and let it blow, the wrong position will make it turn in reverse. Even just trying both will get you there, but maybe pictures on one will suffice.

So in any case this motor does NOT have its minus on the metal body and lots of electrical fans having similar DC motors are designed and connected the same way eye rolling smiley

In reply to # 3543434 by Soapy Thanks, that was all very interesting, but only marginally helpful. My MGB is, as shown, a 1975 model. There are only 2 wires and no wires go into the heater enclosure, so there is not likely to be an internal resistor. The confusing thing is the color of what I presume is the motor ground, coming out of the motor. A photo of the motor is attached.

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