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MG MGB Technical - 1973 MGB Heater System

Well, winters a-comin'!!! Time to finally take a look at the heater system and find out why it isn't working. I won't really be driving the car in the winter, but I would like to get the heat working so I can use it...and be warm... until the snow flies.

I've had this 73 MGB for just under a year now. The heat has not worked at all in that time. That is, turn on the heater switch and nothing happens. I'm hoping it's only electrical and once I get power to the blower the rest of the system will work. We'll see.

So, what do I look at first in the electrical system for the heater. The fuse connections are fine I'll try and find all connectors from there to the blower and make sure they are clean. After that I assume the switch is the next place to go.

And therein lies the problem. How the devil do you get to the swtich. I can get to the headlight switch by assuming the upside down/inside out/backwards looking at the sky position with my head under the dash. Seems though I'd have to pull the dash to get to the heater switch terminals.

If that is the case then I'm not sure where to start. I've read a number of posts on pulling the dash but am still not very clear on how to do that.

Thanks all!
BH Davis
CT
BH Davis

Have a look at http://www.mgb-stuff.org.uk/wn_heaterframe.htm, there are at least 10 possible areas contributing to little or no heat, only one of which is the fan.

Check the water system first, you should get heat without the fan when driving along, the fan is really only needed when stopped or travelling very slowly in traffic. There is a rotary control for the amount of heat, another for directing the air flow to cabin or screen, as well as the switch that simply turns on the fan.

As far as the fan goes do the simple stuff first, like connect a jumper lead from the purple terminals on the fusebox to the green/yellow wire at the heater motor. If that doesn't work try connecting a known good ground to the black wire of the motor. If that still doesn't work the motor is faulty.

If the 12v to the green/yellow *does* work the fan, and you are sure the bullets and connectors in both wires are making good contact, then you are reduced to checking at the back of the switch as there are no other connectors in the circuit.
Paul Hunt 2

Paul,

Seems to be heat coming out when driving so will look into the wiring this weekend.

Thanks,
BH Davis
B.H. Davis

If you can get the blower to run then you are probably good to go for this fall. A complete removal and restore of the whole box, clean and pressure test the heater radiator, clean out the drain tube and add relay to get more power to the motor will make it warm enough to drive most times of the year. Mine blows me out now.
J.T. Bamford

If you really want to boost the airflow you can replace the motor and impeller with one from a Fiero. Definitly moves more air. This does require some modifications to the heater box cover plate as the mounting holes are in a different location and the impeller is a slightly larger diameter.

FWIW

Larry

69 C in restoration
Larry Hallanger

I really do think it is down to getting the existing system working properly, instead of being slugged with 30 years of neglect (and that goes for many areas of the MGB). I know of at least one person who went to the not inconsiderable trouble and expense to remove the heater box and fit an uprated matrix and blower, and was bitterly (ho ho) disappointed in the results. It also depends on what you are used to, and an MGB is never going to be as powerful either heat or volume wise as a modern car, without a lot more than work than an uprated matrix and fan. Both my GT and roadster are capable of burning my feet and shifting mist off the screen in seconds. The GT was my daily driver for a number of years including one bitterly (for the UK) cold period of a couple of months where I would daily pass a dozen or more modern cars with their drivers scratching at the encrusted salt on the windscreen, the MGB screen-washers have never frozen.
Paul Hunt 2

B H,

It's still not clear to me whether your problem is lack of heat at the matrix or failure of the electric fan, or a little of both, but here are a couple of suggestions.

The cable on the temperature control can bind, such that turning it from cool to hot doesn't actually move the lever on the heater control valve. Take a look at the valve, and if it isn't wide open, "help" the control a little by pushing the lever as far forward and under the valve as it will go. I keep mine shut tight all summer and each fall I have to do this little ritual the first time I need heat.

On your '73, you'll find that you can reach the back of the fan switch through the face vents. This involves removing your glove box to remove the face vent plumbing and grills, before you can reach the switch.

Check out Haynes page 235, section 59 and page 175, section 45, paragraph 9.

FWIW,
Allen
Allen Bachelder

You can get an idea as to what is wrong by comparing the flow and return pipe temperatures on the matrix. The flow should be the same as the coolant, the return a bit lower with the fan running, almost the same with no fan. If the return, or even worse both, are cool with the water valve open (the pin in the middle sticking right out at the end of the valve) then there is a blockage in the circuit which could be matrix, valve or hoses. You can forward and reverse flush these alternately until it runs clear (only do this once the engine has cooled down). If both pipes are hot then coolant is flowing OK, but if the fan only blows cool then either the air-side of the matrix is blocked (leaf debris) or there are other problems with the ducting. I get air of 140F/60C coming out of the heater vents, with an ambient of 50F/10C, and as far as I know a standard stat of 180F/82C.
Paul Hunt 2

This thread was discussed between 10/10/2006 and 14/10/2006

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