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MG MGB Technical - Wipers & heater electrics

My 1972 MGB GT wiper motor and the heater motor do not work. Well only intermittently. I've replaced the fuses

The Radio/Stereo works but when I try to switch the heater on or the Wiper - the radio cuts out.

What's happening?

Thanks for any help.

Iwan
Iwan Jones

Speaking of heaters ...
I put the new fuse switch in myself (boy I'm pretty proud of that!), flipped the switch and it worked! But I've got no hot air. The temp. guage says that the engine is 120 degrees when warmed up. Its never gotten any hotter. Do MG heaters run off the water in the radiator? Do they have their own heating elements? If it should heat off the radiator then, after I check the thermostat to make sure its working, should I block a portion of the radiator to get the water hotter? If so, how and how much blockage?

Speaking of the new heater switch, thanks to you folks for telling me about Peter C. at World Wide Auto Parts of Madison, Wisconsin. He got me my switch and my shocks in record time!
Margo Lubbers

Iwan - the 72 feeds the washers, wipers and heater off the accessories position of the ignition switch as well as the radio. That feed has its own in-line fuse between the white/green from the switch and the green/black to the components. If the radio works but cuts out when you turn the wipers on it is probably a bad-connection somewhere in that circuit, like the fuse or fuse holder. Check for 12v along the circuit with the heater motor on. As long as you see 12v the circuit is OK, as soon as it drops you have passed the bad connection.

Margo - the heater works by circulating engine coolant through a 'radiator' which has air blown through it to heat the car/screen. The amount of heat the hater produces is controlled by a knob on the dash. This opens and closes a tap on the right-hand side of the block. Maybe this tap is stuck in the closed position. The stat should maintain the engine at the correct temperature unless the air is well below freezing. Could be stuck open or not even fitted. Alternatively the coolant temp could be correct but the gauge or sender faulty.
Paul Hunt

Iwan: Paul is right on to check all your connections. As for the radio dying on you, it sometimes happens with modern radios/stereos that they have protection circuitry which shuts them down for both over- AND under-voltage situations. Integrated circuitry does not like too great a variance in voltage from its "norm". Sometimes too, the simple fact of too low voltage does the same thing. The low voltage may well be because of voltage loss from too great a load for the connections available. If they are crappy, then they won't pass current as they should, you get lowered voltage, and the radio goes "dead" temporarily.

Margo, Paul is again right about checking your thermostat and temp gauge sender. Get the car warmed up and stick an old candy thermometer into the top tank via the filler neck. That should give you a reasonably accurate temp of the hot water in your car as it exits the engine into the top tank of the radiator. If you have a temp around 180 - 200 F., it should be normal and your temp gauge is a joke. If you are using an aftermarket gauge, junk it and get a new VDO gauge and sender. Or, if you are using the original temp gauge, get the correct replacement sender for your year MGB.

The heater hoses are connected from the rear of the engine head (thru a valve) to the black heater box on the firewall (in to the heater core - a very small radiator) and then back out again to a tube which runs near the valve cover and on to the water pump circuit. Your heater core may be plugged with lots of scale and muck and a good back flushing may help restore flow. Or you may be looking at trip to the radiator shop to have them recore it or at least rod it out clean again. Good luck.

Bob Muenchausen

This thread was discussed between 22/12/2001 and 23/12/2001

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