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Re: To heat or not

Subject: Re: To heat or not
From: John Bartholomew <>
Date: Mon, 11 Mar 96 11:09 PST
Morrice Curet, wrote:
>I have a '70 MGB and would like a more enthusiastic response when I push
>the Heater Fan Switch on the dash.  Has anybody had any experience
>upgrading or changing from a one-speed fan to a two speed fan?  Looking
>in the catalogs, all the motors seem the same, with the only difference
>being the switch.  Thanks.

IMO upgrading the fan to two speed will at best move a little more air, but
likely produce cooler air not hotter ( I assume you are looking for warmth
from the heater?) If it is heat you'r after, you should look at your
thermostat, water pump, fan belt, heater control valve, defroster/interior
control and lastly the heater core itself. These have to be fully functional
for the heater to put out.
The thermostat is pretty easy place to start, about $5 buys a winter 190
Degree thermostat, this will allow the water to rise to a higher temp before
the thermostat lets the water flow through the main radiator for cooling.
This should produce some more heat for the heater core.  You may want to
check the temp gauge to see that it indicates normal after about a five
minute run, giving you conformation that your engine is reaching nominal
operating temperature. If it never gets to NORMAL, your thermostat MAY not
be working correctly. When you have determined the engine is at normal temp,
carefully touch the heater feed and return hoses, they should be pretty hot
to touch (only touch the rubber hose not the metal pipe!!! above all be very
careful.) This will give you a GUIDE as to if the heater core is getting
hot. The hoses should be at a similar temperature if one is substantially
cooler then the heater control valve may not be operating correctly. Check
that the valve is fully open by turning the control on the dash to HOT.
Observe that the heater valve, on the distributor side of the cylinder head,
is fully open. The valve has a curved slot running around the main body.
Running in this slot is a tab that controls the position of the valve plate,
it is moved by a cable coming from the heater dash control. When the tab is
all the way forward in the slot, (towards the front of the car), the plate
is closed shutting off the water circulation, (control set to COLD). When
closest to the heater box (the other end of the slot) the valve should be
fully open (max heat).
There is a possibility that the heater core (heat exchanger) is partly
blocked with sludge, preventing good heat transfer. You may want to flush
the core to ensure it is clear. Attach a garden hose to the lower connection
(heater control) and run water through the core until the water coming out
is clear. This may be easier said than done, I use an old garden hose and
cut it into two lengths, then the hoses have the necessary fittings for
connection to a faucet. Installed the cut ends on the heater making sure the
line from the faucet attaches to the heater valve end. Turn on the water
slowly! a little at a time until a steady flow can be seen. Some water
pressures can be very high and could burst the core, so use good judgement
here. When the water runs clear, shut off the supply and reverse the
connections at the heater and repeat the flushing from the opposite
direction. When completed, reconnect the heater to the engine and try again.
If you have never flushed the core you will be surprised at what come out!  
You may also want to remove the fan motor and check that the air screen, in
the back of the heater box, is clear of debris allowing air to pass through
the heat exchanger. Good luck.   
John D. Bartholomew  Amdahl San Jose CA.  Usual disclaimers apply.
     408-746-6361 - Voice Mail            408-746-8016 - FAX
               Internet id -
 69 MGBGT; 74 MGB; 79 MGB; restoring 76 MGB; 72 MGB daily driver

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