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RE: MGB cooling fan thermostat

To: "Howard Battan" <>, <>
Subject: RE: MGB cooling fan thermostat
From: "Dodd, Kelvin" <>
Date: Tue, 12 Jul 2005 09:28:53 -0700

Short the switch leads together as Mike suggested.  The cooling fans
should then come on.  That determines that the switch is bad, which is a
pretty common occurrence.

Buy a new switch and install it.  FWIW the Moss ones are in Lucas boxes
and are supplied with sealing bush, but the original manufacturer is

Two things to understand as there has been a lot of misinformation about
these switches.  

1.  The reason the switches pop out is low water level.  Steam builds up
in the raised section of the radiator and the switch pops out.  If the
water level is over the switch as it should be, the original bush will
hold it just fine.

2.  Premature switch failure is often due to poor condition cooling fan
motors.  When they start to drag the current required for operation goes
sky high and the switch "canna take it anymore".

The later switch has spade terminals and a retaining clip.  This clip
will not hold the switch in under low coolant conditions as it only has
light spring pressure.  The switch is only available as an OE part,
hence the high price.  Supposedly there is a difference in the radiator
mounting hole, but I have not been able to see one yet.  The diameter of
both switch bushes is the same.  Often people will suggest using a
mounting bush originally supplied by Jaguar which is a bit deeper than
the MGB one, but again the problem is steam build up, not the bush.

This is one of the areas where installing a relay will increase the life
of the switch and the efficiency and life of the cooling fan motors.  On
late cars the factory also installed a circuit breaker on this circuit.

Kelvin Dodd

> -----Original Message-----
> From: [] On
> Of Howard Battan
> Sent: Monday, July 11, 2005 8:05 PM
> To:
> Subject: MGB cooling fan thermostat
> Hey, all, on my '79 MGB, the radiator cooling fans don't come on, even
> though the water gets hot enough to boil over. In the Moss catalog,
> are two thrermostats listed, along with two different grommets and a
> special
> clip for late model cars. I was told by on mechanic to be sure to use
> clip to prevent launching the thermostat at high pressure. My question
> -
> can I use the early model thermostat and grommet with the late mode
> The reason I ask is I was told I probably could, and the early model
> thermostat is much cheaper than the late model.
> Thanks for your help.
> Howard Battan
> '79 MGB
> '57 MGA

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