<VV> Where the thermocouple?

Roger Gault r.gault at sbcglobal.net
Thu Oct 20 09:14:23 EDT 2005

I have the VDO gauge, and have two thermocouples so I can switch between
them.  Currently, one is under a plug, and the other is under a bolt screwed
into the stock thermistor location.  Before that, I had one on each head, in
the thermistor locations.  I don't see a measurable difference in putting it
under the plug or in the thermistor location.  I'd probably put it under the
plug - it's easier.  In my case, I drilled a 1/8" hole through the domed
part of the spark plug boot and cut a slit to the outside edge of the boot
so I could slip the wire in there.  Works fine.

The VDO thermocouple will not work with the stock gauge.  Thermocouples put
out a small voltage that is proportional to their temperature, while
thermistors like the one used with the stock gauge change resistance with

By the way, the other difference between the two systems is that the stock
gauge  / thermistor reads the actual temperature of the thermistor, while
the VDO thermocouple system reads the difference between the thermocouple
and the connector where the thermocouple wire connects to the copper wires.
VDO adds 65 or 70 degrees to that difference to display the temperature on
the gauge.  This doesn't make much practical difference, but you need to
keep a couple of things in mind.  First, you want that connector in some
location that doesn't change temperature much as the engine warms up.  The
inside of the engine compartment above the shrouds is fine for that.  Half
an inch from the exhaust manifold underneath is not.  If you use the stock
thermistor location, be sure to keep the connector away from the heat.
Second, you'll notice that the gauge will read the same temperature in
summer and winter.  That's because it's really reading the temperture rise
across the engine rather than the real temperature.  I actually prefer that,
since it makes it easier to see if something is wrong.


----- Original Message ----- 
From: "Jim Bannister" <jimster1 at earthlink.net>
To: <virtualvairs at corvair.org>
Sent: Wednesday, October 19, 2005 10:46 PM
Subject: <VV> Where the thermocouple?

> Hi All,
> I'm about ready to install a new VDO CHT gauge in my turbo Corsa.  The
> supplied thermocouple is one of the ringy thingy ones that is supposed to
> under a sparkplug in place of the normal gasket.  Is this ok, or is there
> better location.  Does anybody know the result if I were to use the stock
> sender with the VDO readout?
> Jim B.
> '66 turbovert
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