<VV> VDO cylinder head temp. sender

Roger Gault r.gault at sbcglobal.net
Wed Dec 19 21:12:06 EST 2007

What I did was drill a 1/8" hole in the spark plug boot about half way
between the wire and the outside diameter.  If you look at the underside of
the boot it will be obvious where to drill it.  Then I used a knife to cut a
slit from the hole to the outside of the boot (out to the outside diameter
of the boot).  This lets you run the TC wire up through the hole.  Works

No, you can't use the stock thermistor with the VDO gauge.  The VDO uses a
thermocouple (TC), which puts out a tiny voltage that depends on the
temperature at the sensor (see below).  The stock thermistor is a variable
resistor.  Completely different technology.

My pet thermocouple speech:
TCs do not read the actual temperature at the sender like a thermistor.  TCs
measure the temperature difference between the thermocouple and a "reference
junction".  In the case of the VDO setup, the "reference junction" is the
connector where the thermocouple wire changes to copper.  If you use copper
wire to connect from the supplied connector to the gauge (like I did), that
point is at the connector - about 12" from the TC.  If you use TC wire
(harder to handle) then that point is at the gauge.  VDO, like most
manufacturers, sets up the gauge to add approximately 70 degrees F to what
the TC puts out under the assumption that the "reference junction" is at 70

This means that it is somewhat important where you put the "reference
junction".  It needs to be in a place that doesn't change temperature much
as the engine warms up.  Conveniently, the air in the engine compartment
while the engine is running meets this criteria pretty well (especially if
you don't open the Summer/Winter plate).  That air temp is almost exactly
the same as the outside (ambient) air temperature.  What you DON'T want to
do is clamp the "reference junction" to the engine.

So, assuming you have the "reference junction" (connector) hanging in the
engine compartment air, your gauge will read the actual engine temperature
on 70 degree days and be "wrong" on other days.  If it's 40 degrees outside,
it will read 30 degrees high because VDO "added" 70 degrees instead of the
actual 40.  This is actually a GOOD thing.   Unless you're a metalurgist,
you don't know or care what your engine temperature should be.  What you
want to know is how well the cooling system is working.  That boils down to
how much hotter the engine is than the incoming cooling air.  Your TC gauge
will always read that (with a constant 70 degrees added to the real number).
So, Winter or Summer, you can look at your gauge and say, "Ah, reads 330 (or
whatever yours usually reads) degrees at 70 mph.  Life is good."


----- Original Message ----- 
From: "shortle" <shortle556 at earthlink.net>
To: <virtualvairs at corvair.org>
Sent: Wednesday, December 19, 2007 7:42 PM
Subject: <VV> VDO cylinder head temp. sender

> On my newly purchased gauge sender unit, they advise me to put it under a
spark plug or on a head bolt.If I go with it under a spark plug, they advise
removing the gasket from the plug. Would I run wire next to spark plug wire
over top of shrouds or do I run it under top shroud?(not an easy job at this
point).Where do some of you vair heads run this particular sender
unit/wire?Or, would I be able to use a working OE thermister with this
aftermarket gauge?
> Thanks.
> Timothy Shortle in Durango Colorado.
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