Multi Function Temp Sensor ('86-91 5000/200TQ)

["Scott M." <scottmo@teleport.com>]

Francis,

On the 91 200TQ with the 3B 20V engine the Multi-Function
Temperature sensor is located underneath the intake
manifold area on the water pipe. It is a large brass sensor
~ 1 inch diameter, with a 4 pin connector, the sensor
itself will have either 3 or 4 connector pins on it. You
can check the connector pins for corrosion but more often
than not,  the problem is internal to the sensor.

The older 4 pin variety of these sensors as they age
typically would  give intermittent coolant temperature
gauge operation. Many times if you wiggle the connector the
gauge will work for awhile but then eventually it will go
back to being intermittent.

Several years ago I had this happen on my 86 5KT and after
replacing the connector and pins which looked corroded, I
finally just replaced the sensor after getting tired of the
intermittent operation. At the time Halsey Import parts had
the replacement part for ~$40. Most of the newer
replacement parts are the three terminal variety which
supersede the old 4 pin version. My latest 89 200TQ had the
same symptoms shortly after I bought the thing, so I just
picked up a new sensor (three terminal version) and it has
been fine ever since.

If you take apart the old style 4 pin variety you can see
why they are often intermittent, VDO really went nuts when
they designed  this thing. It looks like a simple
temperature sensor on the outside, but if you pry away the
brass swedged lip and pull apart the sensor you will find a
small circuit board with various resistors, diodes etc, and
on top of the circuit board, is a small ceramic substrate
Hybrid circuit  with active components IC's, transistors,
and some screen printed resistors etc. The area between the
connector pins and the circuit board has epoxy potting
material which makes it tough to check these connections.
If you look at the Hybrid circuitry under a micro-scope you
will find that most of the solder connections are cracked
from the thermal cycling these things endure. They have a
small temperature sensitive resistive pellet in the tip of
the sensor and they use a spring to connect this pellet to
the circuit board. I suppose these things could be repaired
but I pretty much gave up at that point. It is a work of
art though.......

I have not looked inside the three terminal varieties, but
I am guessing (hoping) they may have simplified the design
and removed the active components at which point they no
longer needed the 4th (+12V) connection pin.

HTH
Scott M.
scottmo@teleport.com