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4 terminal coolant temp sensor
I recently had trouble with my 4 terminal temp sensor on my 86 5000CS Turbo.
My coolant temp gauge would intermittently go dead. I took the 4 terminal
sensor out and actually took it apart. You would not believe what is inside
this little sensor. There is a hybrid circuit (ceramic substrate) with active
surface mount components (several transistors, resistors, capacitors and an
IC or two). There is a thermistor pellet in the tip with a spring that
connects to the hybrid circuit. It looks like the thermal cycling of the
engine causes all the solder joints to get brittle and crack.
Being the cheap tight wad that I am I attempted to resolder all the
connections but some of the connections near the terminals are covered with
an epoxy potting material. I gave up trying to fix it and just bought
a new one at Halsey for around $42 (VDO brand). The Audi replacement part
($85) from the dealer now only has 3 terminals but replaces the 4 terminal
model. Audi/VDO may have simplified the design which no longer uses the
active components. It might just me a simple thermistor with a thermal swtich
for the overtemp warning. The one I replaced had a date code indicating it
was only a few years old.