How to check coolant temp sensor on 84 4ksq

[Ti Kan]

Adam P. Brodeur writes:
> In trying to fix the cold start problem with my 84 4000SQ, I was told to
> check the coolant temperature sensor.  I'm told this controls the cold
> start valve.  Is the sensor I want to check in the top radiator hose?
> How do I check to make sure this is working properly?

What you describe is called the "thermo-time switch".  Basically it is
a electrically heated-thermocouple switch that is closed when it's cold,
and activates the cold start fuel injector valve when you're cranking
the engine to start.  The thermocouple heats quickly and the
switch opens to stop the cold start valve from spraying.  Also, since
the switch is mounted on the coolant hose flange, if the engine is
warm the switch will not close at all and therefore the cold start
injector won't activate.  The amount of time the thermo-time switch
will run the cold start injector is dependent on the initial temperature.
Also, as soon as you stop cranking (i.e., let the ignition key go from
"start" to "run", the power to the cold start injector is cut so it
stops.

Testing the thermo-time switch involves checking the resistance of the
heat coil to see if it's about right, and checking the switch contact
to make sure it's closed when cold and opens when warmed up (either by
the heater element or externally conducted heat).  Also, check that there
is power to the circuit and that the cold start injector would function
when needed.

Note that this is not the same as the various other "coolant temp senders"
in the engine bay.  These other senders provide signal to the coolant
temp gauge and warning light, and activate/deactivate the cold running
enrichment mode in the fuel injection ECU.  I think there is also another
one that turns off the A/C compressor in case of overheat.

All this is described in the Bentley manual for your model
( http://www.rb.com ).  The specific specs (like opening and closing
temperatures of various sensor switches) are also listed, not to mention
wiring diagrams, etc.  You are advised to get a copy if you want to
work on the car yourself.

-Ti
01 S4 2.7 biturbo (coming)
96 A4 2.8 quattro
84 5000S 2.1 turbo
80 4000 2.0
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