testing coolant temp gauge sender for 80/90/4k/4kq/CGT

[Jpinkowish at aol.com]

Dave,
If your coolant temp gauge is reading low, the sender is really easy to test.
I'm going to assume that the 90q coolant temp gauge sender is identical to
the 4kq sender.
Using a multimeter, these are the resistances you should see:

    TEMP(degF)          RESISTANCE(ohms)
    <68 (20C)                   >1000
      68    (40C)                     1000
     104(60C)                       500
     140(80C)                      180-200
     194(90C)                        75-100
     230(110C)                       45-60

These are from the 4k/4kq/CGT Bentley, page 90.20
Note that you should idle the engine until the cooling fan comes on.  When it
does, the coolant is at 90-95C, and the sender should generate the
appropriate resistance(<100 ohms).  If it isn't, suspect the sender.

Conversely, you can check your dashboard coolant temp gauge by using a jumper
to send 12V from the battery(ignition off!) through the sender wire -- it
should be blue/brown(hey Huw, is this OK?).  I'll assume that this will send
the gauge needle to the top.  If it does, time for a new sender.

Audi # for the 4kq coolant temp sender is 049 919 501B.  The Parts Bin shows
this is a Behr part and gives a price of $20.72US.

HTH

Jan Pinkowish
'85 4ksq
Bristol, CT