[s-cars] irregular temp gauge readings

[Kevin Day]

On Mon, Jan 30, 2006 at 04:29:02PM -0700, Matthew Russell wrote:
> Have replaced thermostat twice now, boiled both thermostats and both 
> appear good. (opening around 87 C).
> 
> HVAC channel 51 reads correctly (86-89 at operating temperature) - 
> meaning the MFTS is functioning, correct?

Not quite.  The HVAC reads the sensor in the hose near the firewall.
Note that it only gets a good reading when the heater is on, so if you
get low numbers on channel 51 in the summer, don't panic.

> Symptoms are that temp gauge will start to rise as car warms up from 
> cold, then will plummet down to almost nothing, then may come back up 
> in 5 minutes or so (time varies).

Two possiblities that I can think of:
- Bad MFTS
- Loose connection from temp. gauge to cluster circuit board

The gauge screws into the PCB.  It's possible (though not likely) that
one of the nuts is loose and you have an intermittent connection there.

You can test the gauge by grounding the MFTS pin in the harness in the
engine compartment.  The gauge should go all the way hot when the pin is
at 0V.

> Is the sending unit for the temp gauge located between the back of the 
> engine and the firewall, in a plastic fitting with coolant hoses on 
> either end of the fitting, held in by a clip?  Is this the likely 
> culprit? Can someone point me in the right direction?

As above, that one is for the HVAC.  The gauge reads the MFTS.  There
are also, in no particular order:
- A two-way radiator fan switch mounted in the lower left corner of the
  radiator
- The after-run temperature switch near the MFTS
- The ECU's coolant temperature sender screwed in to the rear of the engine

However your problem doesn't concern any of those.  I just thought I'd
enumerate all of the ways the Audi engineers allowed us to experience
the joy of aging coolant sensors :)

-Kevin