Under Reading coolant gauge on I5 100

[rob hod]

    Wonder if anyone can help me out on this. Patient is an '88 100 with 2
Litre I5 engine (RT).

    The problem is that the temp gauge is under reading. I have a sneaky
suspicion the thermostat has been removed as the car generally runs cool,
I'll be checking this next weekend.  However there's no doubt that the gauge
reads under, for example the car is running up to a reasonable temp.  in
summer but rarely reads more than 1/4, and for further proof the radiator
fan cuts in with the temp gauge reading around 1/3rd f.s.d.

    Now the specifics of my query are this, The wiring diagram mentions an
'electronic thermoswitch, and the wiring on the car suggests that this is a
brass fitting with a four pin electrical connection under the upper rad hose
connection to the head.  Before I bought the car I think this sender was
replaced as I have an invoice for Pn 034 919 369C "Temp sender" I wonder if
a slightly wrong part was used? If I take a lead from the brown/black wire
on the socket and earth it with the engine off but the ignition on I get a
gauge reading of around 2/3rds. Anyone know if this part is indeed the
'electronic thermoswitch mentioned in the manual?

I have a theory that I may get a slightly higher reading with the engine
running, but presumeably the voltage stabiliser should limit that to a fair
extent.


    In addition there is a small sensor fitted to the top of the hose outlet
fitting which also has a brown/black wire going to it.
    Anyone know if this is the 'overheat' sensor or the actual gauge sensor
?

    By the way the fuel gauge appears to be working and calibrated properly.

    Thanks in advance


    Rob Hod