VDO Oil Temperature Sender Action

My Type IV motor has an after-market VDO oil temperature sender and gauge; the sender is the type which replaces the dipstick.

Worked until recently, now its either reading 0 (at startup) or full-scale (after 1 minute). Does anyone know what signal the sender should be sending. Are they repairable? Are replacement senders available?
tia

[Edited on 30-9-2003 by modulus]

I think that the guage works by measuring the resistance throught the sender.

I do know that the quality of the connections and the quality (and length!) of the wires can make a big difference to the reading: small changes in the resistance of the circuit has a big effect on the guage.

Make sure all your connections and wires are good before forking out for repairs.

[Edited on 1-10-2003 by Purple Martin]