My temperature gauge has been a bit erratic for a while and I’ve suspected the thermostat has been stuck open as the temp gauge takes ages to warm up, never gets past the N mark and then drops very low on long steady runs, if the thermostat is working correctly the needle should get up to temp and stay there. I removed the thermostat housing to find the old stat was stuck partially open so replaced it with a new genuine 88 degree thermostat (ERR2803). A stuck stat is not just an over cooling problem but it was not opening fully either so the engine could potentially get too hot.
With the new thermostat the engine seemed much happier and the heater was very warm. However after a couple of miles the temperature gauge needle was in the red. I have an infra red thermometer that you can point at an object and it tells you the temperature (highly recommend these, very helpful for all sorts of things) so I tested the thermostat housing and it was showing 88 degrees, which is more or less the normal operating temp for a 200TDI so obviously the guage was reading incorrectly. I have a series temp sender in the 200TDI with the adaptor for the TDI engine so that should be matched to the Series guage ok. While opening up the clocks in the dash I found the voltage stabilser was loose in the back, it should be bolted to the back of the speedo as the casing itself needs earthing for it to work correctly. The stabiliser should give a steady 10v to the temp and fuel gauges, as the voltage from the alternator can vary, therefore you’d have varying guage readings depending on the voltage so the stabiliser’s job is to give a steady 10v. As my stabilser was giving the full alternator voltage (13.7v in my case) to the temperature and fuel gauge they were therefore over reading. Once the stabilser was earthed again the temp gauge was reading normal at the correct operating temperature as it should. The bad news is I had less fuel than I thought I had!
Below are pics of the standard series 3 voltage stabliser that should be bolted to the back of the speedo, there are a set of two female connectors and a set of two male. The positive from the fuse box goes to one male spade and one female spade (supplying 10v) goes to the temperature gauge and from there onto the temperature guage. As mentioned before the case needs to earthed for it to work correctly.
While I was at it I decided to upgrade the stabiliser to a solid state modern version that is a bit more reliable and accurate, bought this one on eBay for a tenner – it is a direct replacement & a five minute job to swap over: