Temperature Guage going to full

[Goose407]

1.9 DG 1987. Have owned for 20 years. Temperature guage going to full and red light flashing Replaced temp sensor, replaced coolant level sensor still the same. Remove relay 43 the same. Engine runs fine and rad fan cuts in when it should. Engine not over heating but indicating that it is. Checked all wires that I can see in engine bay all seems ok. I guess it's a bad earth or broken wire, but where do I start. I'm not any good with auto electrics. Any one knows the most common place to start looking? Or recommend any one I can take it to, I'm in Wiltshire near Swindon.

[silverbullet]

Unless the gauge itself has failed or you have a fault at the dash end (pcb ribbon connector is a favourite)
The Haynes manual uses the factory current flow diagrams, permanent live and ignition switched live at the top, earth track is at the bottom. Not hard to read once you've got familiar with the format.

[cobblers]

Does it do this straight away, or only after the engine has been running for a while?
Does the fuel gauge read correctly?

There's a voltage regulator on the rear of the cluster than can fail. A simple way to check it is to unplug the connector from the coolant temp sensor on the engine, and measure the voltage across the two pins. With the ignition on, you should get a reading of 10V.
If your reading is higher, the regulator has failed.

If your reading is lower than 10v, go and look at the gauge. If it's still reading high with the temp sensor unplugged, then you likely have a wiring issue.

[Smiffo]

Not sure if I am thinking about this correct, so someone keep me straight if I am wrong….

Pop 14 pin connector off of dash pod. Leave relay 43 out. Go to temp sensor in engine bay, unplug, and measure resistance between red/yellow wire, and earth.

It should be open circuit. If you have a short, ( prob less than 50 ohms to make LED flash ) then look for that.
If it looks good, fault is at dash pod end.

[Robsey]

Typical values should fall between 50 ohms (hot) and 510 ohms (cold).

Much either side of that indicates a faulty sender, connector or wiring.

[Smiffo]

Typical values should fall between 50 ohms (hot) and 510 ohms (cold).

Much either side of that indicates a faulty sender, connector or wiring.

Good spot. Still could be sensor.

Good evening, Robsey