Inaccurate Temp Gauge

Hi All. I seem to be plagued in recent times with engine coolant issues (my posting “M13A over heating” Dec 23) caused by a failed thermostat and then compounded by installing a new but faulty replacement. During that adventure I replaced ECT sensor, checked water pump, flushed circuit, fitted a first thermostat, Rad cap, the list went on !

My 2003 pre facelift Gen 3, temp gauge is indicating max/off the dial reading and yet I can confirm operating temp is normal using an IR thermometer. All is well on engine performance, so it seems logical to assume the issue is between the ECT sensor and display gauge itself.

The ECT sensor consists of two internal thermistors, one providing data for the ECM and the other data for the cab temp display. The latter is a  yellow/white tracer wire that runs from the temp display gauge through the ECT sensor thermistor to ground.

It is a simple job to just refit the original ECT sensor however I want to take a more diagnostic approach rather than just changing parts, given my previous experience.

The manual provides reference resistance figures for both thermistor circuits but I can only find voltage figures for the ECM side of the connector not for the display gauge circuit.

So my big ask is does anybody know what the voltage should be on that circuit or will some kind soul with a working temp gauge please get their multimeter out and measure voltage on the centre pin of ECT sensor plug.  For those not too familiar with the M13A engine the grey triangular 3 pin plug is located on the nearside rear (UK spec) cylinder head and the wire to measure is Yellow/white trace which is  top centre terminal.

Mine measures to ground 9.25 volts (ignition obviously on), the 9.25 volts surprised me and seems odd I was expecting to see 5 volts..

The other two wires for ECM  thermistor circuit 5 volts with the last pin being a ground to complete the signal back to ECM.


Many thanks for your time
DaveThe only benefit of age is experience.

I looked under the bonnet, but can't identify the ECT sensor. Able to post a photo?

Thank you very much for your response Coil Spring.

I will take a photo asap, but just to clarify on the M13A engine the position of the ECT sensor is, when viewed from the nearside (UK) on the right back face of the cylinder head corner. It’s a little hidden amongst a cluster of connectors, the grey plug is a vertical push/pull fit onto the sensor head.

I back probed mine to measure the 9.5 volts (yellow/white wire), if you disconnect the plug to take the measurement your likely on the next normal start up to have the engine management light come on ! My experience has been this will return to normal on the third normal start up procedure. So maybe back probing is less risky.

Again thank you for your interest.
Dave