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The temperature gauge on the P38 has been going up into the red area on the gauge with the light coming on. The Nanocom shows the temperature at 80 degrees.

I've put a new sensor in and have the same result.

Resistance is .758 Kohms at 18 degrees and 58 ohms at 80 degrees

Voltage is 4.39 at 18 degrees and .2.85V at 80 degrees

There is the same voltage at pin 19 on the BECM so continuity is OK

Should I be looking at the BECM, the wiring or the gauge as the next suspect or are those readings off?

Many thanks as always.

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Just been out and tried on the Ascot which is early enough to have the same single connector sensor as you have. At 11 degrees (according to the HEVAC), the resistance of the sensor is 0.908 kOhms and the gauge, as you would expect, reads cold. Unplugged the wire from the sensor and connected a 62 Ohm (the closest I could find in my box of assorted resistors) between the alternator body, which should give a decent ground and the connector. With the ignition on the gauge read just below the mid point. Which suggests it isn't the sensor but something odd happening in the AD conversion within the BeCM or the DA conversion back in the instrument cluster.

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Thanks Richard. I suppose I could try a secondhand cluster to eliminate that.

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If you've got one. I've got one that I swapped the circuit board over as the driver chips for some of the warning lights died so I had no oil pressure light (amongst others). Just be careful though as swapping the whole cluster can affect the recorded mileage.

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Just had a thought. If you've got 758 Ohms when cold and 58 when hot, presumably the gauge reads correctly when cold? In that case, if you were to put a resistor in series with the sensor, it should bring the gauge reading down. I'd start with something like a 200 Ohm resistor (so giving 258 in total) and see what the gauge reads then.

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That's inspired Richard. I'll get down to the resistor shop in the morning and have a play.

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So, with a collection of resistors I got these results.

disconnecting the green sensor wire and sending direct to earth via a 22 ohm resistor the gauge read high with light on.
With a 68 ohm resistor the gauge read 1/4 to 1/2 full
With a 120 ohm resistor the gauge read just above cold.

Whilst providing a direct earth to the body of the sensor via the alternator body and reconnecting the wire it took a lot longer for the gauge to hit the highest point. Maybe why the later sensors were two pin.

I'm going to pick up a genuine LR sensor tomorrow having had a little lie down after seeing the cost to see if there is any benefit.

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Checking the earth path from the sender to the battery there was a 6.1 ohm resistance. Remade the joint between the chassis earth and the earth cable to te alternator and it's looking like it has sorted it.

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Nice one, I suspect you are correct and that is why they went over to the two pin sensor on later cars. Wouldn't have expected 6.1 Ohms to have made that much difference though.