Temp sensor query

[Paul B]

Ok, so it used to work really well with a stable temperature. After a major drenching a couple of years ago, the gauge started showing a variable temp, eventually stabilising at the correct temp.

I have put in a new sensor FE07798, renewed the earth leads between the gauge and the chassis and renewed the thermostat. The gauge still raises up to 110C or so before dropping back, then moving up and down quite quickly.

Checked the resistance across the sensor which showed, with the multimeter set to 20K ohms, 0.04. On checking the old sensor, it shows 0.4.

If I disconnect the earth from the battery and take a lead from the sensor to the battery earth, the gauge goes straight to max which suggests it is working correctly.

Now I am puzzled. Any suggestions ?

[Clive]

The power goes to the gauge and then earths through the sensor, not the other way round, therefore if you disconnect the wire at the sensor and touch it directly to the block or any other earth point, the gauge should read Max. If this is the case, then the sensor is at fault. These sensors are extremely prone to misread when damp or wet but are not expensive and easily changed.
Another thought is whether or not you need a voltage stabiliser for the gauge.

[Paul B]

Hi Clive
I tested the gauge as you suggest and it went to the max. But my query was also regarding the resistance within the sensor as the old one reads quite different to the new one, although the nominal reading would be about the same.

However, they both showed a varying reading which I am trying resolve, but I am not too electrically minded so am puzzled !

Can a voltage stabiliser easily be fitted into a single circuit ?

[Russ-Notts]

Paul,

The difference in resistance between the old and the new sensor is actually quite small and can easily be accounted for by the contact resistance between your multimeter probes and the surface of the sensor. (E.g. the older sensor surface is slightly more tarnished compared with the new one).

Have you replaced the wire going from the sensor to the gauge? There could be a small break in the cable or more likely a bit of corrosion on the connector or the end of the wire if it terminates on to a spade connector. Everything needs to be nice and clean or it'll add extra resistance into the circuit or a bad joint.

If everything above looks okay so far, then I have a query about what you say, "The gauge still raises up to 110C or so before dropping back, then moving up and down quite quickly." Does the gauge then settle to show a steady temperature or continue to move up an down a lot, even after the engine has been running a good while? The initial rise to 110C and then falling back only to rise again to a steady temperature reading could be due to the thermostat opening and letting the colder water from the radiator flow over the temperature sensor. Thermostats usually have small holes or a bypass to allow some flow of water when the thermostat is closed.

Russ

[Paul B]

Russ, hadn’t thought about the connector which looks clean but may not be inside !

The new thermostat has the proper jiggle pin and is installed the correct way up. The gauge still fluctuates as I drive along with it going between 80 and 110 C. I am confident that the actual temp does not vary that much or that quickly as I have used two sensors and two thermostats with the similar results.

For many years, it was never a problem then after getting caught in a monster downpour, it started playing up. Then I thought that maybe water had gotten into an earth point or something like that, so renewed and cleaned all earths whilst renovating the dash this winter. The issue persists.

Any other ideas ?

[Clive]

Voltage stabilisers are easily installed, however, on reflection, I don't think you need them if you didn't have problems originally. Most new instruments have them built in anyway.

One other way to check the sensor, is to make a fly lead and swap the oil temp sensor wire with the water temp wire and see if the oil temp gauge is behaving in the same way.

[John]

I had a similar issue last year. My problem was solved when I removed the temperature sensor and it was evident I had an airlock in the area where the temperature was located.
On removal of the temperature sensor a tide mark could easily be seen on the sensor with only 3 or 4mm actually being submerged in the coolant. I merely topped off the system with the sensor removed until fluid came out of the locating hole and then refitted the temperature sensor. The problem has not reoccurred. I hope your issue is something similar and simply solved.