The 'E' Type Forum

After some work done inside the temp gauge ( grinding the plastic shield for the led bulb ) I put back the gauge in the dash but now the reading is at the maximum with a cold engine. I checked the input voltage which is 10 V engine stopped and not warm. The temp sender seems ok What mistake could I have made ?
I am sure I am the culprit
The needle is free and at 0 when I disconnect the gauge

Are you pos or neg earth? Try changing polarity.

Hi Yves..sounds like you have a full earth on the input....the temp sender gives a resistance earth that changes with temperature..Steve

The little Smiths gauges don't have "polarity".

What Steve is saying is that you are accidentally earthing one side of the gauge (the other side that is not the 10v green wire) instead of running through the sender which is then earthed into the engine block. Check your wiring and with both gauge wires disconnected, there should also be an infinitely high resistance to the chassis when measuring each gauge terminal.

Simply swapping the wiring might be a bad idea if you now somehow accidentally earth the 10v supply directly. That will make the 10v regulator get very hot very quickly.

kind regards
Marek

Yes, your temp sender wire is either going to earth (ground), between gauge terminal and sender but if it worked before, which I'm guessing it did, it's unlikely to have failed now (never say never though).

If you had the gauge stripped, my guess is that you're gauge outer casing is touching the earth terminal stud instead of being insulated. Once the gauge is refitted, it immediately goes to ground (earth) via dash and will give you the max reading effect.

Thanks all of you for your support, I’ll investigate tomorrow and report here….

I checked the gauge this morning and found nothing wrong but a buzzing sound on the multimeter tester between each terminal and the casing ….! So I unscrewed each terminal to see if there was a short with the casing and tested again …. And you know what…. All is fine after that! I must have made a short contact before….! May be it was the needle touching the casing?

Well done Yves.

Result

breaking news.....! i found exactly what was the issue!
I wanted to have a drive this beautiful sunny cold afternoon but checking the gauges, the water temp was fine but the fuel one not at all ! no panic ... i took out the gauge and found on the bench the mistake . When i opened the casing, i have unscrewed the 2 connectors pins to have an acces to the plastic shield i wanted to grind. Doing that i moved a little the casing holding the studs so i made a short inside the casing that i could'n see . i realized that because i maintained the multimeter plugged and it buzzed as i was refitting the studs screw .
I am happy ! i learned something new for me ( for me formidable !)
Below the photos of the gauge before modification of the blue plastic shield and others...
PS: at the end i took out all the plastic shield to have enough light on the gauge . BTW i have some years ago fitted a dimable button for the dash lights .