Temp gauge

Mornin all...

Read loads of threads on this but nothing really answers me question.

Temp gauge was working fine...changed a bulb in fuel gauge, put dash back together now temp gauge goes almost all the way up (into the red) even when engine is coldish. D'oh ! (_8(I)

Been over and over wiring and cant see anything out of place, earths look ok etc.

Sure its stupidly simple but anyone got any ideas please ?

is the sender wire on the right teminal on the back of the gauge.

yep. there are three wires.
one earth that goes under the mounting braket,
the other two are spade connectors but one side female to male, the other side male to female, so cant get them wrong.

try taking fuel light bulb sounds stupid but thats wot started prob!

After spending a couple of hours attaching various coloured wired to various dials with little success all I can add is that when its wrong the dials pointer quickly rises and sticks at max untill you unplug it again.

I'm still trying to understand it myself.

The wire to the mounting bracket will be the negative for the light in the dial.

Good luck

me too...spent chuffin ages (even let me tea get cold !!) trying to see if anything else had come undone etc but everything else works fine.

I'm wondering if its poss to short out the sender or summit silly like that ?

I don't know enough to know how to test the sender.

I'm really good at assuming things (assumption being the mother of all f$ckups), like surely if the sensor is electric based then if its cold there must be great resistance from the sensor so not much voltage gets through to the dial so the pointer is low and when the temperature rises the resistance in the sensor is less so more voltage gets through so the pointer rises (or its the other way round).

maybe someone on here knows which it is.

If it were me I too would be reluctant to look elsewhere than behind the dash, however if you pull off the temp wire at the engine and it goes to zero then it's a transmittor fault, if it stays the same, and I suspect it will then trace the wire back to the guage, just in case you have chaffed it and shorted it out.

When you pushed the bulb in you sure you never twisted the contacts and shorted it out, the bulb works I take it, if it's not then perhaps you have bent the contacts. It sounds like you can't wire it wrong so check all the contacts again, it sounds like either you have shorted the link to the guage or lost the earth somewhere, sometines the guage will supply the earth for the lost earth causing the fault. On another occasion I have found vehicle to have diodes fitted when they fail they also cause this fault because of feedback through the bulb and guage, but if you take the bulb out it goes back to normal. If the bulb was broke then it would have been ok until you fitted the good bulb and gave it a path to travel.

To check the diode if you find it, use a meter and pass the test current through it, it should only pass one way if both directions then it's broke.

Electrics can take 2 minutes or two days, so hope these sugestions help you.

To the back of erach guage there should be 2 earths - one for the light bulb and one for the gauge. You may find that the gauge is trying to earth through the lightbulb, or some odd effect like that. Asthe husing ofthe gauge is not otherise earthed, try connecting a wire straight from the gauge housing to the body, and then try the gauge. If the needle goes very high, it will also take a long time to show any decrease, as they are damped to prevent the old style wavey needles that change as you go over a small piece of gravel... Also, check under the bonnet that the 'spare' temp sender wire isnt touching something, if your engine is a *J (12,19) engine there is 2 wires, one near the water pump and one near the back of the engine.

Your wires may be brittle (or the sleeve of the wires) if yer landy is old. My 1989 90's wires, if you muck about with them too much the sleeve cracks and the core becomes exposed. You may have yanked a wire (and lets face it theres such a spagetti mess of wires in there, you have to get quite physical with em) exposed the core, which may be earthing. I did that with the temp sensor wire in the engine bay and had to replace the wire as it was shorting on the engine giving similar results to what you say..

Get your multimeter on the connectors on the actual sender unit in the engine to test resistance atr various engine temperatures, then test the gauge itself with the meter, that will confirm whether it is the wires, if so test the wires along their length at various intervals to see where the probelm is (if it hasn't been identified as faulty gauge or sender)

Good luck!
OD

Chances are the gauge is hitting the high red end because the signal cable (the one from the sender) is earthed somewhere.

Test it thus ..... switch on, and let the gauge rise to high red.

Now go under the bonnet and take off the cable from the sender.

If the needle stays UP then the cable is shorting somewhere.

CharlesY

Great, thanks CharlesY.

Tried disconnecting from the sender and gauge goes down. Ordered a new sender now... hopefully will sort it.

Thanks everyone for your help and input. )