Hi
I bought a AC Delco temp gauge to check the other one i have is working right, suspect its reading too high, but the pins in the back are numbered 55 and 75.

usually thay have S for signal wire and I for ignition.

Anyone know which is which, i dont want to connect them wrong way round and damage the gauge. there is also an unmarked pin which i presume is the earth.

Thanks

Normally the gauges have a '+', a '-' and a 'S' or 'G'. I have not seen temp gages with only two contacts. Normally they come with a description of what's what; didn't yours?

No bought second hand, it does have three terminals one isnt named or numbered so i presume its the earth, the other two are as above, it is a ac delco so i thought it would be common for them to number them as such.

Im thinking for some reason 55 would be Ignition and 75 signal, but im not sure, lol

more info needed really as you cant assume you'll get anything useful stamped on the back
what size what shape how does it handle the illumination etc?

Taking a chance and assuming its yer typical 50mm type with tube to stuff a bulb up
for illumination.
I'd take a meter to it and confirm the pin you think is earth is common to the barrel
Then I'd check the two pins which you think are the gauge are insulated to earth/body
If so, I'd next check the resistance between them
If its a bimetal type you may see about 50 ohms possibly less, between them
staying with bimetal types for a minute. Polarity isnt important as its the current (heat)
passing through which makes the needle deflect, so you could apply voltage one side and the sender
to the other with out worrying about polarity.

However, some gaiuges are designed to run at lower then battery voltage and need a regulator
to supply say 8 volts so it really helps to know where its from to try and replicate the correct
working conditions

If its a moving coil type gauge it may not be happy being connected up the opposite way
Id use flying leads and some low value resistors to do some gentle testing prior to fitting

Its strange why they go and use different labelling than normal, heres a picture of the back of the gauge.





My assumption is 55 is live, 75 is signal and the unlabelled one is earth. i took it apart to see if i could see anything but its all incased in metal so couldnt see anything.

reg_K100RS wrote:more info needed really as you cant assume you'll get anything useful stamped on the back
....
However, some gaiuges are designed to run at lower then battery voltage and need a regulator
to supply say 8 volts so it really helps to know where its from to try and replicate the correct
working conditions
...

True... my Goldwing needs a regulator to drop the voltage from ~12V to 7.0V so that the fuel gauge and temperature gauges will work. This is required by many cars. I think that my Jag needs 10V.

The effect is exactly what you said, they read too high with 12v. Also, because the the battery system provides voltages that vary between 12v to 15v, depending on revs, precision gauge readouts need to have the voltage to be a constant, in this case 7.0V.

The cure is relatively simple. see this, it explains it all : http://www.randakks.com/TechTip41.htm

Theres an easier cheaper way Ned
Ideally a 7807 and heat sink
but as theyre hard to find in the UK I've been using a common as muck 7805 with green led between the
ground pin and earth for years and made many for fellow CX owners
A simple, crude but effective solution which is cheap and robust.

The green led has a forward voltage of 2.95V and this added to the 7805 gives 6.95
which runs the temp gauge quite happily.
A heat sink is important as otherwise the IC will overheat and shut down
but thats all I add
As its a crude circuit anyway, I dont even bother with smoothing caps and have
one built into my CX rat/shopping trolleys gauge.
I recently found a supplier of the 7807 in the UK Great! even easier !
and will refurbish old, knackered, regulators using these.
I may even add some smoothing caps to the 'luxury' versions

I actually use LM317t. I've inherited 20 or 30 in a bag the total cost is the price of resistors ...