Real Gauges how to find power/ground etc?

[helloo]

I have some real gauges that are going in my Overhead.  How would I use my multi-meter to determine which is the 5v power for backlighting, power for the gauge itself and ground, etc?  For example I have a fuel temp gauge with a cannon plug deal on the back. 

[jackpilot]

Open it and you will usually be able to figure it.

[Ed]

I have some real gauges that are going in my Overhead.  How would I use my multi-meter to determine which is the 5v power for backlighting, power for the gauge itself and ground, etc?  For example I have a fuel temp gauge with a cannon plug deal on the back.

Here's an example. Different unit, but will give an idea.

http://www.flaps2approach.com/journal/2012/10/28/converting-genuine-b737-audio-control-panels-acps.html

[helloo]

That site has been a great resource and I need to research it more.  I did some research before my post but that site has some info that seems to not make sense to me.

While talking about the cannon plug he states for power to touch a prong and then the outer casing for power (photo on the site and link below).  Then states right below to touch a prong and the lower positive end to the bulb, not the casing.  Seems like he is talking about a bulb vs cannon plug but not mentioning it.  I have not clue. 
http://www.flaps2approach.com/journal/2013/7/21/flight-deck-builders-toolbox-multimeter.html

At that link it states:  Note the black probe is touching the outer casing (earth) whilst the red probe is being moved from prong to prong.  An audible beep from the multimeter will sound when the prong touches the power prong.  It's important to ensure you are only touching one prong at a time, otherwise you will get an erroneous result (click image for larger view).

To check for power, touch one of the two probes to one of the prongs in the Canon plug.  Then, attach the other probe to the lower positive end of the bulb (not the bulb casing).  Move the first probe from prong to prong in the Canon plug until you here a beep.  The beep indicates that this prong is the power prong.

[mickc]

Try not to follow other internet guides, but obtain the proper diagrams for the gauge.   A fairly sound level of electrical knowledge is required to sucessfully interface real gauges and there are several pitfalls.

Take the fuel temp gauge you mentioned above:

1) These require 26v AC to operate correctly as an OEM gauge
2) The indicator uses a Wheatstone bridge to derive the temp, with the sensor requiring a resistance of between 75 and 105 Ohms  for full scale deflection, not a voltage across pins
3) The gauge looks to have been apart before, as there are parts missing from the back - you never know what is inside.  These are faulty gauges 7 out of 10 times, either a dead mechanism or burnt out bulbs.
4) The guide you mention wont help, as most gauges have isolated lighting circuits that do not connect to the chassis.  Also most have case grounds as well as signal grounds just to make it more confusing.

This is not a deterrent, It can be done, and several of us here have done it successfully,  just be aware of the can of worms that can be opened when you interface real parts.  Its about as far away from plug & play as you can get

[bernard S]

you are opening a can of whoopie..  be warned 

[helloo]

Thank you for the replies.  I will have to do more research.  The gauge came with the overhead panel so if I don't end up using it no big deal.