# Testing Gauges



## joedee (Dec 13, 2012)

Hi,
Working on restoring the dash for my 68 GTO. Have the dash all apart and the gauges out. Before I put this all back together I wanted to test the gauges. How would I do that? Also, when I took the gauges apart there was some black putty around the gauges by the face plate. Not sure that is how it should be. I think the last person was just trying to seal it all up?

Thank you,
Joe


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## joedee (Dec 13, 2012)

*Gauges*

I forgot to mention that I only need to test the temp gauge since the needle was hung up and when I opended the face plate it went back to it's home (low) location on the dial.

Joe


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## Rukee (Feb 8, 2007)

I believe if the gauges run on 12v. you provide power to one side and ground out the other post the gauge should swing all the way. If you insert a test light to the ground side to provide some resistance, it should go about half way.


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## joedee (Dec 13, 2012)

*Gauges*

I was thinking about putting 12Volts to it but then I read somewhere that the gauge will fry at that and the max is something much less. 

Joe


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## Rukee (Feb 8, 2007)

Okay then, if you plug the gauges in, with the key on, if you ground out the temp sensor wire the gauge should go to full hot, unplugged it should be full cold, and with a test light on the sensor (the other side of the light grounded) it should be half way.


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## joedee (Dec 13, 2012)

Rukee said:


> Okay then, if you plug the gauges in, with the key on, if you ground out the temp sensor wire the gauge should go to full hot, unplugged it should be full cold, and with a test light on the sensor (the other side of the light grounded) it should be half way.


The Gauges are sitting in the garage on a work bench. I want test the temp gauge before I put everything back together.

Joe


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## joedee (Dec 13, 2012)

*temp gauge still a problem*

Ok Guys I put the gauges back in the car to test them out. All the gauges work except the temp gauge. When I turn the key to run the gauges all go to home (including the temp gauge) and the gen light comes on like it should. When I turn the key to start the temp gauge goes all the way past hot and stay there. All the other gauges worked fine and the gen light went out as it should.I took the wire off the temp sensor and tired it again and the same thing. I checked the circuit boad and it looks ok but I did notice the white strips that on on the gauges all look good except the one near the temp gauge. It is there but looks a little off center as compared to the other gauges. Any ideas what I should check? :confused

Joe


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## BearGFR (Aug 25, 2008)

That happens because your gauge is "seeing" max voltage as soon as you turn on the key. My suspicion is either the sending unit is bad (shorted) or you've got a wiring problem. Gauges are basically 'dumb' volt meters (or ammeters, I forget for sure which). All the magic is in the sending unit. The sending unit "reads" whatever it's measuring (pressure, temperature, fuel level, etc) and translates that into an electrical resistance that varies with whatever it is that's being measured. As pressure, temperature, fuel level, etc. change the resistance also changes. The gauge is just indicating how much "juice" is getting through the circuit after that resitance soaks up some of it.

Disconnect the lead from your temperature sending unit and verify that the gauge doesn't swing when you apply power. Something else you could do, if you have some old junk electronics laying around, is rob a "volume control" (potentiometer) out of something (or go buy a cheap one from a local electronics store - if you buy one try to get one that's in the 0 to 200 ohm range or so - one with a higher range will "work" but it'll probably be very twitchy/sensitive to very small knob movements). They usually have 3 connections. Make some jumper wires, connect one of the "outside" terminals on the control to your battery positive, connect the middle terminal to your gauge lead. You should be able to control the gauge needle "reading" by changing the position of the knob. That'll tell you the gauge itself and the wiring are probably both ok, leaving the sending unit as the villain.
Anothing thing you can test is to use an ohmmeter on the sending unit. With the engine cold, measure the resistance between the wire connector and ground. The reading should be "high" - 100 ohms or so. If it's zero or very close to zero, it's shorted and bad. You should be able to start the engine and observe the resistance slowly dropping as the engine warms up.

Bear


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## joedee (Dec 13, 2012)

BearGFR said:


> That happens because your gauge is "seeing" max voltage as soon as you turn on the key. My suspicion is either the sending unit is bad (shorted) or you've got a wiring problem. Gauges are basically 'dumb' volt meters (or ammeters, I forget for sure which). All the magic is in the sending unit. The sending unit "reads" whatever it's measuring (pressure, temperature, fuel level, etc) and translates that into an electrical resistance that varies with whatever it is that's being measured. As pressure, temperature, fuel level, etc. change the resistance also changes. The gauge is just indicating how much "juice" is getting through the circuit after that resitance soaks up some of it.
> 
> Disconnect the lead from your temperature sending unit and verify that the gauge doesn't swing when you apply power. Something else you could do, if you have some old junk electronics laying around, is rob a "volume control" (potentiometer) out of something (or go buy a cheap one from a local electronics store - if you buy one try to get one that's in the 0 to 200 ohm range or so). They usually have 3 connections. Make some jumper wires, connect one of the "outside" terminals on the control to your battery positive, connect the middle terminal to your gauge lead. You should be able to control the gauge needle "reading" by changing the position of the knob. That'll tell you the gauge itself is probably ok.
> 
> Bear


Hi Thank you for the reply. I did disconnet the temp sensor and the gauge still went to max when I turned the key. From what I can see there is only the temp sensor wire and power that feed the gauge. I am thinking maybe there is a problem with the connection on the printed circuit broad. Maybe one of the connectors is grounding out? What do you think?

Joe


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## BearGFR (Aug 25, 2008)

Ok, if that happened then the gauge is still seeing power from "somewhere". Start with the easy things first. Make SURE you connected the correct wire to the sensor. On my 69 there are several wires that exit the wiring harness at that same spot but only one of them is the right one for the temp sensor. (On my 69 it's the green wire)
Another possibility is that you've got what was originally an "idiot light" car and at some point in the past someone tried to convert it over to gauges and didn't do the job right. The printed circuit board on the back of the gauge cluster, the engine wiring harness, and (I think maybe) the dash harness are all different for "gauge" cars than they are for "light" cars. If you know all that's correct, then the only remaining possibility is a circuit problem of some kind that's sending power to the gauge even with the wire disconnected. Those are the most difficult to find. You'll generally have to start at the back of the gauge and trace backwards towards the other end with either a test light or a meter until you find it. It helps to have a good, readable wiring diagram. I got mine from ClassicCarWiring.com - it's in color, large, and laminated. MUCH easier to read, especially at my advanced age. 

Bear


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## Rukee (Feb 8, 2007)

Maybe the sending unit is working backwards?? What happens to the gauge if you ground out the sending unit wire?


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## joedee (Dec 13, 2012)

Thanks Bear. Yes, the temp wire is green on a 68 (and has the special connector to attach to the top of the sending unit). All the other gauges work correctly and if memory serves correctly the PHS doc's show it started life as a rally gauge car. I will take your advise and start at the back of the gauge and go from there. I hope the problem is at the printed circuit board. I want all the guages (in fact everything on the car) to work correctly like it did when new.


Joe


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## BearGFR (Aug 25, 2008)

Good deal. It's still worth verifying that circuit board. Unless the car's been in your possession since it was new there's no telling what indignities a previous owner might have inflicted on it  You already know "someone" has been in there before because of that goop you found in the gauge, right?

Bear


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## joedee (Dec 13, 2012)

Rukee said:


> Maybe the sending unit is working backwards?? What happens to the gauge if you ground out the sending unit wire?


Good question. Since the gauge goes all the way past hot when the car is started with the sensor disconnected I don't know. I will try grounding the temp wire and give it a try to see what it does.
How could the sending unit work backwards?

Joe


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## Rukee (Feb 8, 2007)

The sending units work with resistance as Bear stated. So they can go from an open circuit(no connection at all) to 0 ohms(fully grounded out) (cold to hot), or from 0 ohms to an open circuit (cold to hot) depending on the application. Sending units for a gauge will make the transaction smoothly, one for a light will be instant at a set temp.


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## BearGFR (Aug 25, 2008)

Something old and musty is rattling around in the dark recesses of my feeble memory... I think this might be one of the differences between the circuit board for gauges vs. the circuit board for lights. I know the traces for where the connector plugs into it are different, and I'm thinking that if you plug the 'gauge' harness connector into a 'light' circuit board you wind up with constant power going to the temp gauge. I think I ran into this when I converted my car from lights to gauges...

Bear


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## joedee (Dec 13, 2012)

BearGFR said:


> Something old and musty is rattling around in the dark recesses of my feeble memory... I think this might be one of the differences between the circuit board for gauges vs. the circuit board for lights. I know the traces for where the connector plugs into it are different, and I'm thinking that if you plug the 'gauge' harness connector into a 'light' circuit board you wind up with constant power going to the temp gauge. I think I ran into this when I converted my car from lights to gauges...
> 
> Bear


Hi Bear,
You might be right but I guess I need to see some pic's of the light one and the gauge one to know for sure. I did remember something last night and this might be the problem. The guy I got the car from told me he changed the alternator and the voltage regulator. I had read that there are different connections for lights versus gauges. I found the instructions for the alternator install with the paperwork he gave me. It appears if you have lights there are 4 connections that are made. Gauges are only 3 (and the wires go to different contacts on the voltage regulator). Well it was getting late and we had a big storm last night so I just looked at how he had it connected and found it appears he wired it for lights. There were 4 wires going to the voltage regulator and 4 to the alternator. Now I need to find out what color wires go where so I can change this and see if that fixes the problem.

Joe


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## joedee (Dec 13, 2012)

Just looked at some pic's of the circuit board from the OPGI site. The one I have is the correct one for gauges. So it must be the wiring on the alt and volt reg. I do wonder why the other gauges appear to work OK and only the Temp is messed up?

Joe


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## Rukee (Feb 8, 2007)

Maybe unplug the engine harness at the firewall and that will tell you which side of the firewall the problem is.


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## joedee (Dec 13, 2012)

I will do that if the rewire of the Voltage Reg and Alt does not work. I also found some info from a firebird site. This could also be the problem if someone changed out the harness at some time.
Here is what that wrote:

Q: Tempature Gauge Problem with Rally Gauges 
I'm having some problems with my Temp gauge. The rally gauges were redone at some point before I got the car by a reputable shop. But the Temp gauge jumps as I start the car but then goes to the cold side and stays there when running. I haven't tested or changed the sending unit yet... Other than that, any suggestions? 

A: I experienced this very problem when I installed my rally gauges. There's a wire that goes to the ignition that was used to test the idiot light for temp when you started the car. I didn't disconnect it at first, and it caused my needle to peg when starting the car. It eventually got stuck. After I removed the wire, the problem was gone. Cutting this wire is mentioned in the FAQs for rally gauge install; I don't remember the color (maybe green) but its the only connection on the harness that had two wires going to one point. I 'll dig into my notes at home if you're still stumped. Sounds like maybe the shop didn't know this little detail.


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## joedee (Dec 13, 2012)

*Temp gauge is working*

The temp gauge is now working as it should. arty: After reading the info from the Firebird site I posted above I checked and sure enough the temp gauge wire went from the sender to the ignition switch and then back out to the harness for the gauges. I cut the wires at the ingnition switch and spliced the wire so it went directly to the guage connector. Turned the switch to start and the temp gauge just went to home and did not jump to hot. Let the car run a while and the gauge started to move as it should. Even though my car came with rally gauges it appears some time in the past someone put a new harness in from a car that had lights versus the gauges. They must have made all the other changes needed but did not know about the change needed on the temp wire. 

Now on to other things like getting car painted and then putting it all back together again.

Joe
:cheers


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