[67428HO]

How do you bench test the rally and fuel gauges on a 68 GTO? Thanks

[Peter Serio]

The gauges in 1965+, 1970s and 1980s GM cars are miniature electromagnets. They are wound using 2 coils in the shape of a "X." Both coils have a common power supply and then branch off to go in separate directions. Post 1965 all of this is neatly contained in a metal can with the dial riveted onto the front. In order to test one of these type gauges you need a good 12 volt DC power supply (I use a jumper pack.) Several lengths of jumper wire with alligator clips on the ends and a variable ohms resistor box. To give you a baseline each gauge needs 12 volts + ground (AT THE BACK OF THE GAUGE) and then you have a "seeker" wire which looks for reference back to ground. (TIP: 1968 GM cars have steel gauge housings) the ground path is thru the steel gauge enclosure via several overlapping touch points ending at the welded metal structure of the car's dash.

On 1970s GM cars that started using all plastic housings for the dash gauges the ground path was re-routed into the printed circuit.


That reference is different depending on the type gauge you are testing. Some go from low or zero ohms resistance to more to show an increase (oil pressure and fuel). The coolant temperature gauge sender starts out at a very high resistance and as the water temperature increases the ohms inside of the sender decreases. This shows you temperature rise on the dial.

The sender(s) are the variable in the test. In order to make any sense out of it all you need a way to imitate each sender as your control. I made a box (re-purposed) out of an old ohms resistor testing box I bought on ebay for around $20. My box gives me access to fixed points; there is a "click" rotary dial on top that represent Ohms resistance. ( I actually have several of these boxes, they are all different.) Kent Moore also made a box that came in a white plastic case. It was made specifically for car dealerships to test car gauges.

1968 Lemans or GTO:

Fuel gauge: zero ohms = E. 45 ohms = 1/2 scale. 90 or 91 ohms = F.
Oil pressure gauge: zero ohms = 0. 44 or 45 ohms = 1/2 scale. 90 or 91 ohms = Full scale.
Temperature gauge: 600 to 700 ohms = Cold. 76 ohms = 1/2 scale. 51 ohms = max overheat.

(NOTE: the above numbers are to the question here; a 1968 GTO.) Other GM cars, and different years; with rally gauges will have different senders and/or marking on the dials; so the #s above may or may not correlate.

After I am satisfied the gauge is right I retest using pressure (compressed air) on the sender for my oil pressure gauge. I used heated water for matching the sender to my coolant temperature gauge. This is the only way that you will know for sure if the gauge is reading true.

[67428HO]

Great info! Thanks!

[alsalp]

Originally Posted by Peter Serio

The gauges in 1965+, 1970s and 1980s GM cars are miniature electromagnets. They are wound using 2 coils in the shape of a "X." Both coils have a common power supply and then branch off to go in separate directions. Post 1965 all of this is neatly contained in a metal can with the dial riveted onto the front. In order to test one of these type gauges you need a good 12 volt DC power supply (I use a jumper pack.) Several lengths of jumper wire with alligator clips on the ends and a variable ohms resistor box. To give you a baseline each gauge needs 12 volts + ground (AT THE BACK OF THE GAUGE) and then you have a "seeker" wire which looks for reference back to ground. (TIP: 1968 GM cars have steel gauge housings) the ground path is thru the steel gauge enclosure via several overlapping touch points ending at the welded metal structure of the car's dash.

On 1970s GM cars that started using all plastic housings for the dash gauges the ground path was re-routed into the printed circuit.


That reference is different depending on the type gauge you are testing. Some go from low or zero ohms resistance to more to show an increase (oil pressure and fuel). The coolant temperature gauge sender starts out at a very high resistance and as the water temperature increases the ohms inside of the sender decreases. This shows you temperature rise on the dial.

The sender(s) are the variable in the test. In order to make any sense out of it all you need a way to imitate each sender as your control. I made a box (re-purposed) out of an old ohms resistor testing box I bought on ebay for around $20. My box gives me access to fixed points; there is a "click" rotary dial on top that represent Ohms resistance. ( I actually have several of these boxes, they are all different.) Kent Moore also made a box that came in a white plastic case. It was made specifically for car dealerships to test car gauges.

1968 Lemans or GTO:

Fuel gauge: zero ohms = E. 45 ohms = 1/2 scale. 90 or 91 ohms = F.
Oil pressure gauge: zero ohms = 0. 44 or 45 ohms = 1/2 scale. 90 or 91 ohms = Full scale.
Temperature gauge: 600 to 700 ohms = Cold. 76 ohms = 1/2 scale. 51 ohms = max overheat.

(NOTE: the above numbers are to the question here; a 1968 GTO.) Other GM cars, and different years; with rally gauges will have different senders and/or marking on the dials; so the #s above may or may not correlate.

After I am satisfied the gauge is right I retest using pressure (compressed air) on the sender for my oil pressure gauge. I used heated water for matching the sender to my coolant temperature gauge. This is the only way that you will know for sure if the gauge is reading true.

Do I need to use a decade box or can I get some resistors for doing a onetime job? Also On a 68 which post is what on a the gauges?

[killakev]

Awesome info. Looks like you could test each gauge with a operating gas sender using the 0-90 ohms it gives.

[killakev]

See the rally gauge pins

[alsalp]

Originally Posted by killakev

See the rally gauge pins

I see the pins but which pins get which jumper etc?

[Peter Serio]

I go by wire color. In all GM cars certain colors of vinyl insulation represent a specific circuit.

Here is the plug that I use to test gauges before I ship them out:

I have used this plug at least 35 or 40 times since I built it about 3 years ago.


This plug is arranged to work to test a set of 68 or 69 or 70 Pontiac A body rally gauges.

You will also need a decade resistance box or equivalent. As well as a 12 volt DC power source. I have ALWAYS use a car battery. I have a fully charged Optima red top underneath my workbench wired to the pegboard with take offs for 12 + and negative. MAKE 100% SURE THAT you ground the metal housing, there is NO ground wire in the plug-in!!! If you run any one of the 3 gauges with power and output to a sender string or resistor box and the steel housing is un-grounded you will fry that gauge, it only takes about 10 to 15 seconds!!!!!

[Peter Serio]

The pink wire is 12 volts , key on battery + power (thru a 15 amp fuse)

GROUND the steel housing, then key on 3 wires for your tests. The dark blue wire goes to the oil pressure sender. The dark green wire goes to the coolant temperature sender and the tan wire runs all the way back to the fuel tank sender. Pretty easy to test. Be for-warned, all 50+ years old GM cars gauge are likely to be in failure mode.

It unrealistic to expect gauges this old with history unknown to be put back into a car and have all 3 them magically spring back to life. They have weaknesses and after many years of use or just sitting in a box out in the garage 50 year old gauges are going to need rebuilt and re-calibrated.

[alsalp]

Thanks for the advice Peter

[Peter Serio]

Hi, I've posted it before on this forum but would you like me to re-post my #s pin-out/ to color as it pertains to my test plug? .......here in this thread.

[alsalp]

Originally Posted by Peter Serio

Hi, I've posted it before on this forum but would you like me to re-post my #s pin-out/ to color as it pertains to my test plug? .......here in this thread.

any information you willing to share is much appreciated. As are your words of caution.

[alsalp]

Originally Posted by Peter Serio

I go by wire color. In all GM cars certain colors of vinyl insulation represent a specific circuit.

Here is the plug that I use to test gauges before I ship them out:

I have used this plug at least 35 or 40 times since I built it about 3 years ago.


This plug is arranged to work to test a set of 68 or 69 or 70 Pontiac A body rally gauges.

You will also need a decade resistance box or equivalent. As well as a 12 volt DC power source. I have ALWAYS use a car battery. I have a fully charged Optima red top underneath my workbench wired to the pegboard with take offs for 12 + and negative. MAKE 100% SURE THAT you ground the metal housing, there is NO ground wire in the plug-in!!! If you run any one of the 3 gauges with power and output to a sender string or resistor box and the steel housing is un-grounded you will fry that gauge, it only takes about 10 to 15 seconds!!!!!

I'm borrowing a decade box. Am I to understand to understand that with regard to your test pigtail, I apply the resistance between the (-) tag black wire and one of the 3 colored (+) tag wires for a given gauge?

[Peter Serio]

I'm borrowing a decade box. Am I to understand to understand that with regard to your test pigtail, I apply the resistance between the (-) tag black wire and one of the 3 colored (+) tag wires for a given gauge?

NO, those wires wrapped with blue masking tape and a + & - sign are for bulb tests only NOT the 3 gauges. I just marked them that way as a reminder to myself. Those blue masking tape wrapped wires are not the gauge wires anyway.

The gauge sensor wires are: dark green, dark blue and tan!!! Those 3 wires exit out from the cluster (each one to a different sender) the sender is your variable and the end point of all 3 senders, one by one is ground. The oil pressure and coolant temperature senders use the engine block as their ground pathway. The fuel gauge sender uses a black wire attached underneath the (read most likely rusted) trunk floor. So bodyground. You want to also check your copper ground straps under the hood, there should be a total of three. Two, one per each side from the back of the cylinder head(s) to the firewall and one from the RF top of the frame to the RF wheel well housing; the black painted steel inner liner.

You would be amazed at how many electrical problems on older GM cars end up being traced back to bad ground(s).

To test you reference variable ohms resistance in-between the proper color wire (for each gauge that you want to test) and ground.

I should have typed this before but just FYI This is not something a beginner should start with, if you mess up the wire connections or run tests with excess resistance, that or with the metal housing or the resistance circuit un-grounded you could very easily damage one gauge or all 3 gauges. Testing gauges is not simple to explain with only words, I could show you but that is part of the reason I do this for a living. People do send gauges in to me for testing.

[Peter Serio]

The big black plastic plug-in on the back of the gauge cluster is arranged so that each cavity is numbered; we have one thru 6 across the top and 7 thru 12 across the bottom. NOTE: The plug-in is also designed, on purpose; so that it will only fit into the back of the gauge housing one way.





#1 lime green (high-beam headlamps on indicator).

#2 Dark Blue with a white stripe (Right turn signal green arrow).

#3 empty.

#4 Gray wire (dash lights, for night-time driving).

#5 Dark Brown ( Alternator charge "gen" light.) This wire runs out to the voltage regulator on the firewall.

#6 Dark Green (Coolant temperature gauge).

#7 Tan (fuel gauge sender wire)

#8 Gray wire (dash lights, for night-time driving).

#9 Light Blue (Left turn signal green arrow).

#10 Dark Blue (oil pressure gauge).

#11 Pink (2 pink wires joined as one at the brass tab); this is the main 12 volt + battery power input to gauge cluster from Ignition switch-thru a 10 or 15 amp fuse, (key on only).


#12 Tan wire with a black stripe (when grounded) this wire turns on the bright-red "BRAKE" warning lamp at the very bottom of your speedometer.


Silver metal steel gauge housing: always to battery ground.

[kk68]

Originally Posted by Peter Serio

I should have typed this before but just FYI This is not something a beginner should start with, if you mess up the wire connections or run tests with excess resistance, that or with the metal housing or the resistance circuit un-grounded you could very easily damage one gauge or all 3 gauges. Testing gauges is not simple to explain with only words, I could show you but that is part of the reason I do this for a living. People do send gauges in to me for testing.

I sent my 50 year old gauges and speedometer to Pete for restore and calibration A year or two ago and they are working flawlessly. Highly recommend him and his work. Reinstalling dash was one thing I’m glad I didn’t have to do twice!

[alsalp]

So my gauges function. I used a decade box and used compressed air for the oil and boiling water for the temp sender. I have a gauge harness on the bench and what seams to be a good gauge circuit board that I carefully cleaned the contacts. The fuel checks out nice and moves nice and slow.

The temp jumps to position whenever voltage is initiated (steady climb as the water heats up) The oil kinda "clicks in into place as I change decade resistance and "steps" up as I vary the air pressure. I assume this is the lack of buffering fluid. I made the tests without the harness or circuit board.

I noticed that when I tried to use the harness or even having just the circuit board attached "the other two" would get "excited" when test a given gauge. What does this mean? The circuit board checks out. Using TU5, 185 deg is between the first two marks. 200 deg is at 1/4 mark, and My propane burner maxed at 206 with reading about in the middle of 1/4 and 1/2 marks.

[Peter Serio]

Any gauge missing it's internal buffering fluid should not be put back into the car.

It sounds to me as if both your oil pressure and coolant temperature gauges are in failure mode and the pointers will soon skew on their support shaft(s) and then detach themselves within a very short time. This is very common problem today mainly because the car was not designed to last forever.

Funny thing though the electrical part of your gauges sounds as if it is still working, it's the mechanical part of those 2 gauges now that has failed. I have seen this exact same problem on older GM car gauges 100s of times before.

[alsalp]

what is the cost to put in buffering fluid?

[Peter Serio]

"I noticed that when I tried to use the harness or even having just the circuit board attached "the other two" would get "excited" when test a given gauge. What does this mean? The circuit board checks out. Using TU5, 185 deg is between the first two marks. 200 deg is at 1/4 mark, and My propane burner maxed at 206 with reading about in the middle of 1/4 and 1/2 marks."

I was not there right beside you when you were doing these tests but I think I know what you are saying. All 3 gauges in the #1 circle opening share a common power supply (key on) thru a 15 amp fuse. And all 3 share a common "reference" ground at the rear steel panel that they are bolted into. However each gauge seeks more information than just those 2 connections. Powering up gauges with a open sensor string can damage that gauge. Both the oil pressure and the fuel gauge in particular. There is no car condition ever that either of those two gauges would see infinity, it's as if their sensor wire was un-plugged. Making tests running 12 volts battery + thru the printed circuit as power MUST include completed sender strings for all 3 gauges. Otherwise you can damage a gauge. A string (1 each per gauge) would be your variable resistance box terminating to ground. That is why I have a total of 3 separate decade resistance boxes on top of my workbench.