I have a question about gauge wiring on my '72 GTO. The previous owner had a '70 GTO that he drag/bracket raced, and eventually wrecked. He bought this '72 after the '70 was wrecked, took the motor out of the '70, and raced the '72 for a while, before he sold it to me. I bought it in roughly 2001, and it has been in (indoor) storage since I joined the Marine Corps in 2004. I recently got it out of storage, and am working on getting it back on the road.

When I bought the car it had an aftermarket oil pressure/water temperature gauge mounted to the console. My assumption was always that the previous owner preferred it for racing and "cool" factor, but I have also wondered if he was forced to put it in if the stock gauge were broken. The stock gauge is is still in the dash, and the gas gauge and GEN light on it still works. It also lights up as normal with the headlight switch. For water temp and oil press it is a gauge, NOT an idiot light. As part of getting the car road worthy, I recently had to change the heater core. During that job i broke the little oil line that goes to the aftermarket gauge, so now I have no choice but to get it out of the car. I bought a new oil pressure sender from Rock Auto, and installed I this evening. The wiring for the stock sender is still hanging there, with the connector. Once I had the new sender installed I put the connector on and started the car. I get no change in the stock gauge, which is pegged at 80 with the key off, the key in the run position, and with the engine running or not. Which leads me to my questions:

1. Is there anything I am missing wiring wise? I did put teflon tape on the threads, and as I am typing this it occurs to me that that could be interfering with grounding the sender. Any ideas?
2. If the wiring sounds correct, is there a smart way to test the gauge, and eliminate the possibility that the stock gauge just doesn't work?
3. For the water temp gauge, I bought the new sender, but I don't see the stock wiring and connector, and I suspect that they are either missing, or have been modified at some point. As a base reference, where SHOULD the wire for the water temp sender come from and route, and what color is it?

I'll appreciate any help anyone can offer.

Joe

ok
guys I am going to need some help here ........

I have 2 beers in me and a porterhouse steak .... so stay with me so I dont fall asleep

first it sounds like a factory gauge car with issues ... is there a fasten the seat belt light at the bottom of speedo ??
72 only

with the key off all my original rally gauge sets read about 1/3 on all the gauges w key off
then turn the key on fuel reads and the other 2 go to 120water and 0 oil

ecause even spare sets in boxes stashedon the shelf working sets read approx 1/3 also

the oil sender is specific for gauges and should be purchased from Lectric limited
https://www.lectriclimited.com/electrical-devices-components

they are ohm'd correct for our dashes....

no teflone tape ...your correct is best

any other positon on a factory gaugeset I would be suspect of...

water temp gauge wire is green

it comes out with the hot wire to the alternator and the voltageregulator wire blue n a white one ? the ones in the plug

may be cut back ...

dont freek out when you start peeling back the wiring to check thinks out ... and find a "Y" soldered wire from the alternator .. thats the splitter for power from the starter to run the inside of your car ..starting from the battery
to the solenoid big stud ..on that stud up comes one of the 3 wires soldered together
1 wire going inside the car for power the the fuse block
the other wire to your alternator... usually they are fine but wrap em nicely with friction tape
https://www.amazon.com/3M-57173-S-10-Temflex-Friction-Inches/dp/B00PGLRN4O/ref=asc_df_B00PGLRN4O/?tag=hyprod-20&linkCode=df0&hvadid=167144081759&hvpos=1o1&hvnetw=g&hvrand=12376547114404004325&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9033507&hvtargid=pla-306709851738&psc=1

it sticks to itself ...cloth based ,, glue doesnt slide with heat.... buy the 3/4 inch also
tears nice but use scissors ....

glad to help if you wanna call to discuss tomorrow or the weekend its all good... 2 zero 6 465 9165
till 11 pm pacific time

Scott

sounds like your dash may have to come out .... the plastic lines are available to repair your mechanical
oil lines also at napa .... or just get a pkg of 1/8th ? ferrulles and use a BRAND new blade to cut theline back an inch
to repair ... slide the nut on slide a ferrulle onyou want about 3/16ths hanging out of the ferrulle

NO kinks or tight bends

gettin sleepy ..........

Hey Joe... (there's a song in there somewhere)
Thanks for your service.

With the gauge pegging like that I'd just about bet that you have the wrong sender. Probably what you have is not a sender but a switch for an idiot light. Those things are either on or off, depending on whether the switch is sensing oil pressure. You can tell the difference by their appearance. A switch looks similar to this:


While a gauge sending unit looks like this:


Don't use teflon tape, your idea about that interfering with the ground path was correct. Also be very careful about tightening the sender to the oil filter housing. It's very easy to get carried away and crack the housing, creating a leak that will find its way to the hot exhaust and cause a fire.

The connection for the water temp sender should be in the same harness bundle that goes to the alternator. It's a single green wire with a spade connector on the end.

Bear

You can get a good laminated wiring diagram for around $20 from https://www.classiccarwiring.com/

You may want to go with aftermarket gauges as the factory ones were not that accurate even when new.

I really appreciate all the responses. As for the aftermarket gauges that are in the car, I have never liked them, and so I am just as happy to have them gone. I definitely have the correct sender unit, and I say that based off of both the pictures above, as well as the fact that I knew there was a difference and was careful to order the correct part. The suggestion about the wiring diagram was a great one, and I ordered one of those last night. I think tonight I am going to pull the sender back out, take off the teflon tape, and try this again with my fingers and toes crossed. I was looking at the Dakota Digital gauges last night, and they are beautiful, and...maybe...someday. Right now I am just trying to get the car back on the road as cheaply and easily as possible, I like the look of the stock gauges. I'll keep this thread posted, and thanks again for all the info.

Joe

Good luck!

Be aware of ground points......

Ok next stop. Do you have any way of knowing if the car was factory original with gauges, or if sometime in its past someone replaced the lights with gauges? The reason I ask is because the 'circuit board' that attaches to the rear of the gauge cluster is different between the two, and the engine wiring harness itself MIGHT be as well (I don't remember that part for certain).

Bear

Bear - I can't find my copy of the PHS documents that I got when I bought the car. I can't say with 100% certainty that the gauges in the car are original, but I do believe that they are. The car is pretty original, I don't see anything under the dash that leads me to believe that the car had had a lot of wiring messed with under there, nor do I think that the dash has ever been out. I am going to go out after dinner and look for the green water temperature sensor wire now that I have a better idea what I am looking for, and also put a little more effort into the oil pressure sender. For now Iam going to operate under the assumption that the gauges int eh car are original.

Joe

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OK gang...I have been looking around for the wire for the water temp sender. There is a large bundle of wires that come out of the fire wall immediately below the brake booster. The best that I can see, there is only one green wire in that bundle. I just undid a bunch of the PO's home made electrical tape wire looms, and sure enough the green wire is cut off and wrapped around itself. Does this sound like the correct wire? A picture is attached. , with the wire right in the center. Don't mind 15 years of grime. I'll get things clean later!

With the oil pressure sender, If I start the engine and take a multi-meter from the end of the new sender, and the other end to a known ground, I do get modulating resistance on my meter. Same with the water temp sender. Does the individual gauge and/or cluster have its own ground? As I said, the fuel gauge, gen light, and gauge light all work (all on the same large round gauge as the oil press and water temp gauges), so I would be amaze if there is a power issue to the oil press and water temp gauges, but I guess I should eliminate that possibility.

In answer to earlier questions, yes I believe there is a fasten seatbelt at the bottom of the speedo, but I dont think I've EVER seen it work. also, I just noticed that my tach is not working either. I know that it used to work for a fact.

1th Indian - Thanks a million for posting that diagram!

Joe

Progress!

After I sent that last post I decided to rough in a wire and have my wife watch the water temp gauge while I tinkered with the wire. Sure enough, the water temperature gauge started responding correctly, so while I still have to finish wiring it in, I think that problem is solved.

It has probably been at least 20 years since the oil pressure gauge moved. IS it possible that the physical needle is "stuck?"

Joe

On my 72, the wire for the stock temp sender originates on the lower left of the factory bulkhead connector.
I believe the grounds are connected on the printed circuit board, but each gauge has its own terminal. It is possible for the individual gauges to have a poor connection to the film on the printer circuit board. (film)

I took my dashboard out, much easier than I thought. The circuit board / film looked clean but was oxidized and needed to be cleaned. I ran some extra grounds anyway. My car doesn't have the Rally cluster. Added a picture of what it looks like from behind. Yours will be slightly different.

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I can't possibly thank you all enough for the help on this. I figured it out this evening. The oil pressure gauge connector was hanging in the stock location, but the other end was cut off, and wrapped up in a home-made 25 year old electrical tape loom. Once I discovered that, I was able to use the wiring diagram and pictures that 11th Indian posted to figure out which wire coming out of the fire wall was correct. I found that, also cut off, and spliced them. What do you know? It worked. I now have functional oil pressure and water temperature stock gauges. Thanks again to all who replied!

Joe