curious problem with my gauges; question about wiring

[thedarkie]

Disclaimer: I have a Ford small block in the car, but all stock gauges are retained and they all work. I didn't do the swap or wiring myself, the previous owner did; hence some confusion.

Car is a 74 260Z

Normally the gauges work fine, but once in a while my tach will go wonky and read 0rpm while driving... at this point the temperature and oil pressure gauges start creeping up, and the fuel gauge starts creeping downward. The temp gauge approaches red and the oil pressure gauge goes up to about 3/4 (normally its around 1/3 at idle). The fuel gauge slowly goes down to 1/4 despite the tank being almost full.

When the tach springs back to life, the other gauges go back to their expected read-outs.

My question is, how are the stock gauges wired? Are they daisy chained in some way, where if one dies or starts behaving erratically the others will follow? Is it possible that when the tach dies, the extra voltage/current/whatever drives the other gauges to false readings? Im hoping someone can at least point me in the right direction to starting a troubleshooting process here. The difficult part is that this only happens randomly and I can't replicate it myself.

Thanks

Edited May 7, 2014 by thedarkie

[Mikes Z car]

My wild arse theory is that the ground for those gauges is loose. That would explain the intermittent aspect and I think why some gauges go up and others go down when the ground opens up and stops grounding. Any dissenting opinions or ideas of where the ground is? Finding the ground point for the gauges would head in a good direction if this theory is correct.

Edited May 7, 2014 by Mikes Z car

[thedarkie]

Gonna post these up for reference and discussion:

Circuit diagram

I don't know squat about electrical but I notice two things: in the first three diagrams, they all begin at the starter which has a ground. This is common to all of the gauges. Secondly, as shown in the last diagram, they also all share a ground at the C-4 junction on a brown wire... this also looks common to them.

Again, Im clueless about electrical. Beyond checking the connections physically, how do I ensure the ground is actually good all the way back to the gauges?

[Mikes Z car]

The pictures are helpful.

Since it is an intermittent issue the approach I would use would be to improve the existing grounds meaning the C4 connector ground and the battery to body ground strap. I am not sure what car model you have but I have seen on a 240Z a ground wire that connects to a ring that looks like a washer that goes under the mounting screw on the cigarette lighter which I guess could be the C4 ground showing in your diagram if your car is wired like a 240Z. That ring could be taken off and scraped clean (The area under the ring could be scraped too) and the screw mounting it tightened. I like to solder any connections I have any doubt about such as the crimp connection on that ring. My car has two ground wires that connect to the battery negative (ground) terminal. The very large one goes to the starter ground and then there is a much smaller one that is screwed to the car body next to the battery on the firewall. On my car this wire that is screwed to the firewall continues on and ends in a ring that goes under a screw on the engine block but I don't know if the wire that goes to the engine block is a stock arrangement. This smaller wire could have the firewall screw removed and the body and screw scraped clean, same thing for any connection to the engine. I am out of ideas, hope some part of this helps.

Someone else might want to chime in.

Edited May 7, 2014 by Mikes Z car

[thedarkie]

It does appear the erratic problem was loose/bad grounds. I cleaned up a few in the engine bay, tightened down the battery with a new hold-down and recrimped some of the terminals.

New question: the fuel gauge is horribly inaccurate and seems to have gotten worse. Previously when filling the tank up it would read half full... slowly going towards empty as the tank empties. Now it goes barely half full, and drops like a rock after very little driving. How do I test the gauge itself? I have it out but Im not sure which wires are the signal, and how to measure that signal? I understand that the sender itself is resistance based, 90ohms empty and 10ohms full (or was that backwards) but how do I test that at the back of the gauge instead of at the tank? Is the signal a single wire or two? Does the gauge read the voltage from that wire?

Any help is appreciated, once again. Thanks

Edited May 26, 2014 by thedarkie

[Mikes Z car]

Great question!

The picture below is the back of a 1972 Z car gas gauge. Ignoring the two large ammeter white wires (not visible in picture) whose gauge I would love to measure one connection is labeled IGN, another UNIT and the third one (the ground) has no label. I just tried connecting a 12 V power supply as indicated on the picture while a 90 ohm resistor was connected between ground and UNIT and the needle moved up to empty. A 10 ohm resistor made the gauge move up to full.

For testing the ground all the way back to the gas gauge an easy way would be to connect a 10 ohm resistor across the two wires (I hope there are only two) at the tank connection to verify the gauge goes to full.

I have noticed from reading posts that most problems with an erratic gas gauge reading are caused by the sending unit being bad. I fixed mine but I think a better approach is what most people do, replace the thing.

I am not sure I answered your questions, if not ask more. I love killing off electrical gremlins.

Interestingly my power supply made a slight noise when the gauge would thermally cycle and I could hear a slight click from inside the gauge when it cycled. I understand it has a thermal strip that moves back and forth but that is all I know about how it works. The needle slightly moved when I heard the click too.

Mike

Edited May 26, 2014 by Mikes Z car

[thedarkie]

Thanks. I was testing the resistance between ground and "UNIT" so I guess I did THAT right! It was showing around 35 ohms but was sporadic. This lead me back to the tank.. I removed the 2 wires from the sender, put them in contact with eachother, and then checked the gauge. Sure enough it went to full. I guess the gauge itself is fine!

With that said, the connectors on those wires going to the sender are JUNK! I tightened the black one up with some pliers and stuck it on there as good as I could. The gauge seems to have gone back to its previous operation. I know the tank is almost full but it went back to showing just shy of 1/2. This is good enough for now as at least I'll be able to tell when Im in the empty ballpark, but I'll have to pull the sender to test further! Just gonna keep driving until I empty the tank

Thanks MikeZ

[Mikes Z car]

thedarkie,

You are welcome. I am not sure if you are thinking of fixing or replacing the sending unit and you didn't ask about this but be aware that on my sending unit I had two poor connections. One was a poor contact between the swing arm and the coil of wire it drags across which was fairly obvious. The other poor connection was between the swing arm and ground. I added a wire as per the blog below to make sure the swing arm was connected reliably to the"contact rivet" which is where the ground wire to the gauge connects.

Mike

Gas Tank sending unit modification - Blogs - Classic Zcar Club

In an earlier post I did I said I didn't know what model car you have but just noticed you stated that in your first post. That will teach me to pay attention. heh heh.

Edited May 27, 2014 by Mikes Z car