Alrighty guys, yet another fuel sending unit/gauge problem at hand! I have searched through both CZCC, ZDriver, ZCAR, and HybridZ and haven't been able to find a solid answer for my problems.

Here is basically the story/symptoms:

A few of you may have seen my thread on my recent purchase of a Fairlady 240Z that sat for 20 years in a covered shed-area in the moutains of Santa Cruz. When I first got the car the fuel gauge was reading about 3/4 at a full tank, however the tank was quite rusty so I had it professionally repaired and sealed by the Radiator Doctor (San Jose, CA, they did a great job for $285). I reinstalled a new fuel tank sending unit, o-ring, and lock plate thereafter. I reinstalled the fuel tank and had all the wires set properly.

Now, the fuel gauge reads only 1/4 and I know that it is at the very minimum the tank has gas up to 1/2 - 3/4 full. The needle also kinda flutters up and down between empty and 1/4.

I've used this small thread as reference:

http://www.classiczcars.com/forums/showthread.php?t=29351

Curtis240Z on a different thread: An emtpy tank sender would read 90 ohms, a full tank 10 ohms. Nissanman is correct in diagnosing the wiring system, just ground the wire and it should read full scale.

What I've done:

- Connected the + and - of my full battery volt meter to both contact points on the fuel sending unit and got a reading of 25-30 OHMs. Which sounds correct to me for a 3/4 full tank. 10 Ohms being full and 90 Ohms being empty.

- Connected the yellow and black wires from the rear electrical harness (the wires that connect to the sending unit, and have black cups). This was to test the fuel gauge, and it read FULL once I did this.

- Tested the current of the yellow wire, which jumps from 0 to 10 to 30... basically all over the place. This could be why the fuel gauge is fluctuating up and down between E and 1/4 full. Makes sense to me anyways... :paranoid:

On the thread mentioned above there was the following post:

Nissan man: Refer to the thumbnail attached:

The gauge terminal of the tank transmitter can be temporarily connected to earth and that should force the gauge to read maximum scale, if everything else is working OK.

As mentioned, there are a fuseable link and a fuse to be checked in the Fuel Gauge circuit.

There is also a Voltage Regulator, shown at the LHS of the gauge, which controls the voltage applied to the gauge.

These have been known to misbehave and cause erratic readings usually on the Fuel and Temperature gauges.

Cleaning the contacts on the VR and all points in the diagram may remedy the problem.

I have attached the thumbnail that Nissanman is referencing. However, I have not been able to find this fuseable link, fuse, nor voltage regulator in the fuel gauge circuit. Have I followed the wire completely around the car? No, so maybe that is the next thing I need to do. When I searched "instrument voltage regulator for 240Z" on Google, I come up blank.

What I, a newbie to electronics, have concluded:

* The fuel sending unit is working correctly (brand new, tested at 30 OHMs, 3/4 full tank).

* The fuel gauge is fine, when grounded (black touches yellow) it reads FULL.

* There is something funky going on between the contacts of the wires somewhere routed from the rear to the front of the car. The fix may be as easy as just cleaning up some connections (rear yellow/black contacts were cleaned) The fuel gauge is never getting a solid reading because of the borderline contacts from the rear to the front.

Questions:

- Plain and simple... what am I doing wrong?

- Where is the location of this fuseable link, fuse, or voltage regulator? Is it under the dash somewhere?

- Is there any other tests I can do?

Any advice would be GREATLY appreciated!

Edited November 18, 201014 yr by spitz17

If that were my car I would temporarily replace the harness ground on the ground connection side of the sending unit maybe with a wire with alligator clips on both ends. Or put the temp ground on the non ground harness wire that goes to the sending unit and drive around and see if the gauge readout is stable. I would be interpreting your experience as an intermittent connection though I could be wrong.

I know this sounds too easy but do you have the sender unit installed upside down?

The only fusible link in a 71 Z is the one between the Starter and the Positive Terminal on the battery. If it blows or is questionable the COMPLETE electrical system is inoperative or intermittent.

While Nissan/Datsun DID use a Voltage Regulator for some of it's instruments in the Roadster (Sports) the Z didn't. I am not positively sure on JDM vehicles, but I would be surprised if the JDM circuitry included one while the US did not.

The fluctuating power may be due to a bad fuse or more likely a bad connection since you mention that when you short Y to B it reads Full.

I'd check your grounds before you pull the sender and verify you didn't slide the float adjustment out of whack. It's not upside down since you're not complaining that it's reading FULLer as you drive the car, but that it never deviates from the no more than 1/4 reading.

FWIW

E

If the electrical issues turn out not to be the problem, it might be that the arm is caught up in the wires that make up the sender unit grid. You would have to remove the unit to determine this however.

Just to clear up the "did you mount it upside down" question... I mounted the sending unit so that the tab with the black rubber or bushing (???) is on the top and the exposed tab directly below it.

Is this correct? I will check the grounds this weekend. Guess I'll have to get my hands dirty and track the system all the way to the front.

I was wondering if this issue ever got resolved. I have been chasing the same thing on my wife's 72 Z. From my work it seems the feed to the instruments is unstable. The supply voltage at both sides of the fuse is good but the voltage at any of the three sending units, gas-water temp-oil pressures bounces from 1 to 6 volts. The feed to the meters is a yellow- red wire but some where it connects to a green feed line, does anyone know where the two wires connect or is it internal to the harness?

Thanks

3 hours ago, Mark S M said:

the voltage at any of the three sending units, gas-water temp-oil pressures bounces from 1 to 6 volts.

That's normal.

The gauges internally modulate the voltages to the sender units. They do that to compensate for ambient temperature and system voltage.

one thing to make sure is the sending unit is installed in the tank so the float moves in a vertical plane. You need to make sure the sending unit is clocked correctly.

Thanks for the responses but they don't explain what I am seeing. When I turn on the ignition the gas gauge will move up to half full then drop down to about an eighth and then start moving up and down between those two points.

Is it just the gas gauge, or do you see the same thing in the temp and oil pressure?

Also, when you say it moves up and down,... is it regular and continuous, or is it sorta random and happens every now and then?

It is only the gas gauge that I am aware of. The needle moves up and down about 10-20 times per minute. I put a temporary ground from a chassis screw that I cleaned well and hooked it directly to the yellow lead at the sending unit. The gauge will run up to full but will then drop down to 3/4 and proceed to oscillate around that point about an 1/8 of a tank in either direction. I have also by passed the sender lead with an external wire from the sending unit to the fuel gauge connector and no change.

Does any one know what the current through the gauge should be for full and empty readings?

All of the gauges will move around a little bit by design (that's the compensation feature I mentioned above), but they should not move around much. If everything is working properly, you can see the needles wiggling around a little if you're really looking for it, but if you're not really watching for it you shouldn't notice it. It certainly shouldn't move around by an eighth of full scale.

And when you ground the yellow wire back at the sending unit, the gauge is supposed to shoot up to "F" and stay there. Solid.

Have you put your meter on the Yellow/Red feed wire from the fuse box right at the gauge? Is the voltage solid steady there. And to answer your question above, I do not know where or how they make that transition from the green wire at the fuse box to the Y/R by the time it gets to the gauges. I suspect it's done inside the harness.

Skipping to the end of the story... It sounds like there is an internal problem with your fuel gauge. You could pick up a spare and give it a try?

To answer your question about how much current goes through the gauge for the different readings, here's something I whipped up for a later year. No guarantee that the numbers are the same for your 72, but I suspect they are. This is sender unit Ohms vs needle position: