Hi guys,  I have a 78 280z.  About a month ago the temp/oil gauges were drift back and forth together in unison from left to right.  Now they do not work at all.  What would have caused this?  Before I start ripping out gauges I want a better understanding of what I am looking for.  

 

The meters have a common power source and a common ground.

Note: I removed removed the image from the post since I tagged the wrong wire for the positive side of the circuit. I should have tagged the blue wire going into the 6 pin connector. The black is the ground for the gauges.

Inspect the pins on the male and female connectors, looking for loose connections and corrosion. Also look for damage on the wiring harness on the back of the gauge and on the dash harness. After you remove the gauge, you may also want to start the car and measure voltage between the green/white and black wires on the connector for the gauge.

Edited September 2, 20222 yr by SteveJ

Those come from the c6 block right?

1 minute ago, gotham22 said:

Those come from the c6 block right?

First, I misread the wiring diagram. It's the blue wire, not the green/white that provides power.

I'm not sure what you're looking at to say C6. Page BE-32 shows the blue wire going through C5 and over to the 6 pin connector for the gauge. The ground wire goes through C1.

I hope that helps to clear things up for you. Make sure you look at BE-32 and ask questions on it if you need to.

I don't know how the internal "voltage regulator" works but Nissan said if both gauges go bad at the same time that's probably the source of the problem.

1 hour ago, Zed Head said:

I don't know how the internal "voltage regulator" works

https://www.classiczcars.com/forums/topic/64427-no-oil-pressure-after-rebuild/#comment-607221

 

I just pulled the gauge theory stuff together in a standalone thread:
https://www.classiczcars.com/forums/topic/67803-how-the-gauges-work/

Have a nagging suspicion that I did that already some time before, but couldn't find anything?   

Since you're on a roll can you explain how the regulator failing affects both gauges at the same time.  Maybe even explain what is happening and if a regulator failure matches the symptoms described.

Nissan didn't mention a regulator for the temperature gauge but I assume that's what the three triangles indicate.  A shared component.

9 hours ago, Zed Head said:

I don't know how the internal "voltage regulator" works but Nissan said if both gauges go bad at the same time that's probably the source of the problem.

Thank you. I didn't remember they had the same VR.

11 hours ago, Zed Head said:

Since you're on a roll can you explain how the regulator failing affects both gauges at the same time.  Maybe even explain what is happening and if a regulator failure matches the symptoms described.

Nissan didn't mention a regulator for the temperature gauge but I assume that's what the three triangles indicate.  A shared component.

Datsun put one regulator device in each of the gauge cases. So the TEMP/OIL gauge has one regulator that is in series with both of the needle moving heating strips, while the FUEL gauge has it's own regulator. Note that the AMP and VOLT gauges do not use a regulator because they do not use a heated bi-metallic strip to move the needle. The AMP and VOLT gauges use a  D’Arsonval permanent magnet method instead of the heated strip, and hence do not need a regulator.

So the TEMP/OIL gauge "can" contains one regulator shared (in series with) by both of the meters, and if that regulator develops a problem, that problem will show up in both gauges.

As for what may be happening... If the shared regulator has dirty contacts and isn't making good contact, both gauges drop to zero at the same time.

Note that the problem may be in the incoming power wiring and not the regulator at all, but based on the cyclical nature of the issue, I suspect the regulator.

When it's stone cold, key just turned to ON, the regulator strip will be it's coldest and the pressure on it's contacts will be at it's greatest. Once that strip warms up and the system moves closer to equilibrium (PWM equilibrium), the contact pressure will decrease. I'm thinking when it's stone cold, the higher contact pressure makes a contact, but once it warms up and the contact pressure decreases, it goes intermittent.

Of course, troubleshooting electrical stuff over the interwebs is never an exact science. But that's my read from a distance.

 

@Captain Obvious, I remember we discussed this some after I posted the video of the fuel gauge bench test. Indirectly you can see the voltage regulator in that gauge start by watching the ammeter on the power supply I was using. When it was cold, the current was high, and then as it reached equilibrium you could see the current fluctuating as the VR pulsed.

 

Yup. It would be cool to run that test again using an analog ammeter in series with the gauge. That way, you would be able to see a better representation of the PWM regulator current.