Fuel and Temperature Readings

[Randalla]

Recently I lost the fuel and temperature readings on my 1972 240Z simultaneously and am wondering if there is a connection. I know on Roadsters there is a step-down voltage regulator that controls both functions, but I'm not sure if there is any relationship between the two on an early Z. Checked the connections at the fuel tank and the temperature sensor and they seem secure. Is there a single fuse somewhere that controls both? Thoughts?

 

[cgsheen1]

The electrical schematic shows that they share a common ground, but I'd expect that to affect your oil pressure gauge too. No step down regulator on a Z car for gauges.

There is a fuse in the circuit but there too it would affect the oil pressure gauge and it looks like your hazard switch as well.

Check the body harness & engine bay harness-to-dash harness connectors under the glove box. You could test continuity of each of the sensor wires (Y fuel, Y/W coolant). Otherwise you're in the dash to figure that out...

 

[Randalla]

Thanks for the reply Chuck. Took the Z for a ride last night and the temp gauge began working briefly and reading properly, before dropping back to zero. Another piece of information I just recalled is last Fall while driving up to Williams the temp gauge went to zero then read properly again two or three different times on the trip. The fuel gauge which has always read about 3/4 after a fill up, all of a sudden, after filling up, read completely full. That only happened once but I thought it was curious. Do you think a bad ground could cause both of those conditions? 

[SteveJ]

It could be a bad connector at the sender for the water temp. I've had that happen before. The wire got fatigued, and it was almost broken off. 

[Randalla]

Steve, do you mean the connector for the water temperature sensor that screws into the thermostat housing?

 

[SteveJ]

The sender for the water temp on my cars have a bullet connector. Anyway, the crimp on that female bullet connector can be subject to wear from the wires moving from the engine vibrations.

[cgsheen1]

On 6/2/2023 at 7:24 AM, Randalla said:

Thanks for the reply Chuck. Took the Z for a ride last night and the temp gauge began working briefly and reading properly, before dropping back to zero. Another piece of information I just recalled is last Fall while driving up to Williams the temp gauge went to zero then read properly again two or three different times on the trip. The fuel gauge which has always read about 3/4 after a fill up, all of a sudden, after filling up, read completely full. That only happened once but I thought it was curious. Do you think a bad ground could cause both of those conditions? 

What Steve said, and - In both cases the connectors at the sensors are very often the problem. Neither are weather tight and although you don't drive in the rain - and Arizona - oxidation does happen. Sometimes wiggling the two connectors on the fuel level sender helps, sometimes they need to be taken off and cleaned. The posts on the fuel level sender too. You can never be absolutely sure but the connectors under the dash are far better protected than at the sensors so I always start there (sensors) and work toward the gauges checking the wiring.

Since your trouble is intermittent, I'd suspect the sensor connectors and/or associated wiring. It's more unlikely but certainly possible: Frayed insulator on wire, to-dash-harness connector, gauge connector, or inside the gauge itself.

 

Edited June 4, 2023 by cgsheen1

[AK260]

To echo the above comments, I lost oil pressure completely during engine running in ride. I pulled over fast and I think a little poo came out too!

The connector looked perfectly well connected - but I took it off and there was:

1) corrosion inside to clean out

2) play - It needed a little reshaping so that it held on more tightly.

Never a problem with it since.

But if both gauges are going nuts exactly at the same time, then you may have another electrical problem!


Sent from my iPhone using Tapatalk Pro

[chaseincats]

Agree that it PROBABLY is the bullet connector on the harness that connects to the temp sending unit - that was my issue a while back as well

[Racer X]

To echo other commenters, grounds are often overlooked in cases like this, and also, the round slip on connectors at the sending units do get loose over time. 
 

Seeing as the gauges share a common ground, it would be the first place to look. Clean to bare shiny metal, and use dielectric grease when reinstalling to prevent further degradation over time.