1977 280z Coolant Temperature Gauge

[ckurtz2]

Hey Guys,

    Back again... On the last gauge to fix on the car. Anyways, the temperature gauge does not work on my 1977 california spec 280z (Oil pressure does). It did work when I originally got it, but one day I started the car and it just did not work (2 years ago). 

Ok so, I grounded out the bullet connector on the t-stat housing and the gauge did not change. So that ruled out the sending unit. I got right to it and pulled everything needed to get to the gauge and pulled the gauge. Simple and easy.

Ok so I first wanted to see if the whole gauge was getting power., and it was. I found that the bottom left pin in the photo was supplying 12V. Sweet. From there on I found that the right 3 pins were continous with the chassis ground. I found that the top left one had 84ohm resistance when testing against the chassis ground. 

Does that sound right? I can't seem to find any electrical diagrams on the 77 showing what each pin for the gauge at the harness does.

To make things odder I tested continuity at the bullet connector and it was not 0ohm from the bullet connector to any pin on the gauge harness connector. This makes me think a wire is bad, but thats all speculative. I can't see how the wire is bad, as I looked for loose connections and deoxed the only connector between it and the gauge that I could find. I also tested the sending unit bullet connector against the manifold, battery ground, and other chassis grounds and I got 1 so it was not continous. Don't know what that means. 

Is there a way to bench test the gauge. Also any thoughts on the info provided helps, I need the group brainstorm. In addition if anyone can tell me what each pin on the connector does that would be awesome. Thank you!

 

[ckurtz2]

http://www.atlanticz.ca/zclub/techtips/wiringdiagrams/F77ZCAR-WIRING1.pdf

Found a wiring diagram. Looks like the wire for the sending unit is yellow and on the middle of the connector one the side where the green(12V) wire is. Problem is my car has both yellow and white where the yellow one is on the diagram.  And yellow and black where the yellow and blue is. 

 

[ckurtz2]

At this point it seems I jumped the gun in posting. But this thread may be good use to someone in the future, so count it as a blog.

With the new knowledge I discovered above I found a way to test the gauge. I plugged the temp gauge back into the car and then turned the key to the on position. I then got my multimeter and disconnected the probe attached to it. I then jammed the probe into the gauge connector where the yellow and white stripped wire was (I figured this must be the sending unit wire). I then hooked an aligator clip up to the probe and then to the chassis ground. And Caboom the gauge shot all the way to 250+ degrees. I believe this confirms that the issue is in fact not the gauge but somewhere in the wire. I will find what went wrong tommorow. I really hate to cut into the harness, but:(

Edited September 21, 2022 by ckurtz2

[ckurtz2]

Ok, so I grounded the yellow wire at the C1 junction on the diagram. That connector ended up being in the passenger side junction box. Anyways when I grounded it the gauge shot to 250 again, so I now know the interior side of the harness is good. So now the problem must be between Connector C1 and the bullet connector. It is one continous wire in this section. I am still at a loss how the wire went bad.

Anyways, I am wondering how I should go about replacing this wire. Should I cut open the protective wrapping around the harness and then remove this one wire. This would be unfortunate, because this yellow wire travels down the passenger side of the car, then through the front of the car next to the headliht scoop, across the front of the car in front of the radiator, through the driver side, and then all the way back to the manifold, then over to the sending unit.

Should I cut the wire at the C1 connector and then add a bullet connector there, and then run a new wire to the sending unit? Should I somehow remove the wire from that C1 connector (idk how to remove the pin from the connector). Then remove it from the harness. Then get a new wire, repin it and put it back in the harness, rewrapping the whole thing. 

I am curious how people do this. I provided a photo where the yellow wire feeds into the juntion box at the C1 connector. Man have I torn apart this car good to get to the source of this issue.

[SteveJ]

It would be highly unusual for a wire to break in a wiring harness unless the harness was pinched hard or pierced. The usual suspects would be where the connectors are attached to the wire. That would be fatigue breaking the strands or corrosion. However, if you wanted to look for a break in the wire, you would need something like this: https://www.amazon.com/Electrical-Wire-Tracer-Continuity-Automotive/dp/B08L6NYQYK

You would detach the C1 connector and clip the red lead to the yellow wire. Then clip the black lead to ground. You would use the probe to "sniff" the wire through the harness, and if you lose signal, that would indicate a break.

The bottom line is that you should inspect the connectors carefully at both ends (C1 and at the sensor).

[ckurtz2]

@SteveJThank you so much for the pointer, I didn't know such a tool existed. That makes life 100% easier. Hopefully I'll only have to remove part of the harness wire and not the full length. Anyways, I am confident it is not the bullet connector, and that it is somewhere in between.

[SteveJ]

You may have misunderstood. It could be at the bullet connector. You might not be able to see it, especially if it's corrosion. Also, there could be corrosion at the connector between with engine harness and dash harness.

[ckurtz2]

7 minutes ago, SteveJ said:

You may have misunderstood. It could be at the bullet connector. You might not be able to see it, especially if it's corrosion. Also, there could be corrosion at the connector between with engine harness and dash harness.

No I understood. I actually snipped the harness about 6 inches before the bullet connector and then aligator clipped it directly to ground to try and eliminate this issue of possible corrosion. Didn't work. In addition, I deoxed and wiggled around the C1 connector (it already looked pretty shiny inside before). So I am feeling pretty comfortable the wire is severed somewhere in the harness. That tool though will be a great help.

[ckurtz2]

Well I am at a loss. Got a wire tracker from harbor freight. https://www.harborfreight.com/cable-tracker-94181.html

        Used it for the better part of an hour and a half, but got no where closer to finding my issue. I connected it to the end of the wire at the sending unit and then grounded it ( I tried grounding it to the chassis, battery, manifold, block, etc). Anyways, it worked in the sense that It gave a tone at the wire. More than two feet down the wire though the tone would dissapear no matter what. In addition, I'd get a tone when scanning the manifold, fuel injectors, fuseable links, car chassis, roof of the car. It was incredibly frustrating. The tone would dissapear before I could reach the C1 connector when I had the tracker hooked up to the wire at the gauge. The tone would also dissapear before even reaching the interior when having it hooked up to the oil pressure sender wire in the engine bay. The oil pressure gauge works, so I know the wire is fine.

Not sure what to do here.

[Mark Maras]

 Take a look at the one star reviews for that item at H.F. They describe the same problems with the unit.

[SteveJ]

I have the HF unit, and I actually used it at work...about 14 years ago.  That was for wiring in a bunch of metal boxes, and I didn't get a bunch of false readings. After a while, I got to where I didn't need it any more. 

Does it look like your harness has been altered/modified? I might have to dig my tracer out and play with it again.

[ckurtz2]

Interesting. No, my harness is 100% stock with no alterations. Let me know if you figure it out. So far I think I am just going to return it. I tested with a multimeter at both sides of the C1 connector and it is continuous, so I am feeling really safe now that somehow the wire went bad inside of the engine bay. I think I am just going to cut the wire at the connector and solder on a new one and run it all the way up to the sending unit. Probably not going to touch the wiring harness itself, I'll just ziptie or electrical tape it to the harness at different sections. Kinda sucks the harness will now look altered, but in reality the car will probably be completely rewired in the future when I have it undergo a full resto.