Help... Temp./Fuel Gauge

[snufy]

71 Valiant Plain Jane Cluster

Got the car a few weeks ago, runs and drives great....

Fuel gauge works from 3/4 full to 1/4 empty consistently. All other lights/gauges works except for the temp. gauge, I've seen it move once from dead to close to "cold" position.

There is no rust on this car, everything is as close to perfect as you would ever find in a survivor more or less.

I poked the sending unit wire with a multimeter, got nothing... tried grounding it, no movement, nothing... tried testing at the firewall bulkhead, the pin beside the purple/violet (temp sensor) wire has a full 12 volts, but nothing from the temp pin.

Everything I've read indicates that they usually fail together due to the regulator but rarely does the fuel keep somewhat working and the temp doesn't?

 

[67Dart273]

the pin beside the purple/violet (temp sensor) wire has a full 12 volts, but nothing from the temp pin.

Sorry don't know what you mean, here

There are a number of failure points.

1....Any of the associated PC board pins, can come loose, break, etc.

2...Temp sender can lose connection through the bulkhead connector.

3...Either sender wire can break connection of course right at the sender itself, the molded connectors.

4...The "socket" contacts for the IVR may not be making actual contact with the board traces. On mine, I had to solder jumpers across

5...The IVR of course can be bad

6...on mine the nuts holding the gauges were loose or corroded. Replace with "real" nuts and work loose / tight a few times to "scrub" the terminals.

You can test with resistors, to get an idea of "how well" it works. (I'll have to look them up) Of course the gauge units themselves can be damaged or inoperative. They WILL burn up if operated long enough with a defective IVR, and pinned "over to the right.

Cluster MUST be grounded. Install a separate pigtail, and bolt to the column support, etc

 

[harrisonm]

First, if the oil pressure gauge works, the Instrument Voltage Regulator (IVR) is OK. The temp sending switch works by varying resistance as the engine warms up. When the engine is cold, current is not allowed to flow through the switch to a ground (the engine). As the engine warms up, there is less and less resistance in the switch and more and more current is allowed to flow through the switch into the ground. Current flows from the IVR, through the Temp gauge and then to the temp sending switch. As current flows through the bi-metal coil in the gauge, the needle moves. With the ignition switch "on", you should have some current in the wire connected to the temp sending switch. I would check that first. When that wire is briefly grounded, a good gauge will move IF there is current flowing through the wire. If there is no current in the wire, you most likely have a bad gauge or a break in continuity most likely either at the bulkhead or at the plug on the back of the cluster. If you can crawl under the dash and test for current flowing into the gauge, that would help the diagnosis.

 

[RedFish]

My best guess is the temp gauge loosing contact where it mounts to the circuit board.

 

[snufy]

It's almost like you guys have done this before....

So I got lucky, after some fiddling around. The gauge works!

However, it isn't accurate, now it will go up to somewhere around 140-160 when hot. It's barely between the markings on the gauge.

When you shut it off, and start it up again, it will bounce to the middle where it more should be, and then take about 10-15 seconds to slowly drift back down to inaccurate reading.

 

[sireland67]

You could try a new sending unit, they are pretty cheap, but take your time removing them from the intake, they might be froze, be gentle so you don't break it off.

 

[snufy]

You could try a new sending unit, they are pretty cheap, but take your time removing them from the intake, they might be froze, be gentle so you don't break it off.

I'm going to try this and see.

Thanks for the help everyone.