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140 temp gauge wonky

AndrewNance

AndrewNance

Amateur hour!
Joined
Oct 24, 2011
Location
Winston-Salem, NC
The water temp gauge in my ?71 is acting pretty strange. Immediately after starting it will start climbing and go all the way to the red. While driving it fluctuates pretty rapidly, or at least more rapidly than temperature actually fluctuates. The sender in the head is new. Do these gauges ever really go bad? Maybe a bad sender?
 
There's a voltage reducer/regulator that supplies lower voltage to the gauges. When cold, it briefly supplies full 12V, so the gauges 'jump', then a thermal switch opens and it gets reduced current via a resistor (I think the same wire wrap that warms the thermal switch).

That can and do go bad. Commonly failing to open, which keeps 12V flowing to the gauges, which peg out and burn out eventually.

https://www.sw-em.com/voltage_stabilizer.htm


This would affect fuel and temp gauges though, so on second thought, probably not that.

Look for issues with the wiring between the sensor and instrument cluster?
 
AndrewNance said:
Fuel gauge is reading normal. The temp gauge does just sit all the way to the right most of the time.
Click to expand...

A common problem with the stock temp and fuel gauges are loose connections.

The 2 gauges are attached to the PC board with brass nuts. These are also the paths of contact for the gauges and the nuts can become loose. If left loose long enough some corrosion can also form and a simple cleaning of the contacts on the PC board might also be necessary.
 
AndrewNance said:
I don?t believe I do. Need to check the back of the cluster and see what?s happening back there.
Click to expand...

If you are going to be experimenting / troubleshooting the gauge, be aware that if you apply 12-volts to it, you'll kill it.

I believe the voltage stabilizer's output is only 5+/- volts.
 
alschnertz said:
If you are going to be experimenting / troubleshooting the gauge, be aware that if you apply 12-volts to it, you'll kill it.

I believe the voltage stabilizer's output is only 5+/- volts.
Click to expand...

Good to know! I?m hoping to find a GT cluster for this car so I won?t go too deep into troubleshooting. I can toss in an aftermarket temp gauge if I need to.
 
This is where I’m at. Does this seem normal? That short blue wire with the female spade connector wasn’t connected to anything. The brass nuts were all snug and relatively clean as far as 900 year old electronics go.

It looks like someone replaced the voltage stabilizer before but cut it so they could slip it under the little mounting screw. Is it supposed to mount under that screw in the long post thing behind it?

WVPX5nRl.jpg
 
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How was the regulator mounted? Some clusters mounted the regulator in rubber so an external (blue) wire was necessary to ground the housing. Other clusters mounted the regulator to a metal post on the board that's grounded so a separate wire wasn't needed.

I see a witness mark on the regulator that looks like it was screwed to the post. If so, the blue ground wire is redundant. Just make sure the post has a good ground.

1069164.jpg
 
It wasn’t mounted at all, just dangling when I pulled the cluster out. So that’s likely my issue I guess. It’s entirely possible that I bumped it loose while trying to change bulbs without pulling the cluster.
 
I swapped back and forth between two clusters and two different voltage regulators and both are giving me completely dead fuel and temp gauges. I’m thinking both of my regulators are just dead as I’ve tried them each on two different pairs of gauges.
 
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If the regulators put out the same voltage as the newer 240 cluster. That is a ten volt regulator. You can use a 7810 three leg voltage regulator in place of the original mechanical type. Input 12v, ground, and 10v output are the three legs. https://www.amazon.com/Quickbuying-...ds=LM7810&qid=1569258797&s=electronics&sr=1-6

Actually whatever voltage output they are. You can use an electronic type. The positive voltage regulators output is designated by the last two digits. So a 7810=10v, a 7805-5v, etc. TO220 type case.
 
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