another temp gauge/fuel gauge/voltage regulator thread

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SlantedMark4

SlantedMark4

'74 Valiant 225 /6
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I have some questions to my '74 Valiant but before I tell what I've done already:
The fuel gauge worked fine but my temp gauge read high (over 250) and went over the visible area of the gauge. When the engine was warmed up, the gauge went to the "cold position". I then changed the temp gauge sensor but nothing happened. Later the fuel gauge started to be "full" at 3/4 and stay on that position until the fuel tank was empty. I then replaced the fuel gauge with a spare one I got from another dashboard aswell as the voltage regulator but nothing happened again. I did drive this way until I rebuilt the dash panel this week.
I've ordered another used temp gauge and installed it. The temp gauge now moves again but it goes straight to "hot as hell" (as I call it) position. I've put in the old sensor again and the same problem with that. With the temp gauge moving, the engine rattles more than before. The needle goes slowly back to "cold" in idle but goes fast back to "hot" when I push the gas pedal a little to raise the rpm.

What's installed now is an used gauge that - the seller said - worked, with the old voltage regulator and the old temp sensor.

So, my questions are:
How can I ground the gauge to see if it's alright?
How can I check if the sensor wiring is bad?
Might the thermostat be faulty and probably being stuck? (It was replaced 1.5 years ago)

I'm not familiar with wiring or electric/electronic system in general...
Like someone said on an old post of mine once: "...electrics are like Voodoo to me..."

Ah yes..:
225 Slant Six, Plymouth Valiant, built on May 17th 1974 (so '74 model year I think), no mods, 3 spd auto
 
Easy way to ground the temperature sensor? Unplug the sensor with the key on, stick a bit of metal like a screw or nail in the sensor connector, touch to engine ground. Should peg the gauge.

To check resistance, unplug the sensor and use a multimeter to probe between the terminal on the sensor and engine ground.
 
I've an update which is a bit concerning:

I did take out my Valiant for a drive to see how the temp gauge acts with a warmed up engine. The first interesting thing is the needle went straight to hot but went a bit back and stayed on the left line in the "hot" section after 10-15 minutes. After 20min, when I put the gear shift (column shift) into R, the temp gauge needle immediatly went down to halfway (or however you call this).

I decided to go home after that but the engine started to misfire and cut out 3 times, of course exactely then when I went downhill. This for sure is a sign of overheating but it had never done this before when the gauge didn't work.

Now the weirdest thing was when I was home I pulled the sensor connector but the temp gauge still moved...? How even is that possible? Might there be a grounding problem?

I haven't tested the sensor yet because the engine/water was hot, obviously.
 
SlantedMark4 said:
when I was home I pulled the sensor connector but the temp gauge still moved
Click to expand...

Do you mean with the car off? It sounds like the wiring has a short. You need a volt/ohm meter to properly diagnose electrical issues, and to be able to get comprehensive, concise help.
 
Demonic said:
Do you mean with the car off? It sounds like the wiring has a short. You need a volt/ohm meter to properly diagnose electrical issues, and to be able to get comprehensive, concise help.
Click to expand...
No the engine was running. I thought it should get no signal with the wire disconnected.
As I said before, I'm very easily confused with electrics.
 
SlantedMark4 said:
No the engine was running. I thought it should get no signal with the wire disconnected.
As I said before, I'm very easily confused with electrics.
Click to expand...
It *should* not get a signal with the wire disconnected. Sounds like the signal wire is touching a ground somewhere along its path. That would explain the erratic things you're seeing on the gauge.
 
Yep, wire disconnected should read fully cold, shorted to ground fully hot. Sounds like it's found another path to ground.
 
MadScientistMat said:
Yep, wire disconnected should read fully cold, shorted to ground fully hot. Sounds like it's found another path to ground.
Click to expand...
What I'm confused about is why did the engine actually get hot? It didn't get hot when the gauge didn't work, now it does. How? Is there a correlation with the wiring or is it just pure coincidence with another fault?
 
The gauge only has the ability to show the engine temperature. It cannot control it, any more than a thermometer can control your body temperature.
 
Unless your wiring harness is in such poor shape that the engine heat finally ruined the wiring, I doubt there's a connection. Even if there is one, you will have to attack the two problems separately.
 
I would get a aftermarket temp gauge so you know where you are really at. You can buy a mechanical one that installs in the upper radiator hose pretty easily.

Depending on how bad you want the oem one, this can be a good solution.

Some folks don’t like the radiator hose senders because they won’t read if there is no water flow. However, I like them because I can see exactly when the thermostat opens. As long as you keep in mind the limitations and keep your radiator full, they are a cheap easy solution to a bad oem gauge.
 
Just a simple temporary work around to take the temp gauge out of the equation.

Remove the plug from the sending unit on the intake manifold.

Clamp your test light ground clamp to + positive battery, them touch the pointed end of the test light to the temp sending unit.

The test light should glow orange on a cold engine, as the engine warms up it gets brighter and brighter.

Can do the same temporary setup with a (side marker light) 194 bulb and socket wired in the same. Gets brighter as engine gets hotter.

Then you can concentrate on the other 2 electrical issues.

I bet it is back feeding through your gauges as it is trying to find another path to ground.

Same thing happens on turn signals that light dim and won't flash, bad ground wires at the sockets back feeding the system making all kinds of strange things happen.

Can use a secondary jump wire for a good ground at the affected gauges to see that it is a ground problem, then go back and clean up or replace ground terminals.

Hint: If any of the connections are hot to the touch after being on for a while, that tells you there is a bad connection there.

Screenshot_20240527-112755_Gallery.jpg



☆☆☆☆☆
 
There's a lot of bad advice in this blog, jumping to conclusions with no troubleshooting at all. If I had that car I would pull the instrument panel out of the car and get things working on the bench. It sounds like your instrument voltage regulator is the place to start. Since it's a Valiant the IVR is external to the gauges and easy to get to. Usually I find the "fingers" holding the IVR are not holding it in well and not making good electrical contact so it has to be fixed out of the car. You'll need to feed 14 volts to the input of the IVR then check the output which is a rather crude pulse code modulated output to emulate 6 volts for the fuel and temperature gauges. If you connect a 10 ohm resistor from ground to the opposite side of each gauge you should get full deflection. If not the circuit is open or you're not getting output from the IVR. If things are working at the bench then you either have wiring issues to the sensors or the sensors are not working properly. In any case, it's a simple series circuit and should not be difficult to fix.
 
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