Fried gauges?

[Kannon Herbert]

Hi guys, I’ve been working lately on a 74’ dart custom, trying to get everything into working order. Lately I’ve been having troubles with the fuel and temperature gauges. I figured this was likely due to a faulty voltage limiter on the back of the cluster, but even after cleaning and replacing the voltage limiter, the temp and gas gauge still do not operate. I’m curious if the gauges are fried. I’ve uploaded a photo of what I see when at operating temp. Do these look fried and bellow the line? I know it’s hard to tell just from looking at the gauges, but from what I’ve heard usually the indicators on the gauges will be pegged far left. Not sure if this is what that looks like, or if they are just not getting anything from the senders, although I find it awfully coincidental that both senders would be disrupted considering these things are known to have their limiter fail and fry the gauges. If this isn’t the case, I’m curious if there’s any way I can test the temperature sender to troubleshoot fixing this issue. I know these gauges are expensive to replace. Thanks!

 

[Mattax]

Can't tell by looking at the face.
This will give you a background on how they work and some troubleshooting approaches.
Thermal-Electric Gauges (Session 227) from the Master Technician's Service Conference

For a '74 the internal resistance might be a little different than the earlier onesbut they work the same way.
Redfish has posted about that - so you can search his posts if that info is needed.

There are also some posts where people have pictures showing the heating wire and bimetal that moves the needle. Those will show you what overheating can look like.

 

[Kannon Herbert]

T

Can't tell by looking at the face.
This will give you a background on how they work and some troubleshooting approaches.
Thermal-Electric Gauges (Session 227) from the Master Technician's Service Conference

For a '74 the internal resistance might be a little different than the earlier onesbut they work the same way.
Redfish has posted about that - so you can search his posts if that info is needed.

There are also some posts where people have pictures showing the heating wire and bimetal that moves the needle. Those will show you what overheating can look like.

thank you. I’ll check that out.

 

[Dana67Dart]

If the guage is in the car you should be able to get a volt meter to the rear posts of the guages. One of the posts to ground should read about 5v. If not the IVR or the power to the IVR may be bad.

With some alligator clips and a 9 v battery you should be able to get needle movement.

NOTE CORRECT POLARITY IF STILL HOOKED UP TO CAR.
DO NOT HOOK 9V BATTERY SOLID TO LEADS JUST TOUCH MOMENTARILY OR YOU WILL DAMAGE GUAGE.

Forgot to mention rear of cluster shown is 67 thru 70 something.

 

[cudamark]

The first test is quick and easy. Disconnect the sender wires and connect jumper wires from the connectors to a good ground. Sit in the drivers seat and turn the ignition switch to the on position. If the gauges start to move, immediately turn the key back off. You now know the gauges work....they may not be accurate, but, at least they work. If the gauges don't move, it can be fried gauges, bad connections, wiring problem, etc. If one works and not the other, the IVR works, but, you still may have a combination of the other problems mentioned above. Getting another used cluster with good gauges shouldn't be all that hard to find. The same gauges (with some minor variations) were used from '72-6 on all A-bodies.

 

[Mattax]

That's a good quick test. Like you said, just do it briefly since there's no resistance.

As far as measuring 5 Volts, not sure that can be done with a typical DVM. An analog voltmeter will show what's going on by the swinging of the needle.

 

[Kannon Herbert]

If the guage is in the car you should be able to get a bolt meter to the rear posts of the guages. One of the posts to ground should read about 5v. If not the IVR or the power to the IVR may be bad.

With some alligator clips and a 9 v battery you should be able to get needle movement.

NOTE CORRECT POLARITY IF STILL HOOKED UP TO CAR.
DO NOT HOOK 9V BATTERY SOLID TO LEADS JUST TOUCH MOMENTARILY OR YOU WILL DAMAGE GUAGE.

Forgot to mention rear of cluster shown is 67 thru 70 something.

View attachment 1715561443

This worked. Looks like the temp gauge is likely fried, but thankfully the gas gauge is still operational. Appreciate the help.

 

[Dana67Dart]

As far as measuring 5 Volts, not sure that can be done with a typical DVM. An analog voltmeter will show what's going on by the swinging of the needle

Op mentioned he replaced the IVR I assumed electronic. Even with a mechanical you can see voltage on a DVM, at least on my 30+ year old Harbor Freight one. Displays erratic voltage readings but you know something is there.

 

[Mattax]

Bench testing the gages and IVR.
The IVR output voltage is shown on the meter to the right with the swinging needle. It's a Sun V.A.T. model CB set to the 8 Volt scale
Fuel gage is connected to an 11 Ohms resistor ( Full) .
The other gages have a single 22 ohm resistor to ground, so mid gage.
Car battery supplying the power

[/QUOTE]

 

[Mattax]

This worked. Looks like the temp gauge is likely fried, but thankfully the gas gauge is still operational. Appreciate the help.

Could be an open elsewhere in the circuit? Even a lose or oxidized PAL nut on the board?

 

[RedFish]

Fuel gauge needle appears to be at its home position. 73 ohms should lift the needle to the 1st/empty hash mark. I dont know if that temp gauge needle has moved at all. That needle doesn't drop as far below the scale/hash marks as a fuel gauge.
With a volt meter at the sender wire... if everything is working and the gauge is passing current you'll see a pulse voltage from a mechanical limiter or steady voltage from a solid state regulator. Three odd looking clips attach the limiter in the printed circuit board. You can loose connection right there. Prove power to and through the temp gauge, then ground the sender wire briefly to test gauge needles full swing. I wont go under the car to the fuel sender until I've proven everything else. If you must pull the panel again, the temp gauge should show approx' 20 ohms post to post. Your fuel gauge approx' 13.5 ohms post to post.

 

[Kannon Herbert]

Fuel gauge needle appears to be at its home position. 73 ohms should lift the needle to the 1st/empty hash mark. I dont know if that temp gauge needle has moved at all. That needle doesn't drop as far below the scale/hash marks as a fuel gauge.
With a volt meter at the sender wire... if everything is working and the gauge is passing current you'll see a pulse voltage from a mechanical limiter or steady voltage from a solid state regulator. Three odd looking clips attach the limiter in the printed circuit board. You can loose connection right there. Prove power to and through the temp gauge, then ground the sender wire briefly to test gauge needles full swing. I wont go under the car to the fuel sender until I've proven everything else. If you must pull the panel again, the temp gauge should show approx' 20 ohms post to post. Your fuel gauge approx' 13.5 ohms post to post.

I tested the gauges again, and it appears as thought the current is being lost through the limiter. I just bought a new one, so this is strange. I will try cleaning the connections.

 

[Kannon Herbert]

I tested the gauges again, and it appears as thought the current is being lost through the limiter. I just bought a new one, so this is strange. I will try cleaning the connections.

Update, the connections have been cleaned on the IVL and it appears to be working correctly. I suppose the fault may be in the senders or at the pin connections, firewall or at grounds at some point.

 

[cudamark]

If you ground the sender wires like I mentioned above, you will KNOW whether it's the sender or not.

 

[mopardude62]

I had this problem on my 1967 Barracuda, the fuel gauge worked but the temp did not. After a LOT of testing and head scratching I discovered the PO had installed the temp sender for the idiot light. I got an old sender from a friend off a parts car that had a gauge and it worked perfectly!