Temp & fuel gauges - what am I overlooking?

[Uncle Bob]

In my 71 duster, neither the fuel gauge or the temp gauge work. All tests have consistently passed. I grounded the temp sender wire, the gauge goes to full. That means the gauge has continuity to the sending unit and the gauge is working, right? I wired a test light in the temp sender wire, and it flashed like it should. That means I've got continuity from the IVR to the gauge to the sender, right? I measured ohms at the sending unit while the car was warmed up, and it measured as expected.

I reasoned that maybe the IVR is pulsing like it should, but maybe it's not pulsing the right voltage or at the right rate. So I bought a digital unit from RT engineering. Put it in, same as before.

All this is consistent with the fuel side - all tests passed, and I have measured ohms at the sender outside the tank so I could move the float arm. Still, neither gauge works.

Also, I've got a ground wire from the back of the circuit board at a ground point to the dash frame.

I've got to be overlooking something, right? I appreciate any ideas on what could be going on here. I'm tired of filling the tank every time I go out. Even if it's just 5 gallons, I'd rather spread my exposure to that shock out a little bit instead of getting slapped with it every time I drive the car.

Thanks,

 

[Super-Cuda]

In my 71 duster, neither the fuel gauge or the temp gauge work. All tests have consistently passed. I grounded the temp sender wire, the gauge goes to full. That means the gauge has continuity to the sending unit and the gauge is working, right? I wired a test light in the temp sender wire, and it flashed like it should. That means I've got continuity from the IVR to the gauge to the sender, right? I measured ohms at the sending unit while the car was warmed up, and it measured as expected.

I reasoned that maybe the IVR is pulsing like it should, but maybe it's not pulsing the right voltage or at the right rate. So I bought a digital unit from RT engineering. Put it in, same as before.

All this is consistent with the fuel side - all tests passed, and I have measured ohms at the sender outside the tank so I could move the float arm. Still, neither gauge works.

Also, I've got a ground wire from the back of the circuit board at a ground point to the dash frame.

I've got to be overlooking something, right? I appreciate any ideas on what could be going on here. I'm tired of filling the tank every time I go out. Even if it's just 5 gallons, I'd rather spread my exposure to that shock out a little bit instead of getting slapped with it every time I drive the car.

Thanks,

Are you using a rallye dash?

 

[67Dart273]

Check the sender resistance. engine cold. Should be at least or more than the yellow print. Then warm it up some, temp resistance should start to come down. Don't discount that the wire end could be "iffy" corroded, loose, etc.

If you overtighten the oil/ temp senders you can damage them

Same deal on fuel. If you have any idea how much fuel is in it you can estimate what the resistance might be. Check that the SENDER (not just the tank is GROUNDED

the ultimate test for the cluster, end to end, is to rig some resistors for the proper test resistances, both gauges use same resistances, and see if they are accurate.

Some problems......

Loose pins at the PC board harness connector
Poor connection where the IVR plugs in. Bad/ innacurate IVR

Corroded studs / PC board where the gauges connect. Loosen the nuts and tighten/ loosen several times. Replace the cheap "tin" nuts with real ones.

No time like the present to invest in a good solid state IVR like RTE

RTE limiter - rte

www.rt-eng.com

 

[Uncle Bob]

Are you using a rallye dash?

Sorry, no it is not a rallye dash.

 

[Uncle Bob]

Some problems......

Loose pins at the PC board harness connector
Poor connection where the IVR plugs in. Bad/ innacurate IVR

Corroded studs / PC board where the gauges connect. Loosen the nuts and tighten/ loosen several times. Replace the cheap "tin" nuts with real ones.

No time like the present to invest in a good solid state IVR like RTE

I've checked the studs and PC board before, will replace those nuts and check the board again. FWIW the way I checked for cracks, loose pins, etc was to trace the continuity. My old eyes will miss more tiny cracks than they find, but the DVM tells the truth.

I installed an RTE IVR in right before I posted yesterday with no improvement.

 

[RedFish]

Very 1st thing that comes to mind is chassis ground. People tend to leave a little ground strap between firewall and engine disconnected at engine service. Then chit doesn't happen.

 

[Uncle Bob]

Very 1st thing that comes to mind is chassis ground. People tend to leave a little ground strap between firewall and engine disconnected at engine service. Then chit doesn't happen.

The engine is grounded at the firewall, and the battery is grounded to the engine of course and also has a ground to the rad support.

The cluster is grounded to the dash frame. The gauge lights, radio, etc are grounded to the same place. They all work.

 

[Super-Cuda]

The engine is grounded at the firewall, and the battery is grounded to the engine of course and also has a ground to the rad support.

The cluster is grounded to the dash frame. The gauge lights, radio, etc are grounded to the same place. They all work.

OK ,Try this. You will need a helper.

1. Disconnect wire from gas tank sending unit. ( Its blue in a 68 Cuda)

2. Have someone turn key to acc position.

3. Put an alligator clip or test lead into the end of the blue wire from the tank and momentarily ground it out to the chassis. Have your assistant monitor if the gauge pointer is moving to full position. Do not leave grounded for very long or you will burn out the fuel gauge.
If the gauge moves, the issue is in the tank sending unit.

4. No movement ? go to the rear wiring harness connector that should be in the driver side footwell. It has the rear taillights, id light , and dome light wiring in it.

Disconnect it.

Find the blue wire in the part of the connector the goes to the rear of the car.

Keep the tank end of the blue wire grounded to the chassis.

Now Using the meter, Put one lead in the end of the blue wire at the footwell connector and ground the other lead to the car chassis. The meter should beep indicating continuity and verifying that the wire from the footwell connector to the gas tank is good. If not, the wire is cut or the connector pins are bad or not making contact.

Let me know how this goes before we go through the next process.

Cheers!

 

[Uncle Bob]

OK ,Try this. You will need a helper.

1. Disconnect wire from gas tank sending unit. ( Its blue in a 68 Cuda)

2. Have someone turn key to acc position.

3. Put an alligator clip or test lead into the end of the blue wire from the tank and momentarily ground it out to the chassis. Have your assistant monitor if the gauge pointer is moving to full position. Do not leave grounded for very long or you will burn out the fuel gauge.
If the gauge moves, the issue is in the tank sending unit.

4. No movement ? go to the rear wiring harness connector that should be in the driver side footwell. It has the rear taillights, id light , and dome light wiring in it.

Disconnect it.

Find the blue wire in the part of the connector the goes to the rear of the car.

Keep the tank end of the blue wire grounded to the chassis.

Now Using the meter, Put one lead in the end of the blue wire at the footwell connector and ground the other lead to the car chassis. The meter should beep indicating continuity and verifying that the wire from the footwell connector to the gas tank is good. If not, the wire is cut or the connector pins are bad or not making contact.

Let me know how this goes before we go through the next process.

Cheers!

I have done that. The gauges go toward full when the wire(s) are grounded.

I have tried every test known to me and they all passed. Grounding the wire, looking for voltage, measuring resistance through the senders. I replaced the IVR. Everything says the gauges should work but they don't.

 

[Super-Cuda]

I have done that. The gauges go toward full when the wire(s) are grounded.

I have tried every test known to me and they all passed. Grounding the wire, looking for voltage, measuring resistance through the senders. I replaced the IVR. Everything says the gauges should work but they don't.

Ok.

Is this what your circuit board looks like?

 

[Uncle Bob]

Ok.

Is this what your circuit board looks like?


View attachment 1716438352

Yes, basically. The car is a 71 but the cluster is a 73 IIRC. IIRC the only functional difference is a FASTEN SEAT BELTS light.

 

[Super-Cuda]

Yes, basically. The car is a 71 but the cluster is a 73 IIRC. IIRC the only functional difference is a FASTEN SEAT BELTS light.

Alright.

Ok, can you take the cluster completely out of the car?

1. Set it up securely on a bench with a 12 volt source. Use your car battery if necessary.

2. Look at the pic of the circuit board I sent you.

3. Looking clockwise from 12 o'clock , pin at 1 o'clock is the wire from the tank. Should be blue.

4. Pin at 2 o'clock should be the temp sending unit wire.

5. Connect a lead from the positive terminal of the battery to the pin at 3 o'clock. You will notice that this is the pin that supplies 12 volts to the IVR. It is then dropped to 5 volts.

6. Attach a ground wire from the gauge cluster to the negative terminal of the battery.

8. You should now have the cluster powered up.

9. Now, looking at the 6 lugs that secure the gauges to the board, counting from left to right, you should have 5 volts at the 2nd and third lug. These are a common point to the Fuel and temp gauges that you have not working.

Lets see what happens here before going any farther.

Cheers!

 

[Uncle Bob]

Alright.

Ok, can you take the cluster completely out of the car?

1. Set it up securely on a bench with a 12 volt source. Use your car battery if necessary.

2. Look at the pic of the circuit board I sent you.

3. Looking clockwise from 12 o'clock , pin at 1 o'clock is the wire from the tank. Should be blue.

4. Pin at 2 o'clock should be the temp sending unit wire.

5. Connect a lead from the positive terminal of the battery to the pin at 3 o'clock. You will notice that this is the pin that supplies 12 volts to the IVR. It is then dropped to 5 volts.

6. Attach a ground wire from the gauge cluster to the negative terminal of the battery.

8. You should now have the cluster powered up.

9. Now, looking at the 6 lugs that secure the gauges to the board, counting from left to right, you should have 5 volts at the 2nd and third lug. These are a common point to the Fuel and temp gauges that you have not working.

Lets see what happens here before going any farther.

Cheers!

The cluster is tipped out right now; I will finish disconnecting it and remove the cluster this evening, . I will check it as you described.

 

[Super-Cuda]

The cluster is tipped out right now; I will finish disconnecting it and remove the cluster this evening, . I will check it as you described.

No problem Uncle Bob!

What I'm trying to do is isolate the gauge cluster from the vehicle completely. This eliminates the car and will help isolate your problem to the cluster only.
You have already eliminated the wiring, the gauges and the grounds. Not much left is there LOL !!

BTW, I noticed that you didn't say if you had connected a ground from the metal dash to the vehicle itself. Should not be a problem as the dash is bolted to the car body but you never know!

 

[Uncle Bob]

You have already eliminated the wiring, the gauges and the grounds. Not much left is there LOL !!

That is the crazy part, isn't it?

I'll go through the wire tracing with the cluster out per your writeup, but I've checked continuity on the connections end to end on the circuit board before and it all seemed OK. I will be surprised if I find anything wrong there - but you never know!

Replacing the nuts on the gauges might help but other than that I'm really out of ideas. Seems unlikely it is both gauges failed at the same time, and both seem to work when the sender wire is grounded. And, the IVR is pulsing the voltage like it should per checking the sender end of the wire.

BTW, I noticed that you didn't say if you had connected a ground from the metal dash to the vehicle itself. Should not be a problem as the dash is bolted to the car body but you never know!

I don't know if I did or not, but there are other things grounded to the dash that work so I think it's good to go there.

 

[67Dart273]

I agree, at this point I would pull the cluster. You need to devise a way to make/ get test resistances, so you can power the cluster and provide test resistances to the gauges. It is unlikely that both gauges failed "the same" unless the failure of a former limiter has damaged both of them.

I recently found some rheostats on egag that would work as well for that

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[67Dart273]

Also, are you aware of the common failure of the IVR contact fingers in the board? The springy fingers that you plug the IVR into sometimes lose contact with the traces. You must solder jumpers across between the fingers and the traces. I hope you also know about the same thing between the harness connector pins and the board. Clean them, use rosin electronics flux, and solder with electronic/ rosin solder, NOT acid "plumbing" solder. I do NOT recommend any "lead free" solder of any type

 

[RedFish]

We need to take a closer look at current path, points of failure too. Male contact pins to copper trace is very iffy. Test probe pushes, all seems fine. Harness connection pulls, contact is lost. We solder OEM boards. Aftermarket replacement boards that I have seen are soldered here.

 

[Super-Cuda]

That is the crazy part, isn't it?

I'll go through the wire tracing with the cluster out per your writeup, but I've checked continuity on the connections end to end on the circuit board before and it all seemed OK. I will be surprised if I find anything wrong there - but you never know!

Replacing the nuts on the gauges might help but other than that I'm really out of ideas. Seems unlikely it is both gauges failed at the same time, and both seem to work when the sender wire is grounded. And, the IVR is pulsing the voltage like it should per checking the sender end of the wire.


I don't know if I did or not, but there are other things grounded to the dash that work so I think it's good to go there.

Ok.

I think once you power up the cluster on the bench we will be able to isolate the problem very quickly.

 

[Uncle Bob]

Also, are you aware of the common failure of the IVR contact fingers in the board? The springy fingers that you plug the IVR into sometimes lose contact with the traces. You must solder jumpers across between the fingers and the traces. I hope you also know about the same thing between the harness connector pins and the board. Clean them, use rosin electronics flux, and solder with electronic/ rosin solder, NOT acid "plumbing" solder. I do NOT recommend any "lead free" solder of any type

I will check that but the pulsating voltage at the far end of the wire kind of confirms the IVR connection is good.

 

[ESP47]

When you pulled the sending unit from the tank and checked the ohms, did you connect it up to the sending unit wire, ground the unit to the car and check the gauge to see if there was movement when you moved the float arm?

 

[67Dart273]

I will check that but the pulsating voltage at the far end of the wire kind of confirms the IVR connection is good.

When you check that, you are not drawing any current. The circuit is open. Intermittents are ALWAYS the worst to find

 

[Uncle Bob]

When you pulled the sending unit from the tank and checked the ohms, did you connect it up to the sending unit wire, ground the unit to the car and check the gauge to see if there was movement when you moved the float arm?

Yeah, I did that.

Not that it matters but the fuel sender is new.

I am thankful that it seems to be a common cause failure for the fuel and temp gauges, it makes troubleshooting easier.

 

[harrisonm]

@Super-Cuda and @67Dart273, what would we do without you? Your commentaries in this thread are awesome.

 

[Uncle Bob]

Ok.

I think once you power up the cluster on the bench we will be able to isolate the problem very quickly.

It's on the bench and I powered it up. The light started flashing on the IVR, there was fluctuating voltage at the 2nd and 3rd studs, and at the 1st and 2nd pins past 12 o'clock. So there is continuity from power to IVR, from ground to IVR, from IVR to both gauges, and from both gauges to the connector pins.

The IVR came with a couple of resistors and sort-of-instructions on using them to test the gauges.

FWIW I saw something today saying to check the resistance across the gauge and it should be 15 Ohms. I don't know if that's right or not but figured it can't hurt so I put the DVM in Ohms mode and touched the probes to the studs. Both the fuel and temp gauges read open.

I've got a few things to do here but I will check back often to see if there are more ideas.