Temp & fuel gauges - what am I overlooking?

[Super-Cuda]

I would say at this point the problem does not exist outside the car, because the gauges deflect if I ground the gauge pins directly (goes toward full scale), or ground them through a 1 Ohm resistor (stops at full scale), or connect the gauges to a couple of AA batteries (stops at approx half scale)

?

It's got me wondering what I overlooked when I decided that both the senders were good - even though there is slim likelihood of both a new temp sender and a new fuel sender to be failed.

Well.. when troubleshooting electrical problems, leave nothing out.

The common denominator to both gauges not working simultaneously is.

1. Grounding.

2. No or improper voltage supply.

3. Both sending units defective. (Highly improbable but not impossible)

 

[Super-Cuda]

Well.. when troubleshooting electrical problems, leave nothing out.

The common denominator to both gauges not working simultaneously is.

1. Grounding.

2. No or improper voltage supply.

3. Both sending units defective. (Highly improbable but not impossible)

Do you have another temp sending unit? This would be the easiest to try.

 

[Uncle Bob]

Do you have another temp sending unit? This would be the easiest to try.

Yes I've got an old used one. I thought about connecting it to the cluster while it's on the bench, and heating it up with a heat gun to see if it moves the gauge. Then I remembered I have the resistors coming today and decided not to bother with it.

 

[Super-Cuda]

Yes I've got an old used one. I thought about connecting it to the cluster while it's on the bench, and heating it up with a heat gun to see if it moves the gauge. Then I remembered I have the resistors coming today and decided not to bother with it.

Not a bad idea!

Be sure to ground the sensor housing if you do this.

 

[ESP47]

It's annoying when you're doing everything logically and still getting illogical results. We've all been there.

These guys have you on the right track but I'll just let you know what my order of operations would be if your gauge doesn't work as soon as your test box is connected.

I'd disconnect the wire to the temp sender and place the test box in the same general spot so the wire doesn't get disturbed too much.

Ground the test box to the intake, check your test box to intake ohms and then your intake to battery ohms.

I'd set the test box to 23 ohms (middle) and then check the resistance at the sending unit wire as it goes into the bulkhead.

Check the resistance under the dash as it comes out of the bulkhead.

Check the resistance into the gauge connector. Should still be in the ballpark of 23 ohms.

You could still have an intermittent open in the wire somewhere but at least you have a baseline that your resistance feed is good as the wire currently lays.

Then check the gauge cluster to dash frame ohms and the dash frame to battery ohms.

Last check would be ensuring that the IVR is showing the same voltage behaviors as it sits in the car that you got with it out of the car. Use the same ground points here as you used when checking ohms in the previous step.

I can't see how it wouldn't work if everything checks out fine. Only other possible issue would be an intermittent open in the wiring under the dash that you are disturbing when doing your checks. At that point I'd check the resistance at the gauge again and start wiggling the harness around to see if you get a fluctuation on your gauge. Might be smart to do the same shake test as you test the IVR voltage as well. Since the IVR is pulsing, you'll probably have to go slower and hold the harness one way for a few seconds and another way for a few seconds to see if you can find a spot where the voltage drops out.

 

[Dartswinger70]

Is there any issues with the senders being incorrect? I ask only because I found out the hard way they aren't always the same etc. in my case it was an idiot light sender along with a gauge and yes it ruined the gauge. But if the senders vary year to year it may be an issue where gauges arent working properly.

 

[Uncle Bob]

I didn't take time to solder or assemble anything - just clipped one of the pots onto a gauge pin and to ground, and twisted the knob. Both gauges went back and forth and stopped in the middle as I turned the knob. I didn't have the DVM in the circuit so I can't say what the range is, but they definitely respond to changes in resistance. I'll see about soldering the pots together and putting them in a project box tomorrow. With fewer alligator clips I will be able to get the DVM in the circuit and try to measure it more precisely.

 

[Uncle Bob]

I didn't take time to solder or assemble anything - just clipped one of the pots onto a gauge pin and to ground, and twisted the knob. Both gauges went back and forth and stopped in the middle as I turned the knob. I didn't have the DVM in the circuit so I can't say what the range is, but they definitely respond to changes in resistance. I'll see about soldering the pots together and putting them in a project box tomorrow. With fewer alligator clips I will be able to get the DVM in the circuit and try to measure it more precisely.

I put it all together again on the bench - actually on a table next to the car.

I connected the pin output to one side of a 100 ohm pot and grounded the other side

I also connected my DVM into the resistor circuit.
When the cluster was powered the DVM read 1. (open), so, I would sneak up on the adjustment and watch the gauge. Once the needle got where I wanted it and stopped moving, I powered the cluster off (removed ground), and read the ohms from the DVM.

Readings from the fuel gauge

E = 40
1/4 = 30
1/2 = 18
3/4 = 13
F = 11

I ran out of time but I will see about getting that to repeat later, maybe with a different pot.

I will also note the numbers from post #3 (74, 23, 10), adjust the pot to that resistance, then see where the gauge goes.

 

[RedFish]

I suppose you did verify your fuel gauge to be the 13 ohm version. I think I stated before that this gauge is not as accurate as it 20 ohm version. I don't have any mfgr detail for these. Having said all that, you numbers are close. The dealers tester,,, 73 , 23 , 10 . You half is off for a 20 amp gauge. I can't say for your 13 ohm gauge. I can tell you that 34 ohms on 20 ohm gauges is 1/4 tank and 180 degree water. 13 ohms is good for 3/4 tank on a 20 ohm gauge.You might verify this with your temp gauge if it's good. You have to be wondering why 73 to 34 moves the needle a quarter scale yet 23 to 11 moves the needle a quarter scale also. The winding wire inside the gauge has a per inch resistance at 68 degrees. As this wire heats up its per inch resistance changes. Now you know.

 

[Super-Cuda]

I put it all together again on the bench - actually on a table next to the car.

I connected the pin output to one side of a 100 ohm pot and grounded the other side

I also connected my DVM into the resistor circuit.
When the cluster was powered the DVM read 1. (open), so, I would sneak up on the adjustment and watch the gauge. Once the needle got where I wanted it and stopped moving, I powered the cluster off (removed ground), and read the ohms from the DVM.

Readings from the fuel gauge

E = 40
1/4 = 30
1/2 = 18
3/4 = 13
F = 11

I ran out of time but I will see about getting that to repeat later, maybe with a different pot.

I will also note the numbers from post #3 (74, 23, 10), adjust the pot to that resistance, then see where the gauge goes.

Ok.

Next step.

What is the voltage reading at the output of the IVR. Measure between the gauge terminals 2nd and third from the left and ground.

Leave the resistors hooked up to the sensor wires.

 

[Uncle Bob]

Ok.

Next step.

What is the voltage reading at the output of the IVR. Measure between the gauge terminals 2nd and third from the left and ground.

Leave the resistors hooked up to the sensor wires.

Voltage is the same as it always had been - variable.

I keep testing and all I'm doing is convincing myself there's nothing wrong in the cluster.

I get an open reading checking the old temp sender. I probed the center stud and the body. Heating it didn't seem to have any effect.

 

[RedFish]

Redfish, another thing I noticed after having numerous clusters apart and refurbishing gauges ( Rallye Gauges only) is that the oil pressure gauge and the temp gauge internals and the windings look identical.

Are they the same gauge with a different face?

Yes and no. Those 2 gauges are upside tight or upside down in the rally panel. If this panel had a oil gaige it would be identical to the temp gauge less the screen.
This case, what have we not considered besides the 12 volt supply? Earlier models had more than 1 "hot in run" supply to the inst' panel. 1 to limiter, another to warning lamps, etc... 1972 or 73, whenever the larger white fuse box appeared, everything that needed 12 volts got it from the same wire and its likely fused. Is any of this relative to the issue? I don't know.

I was asked a question. I answered knowing I have holes in my memory. So now I feel i should correct my earlier statements even though a rally panel is not the topic here. "Are oil and temp gauges the same?" The simple answer is no but... they can interchange and they did. Through some time late 1967 every gauge was completely different. I have crude sketches of all 3 original configurations. Then somebody figured out that if the beam in the temp gauge was shifted far enough and the needle stops were tweaked far enough, it can report oil pressure. Have a close look at any 68 up rally oil gauge and this is what you'll find. Now we can forget again. LOL

 

[Super-Cuda]

I was asked a question. I answered knowing I have holes in my memory. So now I feel i should correct my earlier statements even though a rally panel is not the topic here. "Are oil and temp gauges the same?" The simple answer is no but... they can interchange and they did. Through some time late 1967 every gauge was completely different. I have crude sketches of all 3 original configurations. Then somebody figured out that if the beam in the temp gauge was shifted far enough and the needle stops were tweaked far enough, it can report oil pressure. Have a close look at any 68 up rally oil gauge and this is what you'll find. Now we can forget again. LOL

Thanks very much for the clarification RedFish.

After having lots of these apart I had a feeling there was similarities in the oil and temp gauges but didn't know for sure.

 

[Super-Cuda]

Voltage is the same as it always had been - variable.

I keep testing and all I'm doing is convincing myself there's nothing wrong in the cluster.

I get an open reading checking the old temp sender. I probed the center stud and the body. Heating it didn't seem to have any effect.

Ok.

What are the voltage readings you are getting?

You should be getting 5 volts or somewhere close.

No 5 volts, gauges wont work.

Also, something 67Dart273 mentioned.

Bad terminals in the IVR socket.

Take the IVR out of the socket completely and using jumper wires with alligator clips, temporarily wire the IVR back into the circuit .

Just to be sure your meter is working ok, measure the battery voltage. Should be 12-12.5 volts

 

[Vinny]

But the body wiring must be good because grounding the sender wire causes the gauge to go toward full. Ditto grounding and 12v to the cluster. If there was a problem in the wiring, ground, or 12v feed the gauge wouldn't work at all.

It's all confusing. That's why I tried everything I knew to try including replacing a good IVR to try to get them working before posting for help.


I'm going to give this a try.

Does the temperature sending unit have teflon tape on the threads, thus not making a connection to ground? Take a look at it. Is the clip on ground strap at the tank in place between the sending unit and fuel line?

 

[Vinny]

Voltage is the same as it always had been - variable.

I keep testing and all I'm doing is convincing myself there's nothing wrong in the cluster.

I get an open reading checking the old temp sender. I probed the center stud and the body. Heating it didn't seem to have any effect.

Don't use teflon tape to seal the threads on the temp sending unit. Use pipe thread sealant.

 

[Uncle Bob]

Does the temperature sending unit have teflon tape on the threads, thus not making a connection to ground? Take a look at it. Is the clip on ground strap at the tank in place between the sending unit and fuel line?

It's got that factory red stuff on it.

I used the old temp sending unit and a heat gun to confirm the temp and fuel gagues and the cluster itself isn't the problem. I could keep looking forever, or I could accept it's not in the cluster and put it back in. So I'm in the process of reinstalling it.

 

[Vinny]

It's got that factory red stuff on it.

I used the old temp sending unit and a heat gun to confirm the temp and fuel gagues and the cluster itself isn't the problem. I could keep looking forever, or I could accept it's not in the cluster and put it back in. So I'm in the process of reinstalling it.

Just for the heck of it, after the cluster is back in, use a clip jumper from the brass body of the temp sending unit to the battery NEG, or to an absolutely known good body ground. Let's see what happens. I was in Elevator technician for 43 years. Hopefully your gauge will go high and low, not up and down. LOL Keep us posted.

 

[Super-Cuda]

It's got that factory red stuff on it.

I used the old temp sending unit and a heat gun to confirm the temp and fuel gagues and the cluster itself isn't the problem. I could keep looking forever, or I could accept it's not in the cluster and put it back in. So I'm in the process of reinstalling it.

Bob , are the gauges now working properly?

 

[Super-Cuda]

Just for the heck of it, after the cluster is back in, use a clip jumper from the brass body of the temp sending unit to the battery NEG, or to an absolutely known good body ground. Let's see what happens. I was in Elevator technician for 43 years. Hopefully your gauge will go high and low, not up and down. LOL Keep us posted.

I like this idea!

Redfish suggested making sure of proper grounding in post # 6 and he had a lot of experience with Mopar gauges.

 

[Uncle Bob]

Bob , are the gauges now working properly?

Yes they seem to be working outside the car using a 100 Ohm resistor and the temp sender w/ heat gun. It was all connected to the car battery so it was all powered up for those tests.

Just for the heck of it, after the cluster is back in, use a clip jumper from the brass body of the temp sending unit to the battery NEG, or to an absolutely known good body ground. Let's see what happens. I was in Elevator technician for 43 years. Hopefully your gauge will go high and low, not up and down. LOL Keep us posted.

I will say this - the used sender was hard to get a ground connection on. I ended up with an alligator clip positioned so it bit in between the threads. Just sticking it on there wouldn't ground it, and I never got a ground through the hex part of it. Yes I used 60 grit to clean it up but still no ground through that.

 

[Super-Cuda]

Yes they seem to be working outside the car using a 100 Ohm resistor and the temp sender w/ heat gun. It was all connected to the car battery so it was all powered up for those tests.


I will say this - the used sender was hard to get a ground connection on. I ended up with an alligator clip positioned so it bit in between the threads. Just sticking it on there wouldn't ground it, and I never got a ground through the hex part of it. Yes I used 60 grit to clean it up but still no ground through that.

Damm Bob!

Long hard struggle with this one LOL !

Ok, when you reassemble, verify all connections are good , especially grounds. They may look good but as you said, meter doesn't lie.

I would pay particular attention to dash grounds , especially cluster ground. Also, verify that the metal dash panel is grounded to the body. Meter it as well .

Good luck bud!

 

[Vinny]

Yes they seem to be working outside the car using a 100 Ohm resistor and the temp sender w/ heat gun. It was all connected to the car battery so it was all powered up for those tests.


I will say this - the used sender was hard to get a ground connection on. I ended up with an alligator clip positioned so it bit in between the threads. Just sticking it on there wouldn't ground it, and I never got a ground through the hex part of it. Yes I used 60 grit to clean it up but still no ground through that.

Ok. This problem of yours is now making ME crazy. I just came in from the garage. I have a 340 Duster. I clipped my meter lead to the hex part of the temperature sending unit and I went to the battery NEG and I get a tone. I get a dead ground reading. You must have this, or your gauge will not work. Short and sweet. Please double check that your engine is grounded fully. Vinny

 

[Swinger 340]

Ok.

Is this what your circuit board looks like?


View attachment 1716438352

At 12 o'clock you have the regulator, Clean the contacts or change it out. That is the heart of your panel. This is just another idea, along with the rest.

 

[Uncle Bob]

Put it back together. Heated up the old sender, plugged it in and grounded it, and the dash responded.

Reconnected the new one, ran a ground from the hex (body) to an intake bolt and confirmed continuity between the sender and battery.

I ran the engine with that extra ground connection to where the mech gauge read 160, and the dash gauge never moved. Ditto the fuel gauge. I guess I've got two failed senders, both new, at the same time. I'm skeptical but there it is.