I'm stumped! Fuel gauge help please

[Captain Rust Bucket]

Hello everyone,

I am in the process of replacing the fuel tank in my '65 Newport and have discovered that my fuel gauge is not working. Well, at least I can't get it to work, haha. I'm pretty sure it was working to some degree before I started taking things apart. The float in the sending unit was taking in fuel so the reading was not accurate. After taking the tank out, I was preparing to put the new one in but wanted to make sure my sending unit was working. I was unable to get any reading on the fuel gauge but "empty". While trouble shooting, I realized that there are two wires that the previous owner had connected to the tank. One was the dark blue sending wire that was going to the sending unit and the other was a light blue wire that was connected to the tank itself with a screw (on the front lip). At first I thought the light blue wire was a ground but it turned out to go back to the fuel gauge. Here's where it gets weird, this is what the back of the fuel gauge looked like when I got down there to take a look :

At this point I realized a couple of things, the original pins for the fuel sending wire and fuel gauge feed were missing. The dark blue wire was spliced to the black wire shown in the picture here, the lighter blue wire was spliced to the purple wire shown here and the black feed wire was spliced to the white wire shown here. Also the original voltage limiter was gone. I tested the two that I could reach with a multimeter and got 5 volts on the purple that goes to the lighter blue and 12 volts on the black that goes to the dark blue wire. So today I was able to pull out the instrument cluster after much grief and took some pictures of what I found, hoping to get some help here:

Anyone familiar with this modification? I assume it's a voltage limiter but I don't know how to test to see if it works or if the gauge works and would like some guidance from anyone that has dealt with this. I have the fuel sending unit on the bench too so I would like to test and see where the system is failing but I have no reference for this in the FSM. I have tested the resistance in the fuel sending unit with the multimeter and I got around 80 ohms at empty and around 26 at full. Any help would be greatly appreciated.

 

[MoPar~Man]

You notice on the left side of the circuit board there are 3 slots with jagged-edge connectors. There should be a tin-can module plugged into those 3 slots. The tin-can is a (crude) 5-volt voltage regulator. It provides the 5-volt power source for the fuel guage and usually also the temperature guage. Does your instrument cluster have a temp guage? Someone has created a 5-volt source using what is probably a zener diode and a resistor divider, the fixed resistor is 100 ohms and the variable resistor is probably set close to 100 ohms.

On the far right side of the board is a threaded post, the board and traces in that area show signs of being burned. Is that your ammeter?

 

[CBODY67]

'65 and '66 Chrysler clusters have "HOT" and "COLD" lights. No temp gauge.

 

[Captain Rust Bucket]

You notice on the left side of the circuit board there are 3 slots with jagged-edge connectors. There should be a tin-can module plugged into those 3 slots. The tin-can is a (crude) 5-volt voltage regulator. It provides the 5-volt power source for the fuel guage and usually also the temperature guage. Does your instrument cluster have a temp guage? Someone has created a 5-volt source using what is probably a zener diode and a resistor divider, the fixed resistor is 100 ohms and the variable resistor is probably set close to 100 ohms.

On the far right side of the board is a threaded post, the board and traces in that area show signs of being burned. Is that your ammeter?

Yes, I realize that the original voltage limiter is missing. My instrument cluster does not have a temperature gauge. Just the cold and hot lights. I think you're right about the 5-volt source using the resistor, my question is: how does this affect the interface with the fuel gauge and the fuel sending unit. I tried testing the system on the bench by connecting the positive battery terminal to the post that would be the fuel gauge feed, then the 5-volt output to the sending unit and the negative post on my battery to the body of the sending unit and I couldn't get any movement out of the needle. Yes, that is the ammeter in question. It burned out one of the first times I tried powering up the car and I bypassed it by putting both wires on one terminal so I could get power to my ignition.

 

[MoPar~Man]

The sending units have one insulated terminal, on the round mounting plate. The fuel line passes through the mounting plate. You have a new sending unit? Is it mounted in the tank yet? Measure the resistance between the mounting plate and the insulated terminal. I think it should be about 200 ohms in the empty position, not sure about the full position but maybe 25 ohms.

For the guage to work, the round mounting disk or plate needs to be connected to ground. The factory way was a flimsy clip-strap that spanned from the metal fuel pipe coming out of the mounting plate to the next metal segment of the fuel line, which presumably is held somewhere against the chasis with a screw. The better way is to attach a wire directly to the mounting plate and run that wire to any nearby body / chassis screw that makes good / clean rust-free contact with the body. The previous owner seemed to think that a wire attached to the gas tank body could or would act as a suitable ground to the sending unit - this might work because the locking ring will make contact between the tank and the sender mounting plate.

 

[Captain Rust Bucket]

The sending units have one insulated terminal, on the round mounting plate. The fuel line passes through the mounting plate. You have a new sending unit? Is it mounted in the tank yet? Measure the resistance between the mounting plate and the insulated terminal. I think it should be about 200 ohms in the empty position, not sure about the full position but maybe 25 ohms.

For the guage to work, the round mounting disk or plate needs to be connected to ground. The factory way was a flimsy clip-strap that spanned from the metal fuel pipe coming out of the mounting plate to the next metal segment of the fuel line, which presumably is held somewhere against the chasis with a screw. The better way is to attach a wire directly to the mounting plate and run that wire to any nearby body / chassis screw that makes good / clean rust-free contact with the body. The previous owner seemed to think that a wire attached to the gas tank body could or would act as a suitable ground to the sending unit - this might work because the locking ring will make contact between the tank and the sender mounting plate.

I'm using the old fuel sending unit, not mounted on the tank yet. I'd like to bench-test it before I start putting things together. I did measure the resistance and I got 80 at empty and 26 at full.
In regards to the ground for the fuel sending unit, am I correct that I should be able to run a jumper wire directly to the negative terminal on the battery from the mounting plate to bench-test the system?

 

[MoPar~Man]

Your fuel guage has 3 terminals. I'm looking at the '65 service manual. Somewhat confusing - the Imperial shows a 3-terminal guage, with one of the terminals going to "ignition circuit". This also has a "fuel relay". Not sure how that's supposed to operate, unless there's a relay energized by the key in the run position only. But still no clue what the 3'rd terminal on the guage is for.

The circuit diagrams for the Chrysler models only show a 2-terminal guage. Where does the center post on your guage go on the pc board?

The posts on your guage (the 2 outer posts) have unequal length. One is significantly longer than the other. I don't know what that's all about.

Yes, to complete the circuit, the sending unit mounting plate must be connected to ground, so for bench testing you would connect it to the negative battery terminal. When you do make that connection, and turn the key to the run (not start) position, I would have a volt meter on the longer post on the guage (so measure between that post and ground). You don't want to see 12 v on that post. Hopefully it's only 5 or 6 volts. If it's higher than that, turn the key off quickly or you might burn out the sending unit or the guage.

 

[Captain Rust Bucket]

Your fuel guage has 3 terminals. I'm looking at the '65 service manual. Somewhat confusing - the Imperial shows a 3-terminal guage, with one of the terminals going to "ignition circuit". This also has a "fuel relay". Not sure how that's supposed to operate, unless there's a relay energized by the key in the run position only. But still no clue what the 3'rd terminal on the guage is for.

The circuit diagrams for the Chrysler models only show a 2-terminal guage. Where does the center post on your guage go on the pc board?

The posts on your guage (the 2 outer posts) have unequal length. One is significantly longer than the other. I don't know what that's all about.

Yes, to complete the circuit, the sending unit mounting plate must be connected to ground, so for bench testing you would connect it to the negative battery terminal. When you do make that connection, and turn the key to the run (not start) position, I would have a volt meter on the longer post on the guage (so measure between that post and ground). You don't want to see 12 v on that post. Hopefully it's only 5 or 6 volts. If it's higher than that, turn the key off quickly or you might burn out the sending unit or the guage.

I was confused by the same thing. I am thinking now that maybe they tried adapting the fuel gauge from an imperial to this Newport I'll have to look at the FSM for the Newport and try to figure out if that could be the case.
I don't think the center post is connected to anything else on the board. I think it's just for the fuel gauge. It is the voltage regulated 5-volt output that is supposed to go to the sender, I think. You can see in one of the pictures I posted that they actually had to drill a hole in the board to allow that post through. It looks to me as if two of those posts were cut off with side cutters to allow for clearance with the speedometer (refer to the first picture of my original post). I will definitely look into this imperial fuel gauge and see if there is something I can learn. I appreciate your help MoPar man.

 

[Big_John]

I am thinking now that maybe they tried adapting the fuel gauge from an imperial to this Newport

That's a newer aftermarket gauge that's replacing the original. Not an Imperial or other OEM.

Being an aftermarket gauge, there's a 99.9% chance it's made to run on 12 volts rather than the usual 5 volts.

The piece with the yellow paint is a potentiometer that can be adjusted with a screwdriver, but don't touch it until you get everything else straightened out.

 

[CBODY67]

It kind of appears that the prior owner/modifier was not fully aware of Chrysler's wiring and voltage limiter issues?

To me, using the fuel tank for a ground is not electrically good. Grounding the sender through the fuel line (as OEM) should work better. What might work even better would be to ground the sender via the car body. The tank has an insulator between it and the body, although the retaining straps might be a bit troublesome. Is the sender even a Chrysler OEM part?

Might be best to consult the Chrysler FSM for wiring schematics, related color codes, etc. rather than not. THEN see what is different on the car and go from there.

@Big_John makes a great point about the aftermarket items, including the manual voltage adjustment pot! Which would be a great place to check to see at what voltage is adjusted there.

That wire going to a "ground" on the tank's lip, attach that to the body instead and see how things work.

CBODY67

 

[CBODY67]

An Imperial unit? Check the FSM to see IF any wires might be different. Imperials usually had a "Master Gauge" warning light to indicate "out of range" readings on the instrument panel gauges. The schematic should show any additional wiring. OR perhaps the Imperial dash gauges had an extra terminal on them to run THAT light?

CBODY67

 

[Big_John]

An Imperial unit? Check the FSM to see IF any wires might be different. Imperials usually had a "Master Gauge" warning light to indicate "out of range" readings on the instrument panel gauges. The schematic should show any additional wiring. OR perhaps the Imperial dash gauges had an extra terminal on them to run THAT light?

CBODY67

It's an aftermarket gauge. Look at the construction. Chrysler never made their gauges like that.

Here's a '68 Imperial gauge with the typical OEM gauge movement.

 

[Big_John]

@Big_John makes a great point about the aftermarket items, including the manual voltage adjustment pot! Which would be a great place to check to see at what voltage is adjusted there.

That would be a resistance pot.

When the gauge was built and calibrated, that would tweak it to read correctly. The yellow paint seals it.

This is the adjustment pot on my aftermarket gauges in both my Chryslers. And just to show how it's done, here's my decade resistance box hooked up to the gauges to check them and the salvaged 12 volt computer power supply.

Of course, the factory would be doing this with a slicker, more professional setup, but the process will be the same.

 

[Captain Rust Bucket]

That's a newer aftermarket gauge that's replacing the original. Not an Imperial or other OEM.

Being an aftermarket gauge, there's a 99.9% chance it's made to run on 12 volts rather than the usual 5 volts.

The piece with the yellow paint is a potentiometer that can be adjusted with a screwdriver, but don't touch it until you get everything else straightened out.

I think you have cracked this case @Big_John !!! That would make sense why I was getting 12 volts to the fuel sending unit wire and 5 volts on the one that I thought was ground. Now I just have to figure out the polarity and test it out. Much thanks to both of you gentlemen! May God bless you richly
I will post an update after some tests today.

 

[Big_John]

I think you have cracked this case @Big_John !!! That would make sense why I was getting 12 volts to the fuel sending unit wire and 5 volts on the one that I thought was ground. Now I just have to figure out the polarity and test it out. Much thanks to both of you gentlemen! May God bless you richly
I will post an update after some tests today.

Thinking about it, and it's usually dangerous when I think, there was a company that was repairing Mopar gauges by replacing the internals with an aftermarket gauge. I'm thinking they were using VDO gauges, but I could be completely wrong. I can't remember the name of the company.

edit: Possibly these guys » Gauges & Clusters Instrument Specialties

It could have been done by a previous owner too.... Who knows?

 

[Captain Rust Bucket]

Praise God it's working!! I took a short video to show how I wired it up on the bench. I just have to adjust the pot to get more accurate readings and I should be able to slap it back on the car. Ahhh time for a beer! Cheers gents thanks for your help.

 

[Captain Rust Bucket]

I claimed victory too soon. It looks like there is only enough adjustment to set either empty or full. I am not able to adjust the full range. I only get about a quarter of the full range of movement out of the needle while moving the arm of the fuel sending unit from empty to full. @Big_John is there a way to adjust the end points on the dash pot or do I have to get a new fuel sending unit?

 

[MoPar~Man]

First thing - I highly doubt you've got an aftermarket guage. It fits too well in the cluster housing, it looks like the exact correct guage from the front. The face plate has the correct contours to be attached to the cluster. I don't know the visual diff between a Chrysler vs Imperial fuel guage for any given year. Is it possible that for '65 they were similar? Someone altered it to fit your cluster, and to make it work without having the 5V limiter.

According the the '65 manual, the Imperial fuel guage contains the 5v limiter.

Regarding the sending unit, I think there are units that go from 200 to 25 ohms and others that go from 80 to 25 ohms.

To make your guage work with your sending unit, I'd need to connect your guage to a controlled voltage source, a lab power supply if possible, and see at what voltage the guage registers at the empty mark and the full mark. Then use an ohm meter to measure the sending unit at zero and full. From that it's possible to create a compensating resistor network to have the guage register correctly.

 

[Big_John]

I claimed victory too soon. It looks like there is only enough adjustment to set either empty or full. I am not able to adjust the full range. I only get about a quarter of the full range of movement out of the needle while moving the arm of the fuel sending unit from empty to full. @Big_John is there a way to adjust the end points on the dash pot or do I have to get a new fuel sending unit?

The pot won't change the span of the gauge. For example, if you were to set the low end 1/4 of a division higher, the upper end of the gauge will also increase 1/4 division higher.

Edit: I should have watched the video first LOL. You may be able to adjust that. Adjust for the low point and try it. Judging from the bouncing gauge, I think it's bouncing off the stop at full range. But first... Check the sender.

What I think you could do is to check the resistance of the sender. It's tough to get a mid-range but you can see full and empty.

10 ohms full.
23 ohms mid range
74 ohms empty.

That would make sure your sender is good.

Since it is an aftermarket gauge, it may be set up for a different range sender.

Did this ever work right? Since someone has swapped in this gauge, I'm wondering if it worked when they were done.

The ultimate solution might be finding the correct gauge and/or a new sender.

 

[Captain Rust Bucket]

First thing - I highly doubt you've got an aftermarket guage. It fits too well in the cluster housing, it looks like the exact correct guage from the front. The face plate has the correct contours to be attached to the cluster. I don't know the visual diff between a Chrysler vs Imperial fuel guage for any given year. Is it possible that for '65 they were similar? Someone altered it to fit your cluster, and to make it work without having the 5V limiter.

According the the '65 manual, the Imperial fuel guage contains the 5v limiter.

Regarding the sending unit, I think there are units that go from 200 to 25 ohms and others that go from 80 to 25 ohms.

To make your guage work with your sending unit, I'd need to connect your guage to a controlled voltage source, a lab power supply if possible, and see at what voltage the guage registers at the empty mark and the full mark. Then use an ohm meter to measure the sending unit at zero and full. From that it's possible to create a compensating resistor network to have the guage register correctly.

I agree it seems too good to be aftermarket but it's definitely not the original and it's working on 12 volts for sure, so who knows. I tried adjusting the dash pot to no avail. It will have do for now until I can replace some of these parts. For now, "empty" will be half tank and "full" will be just over 3/4 on the gauge haha. I'm excited to get the new fuel tank in so I can drive the car.