Fuel gauge gone goofy

[Rich69shortfleet]

So, is it gauge, sending unit or wiring?

Had the cluster out recently but everything worked fine after reinstall. When I filled the tank recently it looked fine. Then the gauge looked like this when I stared it up the next time. I've logged about 30 miles since and it moved a bit but is still way off scale. It has gone way off scale before after sitting but always corrected when started.

I'll check the connections at the cluster but not sure what else to check. And since the needle has moved it is connected to something. All parts original to the truck.

[Rick Bollinger]

Your fuel gauge wire plugs into your fuse box if I remember right. It is a tan wire I believe. Verify this as I am just going from memory. Check to make sure it is getting good contact there as well. I think it is just a floating ground that goes to your sending unit in tank. Someone correct me if I'm wrong and will pull this down. I don't want to put out bad info.

['68OrangeSunshine]

Originally Posted by Rich69shortfleet

So, is it gauge, sending unit or wiring?

Had the cluster out recently but everything worked fine after reinstall. When I filled the tank recently it looked fine. Then the gauge looked like this when I stared it up the next time. I've logged about 30 miles since and it moved a bit but is still way off scale. It has gone way off scale before after sitting but always corrected when started.

I'll check the connections at the cluster but not sure what else to check. And since the needle has moved it is connected to something. All parts original to the truck.

When the red needle pegs to the right -- past Full and almost at 3 O'Clock -- usually the tan wire has come loose from the fuse block. Check that first.

Fifty-Plus year old wiring is brittle. When you had the Instrument Cluster loose, possibly the Tan wire was compromised. Check for breaks or opens inside the insulation. Or a loose connection at the back of the Fuel Gauge.

If the float on the end of the sender arm gets soggy, fills up with fuel, or falls off inside the tank, then the gauge would read Empty. They don't Fall UP.

[Getter-Done]

I will add one thing picture below for reference.

Is the ground clip attached and have a good contact?

[Rich69shortfleet]

I'll be checking these things tomorrow or Saturday.

[Rust_never_sleeps]

I couldn't remember which way it goes, so pulled up a search, found this:
https://67-72chevytrucks.com/vboard/...d.php?t=609792

short story is: ground out the gauge and it should show empty, which would mean the gauge is working
That means it's seeing high(possibly infinite) resistance and either the connection to the sender has a fault, or the sender itself has a problem
Should be a 90Ω sender, but don't trust me on that

[Rich69shortfleet]

Ok, here's what I know so far.

1. The ground wire is firmly attached and has a clean connection.

2. I disconnected the fuel gauge wire at the fuse panel and the gauge goes even farther past full.

3. I reconnected the fuel gauge wire and grounded the sending unit wire and the gauge goes to empty. No farther, just to empty.

I cleaned the connection at the sending unit and reconnected things and the gauge goes back to the a bit past full. Given how much fuel I have and experience, the needle should be part way between full and 3/4s full.

So, it appears the gauge is working, no confirmation it is accurate, but it is reactive.

Next test I suppose would be to pull the sending unit and see if it is stuck and then see if moving it through its range of motiion appropriately moves the gauge. Another test would be to enlist my electrical guru friend and put a varying amount of 0-90 ohms through the sending wire to see if the gauge acts appropriately. It might be that the resistor on the back of the gauge is failing.

[Rust_never_sleeps]

Yep, gauge probably works
If you want you could hook up a MM to the sender and read the resistance, but it's not going to save much effort over pulling it

[Rich69shortfleet]

In the event I need to replace the sending unit, which would I need? There is a 3/8" line version and a 5/16" line version. My truck originally had a 307 so I'm guessing the 5/16" version is what I need? Truck has a 400sb in it now but I've never had a fuel starvation issue.

[PbFut]

I would do a techron treatment first. Without going into too much detail an online group i was following was having issues with fuel guages in the BMW 850. It was sort of a epidemic. Most but not all found a treatment or two of the techron fuel additive corrected the problem. If you do need a sender unit replacement. I picked up a couple pointers. The worm gear type claps are easier to use and remove but are much less reliable. The clamps with the small nut and screws with a solid band are much more reliable with more even clapping force. Some hose are advertised as fuel line and resistant to fuel on the inside. A weeping connection some how reacts differently on the outside and the fuel will break down the hose on the outside.

[VetteVet]

Originally Posted by Rich69shortfleet

In the event I need to replace the sending unit, which would I need? There is a 3/8" line version and a 5/16" line version. My truck originally had a 307 so I'm guessing the 5/16" version is what I need? Truck has a 400sb in it now but I've never had a fuel starvation issue.

Your tests indicate that the gauge is working and the dash is grounded.
Also the problem is between the fuse panel and the sender unit.
Check for continuity on the wire from the fuse panel to the sending unit (zero resistance), If I remember right an open circuit on the ground side of the circuit will result in the gauge staying at it's last reading.

You can slave in a wire from the fuse panel to the sending unit and check the gauge for the correct reading. If it remains the same , then pulling the sending unit and checking the arm for being stuck would be my next test.

The question on the fuel line diameters is, the 5/16th is for six cylinder engine and the 3/8ths is for v/8s

If you pull the sending unit you can connect a volt meter between the flange and the wire terminal and check the resistance by moving the arm up and down to read between 0 and 90 ohms,

[Rich69shortfleet]

Haven't had time to do any more work on the truck. Have had bigger fish to fry lately and spare time has been at a premium. I don't drive the truck daily or anything, so the urgency to get back at it has been low. I should have my other project done this weekend and the wife's car needs resolved this weekend, so I should be able to get back to the truck next week.

I wish my bench seat back was able to fold forward, even with the seat all the way forward getting to the tank and sending unit is slightly cramped. And taking the seat out isn't really an option because I have no help and that damn thing is heavy and awkward. I took it out and put it back by myself last Summer, never again will I do that without help.

[Palf70Step]

I might have missed it, but make sure you have a good ground at the sending unit, besides the one at the dash gauge. That is what was bad on mine when it showed that. Also, if replacing the sender I would go with the 3/8, regardless of what you have.

[Rich69shortfleet]

Finally got a chance to test the sending unit. I pulled it out and on the bench it provided 3 to 200+ ohms and bounced up and down through the float movement range. I took the rheostat cover off and everything looked intact but a few of the windings were out of line. I tried to straighten things out and when it was at first back together it seemed to run from 2 to 90 ohms but it soon went back to whacky readings.

So, I'm going to get a new sending unit.

['68OrangeSunshine]

Originally Posted by Rich69shortfleet

Finally got a chance to test the sending unit. I pulled it out and on the bench it provided 3 to 200+ ohms and bounced up and down through the float movement range. I took the rheostat cover off and everything looked intact but a few of the windings were out of line. I tried to straighten things out and when it was at first back together it seemed to run from 2 to 90 ohms but it soon went back to whacky readings.

So, I'm going to get a new sending unit.

Definitely time to R&R the Sender.
Once installed, take notes on how it reads.
They never duplicate the quirks of the old one, but are quirky on their own.
Some read low, but have more gas in the lower half, and some read too high.
Never an accurate steady read from sender to sender.
That's how it is.

[GMCPaul]

Originally Posted by Rich69shortfleet

Finally got a chance to test the sending unit. I pulled it out and on the bench it provided 3 to 200+ ohms and bounced up and down through the float movement range. I took the rheostat cover off and everything looked intact but a few of the windings were out of line. I tried to straighten things out and when it was at first back together it seemed to run from 2 to 90 ohms but it soon went back to whacky readings.

So, I'm going to get a new sending unit.

There are at least 3 manufacturers of the fuel senders for 67-72's. 2 use cold solder lead joints on the brass floats the cold solder uses temperatures below 450°C and while good for electronics is poor for sealing and 1 uses a hot solder joint which uses higher temperatures to join metals and requires flame for brazing or carbon arc tools which creates a better seal. The 2 manufacturers using the cold solder joints fails in short order and fills with gas causing a empty gauge reading since float then sinks. We of course carry the hot solder version. When the cold solder version begins failing gauge begins reading lower than it should and once float fills completely reads empty. Gauge is still good other than the float which can be replaced to fix issue.
https://gmcpauls.com/product-categor...-tank-related/
Paul Jr @ GMCPauls

[Rich69shortfleet]

Or I suppose I could just silver solder the float again myself. I've had to do that to attach gas tank filler necks on new fuel tanks.

[Getter-Done]

Originally Posted by GMCPaul

There are at least 3 manufacturers of the fuel senders for 67-72's. 2 use cold solder lead joints on the brass floats the cold solder uses temperatures below 450°C and while good for electronics is poor for sealing and 1 uses a hot solder joint which uses higher temperatures to join metals and requires flame for brazing or carbon arc tools which creates a better seal. The 2 manufacturers using the cold solder joints fails in short order and fills with gas causing a empty gauge reading since float then sinks. We of course carry the hot solder version. When the cold solder version begins failing gauge begins reading lower than it should and once float fills completely reads empty. Gauge is still good other than the float which can be replaced to fix issue.
https://gmcpauls.com/product-categor...-tank-related/
Paul Jr @ GMCPauls

Thanks for sharing this info.

It's good to have a down to earth vendor on this forum.

['68OrangeSunshine]

I switched to the Jeep part, plastic floatball.

[custom10nut]

Originally Posted by Rich69shortfleet

I'll be checking these things tomorrow or Saturday.

Also when you do get the issue resolved, do yourself a favor and run a couple ground wires from the metal portions of you instrument panel to the main dash panel.
A few years back, I was having fits with the dash mane outer lights, and it all centered around poor grounding.
I put the additional direct grounds in, and haven’t had an issue since.
Good luck with solving your issue

[Rich69shortfleet]

Originally Posted by GMCPaul

There are at least 3 manufacturers of the fuel senders for 67-72's. 2 use cold solder lead joints on the brass floats the cold solder uses temperatures below 450°C and while good for electronics is poor for sealing and 1 uses a hot solder joint which uses higher temperatures to join metals and requires flame for brazing or carbon arc tools which creates a better seal. The 2 manufacturers using the cold solder joints fails in short order and fills with gas causing a empty gauge reading since float then sinks. We of course carry the hot solder version. When the cold solder version begins failing gauge begins reading lower than it should and once float fills completely reads empty. Gauge is still good other than the float which can be replaced to fix issue.
https://gmcpauls.com/product-categor...-tank-related/
Paul Jr @ GMCPauls

I received my sending unit today (I ordered before you posted). How do you tell the difference between the various types? I'd like to know if I need to resolder the float.

I tested the unit and it provides 0-90 ohms through the range of motion like it should.

[Rich69shortfleet]

Or I suppose I could just swap the OEM float over to the new sending unit.....

It has held up for 56 years so far.

[Rich69shortfleet]

I connected the new sending unit to the truck and the motion range of the float arm takes the gauge through the expected range. I plan to install the new sending unit tomorrow.

[Rich69shortfleet]

New sending unit is in. It reads different than the original, but that was expected. At least it will move throughout a range unlike the broke original.

[Rich69shortfleet]

Damned if you do and damned if you don't. Due to comments earlier about the reproduction floats often being bad (never did get a response to my question about how to tell the good ones from the bad ones) I installed the new sending unit with the original float attached. I needed the truck today and when I started it up the gauge was showing just below empty. I know it had about 3/4s of a tank. After I got done with my errands I pulled the sending unit and sure enough, the float was full of gas. I put in the reproduction one that came with the sending unit and it is working properly again.