The Stovebolt Site Forums The Shop Area The Electrical Bay New fuel guage pegs

When I bought the truck the fuel gauge did not work. I followed all the testing procedures on this site and determined when the previous owner converted to 12v he did not step the voltage down to 6v for the original gauge and fried it. I bought a new 12v replacement. When I hooked everything back up the fuel gauge pegs not to the full mark but completely off the chart to the right almost pointing straight down, almost 180 degrees opposite a half tank. When there is no power the fuel gauge just floats around somewhere around a half tank. Did I reverse the wires to the fuel gauge when I installed it? Could I have a miss match on the ohms resistance between the sender unit and the new gauge. Sender looks new. The lowest scale on my ohm meter is 200. I know I have between 3/4 and full tank of gas. Reading across the two sender unit terminals I get a reading of 1.2. This is with the battery disconnected.

My guesses:

1. You might have a bad ground - run a ground wire from a post/nut on the sender to a clean ground on the frame.

2. You might have a sender with an ohm rating that does not match the ohm rating expected by your gauge.

3. Or, both of the above.

http://www.tanksinc.com/index.cfm/page/ptype=results/category_id=159/mode=cat/cat159.htm

Sounds like your gauge is bad. With no power, it should read less than empty. With no power applied, the sending unit is completely out of the picture.
Carl

If I tilt the gauge left or right (disconnected) the needle tilts either direction. I don't think it is bad.

I just now saw that you have a 12 volt replacement gauge. I have heard that they don't go to zero with the power off. (The original 6 volt gauges do.)
Someone else should be along soon who knows what to expect from the new gauges.
Carl

all I know and may not applied to what you are doing or have done but the ohms need to be correct to your sending unit and gauge, I installed an after market sw on mine and someone had mixed up the suppose to be correct one in the box and it wasnt the correct match.. I doubt you are having the same problem but just a thought.. I think maybe 30 ohms

Miss-match is what I think is the issue. I hope that is the issue. I can fix that easily. Can someone tell me how to test the ohms resistance of the sending unit. I tried checking it measuring it across the two terminals on the sending unit but that did not work.

I found information on testing the sending unit. First, I was using the correct method to test the sending. My mistake was the interpretation of the results. When I put my meter across the two terminals with the scale set to 200, I was reading 1.2. I did not realize the ohm reading was in decimals. If what I saw on a You Tube video is correct, 30 ohms resistance would read 0.3. The sending unit is still in the tank and I know I have close to a full tank. Once again, assuming I now understand how to read the meter, 1.2 would translate to 120 ohms. This tells me the sending unit is a more modern unit and probably operates on at 0-90 or even a higher range. I am going to test one more time. If I get the same results I will pull the unit and get the full range of ohms resistance by moving the float arm between the empty and full range. If I am correct and the current ohms resistance on my unit is currently 120 ohms that would explain why the needles pegs way past full. I think I will be ordering a new unit.

I just tested one today. You need a digital multimeter capable of reading the actual ohm value. But even if you do not have that, the problem with analog is, as you move the arm up and down slowly, the wiper can skip and cause erratic behavior. The older the sending unit is, the more likely this is to happen. Usually cleaning the mechanism will get the wiper working. I use electronic contact cleaner. Amazing the difference after cleaning.

I would think you are right though, you need a new 30 ohm sending unit.

I have not ordered a new sending unit yet. I am still concerned about the behavior of the new gauge. When I installed it, the needle on the gauge swings past full and buries the needs almost 180 degrees opposite the half tank mark. If referencing a clock, the gauge should read between about 10 o'clock for empty and about 2 o'clock for full. My needle goes to about 5 o'clock, way off the scale. Nothing changes when I disconnect the sending unit. The needle does swing back when I disconnect the lead to the gauge. Do I have a bad (new) gauge, do I have a bad (miss-matched sending unit, should be 0-30 ohms unit) or could I possibly have the two wires on the gauge reversed? With a properly working sending unit would the gauge come back into range?

Did you run a new/test ground lead from an attachment bolt on the sender (which is attached to the gas tank) to a good clean ground?

"My fuel gauge only reads full.

If you cannot make your gauge read anything but full then depending on your ohm range you may have one of the following problems:

If you have a 0-30, 0-90 or 10-180 Ohm sender then the sender may not have a proper ground or there is a break in the sender lead wire."

http://www.fifthaveinternetgarage.com/tech_tips_gas_gauge.php

The sender has a ground wire under one of the attachment bolts and it runs about 8 inches to the back of the cab were it is fastened to the body. It looks like a good connection. When I remove the wire from the sending unit connection post it has no effect on how the fuel gauge reads. I would have thought that with a full tank and no resistance (0 ohms) the fuel gauge should go to full, not way off the scale. Doesn't removing the lead from the sending unit simulate a 0 ohm reading and as the resistance increase the gauge should start dropping toward empty at about 30 ohms resistance? The whole thing with the gauge reading of the scale is what has me stumped. This could be normal and a new sending unit might resolve the issue, I didn't know if anyone had any experience with this?

0 empty

30 full

Okay, so there is one more test I can do. If I ground the wire at the sending unit it should simulate an empty tank and my needle should go to empty. If nothing happens I can test continuity of the wire between the fuel gauge and the sending unit. If the wire is good and grounding it does not change anything thing I am at a loss other than a bad (new) gauge.

I can shed some light on your problem after this weekend. I installed a 12 volt fuel gauge from Classic Parts and it is not usable. The needle goes about 180 degrees past full. WAAAY out of bounds. They must have a bad batch of gauges. Send it back and tell them what its doing.

You should be able to put 12 volts on the power side of the gauge and it should read right at FULL, maybe just a tad more. Taking power off the needle should go to EMPTY maybe just a tad less. Until they tell us what we are supposed to do with a needle that rotates all the way around, let alone what it will do with the actual readings, we just have to go with another vendor or wait until they fix the problem.

Also, Zero Ohms is not ground. So to test the gauge by itself, put 12 volts on it.. reads full. Test the sending unit out of the truck.. goes from zero ohms to 30 or so ohms with no erratic behavior as you raise the arm up and down. With both behaving normally, it WILL work great. If it does not, be sure the sending unit ground is at the same ground potential as the gauge.

Thanks. I will contact them.

I am not surprised by their response. They are telling me I have a bad ground. I don't think this is the case but I am finding the previous owner did some questionable things to the vehicle. First thing according to the Classic Parts technician, I should only get needle movement when I turn the ignition switch on. Right now, as soon as I hook the battery up the needle buries regardless of the position of the switch. This tells me the feed to the gauge is hot all the time, bypassing the ignition switch. They also told me the ground wire I have from the sending unit to the back of the cab might not be good. They told me because of the rubber bushings on the cab mounts I might have to run a ground to the frame. I will trace the wires one more time to make sure the feed from the sending unit is wired through the ignition switch and I have a good ground. He also told me a bad ground will allow the needle to swing way pass the full mark the same as disconnecting the sending unit completely because the needle will bury when there is an open circuit. However, I will test it one more time with an extra ground clipped to the frame. He also told after verifying the ground, if I ground the sending unit the gauge should drop to empty. I will have an update tomorrow.

Has to do with their fuel gauge being bad if it goes all the way around to the down position when you apply 12 volts to it. I tested it using a bench power supply first before ever putting it in the truck. The gauge is bad if it does not behave the way I described above. Since I have one that is doing it too, its BAD. To get a good idea if you need better ground, just put the ohm meter on the dash (an unpainted area) and the gas tank. If it reads a short, the ground is fine. This isn't about how it behaves in the truck, its how it behaves!

When you talk to them again, explain that there is no need to add the sending unit behavior into the conversation or the grounding issues. 12v to the meter should not cause the needle to point directly down (180 degrees)! They should know this. Tell them to compare one of their 6 volt gauges to see how it is supposed to work.

In the services portion of the Links page, there are a number of listing for shops who rebuild / repair various gauges.

Perhaps one of them might be willing to give you some insight.

Latest update. I removed the gauge and bench tested it. This is where it gets frustrating. Works as it is suppose to. With 12v across the two gauge terminals the needle dropped to empty. I then connected the fuel gauge to the power wire and sending unit lead. I turned the key on and the gauge works as expected. At this point the gauge is not mounted in the cluster, it is hanging loose. I now install the gauge back into the cluster, hook up the two wires and it is working. At this point the instrument cluster is hanging loose. I scratch my head and figure I must have had a loose wire somewhere. I drop the instrument cluster back into the dash tighten up the 3 nuts, turn on the ignition and the needle swings around to the right and disappears. WTF. The gauge is clearly working and it matches the sending unit. I was getting about what I expected on the gauge based on the amount of gas in the tank. I only have one other idea. On the back of the gauge it has a copper plate on the back of the circuit board facing the cluster. I assumed this was suppose to ground to the cluster housing. The gauge came with a piece of fiber board that the two post fit through. I wonder if that is suppose to go between the gauge and the cluster housing to prevent it from grounding to the case. The gauge did work just dangling from the wires and not touching ground. I know when the cluster housing is bolted in the case it is grounded and now so is that copper plate on the back of the gauge. The only other thing I know to try is to take it out for the 4th time, slip that piece in there to insulate the gauge and try again.

The gauge needs to be grounded to work


On the back is a dab of solder that needs a ground path

Make sure the cluster is clean for a good ground


You need to Ground it for a bench test also

Well, it must be a different problem than I am having. I am thinking it might be exactly what you are describing, an insulator issue. Glad you got it working! I thought from what you described your needle did not stay within E and F when applying power. I must have just got a bad apple.

My first test was after installing it, so it did go out of range and stayed out of range. DADS50, when I had the gauge hooked up and just hanging from the two wires it worked fine. The old one clearly was grounded when I removed it. there was no insulation between the gauge and the case body. Could the design have changed with the modern 12v replacements? I am going to call ClassicParts again to see what their technician says about this little twist in the drama.

do like I did, get you a sw gauge and sender...
it is not an org truck any more any way,, just a thought..
I replaced mine after running out of gas after messing with it for awhile..this was in a hot July summer.lol


good luck

It keeps getting stranger. The ClassicParts technician agrees with DADS50 and does not understand how I could bench test the gauge without grounding the solder plate on the back. I just know I hooked a power supply across the two post and the gauge went to empty. According to him the procedure is to connect power to the full or positive side of the gauge and then connect the negative side of my power supply to the solder plate on the back of the gauge. This should cause the gauge to read full. If I then extended the ground to include the sending unit post the gauge should then drop to empty. He did not think the gauge should work without the solder plate being grounded. I was going to trouble shoot it one more time. If I have no luck they said I could return it. They would bench test it. If defective they would send me a new one. If it is not defective they would send it back. Deve, at least it looks like they are willing to work on making things right if the gauge is bad. There policy states no returns on electronics, so I was afraid they would tell me tough luck on the fuel gauge.

Have not followed the whole thread

Have you tested the sender?

Remove from tank and meter it
....o empty. 30 full...

Since your cluster is out. Hook everything up on a bench and test
....example... remenber to ground the cluster for the test.


Or you could go to radio shack and test with ohm resisters

....example. 30 ohm test....


Don not run power to the wrong post
You could fry your gauge
`


...pic...

Yeah, I haven't had recurring problems with Classic. I even use their part numbers in my How-To articles. But I am flabbergasted at how a fuel gauge can point straight down from its mounted position and be okay.. so to your point..

go back to your bench.. put the power supply on the power terminal of the gauge and the negative to that smooth point under the gauge that needs to be grounded. Reading should be FULL. If it reads full the gauge is good. Install it and don't look back. Dads has a good method there. Good luck!

It continues. I installed a new sending unit to go with the new fuel gauge. I have a full tank. Now my gauge shows about a 1/4 tank past full. If I run a jumper from ground to the sending unit post at the sending unit the gauge drops to about 1/4 tank (should drop to empty). I am curious to see if after running the truck and the fuel level drops if the gauge drops (measuring off scale by about a 1/4 tank) Any thoughts? I can't think of anything except the new gauge is bad. I hate the thought of pulling it out again, for the 5th time.