LMC Truck: How to Diagnose a Non-Functional Fuel Gauge Transcript
Hi, I’m Kevin Tetz working alongside LMC Truck to help you with some technical information and videos that help you restore your truck.
In this video, we're going to be showing you how to diagnose a non-functional fuel gauge. The fuel system is one of the most important driver communications in your truck. Fortunately, it's a relatively simple thing to troubleshoot and to repair. I’ll also show you how to bench test an original fuel gauge, a brand new one, and a fuel tank sending unit. Wiring overall can be intimidating and troubleshooting can be just as puzzling. But if you break it down into some simple steps, you're going to be able to diagnose your own system and easily get it fixed.
LMC Truck has everything you need to repair or replace your fuel communication, or any other gauge delivery system: new sending units, new stock replacement gauges, aftermarket gauges, even a replacement printed circuit board with or without the tack if that's what turns out to be the problem with your truck and you want to keep it original. If you need a speedometer, if yours is broken, or if you just feel like starting from zero, or even if you just need a cable, all you got to do is measure the original an LMC Truck will send you exactly what you need.
Now normally wiring problems are high on the meter, but when you isolate the fuel system by itself it's pretty simple. So, on a difficulty meter, I’m going to give this a three out of five.
Not a lot of tools are required for this project. You will need a 12 volt power source, some simple hand tools like some screwdrivers, you're going to need a multi-meter with an ohm scale, but it's not that complicated to use. Some jumper leads will help, as well as a test light and maybe even some dielectric grease for your electrical connections. I’m also going to use this cool tool that I got from the LMC Truck catalog.
With the correct tools you want to give yourself a couple of hours to get this test done. So troubleshooting is a process of elimination. You're trying to figure out what doesn't work by confirming what does all along the chain of your system. And that chain starts with a 12-volt power source. We're using a bench top, so we're going to test our battery right here. Let's make sure our meter works first. And the way we do that is by confirming whether it's on a voltage setting, the right range setting, and you touch your leads together. If it zeros out, that tells me that both leads are connected properly and we've got communication. So now we can test the battery. Negative to negative, positive to positive. And I’m seeing 12.38 volts. We got a good hot battery. Now we can move on to test the gauges. We've confirmed that. Let's move forward.
We're going to test out a brand new gauge. But in with the gauge comes a note that explains that on the back side of the gauge what looks like the original style resistor that came with the original gauges is now just a spacer. They've upgraded to more modern electronics in this gauge, and the resistor is no longer necessary. So make sure that if you're switching to this gauge, you don't switch the resistor. Keep the spacer and you'll have a good accurate read for a long life. So we're also going to use a sending unit that I’ve confirmed that works. It's a brand new sending unit and we need to create a little false circuit in here to verify the gauge accuracy.
The next thing we're going to do is actually verify that electric signal moves the needle. That's pretty easy. On the gauge there's, a ground terminal at the bottom. Our ground is attached and our positive lead from the 12-volt battery goes to where the key-on circuit would be. We can see that the gauge is traveling, and it keeps on traveling. The reason it doesn't stop is because the gauge measures resistance that comes from the sending unit. With nothing hooked up, it'll just keep on moving to an over-full position. But we've affirmed that the gauge works and responds to an electrical circuit. It just is not complete without the sending unit factored in.
Now we're going to add the sending unit into the system. We'll get some communication. So we're going to add another ground, which is going to act like a chassis ground. Your gauge would be grounded to the chassis through the dash. Your fuel sending unit is grounded through the chassis ground with the tank mounts, so we'll complete that part of the circuit by adding the ground. The one last piece of the puzzle is connecting this terminal, which in the wiring system, goes to the sending unit terminal. We're going to replicate that with the yellow lead. And now we should see the gauge return back to its normal position. There it is, it reads empty.
With our wiring circuit complete, the gauge reads dead empty and that's because the float is at the bottom level where it would be in the tank sensing that there's no fuel in the tank. So now we're going to verify that the float actuates the gauge by simply moving the arm up. So we'll go to about the halfway point. We can see the gauge slowly traveling up just like it would in the vehicle. It should settle somewhere around the half full mark. And of course this depends on the fuel volume, and the shape of the tank, and all kinds of things, but we're pretty close to half right there. So to me that translates half a tank, halfway position on the sending arm. And we're going to go all the way up to the full position and just let it sit there. We can watch the gauge and it's going to stop at the full position. So we verified we've got good 12 volt power source. We verified that that power source moves the gauge needle. We've added in a known commodity here with a sending unit that works, that's accurate, and we've added that into the system. So we've literally made a mirror system of the vehicle sitting on a bench top. We know our gauge works, and double bonus we've verified that our sending unit works, and it's reasonably accurate. So now the next thing to troubleshoot will be the wiring between the dash and between the fuel tank, if you've still got a non-working fuel gauge.
So we know our new gauge works now we're going to test the old vintage gauge, which is still mounted in the cluster and still has the circuit board on the back side. So take a look at the pin configuration. It's exactly the same three pins. We've got a ground on the bottom, always. On my left is key-on voltage, and on my right is the sending unit lead. So what we're going to do is hook everything up, sending you to ground, battery ground or chassis ground, key-on voltage, and finally the sending unit. Reconnect the battery, and because the sending unit is in the down position or the lowest position, we're now at zero level on the fuel gauge. So we'll go through the test. Now we're out of gas. We'll put it to about the halfway point, half full, wait for the gauge to catch up and we've got about a half a tank. I’ll put it in the full, or the highest float position, and we're gonna see that the gauge is functional. It works. It looks horrible, but it actually works.
And just in case you've tested the gauge like this, but you still get a no-read situation, even though everything else looks like it's right, there's one more thing to check. And that's the actual printed circuit board on the back of this cluster. I’m going to lay this on its face, but I’m putting something underneath just to make sure that I don't lay the cluster down on the gauge needles and protect them. Now, right here, you can see where the circuit board makes a connection to the main chassis harness. Back here is the printed circuit board. It's a very simple printed circuit board, which is kind of neat because we can see all of these tracers and find out where they go. For instance, this guy here goes right to this light. That's for the 194 bulb that's in the back that illuminates at night when we've got our lights on. Simple to trace that. Goes right here. Sometimes what happens is that these can either break inside, in between the two layers of plastic, or the terminals themselves, that you can see right here. Sometimes they get corroded. So it's a simple troubleshooting procedure to just trace this guy. This is our sending unit lead, so we're going to take that one. We're going to clip him on there. And now this is our key-on 12 volts, goes right there. Make sure you don't touch the two alligator clips. And now, effectively what we've done is jumped the circuit from here to here. So now we'll flip the cluster over and we'll verify that our gauge works, which will tell us that the printed circuit board is working. Now, right away, our tank shows empty. That's a good sign. It's not over, or it's not under. So it's staying the same. So now we'll actuate the arm on our sending unit. Go to the middle position, and it's climbing up. Go to the complete full position and we'll make sure that it rests properly. But, essentially, that's another way of troubleshooting. Like I said, we're eliminating things that could possibly be wrong and interfere with a completed electrical circuit and this is not one of them. So we'll just keep on going and that tells me that the gauge works, the printed circuit board works although it's 40 years old. Incidentally, if you need another one, if your PC board is out of commission, you can get one with a tack, without a tack, or like we have with gauges without a tack. So regardless of what's gone wrong in your chain of electrical circuit, LMC Truck, well they've got the fix.
This is a fuel tank from a '71 C10 pickup. Here's the sending unit, and here's the stud that we've been connecting to the back side of the gauge to complete our circuit. Now the ground in this fuel tank comes from a chassis ground, which is the tank actually strapped down and bolted to the cap. So what we're going to do is pull this original gauge, inspect it, we're going to test it, and see if perhaps that maybe it needs replaced, if there's any reason to do that.
To remove the sending unit I’m going to use this tool that I got from the LMC Truck catalog, Now typically, you take a big flat screwdriver and a hammer and tap this ring out in a counterclockwise motion. Sometimes it's not the most graceful thing. This tool really make short work out of it. You make sure it's centered, and in the tabs, and it's just that simple. The retaining ring comes out and now we can pull our sending unit out.
You've got to be very careful when you're fishing out your sending unit so you don't tear anything up. Typically I’ll pull the sock out first, which is a nickname for the pre-filter, and then you can get your float, and it comes out like that.
The first thing I’m going to do is to test the sending unit. Now this works much the same as a dimmer switch in a house circuit. It measures resistance as the arm actuates, because of the float, because the level of fuel in the tank. It actually sits like this, and when the tank's full the floats all the way up. So we're going to test this with our multi-meter set on the ohm range, since we're testing resistance and not current itself. So we'll send the red lead to the contact that goes to the back side of the gauge, and we'll ground which replicates our chassis ground, and we'll take a look. So this sending unit is a 0 to 90 ohm range. So I want to make sure that I make this point. It's a range. It's not a precise measurement, or a precise instrument, so it's going to give us an idea of 0 to 90 ohms. 0 being the empty tank reading, 90 ohms, more resistance, being the full tank range. So we're going to go through the motion of the arm and it doesn't move at all. It went from 3.9 ohms, yeah it's all over the map. So we're not getting an effective circuit here. So there's something wrong, horribly, with this sending unit, other than how it works. But let's take a closer look.
The first thing that shows me that this needs to be replaced is the pre-filter. It's just toast. It's coming apart and that's going to do absolutely no good at all. The other thing, this is probably the reason that we didn't get a good reading, is because the isolator has come loose. This is rattling around and it's probably grounding. You can see it actually hitting the side and it's grounding out the circuit, so strike two. Strike three, there's a little bit of rust on the outside. If it's rusted on the outside, it's probably rusted on the inside of the pick-up tube. So not only does it not work, there's some other signs and other good reasons to replace your sending unit.
Now we're going to test our new sending unit. Red lead to the center stud, black lead replicates a ground, and chassis ground. And right away we can look at our meter, and it's three ohms. Now we've got a zero to 90 ohm range. It's not a precise measurement instrument. It's a range. So we've got three ohms at the base, at the negative, or empty position, and it goes all the way up to 98 ohms at the full position. That's very close. At the halfway point, which I can see in the range here we're about 55-ish. That tells me that we've got an accurate reading from this sending unit throughout the range of empty to full. Different gas gauges read differently and that's part of one of the reasons why, but that's going to give us a good accurate range, and a good reading, and a brand new sending unit that's ready to install into the tank.
Installing a sending unit is very straightforward. Resist the urge to use the original gasket just because it might be a little bit easier to put it in. Use the new gasket and it always goes in first. Now we'll go in with our float. Carefully follow that with the pre-filter, finds its happy home. The ears find the slots. We'll bring our locking ring. Line up the tabs in the tool. Give it some pressure, and it's in. Simple as that. If you verify that your gauge works, and that your sending unit works, and that all the connections around the instrument cluster are fine to the chassis harness, well the next thing that could go wrong is the connection between your gauge and your fuel sending unit.
Here's how to do a simple continuity test on wires. Our multi-meter has a continuity setting. You can hear that little tone. You can verify it zeros out on the gauge and it creates a tone. With them separated, the tone goes away. So you can test the connection of any given wire by connecting one end to the positive lead, one end to the negative lead. And now you know there's a good connection that travels all the way down this wire and all the way back up to this connection. You've just done a continuity test, and you can test each individual wire hitting between your gauge and your tank this way.
If you're feeling good about the function of your gauges but they're looking a little bit shabby here's a couple of tips. A soft bristle brush works fairly well on dust. If you find, like we do, that there's some more debris that needs cleaning, a clean microfiber and some glass cleaner. It does a nice job. I’m just cleaning up that gauge face. Be very careful around your needle. I don't recommend this, but we can move it just a tiny bit. Just be very careful, don't snag it on the microfiber. That's much better. Can't do much about the screen print on here if it's damaged a little bit. Ours is okay. It's pretty good.
LMC Truck offers this gauge needle paint. Make sure it's shaken up for a couple of minutes. Paint settles and it turns to gum on the bottom, so make sure that it rattles for a couple of minutes. The paint comes with an applicator, which makes it really convenient. But paint runs, so we want to be careful. Here's an easy way to mask off your needle you don't get any paint on the gauge face. One coat covers really well with this stuff. I like it. There we go. You let that dry a little bit. See if it needs another coat. That looks pretty dang good. That is restored.
Breaking down the fuel system into subsections that you can test makes diagnosing a faulty fuel gauge very simple and you don't have to spend a bunch of money on expensive tools. Chances are you probably already have them in your garage. Once you figure out what the problem is, well, then you can go to lmctruck.com and they'll send you whatever you need to restore or to repair your truck and keep it on the road.
Thanks for watching.