How to Diagnose a Non-Functional Fuel Gauge | Kevin Tetz with LMC Truck

[Music] [Applause] hi i'm kevin tates 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 azure 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 multimeter 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 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 kion 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 attack without attack or like we have with gauges without attack 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 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 multimeter 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 pickup 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 into 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 multimeter 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 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 you

Channel: LMCTruck

Views: 71,912

Rating: 4.8663483 out of 5

Keywords: LMC Truck, truck restoration, Kevin Tetz, troubleshooting, installing, fuel gauge, sending unit, replace sending unit, how to install and test gauges, dash gauge testing, fuel system troubleshooting

Id: Bq3YfCLq4So

Length: 19min 20sec (1160 seconds)

Published: Thu Oct 08 2020