Take the mystery out of GMC fuel gauges. How they work, and what to do when they stop working. This applies to many GM vehicles, though specifically we are addressing Square Body trucks, both GMC and Chevy, 1973-1987 model years.
Great video. Once I had gathered the basic idea of how to circuit works from your video, I was able to repair my non functioning gauge in just a few minutes. Thank you greatly
Thank you for explaining how everything works. It's really beneficial to have some insight on these symptoms. Even more helpful is that I'm experiencing the same symptoms you described at the very end of the video.
Hands down best video I've seen on fuel sending unit. I have 3 wires comming d From my passanger side so. Not exactly what I needed. But still very informative and helpful. Great job sir. Thumbs up
Alas, too many folks (and I fear the RU-vid algorithm) think that methodical details are dull. Not to worry, I plan on continuing with my (complete lack of) style!
Can’t thank you enough for posting this. 🙏🙏 This info that you provided is truly invaluable!! My truck does the same as what you mentioned that yours did, the fuel needle only goes up to about between 1/4 - 1/2 tank when my tank is full. So I’m assuming that my issue is also the resistor like you mentioned in your video. Thank you again for the video!!
hey I've seen a lot of you tube video's on square body fuel gauge but you did such a great job on this, I had to leave thumbs up & a comment.( first time ever) great job. five star.. I hope you earn a lot from this. thanks
Thank you very much for the repair tip. It helped me a lot with my 1984 GMC K 1500 Stepside. Two resistors for less than €1 and the display works like new. Greetings from Germany
I appreciate this, I want to get a original tach to go in place of the big fuel gauge on mine and put the small fuel gauge where the clock is, anyway if I find them I wish I could ship you my cluster for a rebuild. Thanks again! 👍🏼
You did a wonderful job in this video. I appreciate you for posting it. This is the most thorough explanation of how to address fuel gauge issues. I have a square body and detail is hard to find in repair videos. You need to make more! Don’t worry about how many views or subscribes, just put out the content and give it time. Following now!
Glad it was helpful! Thanks for taking the time to give me feedback. I do have lots of square body videos in the works, all from the perspective of an average guy with a realistic budget. I also have a "fleet" of vehicles that is ever-changing, which gives me the chance to shoot a lot of stuff. I'm trying to slow down and give details that many other channels seem to gloss over (no doubt my videos can be a bit dry in comparison). If only my editing skills were better, haha! Or at least faster - there is much more content in the pipeline, waiting for me to get it ready. More content soon, I promise.
@@hardscrabblegarage Looking forward to it. And don’t worry about the production-content is king for those who are serious. Sometimes I get tired of “slick” anyway. I’m an average guy with a square body who has no mechanic experience. I ordered some dash parts from LMC Truck and saved this video to guide me as I restore my cluster. Your video will help because my fuel gauge only goes up halfway even when the tank is full.
If you remove the clear cover, black plastic, and then screws holding the speedometer in you can remove the cable much easier(through the hole by the headlight switch). Once the speedometer is out the cluster is much easier to remove. When the cluster is back in place you can install the cable then turn the speedometer slightly to line it up with the holes.
Question on the connector. The brown wire off of the sending unit, does it go to the single wire on the back #7? Or #18 with the 2 brown wires as well coming out?
I've been study how gauges work on 1986- mid 90's I took apart a speedometer cluster just for study from a 86' olds delta 88 and the back of the cluster looks exactly what your showing. I had never seen anything like that . The harness plug fitting in that recessed slot, and just pushing against the ribbon wire circuit . I guess for the light bulbs. I've replace the 2005 pontiac speedometer and gauges steppers But I guess the older inst. clusters use some round thing, there not steppers and there not soldered. and prob. don't break like steppers either. I'm reading in my factory 90 bonneville service manual. that the gas gauge used, it say's the pointer on the fuel gauge is moved by magnetic fields , of two coil's. the coil's are at right angles to each other. battery voltage is applied to the to the "E" coil ( for empty I guess ) & the circuit divides at the opposite end of the coil . One path continues to gnd through the "F" coil. another goes to gnd through the variable resistor of the fuel gauge sender. when the tank is low the resistance of the sender is low. A large flow of current passes throu the "E" coil and the fuel gauge sender resistor this movesa the pointer towards "E" on the scale.when the tank is full the sender resistance is high. More current flow's through the "F" coil, movin the pointer towards "F"on the scale. Say's with the 2 coils operating the pointer, the gauge is not effected by the changes in the voltages of the system
Weird question but what the heck… I had my whole dash Cluster apart for a restoration on it and I happened to completely spin the fuel guage needle 360° around while wiping the guage off. Would this change the readings on the fuel level?
Good video I got a ? I filled my 85 Pontiac stays on full for 3 day and fuel level drops slowly I’d say bout 1/2 should the gas gauge move from half to full as the gas slushes around?
Great tips. Maybe you can help me out. I have an 1982 GMC Sierra Classic 1500. I can't switch to the passenger tank as of yesterday. I replaced the switch, nothing. I hooked up a new fuel tank selector valve, nothing. I filled up both tanks and the gauge works perfectly on the driver's side tank. I drove around the gauge drops as usual. I just can't switch to the right side tank. Any help would be appreciated. Thanks!
I have a 6.2 cucv d30 (k30 army) would you know if a civilian type cluster would be plug and play to get regular guages instead of idiot lights? I do know I have 2 alternators
I’ve got a 1976 Chevy Itasca on a p20 chasis. I can not find out what the green wire is. After I got a new alternator put it my clusters all spin crazily. Any advice?
I bought a ‘72 C10 many years ago. The seller told me occasionally the gas gauge quits working properly and he had a magnet stuck to the kick panel and said just take that magnet and touch it to the two leads on the back of the gauge and it will start working correctly. It does happen occasionally and the magnet touched to the back of the gauge does correct it.....Any idea on the theory that is taking place?
I am having an issue can't figure it out my fuel gauge at full goes lil past quarter tank but will go to empty I disconnected the wires at sending unit and it went to full grounded went to empty so replaced the sending unit still same problem gunna pull the fuel gauge today see if that's the problem but I don't have a meter now got damaged when I moved
Nice video. Pretty sure my sender is what's got my fuel gauge all screwed up on my 86 K2500. While mine has the same fuel gauge as yours, do you know if the smaller fuel gauge that takes the place of the clock in the clusters with a tachometer use a resistor like that in yours, or do they have a built in one? If so and it turns out mine needs a resistor I may just buy a second kit to convert my 86 over to a factory style tach if I can't find one 90 ohm one.
To the best of my knowledge, all the fuel gauges from this era used the resistors. I myself have handled a small (tach version) gauge with the resistor. Note that new, repop gauges probably won't need the resistor, as it is built-in. I've seen new ones with what looks like a dummy spacer where the resistor would be, but only in pictures. I've always been able to get the old factory gauges to work, knock on wood! Best of luck! I want to do a factory-look tach swap, too, which means sourcing a small fuel gauge.
@@hardscrabblegarage LMC sells an entire kit to swap it over to a factor style tach. Also I pulled out my gauges I got to go into the tach swap kit that I have for my 85 C30 dually and they all have a piece of plastic on the back of them that mimics the look of the resistor. I'm pretty much going to use the kit and build a completely new cluster for my 85, with the exception of the Choke/4 Wheel Drive and Brake/Seatbelt lamp lenses. Those will have to come from the old cluster.
I had two 1980 Corvettes and 181 and everything is exactly the same there the circuit boards other than my shift lever is in the console but I was surprised to see that the 85 mile an hour speedometer is also on the truck because that's all the Corvettes have. I sent one to ecklers and had to change to the 165 mph speedometer it came back with 7 miles on it for testing purposes. But trying to find the output gear for the speedometer cable it seems that the Corvette models have about 10 different ones you would have to know the exact ratio of that transmission and your rear end to make it work or it's going to be off I got one through Summit and it was about 10 to 15 miles an hour off. If you look it up they have a variety of colors of the little gear that's permanent in the transmission that's where the big difference is at. I tried buying different ones at O'Reilly's they have them they're very reasonable the 200 mph speedometer but you would have to buy an adapter cable from them to put it into the car they're not the same they won't bolt in and then you're going to have trouble with the transmission gear probably not worth your trouble
Well what I was mentioning was the Corvette and the replacement speedometers from O'Reilly's they hook up different the cable you'd have to buy a cable but I had an 82 Chevrolet pickup the turn signals were down in the front bumper the only year that it's that way four-wheel drive truck very nice I ended up putting a 454 in it didn't matter about the transmission I could milk the tires after the big engine went in it was modified a little with the RV cam Edelbrock manifold Edelbrock carburetor and a different kind of distributor and it had no smog on it at all the state of Nevada registered that halftime truck as an 8500 lb class truck for 5 years but is the emissions got higher harder rather they wouldn't do it so I sold it off to another County where they did not have emissions and still don't have emissions Nevada has 17 counties only two of them require emissions
Yes, some of the other gauges have these resistors on the back. Be aware! The battery/charge gauge in some trucks was an ammeter (current meter), and others are voltage meters. They are not interchangeable, and if one tries the wrong combo fireworks will ensue.
Technically they are wired in parallel, not series. If in series the 2 180ohm resistors would actually measure 360ohms. In parallel they come out to 90ohms. And no, resistors are not directional, like electrolytic capacitors, so it doesn't matter which direction you install it. You do need to put it on the same contacts that held the factory porcelain resistor.
I have a 1980 C10 Big 10 Silverado, None of my gauges are Working. I purchased a new printed circuit board, everything is grounded and working , new wiring harness, fuses .lights etc..., the gauges are 40yrs old. Do the ceramic resistors go bad ?And if they do. Will they cause the gauges not to work? Also do you know what the part number # for the case ceramic resistors. I would like to purchase some.please advise. Any info would be greatly appreciated. Thank you Robert. One more question, I call lmc and ask them if I purchased new gauges would they have resistors applied to the gauges. They didn't know if they did. Anyway do you know if they do? Thanks again.
Keep plugging away at it, you'll eventually get them working. Yes, the ceramic resistors can go bad. And yes, it is possible 40 year old gauges are bad, too! Now, they can also work fine... much depends on condition. Have they been kept dry? Now imagine a truck with a missing window, sitting in a junkyard, open to the elements... different entirely. No idea on the part numbers for the original resistors, sorry. You can Google GM gauge resistors and get many hits, but only a few are links to sellers, and they are often out of stock (which is why I made my own). You should be aware, many repop gauges don't need the resistors at all, it's built-in to the design of the new gauge. This is not always true, but it is often so. If you flip the cluster over and measure voltage on the pins of the gauge, but it does not even move? Likely a bad gauge. Are you sure about the ground being good? It is possible for all of the gauges to fail, but not that common (in the absence of exposure to the elements, flooding, etc.) Good luck!
@@hardscrabblegarage thanks for giving me the information I greatly appreciate any advice. I'll keep on working on gauges. Got a question? The pig tail that goes into the back of the instrument cluster with different color wires, the 3 black wires are ground wires,Correct? Now, I'm I suppose to ground one wire under the dash , too sheet metal and the other ones run it through the fire wall and then attach to the engine block.
The fuel gauge-specific ground mentioned in the video is located on the inner surface of the frame's c-channel, a bit forward of the gas tank. The wire connects the sending unit on one end, to a grounding bolt on the frame a short distance away. At least on 1984 models the wire is black, passes through a hole on the frame, and bolts to the inside surface of the frame. Just disconnect it and clean it up. Afterward you can use a DVM to check the resistance to ground from the ground terminal on the sending unit (one probe on the terminal, the other on any clean, bare-metal part of the frame).
Hello Like to know if you can help with the dash cluster swap I have done. I purchase a electric type speedo dash cluster that looks the same the older dash clusters. I have done a LS swap in my 1984 c10 chevy truck. The 2005 5.3 chevy engine with a 4L60 trans are electric speedo setups. No cable feed. I purchase a dash cluster out of a 1990 model truck that they only made these dash's from 1989to91in all new chevy trucks suburbans GM getting away from cable feed speedos in the 1990 models, and so on The a dash looks and works good . But I'm having a problem with the speedo wire connection. The transmission speedo only has to a two wires connection should be easy enough to plugin. I run them two wires to the back of the cluster plugin. Not sure if i should go to the PMC ECM first, because the way I have it hookup it's not working. Hope you can help. Thanks
OK, you have lots going on there! Unfortunately, any help I can give is limited for a few reasons. You say you have an aftermarket cluster of some kind... without knowing what one its a mystery to me. Even then, my knowledge of your particular aftermarket gauge cluster is going to be lacking. I would imagine that any reputable supplier will have a tech support line. I guarantee you are not the first person with an LS swap to use their gauges - just reach out to them! As far as the LS stuff goes, and using it with the 1990 cluster, the vehicle speed out (pin 50 of the green connector) is I think what you are looking for. I'm far from an expert on late model stuff, though. Take a look at lt1swap.com/2004vortec_pcm.htm
I have a 1974 shortbed stepside chevy x10 and put in a new float and sending unit. Cleaned the ground and when I turn the truck on the gas hand goes to the 3:00 position. It is not going back as long as the truck is on,after a while it will go back to almost 3/4 of a tank. What could be the problem? Oh when I pulled the gauge out someone had put 2 rubber caps on the terminals,why would they do that? Stumped,need advice.
When it goes to 3 o' clock it indicates there is likely an open circuit in the wires somewhere. As in a broken wire, bad connection, etc. Follow the signal wire from the float assembly all the way to the gauge, verifying each connection with a multimeter. Good luck!
I'm currently fighting the same issue. My K10 has dual tanks and recently goes past full to the 3 o'clock position. Did you ever figure out where your break was?
I'm not sure, but it could be the gauge itself. You might try taking it out, and squirting it with a cleaner called DeoxIT, the aerosol one. I use it on the volume pots of vintage stereo gear, so it should be safe. Don't get it on the plastic, or the face of the gauge, it might screw them right up! Spray the inside... if it's just dirty, that might help. Otherwise, maybe a new gauge. Hope this helps!
So something weird is going on, new sending unit, cleaned grounds, but gauge sits at overfull and if I turn on hazards it pulses the needle slightly. Also when I disconnect my sending unit wire from the tank my voltage light just gets more dull (dummy lights). Need help😂
Well, that really is strange! A couple of things: 1) Test just your meter, out of the truck. Apply the correct voltage... is the meter itself OK? 2) The thing with removing the sending wire affecting the voltage in the whole truck - it sounds almost as if it's trying to use the sending unit ground as the only ground point in the whole truck. With the lights dimming like that, I'd bet another ground is either dirty/rusty or missing. There should be one by the battery to the body, an engine grounding strap somewhere, and depending on model specifics maybe another one or two (gas tank sending unit, etc.). Check and clean all grounds! Also, watch the video again. If the meter sits overfull at 3 o'clock it tells you something.
@@hardscrabblegarage thank you man, I appreciate the time you took to write this! So it seems like the issue was there’s a ground wire on my filler neck that was missing, got a new filler neck and wire and now the gauge seems to be almost accurate. I know I have a tiny bit more than half tank and it’s saying little less than half. So now I’ll do what you said and check the resister that goes on the back of the gauge itself!
That could be bad wiring, a bad meter, or a bad sender. Do the troubleshooting like in the video, open the connection at the sender, then ground it out, and watch what the gauge does. Good luck!
@@hardscrabblegarage I bought another guage and then it decided to work before I put the new one in but still swings erratically sometimes if not full but I'm going to pull the cluster to change bulbs soon and I'll go ahead and swap it out .
Brother if you're going to give advice about resistors, you need to learn the difference between watts and ohms. Also, instead of "wiring up a couple in parallel", just use one of the correct resistance.
Uh, brother, I used to service fire control radar, troubleshooting to the component level. "Wiring up in parallel" two equal resistors is exactly the same as using a single resistor of half the value. As in EXACTLY the same. Total resistance = 1/R1 + 1/R2 ... +1/Rn. This is like the first thing you learn in a basic electricity class... but thanks for your input.
