tegra1027 Posted March 26 Author Share Posted March 26 7 hours ago, mowerman said: Hell, I’m scratching my head trying to figure out how you got so much accomplished and so little time. Great progress thanks for sharing. Truck Looks amazing.bob … Thank you! I have a team here, so it's not just me! It has definitely been a lot of work though! 15 hours ago, mechohaulic said: both wires should be a ground circuit . positive feed will be at the gauge . ground in == ground out to gauge. as the fuel float in tank raises and lowers the rheostat inside on the sender changes "continuity"= short version with wrong terminology, 12-volt ground (black wire) going in/ on sender may be 8 volts out sender (red wire) resulting in gauge not fully grounded and show half tank. taking the red wire to frame providing it does NOT have power and does show as a ground the fuel gauge should jump to full. KEEP IN MIND THIS IS OLD SCHOOL system!! can NOT say it's the same on all this modern computer stuff. I'm trying to put two added brake lights on my 2021 jeep gladiator and I find out it has to be calibrated at the computer. 3 simple tiny wires . checking system having rs light off with my lawn tractor battery ; think two wires touched. now there are only 3 of 4 led lights working on right side and the dash keeps telling me signal light out. point is staying on topic todays electronics are all screwed up. I couldn't agree more! If you think that is bad, do some electrical diagnostics on an RV... That's some complicated stuff. Thank you for your help! 17 hours ago, JoeH said: Wire it up black to ground, red to sending unit and see what happens. Sounds sketchy,,, I like it. Quote Link to comment Share on other sites More sharing options...
mowerman Posted March 26 Share Posted March 26 lol.....gotcha,,,,takes most of us years,,,just to find time to start anything,,,,we did have a girl here,,,while back,,went thru a B model,entirely,in like a year or so complete restoration,,,,we were all floored,,,but she had husband and son,helping out...bob 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 26 Share Posted March 26 I'm not sure if this is the same topic I responded to in another thread, but grounding to the tank may not work.. the tank is insulated by the rubber at the mounts and under the straps.. 2 Quote Link to comment Share on other sites More sharing options...
tegra1027 Posted March 26 Author Share Posted March 26 7 minutes ago, Joey Mack said: I'm not sure if this is the same topic I responded to in another thread, but grounding to the tank may not work.. the tank is insulated by the rubber at the mounts and under the straps.. So the black wire that is coming from the truck should be tied into the gauge, correct? Should I ground that to the truck somewhere? Quote Link to comment Share on other sites More sharing options...
JoeH Posted March 26 Share Posted March 26 If I read @mechohaulic right, it's not supposed to ground at all. One wire is 12v "hot" coming in, then it goes through the sending unit where the resistance changes based on fuel level, which drops voltage down. The other wire goes back to the gauge with "weakened" voltage, then the circuit grounds *after the gauge*. 1 1 Quote Link to comment Share on other sites More sharing options...
JoeH Posted March 26 Share Posted March 26 One wire gets mounted to a screw on the sending unit, the other gets mounted to the terminal on the . middle of the sending unit. 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 26 Share Posted March 26 JoeH,, he has the updated sender,, it is sealed plastic, and not made the same way as the one in your MR. Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 26 Share Posted March 26 3 hours ago, Joey Mack said: I'm not sure if this is the same topic I responded to in another thread, but grounding to the tank may not work.. the tank is insulated by the rubber at the mounts and under the straps.. the reason for separate ground on the sender is because of the rubber straps which will cause the tank to loose "common " truck ground IN MANY CASES. the center post on old school electronics was technically a ground wire to gauge. in days of gas driven units it wasn't always the best idea to have a live 6/12 volt power to a gas/diesel tank. think of a scale model choo choo train in a house. as the operator turns the control =basically a rotating reostat switch the choo choo goes faster and faster and faster;; still with me?? now as the fuel in the tank of a truck lowers the floating sender lowers also. this causes the center post on sender to have lower ground voltage. lower ground to the gauge causes the needle to read lower. if your headlights have a bad ground don't the light get dimmer????? using a meter on the head light with bad ground, will you not get a lower then 12 volt read out??? the fuel gauge gets a lower ground reading causing it to fluctuate. I did say earlier it was a short version using improper technology. what do you expect from an ole man explaining old school electronics. Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 26 Share Posted March 26 don't forget there was a common fix for a fuel tank gauge== it is called a clean stick behind the seat. also very accurate. 2 Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 26 Share Posted March 26 3 hours ago, JoeH said: If I read @mechohaulic right, it's not supposed to ground at all. One wire is 12v "hot" coming in, then it goes through the sending unit where the resistance changes based on fuel level, which drops voltage down. The other wire goes back to the gauge with "weakened" voltage, then the circuit grounds *after the gauge*. both wires were ground wires which is why original post stated the two wires checked out as ground. one wire (black ) would be a common ground going to sender mounting. second wire (red) is also a ground but going to the fuel gauge independent of chassis ground. it regulates the ground circuit going to gauge. the "hot "live wire is connected to gauge in dash cluster. sorry for confusion. Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 26 Share Posted March 26 1 hour ago, Joey Mack said: JoeH,, he has the updated sender,, it is sealed plastic, and not made the same way as the one in your MR. there's those two dreaded words :::"updated and plastic". purposely stated explanation given was based on old school way before the update and plastic became common. old school didn't need modules/ resistor/ computers and others. Things worked fine TOO. Quote Link to comment Share on other sites More sharing options...
JoeH Posted March 26 Share Posted March 26 My 1979 r686st. Hard to see what's going on but black from the truck harness grounds to screw, orange from pigtail mates to same screw. Other wire on pigtail butt connected to other wire from truck harness. My 1995 RD688S, has a type with 2 screw terminals. Quote Link to comment Share on other sites More sharing options...
JoeH Posted March 26 Share Posted March 26 Better angle of the 1979. 1 Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 27 Share Posted March 27 checking the wires in all pictures no matter what style plug, they all should still be ground wires meaning none have positive voltage. picture having a sender with two terminals would still be just grounds. newer "updated" unit could be made cheaper eliminating second terminal putting chassis ground wire on one of the mounting screws. one version had a spade terminal on the sender housing also. picture of '79 basically a push on plug having one wire from plug to sender ground other wire going into harness would go to gauge 1 Quote Link to comment Share on other sites More sharing options...
JoeH Posted March 27 Share Posted March 27 I think I have at least 3 different style sending units on my fleet. 1 Quote Link to comment Share on other sites More sharing options...
Joey Mack Posted March 27 Share Posted March 27 highly possible. I think there is even one that is just a straight stick with a sensor in the stick.. I prefer the ones with the arm and potetchiometer... Sorry about my spelling.. Quote Link to comment Share on other sites More sharing options...
AlleyKat Posted March 28 Share Posted March 28 I think Mechohaulic is on the right path here. I believe it is a "Variable Resistance Ground" system. The better the Ground, the higher the gauge reads. I am assuming the truck has a separate gauge that is not part of a instrument panel with a circuit board. I am not familiar with Superliners. Would love to have one someday though. I had similar problems on my "93 CH613. This is what I did to troubleshoot the system. It sounds like you did a continuity test on the wiring from the gauge to the sending unit. If you have "0" Ohms resistance, the wiring is good. The gauge likely has three pins on it. Power, Ground, and Signal. The Signal pin would be the wire returning from the sending unit. To test the gauge, remove the gauge from the dash and connect 12v to the Power pin and ground to the Ground pin. Then momentarily jumper the Signal pin to the Ground pin and the gauge should read Full. If not, you likely have a bad gauge. To test the gauge and wiring, reconnect the gauge and disconnect the plug from the sending unit and momentarily connect a jumper across the two pins, the gauge should read full. This means the wiring and gauge is good. To test the sending unit, reconnect the plug and physically move the float up and down, the gauge should respond accordingly. I removed the sending unit from the tank, it was easier for me to work on it. If all of those check out and it doesn't read correctly, I suspect there may be a mismatch between the sending unit and the gauge. My fuel gauge only reads to 5/8 full on a full tank. Sending unit from Mack and Fuel Gauge from PAI. I suspect there is a mismatch or calibration issue going on. I was lucky enough to get a complete wiring diagram from my local Mack dealer, it was very useful in tracking all of this down in the truck. Hope this helps and Good Luck. The truck looks awesome!! 1 Quote Link to comment Share on other sites More sharing options...
Popular Post Joey Mack Posted March 28 Popular Post Share Posted March 28 You can adjust the sending unit. If it reads too low, bend the arm down a little, too high,, bend the arm up a little, put it back in and check it. It's important to make sure the sender is clocked right. So match mark it before removal 3 Quote Link to comment Share on other sites More sharing options...
tegra1027 Posted March 28 Author Share Posted March 28 Thank you everyone for your help and discussion on the sending unit. @JoeH thank you for the pictures! That helps a lot. For the hood hinge, we replaced all the steel except for the hinge itself. We did sandblast and refinish it however. We decided to basically recreate what was already there. I'm not thrilled with the gap between the hood and the cab, but it isn't terrible. We adjusted as much as possible in the ball joints, but the condenser was our limiting factor. We wanted to keep the reinforcement for the fenders so they didn't crack. The only thing we decided to do differently was to keep the steel exposed vs encapsulating it in the fiberglass. I had a picture before the hood was installed but I can not find it. Sorry Quote Link to comment Share on other sites More sharing options...
tegra1027 Posted March 28 Author Share Posted March 28 2 Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 28 Share Posted March 28 as stated by one of the BMT master Techs :: the sender must be clocked right . have seen gauges which stopped mid way due to sender installed at wrong position, larger barrel tanks were good for this. float will go to empty spot= float up to just above 1/2, higher then 1/2the float gets stuck internally on inside barrel . adjustable senders were made too long to reach bottom then install with float angle side to side not front to back. 1 Quote Link to comment Share on other sites More sharing options...
mechohaulic Posted March 28 Share Posted March 28 On 3/27/2024 at 4:27 PM, Joey Mack said: highly possible. I think there is even one that is just a straight stick with a sensor in the stick.. I prefer the ones with the arm and potetchiometer... Sorry about my spelling.. ok !! now what the heck is a potetchiometer??? don't be using these big words on us old people. LOL 1 1 Quote Link to comment Share on other sites More sharing options...
HarryS Posted March 29 Share Posted March 29 16 hours ago, mechohaulic said: ok !! now what the heck is a potetchiometer??? don't be using these big words on us old people. LOL I'll confess to having to google. Interesting to know. 1 Quote Link to comment Share on other sites More sharing options...
mowerman Posted March 30 Share Posted March 30 On 3/29/2024 at 7:57 AM, HarryS said: I'll confess to having to google. Interesting to know. I use Google all the time especially YouTube I had to watch a video with two real young kids to figure out how to hook up the linkage on my mini bike ha yes, the list goes on…. Bob 1 Quote Link to comment Share on other sites More sharing options...
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