1983 thundercraft 470 merc 170hp fuel gauge & sending unit question

[fmalott]

i bought a new sending unit because the old was no good to verify that i benched tested the old one and no reading benched tested the new one worked good. i then i tested the fuel gauge the way the manual says to do it and it worked good. so one would think everything should work but it didn't as soon as i attached the positive and the ground to sending unit the fuel gauge went right to full and i know for a fact the tank is only a 1/4 full so i pulled the sending unit out and tested it out of tank and same thing i attached the positive and the negative and the gauge went right to full, i then lifted the float to see what the needle would do and the needle moved farther ahead about a 1/8" as long as the wires are attached it stays at full. i checked all the wiring and everything seems to be good i have never touched the wires and it don't look like the previous owner did any wire work. So any input would be nice

 

[Silvertip]

Re: 1983 thundercraft 470 merc 170hp fuel gauge & sending unit question

Please use the SEARCH function in this forum. Use the search terms FUEL GAUGE PROBLEM. There are about 60 or more posts dealing with problem and nearly all of them have a diagram of the sytem. The gauge works because you tested it. The sender reistance measures right because you tested it. That leaves a short in the sender wire (pink) to ground or some other wiring error. Follow the wiring in the schematic and it will work.

 

[fmalott]

Re: 1983 thundercraft 470 merc 170hp fuel gauge & sending unit question

Please use the SEARCH function in this forum. Use the search terms FUEL GAUGE PROBLEM. There are about 60 or more posts dealing with problem and nearly all of them have a diagram of the sytem. The gauge works because you tested it. The sender reistance measures right because you tested it. That leaves a short in the sender wire (pink) to ground or some other wiring error. Follow the wiring in the schematic and it will work.

here is where i'm at. i did a continuity test on the wiring from the gauge to the sender i took both ends of the positve ohms went to 0 took both ends from ground ohms went to 0 the wiring is not the issue. i made sure the tank was grounded i did an ohms test it showed the proper level of fuel from the tank ground or the sending unit ground so not the issue. getting back to the gauges i did a continuity test on every wire on all gauges(took me a few hours) ohms went to 0 so there is no short coming from all gauges. now the fuel gauge did something strange (i think!) when i was doing further test i did the normal test unhooked the "S" wire and grounded the "S" terminal out and it pegged out to full just like it's suppose to. so moving on to other test i attached the positive & ground to the sending unit and i did the ohms test on the ends of the wires that attached to gauge (but not attached to gauge )and the fuel level showed 120 that's where i figured the feul level should be so i attached the positive wire to the gauge's "S" and ground to a good ground and tested again (ignition off) and the ohms drop to 58 and when i turned the ignition on there was no reading at all from the "S" treminal. is this suppose to happen to the ohms reading when i did the last test and is the fuel gauge no good even though it passed the test grounding out the "S" terminal or is the ignition switch no good?

 

[fmalott]

Re: 1983 thundercraft 470 merc 170hp fuel gauge & sending unit question

i have been looking at gauges some that are for sale say 90 ohms empty 10 ohms full then others say 30 ohms full 230 empty. how do i know which gauge my boat has is there a test for that

 

[Silvertip]

Re: 1983 thundercraft 470 merc 170hp fuel gauge & sending unit question

A lesson in RESISTANCE (ohms) measurement. You NEVER test resistance on a powered circuit. That's what a voltmeter is for. Your sender has a resistance range of 33- 240 ohms. The gaujge must match it. I'm also betting that your testing is flawed leading you to believe circuits are ok when they are not. You tested the gauge and it works. Why replace it. You tested the sender and said it was ok. That leaves wiring. So one of two things is going on: 1) incorrect testing, or 2) bad wiring.

 

[fmalott]

Re: 1983 thundercraft 470 merc 170hp fuel gauge & sending unit question

A lesson in RESISTANCE (ohms) measurement. You NEVER test resistance on a powered circuit. That's what a voltmeter is for. Your sender has a resistance range of 33- 240 ohms. The gaujge must match it. I'm also betting that your testing is flawed leading you to believe circuits are ok when they are not. You tested the gauge and it works. Why replace it. You tested the sender and said it was ok. That leaves wiring. So one of two things is going on: 1) incorrect testing, or 2) bad wiring.

i hear what your saying i'm just not understanding the resistance when i tested the sender i unhooked the pink wire from the gauge S terminal and unhooked the ground when i tested the resistance it read at the end of those wires it read 120 ohms that's a good reading i figure that's the fuel level as soon as i hooked the pink wire back to the S terminal and hooked up the ground i tested the resistance and it dropped to 58 ohms and when i turned the ignition switch on there was no resistance that's the part i'm not understanding is this suppose to happen