My fuel gauge is off by about 1/4 tank. When I fill the tank up, it shows as 3/4 full. When the low fuel light goes on, I only get as much as 14 gallons in the tank.
I bought a spare on eBay and it’s actually not working at all…oh well…
Is there anyone that knows how to diagnose these things? I opened one up in hopes of finding a bad solder joint, but all “looks” good to the eye.
That’s all? 1/4 tank. ON my XJ6 one take reads full and the other reads empty. The weird thing, is that I’ve put only a couple gallons in the tank, to get it running, now I want the tanks dry so I can take them out. The car just won’t run out of gas!
generic gauge works
The sending unit is a Potentiometer of sorts. Not knowing what the orientation of the sending unit is on the XJ-S (less resistance means full or the other way around) , but when fully extended one direction will have more resistance (bust out the ohm meter) than fully extended the opposite direction. Which ever has the least amount of resistance should be it equates to when full or empty - again it all depends on the orientation of the gauge. if your gauge is reading 25% less fuel than what is actually in the tank, then:
LGO is wire color Light Green Orange
Sorry to be so vague. but in a nutshell that’s how they work.
Hope this helps some.
Your post doesn’t say what year/body type you have.
I pulled the sender out of my 94 4.0 tank which worked great.
Let me know offline if that will help you.
Steve 650.455.1110
Thanks, however I am not dropping my tank for this problem. That is a ton of work for a minor issue such as this.
Now when the gas pump goes…
I am pretty certain that the tank can stay in the car to change the sender.
You’ll just need to remove enough gas to do it.
Then you can remove the back panel.
Sender is behind the round port.
Unscrews after some tapping CCW.
I thought the way to bench test an anti slosh module was to lay it on the bench and hit it with a hammer.
No need to drain tank. My 94 coupe has the module mounted on the left side of the trunk behind the panel. A twenty minute fix.
Gordon
I replaced the sender, no change.
The gauge has gotten worse. When full to about 13 gallons burned, the needle goes from all the way past “F”, to all the way pegged at E.
I think I have to get behind the dash.
Did you ever replace the anti slosh module? Can’t tell from the thread. Seems to me it would be a quicker possible fix than pulling the dash apart.
Gordon
I never changed it. I jumped it and nothing changed. I figured it wasn’t the module…
I’m gonna try jumping it again. Green to green/orange, I believe?
I tested and repaired my anti slosh module
a few things can go wrong in this, but mostly the 2 “big” capacitors (they loose some capacity or can short out) and/or the transistor
if the gauge reads too low, and this gets worse with time, it can be the said antislosh module, or contact corrosion to the dial
first test is to disconnect the green sensor wire and short it to ground, then start the engine : the gauge should rise quickly above the max level
then disconnect the ground from the green wire : the gauge should drop to empty and the
low fuel warning light show on the dash
I thought bench testing of the anti-slosh module meant leaving it on the bench and going for a drive without it.
I would consider the tank sender unit to be a suspect as well. They’re not too expensive. The catch 22 is that you need to know that the tank is nearly empty before you can change it out. My sender unit failed and was always reading empty.
checking the gauge full displacement is easy :
shorting the green sensor to ground should move the needle above the “full” mark
adding a resistor of about 250ohms should have the needle on “empty”
a resistor of a few ohms should point the needle on “full”
you can do the same testing at the module connector, getting the module out of the equation
yes : green and green+orange
or if you have doubts : central pin and the only pin @ 90°
I swapped out the module with one I had on hand. No difference.
I jumped the two pins. No difference.
I don’t think the module has anything to do with this issue…
I swapped out the float assembly with a new one, and I think I’ve successfully eliminated the module.
Nothing left but to get behind the dash and see what might have gone wrong.
My FIL once asked me to help him with the fuel gauge on his Chevy. IIRC, when empty it read empty, when full it read 1/4 tank. He had pulled everything apart, carefully measured ohmages everywhere (he was an EE). Of note, the gauge itself had something across it that looked kinda like that thing in the picture with the 55 and 75 on it. I told him Chevy doesn’t put things on cars for nuthin’, that thing was supposed to do something. Taken off so it could be checked separately, though, it read open circuit. So we drew up the schematic of the circuit, and after pondering on it a while we figured out that if that little bridge was a resistor it would make the gauge read more full when it was full. He said “How do we know what the resistance should be?” and I answered “You’re the EE here! Figure it out!”
So he sequestered himself in his room for a while and eventually came out with several pages of squiggles and announced that he thought that resistor should be 80 ohms. So I bustled him into the car and we drove down to Radio Shack to look over their selection of resistors. Of course, nothing in 80 ohm, had to buy a couple that could be combined. Got big ceramic things, as we had no idea how much wattage it would need. Went back to the house, attached these cobbled-together resistors onto the gauge, and reassembled everything. Gauge worked perfectly.
He couldn’t leave that alone, of course. Sat out in his garage and spent an hour carefully scraping the coating off that bridge piece we had taken off. Under the protective coating, he found: a resistor with a broken lead. Checked the resistance bypassing the lead, and it checked out to be: 79 ohms.
Same thing here? No idea. But if I were you, I’d take that 55/75 thing off and check it. And ask someone else here to do the same with theirs. See what we learn.
Post 55 to Post 75 : 124.3 ohms (consistent)
I wish I had done the same with the new gauge.
Other readings of note
Post 55 to Copper : 283ohms
Post 55 to Silver : 252.7 ohms
Post 75 to Copper : 153.5 ohms
Post 75 to Silver : 128.8 ohms
The above readings (excluding Post 55 to Post 75) are wildly inconsistent though. Every time I reconnect on a different are of the same post, I get different readings.
Copper Post to Silver Post : 60 ohms (consistent)
