Fuel gauge question

[91merc]

Can somebody tell me how exactly the fuel gauge works on an 85 MGM? I know that it works by the float system but for some reason the gauge takes about 100 miles to go from F to 3/4, but only takes 20 miles to go from 3/4 to 1/2 tank. Can someone explain this?

[friskyfrankie]

Could be the float sticking and not moving slowly to register properly.

[Arquemann]

Mine does kinda the same, first half lasts way longer than the second. Also I got like 5/4th on the gauge since full tank is like a quarter beyond F
Also when it hits E there's like 20 liters left in the tank. My sending unit (and tank generally) is clean, so I doubt there's too much stickiness in the float.

[91merc]

Also does anyone know why on these cars the gas pump always clicks off 2 gallons short of being full?

[Tiggie]

The wagon tanks are skinny in the middle and that causes this issue. Sedans are more rectangular, so in theory wouldn't be as much of a problem. Could be a problem with the circuit board. Here's what it looks like:



I've questioned whether or not the angle caused by high or low fuel levels would change the speed of the arm movement but can't seem to come to a conclusion that would cause it.

[gadget73]

Gauges are also usually calibrated to do this on purpose. Its probably anecdotal but I read something about an 80s car with digital dash, maybe a Caddy, but Lincolns worked the same way. The gauge read accurately, so when it was 1 gallon down from full it said so. People complained about poor fuel mileage, even though it was fine. The next year the gauge wouldn't start ticking down until it was 2 gallons down from F and suddenly the fuel economy complaints were "fixed". I don't know if thats BS, but I can tell you my Continental reads F when full, and the next step down is 20 gallons. Its a 22 gallon fuel tank and never indicates 21 gallons. That gauge is DOB accurate too, when it died at the fuel pump claiming 3 gallons, it took 18.6 to fill.

The Towncar will go an easy 50 miles before the gauge drops from the corner down to F. 1/2 tank takes 10 gallons, its an 18 gallon tank. E is absolutely E though, none of that 'I got another 25 miles' crap in that one. You can re-calibrate it if it bugs you too much, just bend the arm on the float a bit.

assuming the sending unit isn't just worn out, which is entirely possible. The IVR, Instrument Voltage Regulator, also has some effect on all this. Its a mechanical thing, basically works just like a blinker stat. It provides pulsed voltage that averages out somewhere around 5 volts i think. The gauge itself also works on heat, which allows it to respond slowly rather than going bonkers every time the fuel sloshes in a turn or bump. The current flow through the sending unit changes how hot the gauge gets, and thats how the fuel level is indicated. If the IVR is whacky because its 40 years old and tired of pulsing on and off the voltage gets off and the gauge reads weird too. The IVR is that metal can thing on the back of the cluster. Modern solid state replacements exist that just plug in to the original location. They do away with the mechanical pulsed contacts thing and just use a modern solid state regulator that provides stable output voltage. Can also DIY one with something like an LM7805 and a small capacitor.

[91merc]

Usually I can squeeze in about 10 gallons at a half tank, if these cars have an 18 gallon tank would that mean the gauge is off? The pump usually shuts off around 8.5 gallons and I’ll squeeze an extra 2 or so in it

[gadget73]

Mine is dead on 10 gallons from 1/2 to full. Yes its an 18 gallon tank. I suppose if you really wanted to you could tweak the arm to change the gauge calibration, but honestly if its consistent I'd just leave it alone and just be mindful of when you need to put gas in the tank.

[sly]

It's a bit of a curve and not a linear sweep, so yeah. It's HORRIBLE in my Expedition. A hair above 1/4 tank on the gauge is actually half a tank in fuel. The gauge races down to 1/4 and then takes forever to get to empty. My vics on the other hand do about the same as your car. it's about 55/45 top half, bottom half.

[91merc]

The IVR, Instrument Voltage Regulator, also has some effect on all this. Its a mechanical thing, basically works just like a blinker stat. It provides pulsed voltage that averages out somewhere around 5 volts i think. The gauge itself also works on heat, which allows it to respond slowly rather than going bonkers every time the fuel sloshes in a turn or bump. The current flow through the sending unit changes how hot the gauge gets, and thats how the fuel level is indicated. If the IVR is whacky because its 40 years old and tired of pulsing on and off the voltage gets off and the gauge reads weird too. The IVR is that metal can thing on the back of the cluster. Modern solid state replacements exist that just plug in to the original location. They do away with the mechanical pulsed contacts thing and just use a modern solid state regulator that provides stable output voltage. Can also DIY one with something like an LM7805 and a small capacitor.

is this why sometimes the fuel reading will be slightly different in the morning than it was when I park it the night before?

[sly]

Yes. And the anti-slosh module. As for bodging in a 7805, YMMV. Might need a heat sink bolted on. I don't remember what the current draw is on all that... I don't think it's too much for a standard 7805, but I'm not sure it'll run cool either.

[Arquemann]

Btw, Dennis Carpenter sells a modern IVR for "77-86 Ford Trucks & Cars" for a whopping 24 dollaridoos.
Dunno if it fits but they are available.