wtf low fuel light? And range. You too.

[E30Nova]

I usually drive my cars until the low fuel light comes on. Bad for the fuel pump, I know, but that's how I am. Moving on.

In my 84 325e, that worked fine because once the needle hit the red part of the gauge, the light would come on. But on my 88 325is, the needle gets to the bottom white line under the red part, and the light still isn't on. I've had the cluster out a few times, and while it was out I swapped the bulb with a known good one. Didn't help.

So last night I did some searching and found out how to test the bulb. So today I pulled the rear seat and jumped the sender and sure enough, the light came on.

So when the hell does the damn thing come on? Clearly it works but I'm not going to keep driving when the needle is already at the bottom of the gauge.

Also, what sends the info to the obc? When I had the needle at the bottom of the gauge, I hit the range button on my obc and it said 37 miles left. I kept driving and after about 5 miles it actually went up to 38 miles left.

Maybe my sender is bad, actually. I jumped one of the outer terminals with the middle terminal and the gauge went to 3/4 full. Or maybe my fuel gauge. I have some extra gauges. I might try swapping them in.

Matt

[jlevie]

The light will come on when there is approximately two gallons left in the right side of the tank. This car has two sensors (left and right sides of the tank) connected in series with the warning light on the right side. Fuel is lifted by a siphon operated by returning fuel from the left side of the tank to the right side where your 84 had a cross over tube to equalize fuel. A failing level sensor can result in the gauge not accurately reflecting fuel level.

Unlike your 84, this car has only a single high pressure pump mounted in the tank. That pump is cooled by fuel, so frequently running the tank below 1/4 can result in premature pump failure.

The range reported by the OBC is a function of the fuel level and recent consumption rate. Unless cruising at a constant speed what it reports for range should considered optimistic, at best.

[Jean]

What Jim said....but I wanted to add that the thin fuel level sensor has 3 pins on it. 1 is common/ground, 1 is for the gauge itself and one turns the light on/off.

It's possible that it's half way working.

[E30Nova]

Yeah, lately the gauge hasn't been reading full when I fill up the tank. When the fuel handle/pump thing clicks, the gauge reads between 3/4 and the big line under F on the gauge. If that makes sense.

So the pins on the sensor, if I jump the ground pin and the gauge pin, should the gauge read full?

I'm gonna try swapping a different gauge in tomorrow and see if that makes a difference.

And Jim, the light comes on when there's ~2 gallons left? The car manual says it's a 16.4 gallon tank, and when I fill up it usually takes 12-ish gallons. But that's always when the needle is at the bottom white line under the red. So something's definitely not right.

Matt

[Jean]

Not if you have TWO sensors as they are wired in series. If one sensor is bad then you'll get an off reading...both have to be in working condition. You can test each one with an ohm meter.

[E30Nova]

Problem is I don't know the specs. I didn't see any in the ETM and there's nothing in the Bentley that's specific about gauges. :(

Matt

[Jean]

about 5ohm to 70or so ohms is the range....the switch that turns the low fuel light on/off, is jsut that....it will be ON or OFF. You can turn the sensor upside down and hear the float inside of it move...you can measure it while it's moving etc.

[dnova89]

Wow I didn't even know e30s had low fuel lights LOL, obviously mine isn't working then... oh well.

[craiggroves91]

Mine hasnt worked yet either. Don't know if its cause i dont wait till its low enough or somethings wrong

[jlevie]

Yeah, lately the gauge hasn't been reading full when I fill up the tank. When the fuel handle/pump thing clicks, the gauge reads between 3/4 and the big line under F on the gauge. If that makes sense.

So the pins on the sensor, if I jump the ground pin and the gauge pin, should the gauge read full?

I'm gonna try swapping a different gauge in tomorrow and see if that makes a difference.

And Jim, the light comes on when there's ~2 gallons left? The car manual says it's a 16.4 gallon tank, and when I fill up it usually takes 12-ish gallons. But that's always when the needle is at the bottom white line under the red. So something's definitely not right.

The gage not going to full strongly suggests a level sensor problem. If you pull the sensors out of the tank, allow all of the fuel to drain out, and turn them upside down you'll hear the floats go to the top of the housings. At which point the gage should read full. It should also read full with both sensor shorted.

The sensors are at zero ohms for full and 58-60 ohms for empty. So a bit of work with an ohmmeter will tell is one (or both) of the sensors are producing valid readings.

The 2 gallon warning light point is what I've observed on a few tests. I don't know what the actual spec is, but it is probably close to that. The way I tested it was to drain all the fuel from the tank and add fuel one gallon at a time to see when the light went out.