Fuel Gauge

[Jehu40]

My fuel gauge gauge has stopped reading correctly; it goes down to about 1/4, and at that point it runs out of fuel. I dropped the tank and tested the sender unit but it's working fine. Anyone with this expererience or ideas?

[saskak]

My fuel gauge gauge has stopped reading correctly; it goes down to about 1/4, and at that point it runs out of fuel. I dropped the tank and tested the sender unit but it's working fine. Anyone with this expererience or ideas?

When it runs our of fuel, does the light come on? Also, when you fill up, does the needle go beyond the usual upper position? If yes, then it could be that the needle slipped, but I would be surprised if that has happened, they are really tight on. You could take the needle off and re-position it (https://www.youtube.com/watch?v=wSVLBpksyBs) and check if it is moving without any resistance from the top to bottom. There could be something wrong with the motor as well at the back of this needle. Do the rest operate as intended, to rule out cluster problems.

[Jehu40]

Thanks Saskak. The light doesn't come on. I didn't think of the gauge slipping but that would explain it. Yes the other gauges are working fine.

[Jehu40]

Update: I plugged in the sender unit and the dial went up to nearly full, but not quite. Now the needle won't move from that position at all after moving the resistor about, and even with the sender unit unplugged. Definitely electrical?

Or could be needle sticking?

[saskak]

Update: I plugged in the sender unit and the dial went up to nearly full, but not quite. Now the needle won't move from that position at all after moving the resistor about, and even with the sender unit unplugged. Definitely electrical?

Or could be needle sticking?

It is fairly easy to remove the instrument cluster. Wait for BSI to shutdown (everything goes dark), pull towards you the upper cowling. Unplug the cable at the back and there is one bolt holding it. The front plastic cover of the instrument cluster is also easy to take off. Then you can move the fuel needle with a finger and see if it is sticking and moving freely. It's either needle sticking or faulty motor.

[Jehu40]

Many thanks again - that's great. On reflection the needle does return to zero when the ignition is off. One other thing, the sender unit receives 5.18 volts with the ignition on - I don't know if that has any significance.

I'm actually in the process of changing the back axle so I took the opportunity to drop the tank and check the sender unit when I was at it. I'll go into the instruments when I've finished that and give an update in a few days (that's how slow I am!)

[Gryffindor]

If it's still possible, open the sender unit up and check it for gunge or grit stopping free movement of its inner parts. Give it a clean out.

[oilyrag]

I haven't taken my pump unit out/apart, but on a couple of other senders I've played with (other cars) the warning light 'switch' , within the tank sender assembly , is independent of the fuel tank level sender. So it should be coming on even if the gauge is sticking at 1/4 full unless the sender float is for some reason failing to drop and trip the switch . That also might suggest that the dash gauge is not at fault.  
Unplugging the sender may give 'odd' results' - I've had a car where the gauge is also driven downwards, as well as up - presumably gives greater stability, but it was a 1964 USA car that managed 12mpg!

[Jehu40]

I checked that the sender unit was giving a linear resistance across its range. Also, the dashboard gauge did eventually respond, if unsatisfactorily. Before I took the sender unit out the gauge did work to some extent, but now it seems to e "freezing" in one position, as if it has a memory issue.

[Gryffindor]

Assuming the sender sends an analog signal (voltage level, 0-12V or whatever) to the BSI and then the BSI sends digital data to the gauge on the vanbus, I would guess that the analog signal has an intermittent connection or poor connection, resulting in sporadic voltage readings going to the BSI, and then (at a guess) the BSI is 'remembering the last good reading it got from the sender.

I could of course be very wrong.

To BSI experts: is it possible to identify the pins on the BSI which the sender is connected to, and then experiment by feeding in a variable voltage from a small 12V battery and potentiometer, to see how the BSI responds?