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Old 21st May 2022, 08:57   #1
Sinc1964
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Default Fuel Gauge Stuck

Quick question,

My fuel gauge never moves, even when the ignition is off or battery disconnected.
It always shows 1/3 full.

If it never moves with the power off, does that indicate that the gauge itself is faulty...?

I have had fuel gauges that have stuck due to issues with the sender units, but thay have usually returned back to zero when the power was off.
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Old 21st May 2022, 09:31   #2
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Probably the slider in the tank has jammed due to wear of the segments.
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Old 21st May 2022, 10:19   #3
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Quote:
Originally Posted by COLVERT View Post
Probably the slider in the tank has jammed due to wear of the segments.

The OP says:

...............My fuel gauge never moves, even when the ignition is off or battery disconnected.

It always shows 1/3 full.

If it never moves with the power off,.........


Looks like the stepper motor is faulty, with ignition off or battery disconnected it should fall to zero.

When new motors are fitted they sometimes need the needles to be aligned, however if it is stuck in one spot when power is removed and never moves from the 1/3 full no matter what the fuel level is, it looks like the motor is Kaput.

You can go into the test mode, instrument cluster test, to cycle all the instruments, if it stays where it is it is pretty conclusive.




See here for entering test mode, it has to be done exactly as stated:


https://www.the75andztclub.co.uk/for...ad.php?t=55585
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Old 21st May 2022, 17:28   #4
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However if it's jammed, as I mentioned, it's never going to move from where it is now. With the ignition on the gauge on the dash will always read the same.---Taking it out is really the only way to find the fault.-----

Last edited by COLVERT; 28th May 2022 at 12:22..
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Old 21st May 2022, 23:13   #5
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Quote:
Originally Posted by COLVERT View Post
However if it's jammed, as I mentioned, it's never going to move from where it is now. The gauge on the dash will always read the same.---Taking it out is really the only way, really, to find the fault.-----

Nope wrong, sorry !



No matter where the float assembly is, as soon as the ignition is turned off the needle should drop to zero, the stepper motor always returns to zero,its park position, likewise with the rev counter and speedo !


Rather than faffing about dragging the pump / float assembly out to check, the easiest and quickest way to initially diagnose if it is the gauge the dial or other problems is, as I mentioned earlier, to use the onboard diagnostics to cycle all the dials from rest position to full scale, if it passes this, the fault may be somewhere else.


These modern stepper motor instruments work nothing like the old magnetic ones, and even with those, if the float was stuck at say, half full, the gauge would rise to half full with ignition on, regardless of the actual level, due to the current from the resistive path at that point, providing the magnetic field to pull the needle round to that position and hold it there.


However, once the ignition is turned off, there is no current flowing through the resistive path, the magnetic field collapses and the needle is returned to its rest / park position by the hair spring.


Unlike the old gauges, there is no direct connection between the tank sender and the gauge, though both use the same info, varying voltage, in the new systems this voltage is processed and is then sent to the stepper motor to move the needle to the correct position via x number of increments of the motor.
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Old 21st May 2022, 23:21   #6
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Cheers, really appreciate that. It got me puzzled when I disconnected the battery when replacing the alternator, and noticed the fuel gauge never returned to zero.
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Old 22nd May 2022, 07:44   #7
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Karl; let us know how you get on with the instrument pack test mode won't you. This is a very unusual condition.

I'd also like to acknowledge Ducati750cc's clear and comprehensive explanation of gauge operation!

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Old 22nd May 2022, 09:49   #8
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Hope to get chance this week. If the actual fuel gauge is at fault, I assume its not as simple as a straight swap with a working set of clocks.....old skool style...
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Old 22nd May 2022, 09:56   #9
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Quote:
Originally Posted by Sinc1964 View Post
If the actual fuel gauge is at fault, I assume its not as simple as a straight swap with a working set of clocks.....old skool style...
Correct. The car details incl Vin number, mileage etc are stored in several locations including the eeprom (chip) in the clocks. You can either swap the eeprom from the old clocks to the replacement ones if you are good with a soldering iron or involve a T4/TOAF. The replacement clocks would have to be "virginised" (all data removed) before they can be reprogrammed.
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Old 3rd June 2022, 10:06   #10
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Finally managed to do the diagnostics check....so thanks to the post regarding how to do it.

When performing the diagnostics 2.0, all warning lights illuminate etc, all gauges do the correct sweep....EXCEPT THE FUEL GAUGE !

The fuel gauge warning light comes on and off, but the fuel gauge (which is currently stuck showing approximately 1/3 full), moves about 1mm up and then back 1mm to return back to 1/3 full.

As previously mentioned, the fuel gauge also does not return to zero when the ignition is off, or the battery connected. It stays showing 1/3 full

Anyone have a clue why this is....never come across one which wont return to zero without power.

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