Fuel reserve light / gauge

[hot66]

My fuel gauge has always been spot on . Usually gets to the bottom mark and the reserve light comes on. ( I’ve measured it at this point of having about 3 gallons left.

For some reason today the reserve light came on at the 25% mark on the gauge .

I’ve removed the gauge and all wires are connected ( although there is a earth spur with no wire to go to it and look like never has been)

I’ve removed the sender and all is moving freely . Measured resistance ..2.5 ohms full and 67 empty ( checked by turning it upside down and back again when out of tank )

With fuel in at the moment it is showing about 65 ohms resistance and this is reading 25% on gauge.

Only thing I’ve done recently is had the gauge out of the dash to try and find a rattle ( couldn’t find anything )

So, question , does the reserve light get triggered by the sender position or by something in the gauge ? As in, am I actually on reserve with the light on but the gauge needle is lying to me saying I’ve got 1/4 tank full.

If so, what would cause the needle to loose calibration ?

Or , do I actually have 1/4 tank of fuel but something inside the gauge has triggered the reserve light ?

[hot66]

It appears the low level warning light is triggered from the sender unit .

So need to work out now wether the needle incorrect or the light is and why

[hot66]

Bit more reading up, the reserve light is activated by the float in the sender, which implies it must be correct as the float moves up and down freely and if it is activating the reserve light must be bottoming out … which means for some reason the needle in the gauge is lying

[PeterK]

So 65 ohms is almost empty, suggesting that you are at the low level but the gauge is still showing 25%. I would start by cleaning the gauge connections as that’s the most recently touched item

[Mick Cliff]

Only thing I’ve done recently is had the gauge out of the dash to try and find a rattle ( couldn’t find anything )

Nothing to do with the gauge or light but many years ago during a trip to the Nurburgring my 911T developed a rattle that emanated from the dash area. Once home i searched for weeks trying to find that damned rattle, to no avail. The rattle was quite metallic and solid sounding. I gave up and just accepted that there was this rattle!

A bit later on I had the T up on the ramp and found that the bushes in the rear trailing arms were 'shot'.
On a '73 T this means engine out to get at the bushes.
Inconveniently I had a minor stroke before I could drop the engine so had to bide my time. 85% recovery from the stroke was a signal to do the work.
All went well and everything was reassembled.

Test drive - no rattle! So it was the trailing arm rattling on the mounting bolt - at the oposite end of the car to where I could hear the rattle!

Worth a look James?

[hot66]

Thanks Mick. It’s just a kind of loose washer noise so nothing I’m too worried about

[hot66]

So, more investigation.

With the sender out of the tank , turned upside down so float is at top and gauge is reading full

With sender turned correct way up and float physically at the bottom of the sender, the reserve light is on but gauge reads 1/4 full

So, doesn’t appear to be a fault at the sender end of things

Checked all the wiring at the gauge and all seems good. Removed and cleaned up the behind dash earth for the loom.

Interestingly removed the earth from the fuel gauge and turned on the ignition and the gauge still reads the same 1/4 full as it does when earth connected

So I’m thinking somewhere or some how there is some new resistance in the wires between the sender and the gauge , OR maybe the gauge itself is playing up ? ( although when ignition turned off the fuel needle fully drops to 0 )

Any ideas ?

[Nine One One]

What happens if you connect a new wire as earth to the back of the gauge and a good earth point on the car?

[hot66]

Nope I hadn’t … not sure why I hadn’t even thought of doing that !

so, just popped out to the garage and ran a separate earth … and great news it worked correctly


Now, in a weird twist of fate, I reconnected the original earth wires and the stupid gauge is now working correctly ! I’ve had those wires off and on about 20 times now so no idea what’s happened , but either way it’s back to normal and working again

Thanks for the prompt to try and direct earth . At least if it happens again I know what the problem is ( electrics are not my thing )

[Nine One One]

Glad to help, glad it worked!
You may find the good earth has actually reset the gauge, if it had a ‘sticky’ point, never being run down to almost empty by itself. You could always put the gauge back with a double earth to save stripping it all out again?

[Ian Comerford]

This has been a really useful thread and pointed me at a couple of options for my similar issue. I hadn’t considered that wiring could be the problem, had ruled out the sender as being at fault and assumed the gauge to be faulty in some way. Hopefully fiddling around with wiring, running a new, temporary earth etc will work and I can stop resetting my trip meter at every refill to gauge when to fill again. Thank you to all who contributed, really useful.

[911hillclimber]

On hols atm so just read this.
My 73T has a condition that at a low fuel level it will switch on a bloody big red light in the fuel gauge when cornering, so fuel surges, sender drops and this light comes on.
It goes out as soon as the car becomes stable.
With the light on you can hear a fluid noise as if the pick up in the tank is gulping air.

Engine is a 3.2 and simply carries on running perfectly during all this.

Is the light the reserve light, but there is no actual reserve in the tank to be switched, so just a Low Fuel indicator.

Car is a German market car when new.

[Nine One One]

Yes it is a LOW LEVEL FUEL indicator light...........time to top up with fuel

[hot66]

so update to this .. stupid gauge reverted to reading incorrectly again.. so I ran an additional earth then disconnected and reconnected power and it read correctly .. this was a few weeks ago in the garage .. took it for a run on sunday and the garage is once again reading incorrectly ( light comes on whilst showing just below 1/4 )

at this stage I think the next step is to open the gauge up as it doesn't appear to be a wiring issue

[911hillclimber]

My 73T original gauge/sender has the big red light come on with 1/4 tank and cornering hard to the right.
I guess the sender drops with the fuel level surge.
It comes on constantly when still and little fuel left, ie fill up asap.

Has done this for 35 years now consistently so guess it is designed this way.