reversing fuel gauges readings? 49522/5 and 42139 8 HP

[JonE]

Can anyone give me identification on either of these please... marked with 6V on the Smiths face. 
I hope the former might be something from before they swapped the resistor connections around - i.e. perhaps earlier.
Still trying to attend to mismatched early sender and what I think (latter) is a late gauge.

[JonE]

I've changed the title slightly so this is more useful to others when searching.
Suffice to say, I swapped out one for the other and they BOTH exhibit reverse gauge pointer in relation to fuel in the tank! I'm presuming these are both 'late', and its probably a '31 van tank with a sender unit.

Someone mentioned before about swapping the resistor wiring around to correct this. I opened up the sender and the resistor wires are tiny. I wondered whether I can just reverse the wiring and earth to the sender, if I have a thick silicone gasket and perhaps replace fastenings with nylon bolts? As I type this it seems silly, but I'm asking if its technically possible, or would it result in some other danger in the circuit?

Is there any other easier way that micro-surgery of the sender wires?

[John Cornforth]

Hi JonE

Don't do anything drastic just yet !  I have to go and walk the dog, and I will try and formulate a longer reply with your options !

Watch this space.

[John Cornforth]

Hi JonE

I have now had time to delve into this. Assuming you don't want to change or modify your early "minimum resistance when full" sender, these are your options:.

Stick a reminder label on the gauge that Full=Empty and Vice Versa

Find a matching early Gauge.

Run a late Gauge with the battery connected to G (the shell) and earth connected to B. Tank sender stays on T. You would have to insulate the gauge shell from the dashboard, or modify it to remove the internal connection to its shell and bring the battery feed out separately. Although the gauge will now read in the correct sense, the scale will be cramped up at one end. The cramping is because the two coils have different numbers of turns to balance their effect on the needle, and running the gauge "backwards" means that the coil with more turns is in the "wrong" circuit position and has an unwanted dominant effect.

Modify a late Gauge internally by swapping the physical locations of the two coils from one side to the other, whilst preserving their electrical connections. You might need to extend the coil tails so that they reach, and it's a fiddly job with fine wires. Each coil is held in with a single tiny stud and nut in the backshell, and these are slotted for calibration. I'd suggest just putting them in the centre of the slot to start with.

Disclaimer (!) I present this in good faith, but have not tried the above myself. If you do experiment, please fit a temporary 1 amp fuse in the battery supply in case of errors. These gauges normally draw something between 140 and 360 mA at 6 volts.

The following methods suggested over the years (it's not a new problem!) will NOT work:

Changing battery polarity

Wiring the sender the other way round and insulating it from the tank

Fitting resistors in various places

[JonE]

Ha - you forget one - "de-wire and find a stick"! (I did today and it worked really well... I was thinking of the weight saving possibilities of losing two electrical items and all that wiring)
That is fabulous to be able to see those together, thankyou John. Realistically, fiddling with original items is probably a silly idea when an early will turn up. Indeed, someone may be in the reverse situation if they read this in months to come. What would be interesting is to know the face descriptors on an early one... as we know that two listed above are now late.

Either of the above "lates" are available - working well - for an early swap should anyone have one...

[Roger]

JonE,
Silly idea, but could you reverse the bend on the float arm so the sender wiper works in reverse!

Roger