Petrol Gauge

[chrisgriffiths]

Hi Everyone !!
I could do with some advice please.. I've been having trouble with petrol gauge on my 1967 1098cc saloon with a black faced speedo (just in case it makes a difference)
initially the gauge didn't go over the quarter mark, even when full. I did the usual checks and fitted a new sender. Still no joy, so I fitted a new voltage stabiliser the gauge then appeared to go up and when I topped up the tank it went straight to 'full' I thought I'd cured/sorted it.
The thing is that the gauge shows full all the time even when the ignition is off and the keys out. It used to rise when you switched the ignition on. Should it be like this or maybe I've got the wires on the wrong terminal on the stabiliser. Help/advice would be appreciated

Chris

[simmitc]

You've got the wiring wrong. The gauge should read empty with the ignition off, and normally the the ign on. There might still be another fault. With a 12 volt supply to one terminal and an earth to the other, the gauge should rise slowly yo show full deflection. No circuit, it should show empty. If that test fails, then the gauge is faulty. Now repeat with the voltage stabiliser in the circuit. You should get the same results. You can also use a meter to check output from the stabiliser. Once the above is working correctly, then connect the sender unit and check gauge readings/operation. It's a process of elimination as to which bit(s) is/are failing; but it is usually the sender unit.

You were correct to describe the type of car - the earlier works worked in a similar way, but with an instant reading rather than rising slowly. The sender units for early and late gauges are different. Although both will work ,the readings will be incorrect. Good luck with sorting it, it sounds as though you are on the right track.

[chrisgriffiths]

Ok Cheers....
I appreciate your help. I'll give it a try (prob in the next couple of days) and let you know how I get on

[catswhisker]

It's just possible that the needle has 'jammed' on full, I would see what it does with the battery disconnected first.
The gauge would show full if the wire from gauge to sender is touching earth but only with ignition on.

[chrisgriffiths]

Hi,

Ok cheers. I've tried disconnecting the battery and the needle drops quite smoothly.
I've tried moving wires in all combinations on the terminals and its still the same ..showing full even with the keys out.
I'm beginning to wonder whether the replacement voltage stabiliser I've got is faulty and the current is going straight through it unregulated

Ta for suggestion though

[simmitc]

OK, the VS must be getting its power from somewhere - with the ign off, there should be no power to the VS, and without power, the gauge cannot read anything. So, looking at the VS, there should be three green wires on one terminal. One is the feed from the fuse box, which itself is fed from the ign. The other two go to the wiper switch and the warning lights & heater switch. There should be a single shorter light green wire from the "I" terminal on the VS to the gauge; and then a green/black wire from the gauge to the sender unit.

If the gauge is getting power with the ign off, then it must be coming from one of the wires on the VS. Does your heater blower work with the ign off? Do the wipers work, are any warning lights on? Could there be a short in the fuse box from the unswitched to the switched side? Hope that helps. Happy hunting.

[chrisgriffiths]

Hi,
Thanks for the info on the wiring , When I've checked for a live wire I found a fourth wire fed from the fuse box. I think the previous owner must have put that in for a feed for an extra dial or something (no longer there).
Ive disconnected the wire and insulated it. I also decided to go back to the start and put the old vs back in. When I've done that I also have the wires as you described them exactly. The result was that the gauge does not go above the quarter even though the tank is full. This was the original fault I was trying to cure.
I'm thinking now that possibly the vs I purchased is either faulty or from wrong type of car. I've ordered another one and hope it arrives tomorrow when I can try it.

Cheers for the advice its appreciated to know I'm go along the right lines (I think I am anyway)

[simmitc]

Good progress, one more check to make: With the setup working as above (1/4 full reading) go to the boot and disconnect the green/black from the sender unit, Connect the wire directly to earth (ground it on the car). Now turn the ign on and see what reading you get. If it shows full, the the sender unit has failed. If it still shows 1/4, then it could be the vs.

[chrisgriffiths]

Hi again.....

I carried out the check on the sender and the gauge didn't move so then I tried replacing the wire to the sender still the same. Luckily then the second new VS arrived in the post and ...Bingo its working perfectly.

Thanks to all with the advice its much appreciated

Cheers

[catswhisker]

Nice one ! glad you got it sorted. I love a Minor electrical mystery .

I have done some reading on MG sites about voltage stabilisers, very interesting, apparently they switch
several times per second making test meter readings virtually impossible. It is possible to strip out the old parts
from the vs and convert it to solid state by soldering in a 10 volt regulator (TC 7810 CZ) Might have a go at this
just out of interest with a faulty one.