MKV Instrument Repairer

Hello friends,

The fuel gauge on my MKV is quite miraculous in that it always shows “Full”!

Seriously, the car is back on the road and it’s so lovely to drive, however the gauge needs to be looked at. As I said, it shows "Full’ with ignition on, though can take a minute or two to jump from empty to full. I know that the sender in the tank is working properly as it has been looked over, and the warning light comes on when the tank is low, so I’m assuming it’s something sticking in the gauge mechanism. We have a great repairer here in Melbourne Howard Instruments but they now have a more than eight month turn around. Does anyone know of a reliable, quality repairer in Australia or even overseas with a better turn around time?
Cheers,
Tim

You may be able to fix it yourself.
If it only jumps between empty and full, and never in between, there is a possibility that you have connected the wires wrong.
Assuming your sending unit in the tank is connected correctly, and changes its resistance as the float moves, if the needle jumps a minute or two after you switch on the key, it suggests that there is a drag on the armature inside the gauge.
The needle is on the shaft of an armature in the center, which swings past two magnets. The magnets must be in close proximity to the armature, but not touching it. The magnets are adjustable by two nuts on the back.


You could maybe do this in the car, but probably easier with the gauge, sender and a battery on your workbench set up in a temporary circuit so you can move the float and watch the needle move as you adjust the magnets.
This is what a repair shop would do.
In the USA there is Nisonger Instruments in New York and Palo Alto Speedometer in California.

1 Like

Hi Rob,

Thank you for the message and the helpful information.

The Service Manual must be incorrect as it only mentions one wire and not two going from the tank to the gauge?

Given that the low fuel warning light is accurate then I would assume that the wiring (two wires) is correct? Is that the case? I’m guessing that the gauge has a drag on the armature inside the gauge, as you said. Given that the car hasn’t been in regular use for quite a few years it seems likely that having a current flowing through the gauge after such a while is highlighting a stiffness or a poor adjustment.

The sender unit in the tank is free though is it possible that it could be open circuit causing the gauge to read Full? It might also be a fault in the wire from the tank to the gauge causing a path to earth?

With my thanks,

Timothy Fox
Timothy2963@gmail.com

Hmm, not sure which page you are reading, but the wiring diagram shows 2 wires on the sender, green with yellow and green with black. Although my '38 has only one, because it has no low level light. Have you read page C22 of the Mark V Manual? Open circuit should make the gauge show Full. Possibly you have a bad connection somewhere.
image

Mark V fuel gauge has three wires coming into it. Two come in from the tank sender unit, wires 19 and 24, and there is wire 17 for power. Also on the back of the gauge is a small lead with a blue cover running closely across the back of the gauge case to the electrically-insulated outer casing of the low-fuel lamp fixture. When no voltages are applied to the gauge unit mounted in the dash, the needle should nestle on the empty-level peg when gauge is mounted in dash. If the gauge is loose from the dash and rotated so it would view upside down from the driver seat, the needle should go to “full” reading without any apparent friction dragging on the needle when moving from empty to full. Chances are good your wiring circuit is not complete in some way. In my limited experience, I’ve never seen one of these gauges fail but I have seen wiring problems arise.

Dear Roger and Rob,

Thank you both for your messages.

Gong on what both of you have said I now think it’s highly likely that it will be a wiring issue. An order was placed a few months ago for a new wiring harness to replace the non-original one, so when fitted that will no doubt remedy the problem. I will still look at the gauge to make sure that the wires are secure and trace the wire from the sender as far as I am able.

Best wishes,
Tim