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MG MGA - Fuel Gauge Jitters

Guy’s I am getting all the electrical bugs worked out on my recent almost completed 2 year restoration. I noticed with my new Moss sending unit and rebuilt fuel gauge that my needle bounces from full to empty with the slightest movement of the car. When stopped it appears pretty accurate. Does everyone have similar results and should this be expected from a car that’s fifty years old?


WMR Bill

There is no damping in the MGA gauge, so it will vary as the fuel sloshes around. However, it should't be jumping from full to empty unless you are doing some violent acceleration or deceleration. Perhaps there is a loose or frayed wire that is opening or shorting. There is also the possibility that the new sender is defective, or less likely a loose wire in the gauge.

PS Your car is looking really good.
Jeff Schultz

The fuel gauge is not electrically damped, so the needle will wobble a little in harmony with sloshing of fuel in the tank. If the needle bounces around the ends of scale when it shouldn't, like a light switching on and off, it means you have an intermittent electrical connection somewhere.

Start with the circuit diagram shown here: http://mgaguru.com/mgtech/electric/circ_f1b.htm
Check for loose connections on the fuse, Green wire(s), and Green/Black wires including the bullet connector near the starter switch in the engine compartment.

The fuel tank, mounted in rubber straps, is originally grounded through the steel fuel line to the fuel pump. If the steel line has been re-connected with a rubber hose you may have lost the tank ground connection. This can be restored by adding a ground wire from one of the sender unit screws to a grounding point on the frame nearby.

To test the sender unit use an analog ohm meter (swinging needle type not digital). Disconnect the wire from the sender unit, connect an ohm meter from sender terminal to ground, set meter on 100 ohm scale, and shake the car. Depending on fuel level, the meter should read somewhere between 0 ohms (empty) and 70 ohms (full). When fuel sloshes and the sender float wobbles up and down the needle on the meter should waver gently in harmony with movement of the float. If meter needle movement is jerky or eratic, the sender unit has intermittent internal contact, needing to be rebuilt or replaced.

If the sender unit and fuse and wires are okay, you finally get down to a faulty fuel gauge. Then you can go here: http://mgaguru.com/mgtech/electric/fg_01.htm








Barney Gaylord

I had a similar problem with fuel gauge. Intermittently while driving the needle would start to violently gyrate, pinning against the rightmost bumper pin for a split second then falling back down, etc. I figured the problem had to be in the sender unit and, not wanting to be have the car out of order while I messed around with it, I decided to get a known working sender unit and just swap them. Since I'd heard stories of the aftermarket units being of inferior quality, I went in search of a reasonably priced, working, original unit and soon found one on eBay. The swap over was easy and now my fuel gauge works normally.

So what was the problem with the old sender unit from my car you ask? Well if a picture is worth a thousand words, two pictures are worth more than I car to type. Here's what the inside of the original Smiths unit looked like...



Andy Bounsall

...and here's what the inside of the after-market unit I removed from my car looked like. Enough said!



Andy Bounsall

Andy, How right you and Barney are! I took my replacement out and threw it in the parts box. I got the old one out and dissembled it per Barney's site and sure enough the little wire to the post is broke. I pulled it out and uncoiled the wire and re-soldiered it. It still comes up 70 ohms, Problem is it is still slamming from one side to the other when the car is moving. When it is still it reads fine! I think the sliding contact arms are rising above the coil wire I removed. Does anyone know of a company that re-builds these units?
Bill R

Bill, I spent last winter mucking about with sender units including a new Moss one ( look at Barneys site about that). I believe that one possibility for the flickering (NOTE not swinging as the fuel slops about in the tank but actual flickering)is that the contact between the float arm where it goes into the casing and the casing itself (which forms the earth path)becomes intermittent. Namgar did an article about this where a chap overcame this by soldering a length of braided copper wire onto the float arm and then attaching it to the body of the sender unit with a small self tapping screw. It was Tech session Volume 17 Number 4.
If you have time and patience it might be worth a try.
David
D C GRAHAME

Bill
I had a similar case but traced the fault to the guage and not the sender (they are not all bad!!).
On taking apart the guages I found that one wire had become disconnected from under a nut and was just resting on the contact. Obviosly this is not a good contact and so the guage went from empty to correct level while driving. When static it was either correct or empty which could be changed by tapping the dial :-)

Neil
Neil Purves

This thread was discussed between 19/07/2007 and 25/07/2007

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