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MG MGA - Fuel gauge only shows half full

The original Jaeger fuel gauge on my 1960 Coupe only shows half full even when the tank is full. It works and shows "E" when the tank is empty. The maximum resistance at the fuel level sender in the tank is 48 ohms when full. According to Barney it should be reading 68 ohms when full. My question is can I follow Barney's procedure for recalibrating the fuel gauge but use 48 ohms as maximum resistance instead of 68 ohms?

Thanks Andy
Andy Preston

Andy,

What's the resistance when the tank is half full? Adjusting the gauge is as Barney says, but it's unlikely that your sender is 'off'. More likely that it's no longer linear. Run or pump it dry, then add the fuel back in a gallon or two at a time, checking the resistance each time. Another impediment to accurate fuel gauges is extra resistance in the wire between the sender and the gauge. Best choice is to measure the resistance at the gauge (without the gauge connected) between the ground wire and the sender wire. You might even be able to do that at the gas station without looking too silly. As a bonus, you can set empty to be about 1 gallon before fumes.
Dave McCann

Clean up the connection at the tank. Then check the connections at the bulkhead below the starter switch. That may bring back your missing 20 Ohms
dominic clancy

48 ohms may be too far out of range for successful recalibration of the gauge which was designed to work with 70 ohm sender unit. Time to investigate what's wrong with the sender. You might end up rewinding the resistance wire on the bobbin inside. In recent years new ones have been such junk that it is usually better to repair the old ones.
Barney Gaylord

Many thanks for your comments. I'll start pumping the tank dry and getting resistance readings from empty to full and see where the problem lies. BTW should the bobbin (rheostat) unit be dry or does gas seep into the small casing?

Andy
Andy Preston

The steel coil wiper axle passes through the diecast housing into the tank and the float is attached to the other end. Where the axle passes through is a tight fit but not fluid proof and as the petrol level with a full tank completely covers the housing petrol can find its way in to the inner coil area. It can't get to the outside world unless you have poor gaskets on the sender cover or the brass coil connector screw. I remember worrying about this and the potential for an explosion if there was an intermittent electrical contact in the housing. It can't be a problem though because you never hear of people's tanks blowing up from this!
Mike
m.j. moore

I pumped 11.5 gallons of gas out of the tank and removed the sender unit from the tank. The housing for the bobbin rheostat is not fuel proof and fuel does get into there.

On the workbench I verified that the sender unit is reading 1 ohm when at the top of the bobbin (tank empty), 38 ohms when half way (tank half full) and only 48 ohms when at the bottom (tank full). It should be reading 70 ohms with the tank full, according to Barney. The resistance readings were linear from empty to half and then non linear between half and full. The windings on the bobbin looked good and there didn't seem any obvious faults. But there must be a problem in the bobbin somewhere.

However, I thought I would see if I could make any improvements by adjusting the magnets on the back of the gauge. On the workbench I had the gauge attached to the sender unit and a 12V battery source. Using Barney's magnet technique I adjusted the E and F magnets to get the fuel gauge to read between E and 3/4 using the sender unit as the resistance. I thought this worked well and that I could live with the fuel gauge only showing 3/4 full, at least it was an improvement from before. I refitted the gauge and float/sender unit back into the car and refilled with gas. When I turned of ignition the gauge showed 3/4 full and I thought that I had fixed it, however when I started the engine the fuel gauge needle continued to climb and went to the right hand stop.

On the workbench I had 12.6 V battery source, in the car and with the ignition on I have 12.6 V as well, but with the engine running I have 14.5 V. I will remove the fuel gauge again and see if I can adjust it in the car to 14.5 V and see how that works. I believe that I have turned the fuel gauge into a volt meter of sorts.

If I install a voltage stabilizer in the fuel gauge circuit will that fix my problem? How difficult is it to rewind the sender bobbin? It looks very difficult.

Andy

Andy Preston

Since I finished writing my previous posting I went back to Barney's site and read the section dealing with rebuilding the sender unit. I have now realized that the problem is broken insulation on the sender unit rheostat just past the half way point which is shorting out the rest of the rheostat. It doesn't matter how much I adjust the fuel gauge magnets I cannot fix the problem that way. I either need to buy a new sender unit or try and repair the rheostat on mine. I'll try a new unit first from SF and see how it works.

Andy
Andy Preston

A new gauge is certainly going to help. However, if your old gauge is accurate between E and 2/3 then, it should probably be good enough if you don't mind not knowing where the fuel is between 2/3 and F.

I'm surprised the gauge changes that much between 12.5v and 14.5v. Maybe, it's a byproduct of how you have the magnetic poles adjusted. (14.5v; You have an alternator in that car? I thought a generator shouldn't put out much more than 13.5v.) Regardless, just recalibrate the gauge using a reference 14.5v instead of the 12.5v. Where to get 14.5v? OK, good question. Other than running lines from a running car, I can't think of anything. Probably a lot of trouble.

Consider calibrating the gauge using 0,34,68 ohms as E,1/2,F, (and use your 12.5v bench power) and then see how it works with your old sender. If it's good up to 2/3 and then flat (both running and just ign on), then go with that.

I can't imagine how broken insulation on the fuel gauge wiring makes the sender more of a safety concern (the moving pickup should already be in contact with the fuel), but you're probably better all around to just get a new sender. Note, that if you do the paragraph three calibration, when your new fuel sender arrives your gauge should already be ready (I'd probably double-check the calibration of the new sender before install, you know, because it's easy).
Dave McCann

Dave, many thanks for your input. I did exactly as you suggested and reset the fuel gauge on the workbench to 0,34,68 ohms at E,1/2,F. I reinstalled it in the car and as you summized it now reads from E to 2/3 full, which is fine with me. I can live with that especially given the problems that I've heard with the replacement sender units that are available.

Thanks again; BTW will you be at the annual dinner this year?

Andy
Andy Preston

This thread was discussed between 16/01/2010 and 25/01/2010

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