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Bill E, Maple Grove, MN
Bob,
I am not sure why a voltage compensator would help but do have an educated guess. The voltage compensator is a bimetalic switch that opens and closes to control voltage. Without some damping circuit, this would have a square wave output. Since the output of the voltage stabilizer is a reasonably constant 9 or so volts, it must include some storage/damping curcuit on the output side. This circuit may dampen the voltage swings from the tank sender as well although I don't know the details of the two circuits.
My neighbor and I just finished calibrating his guage. Somewhere, I believe from Barney Gaylord's sight (
http://www.ntsource.com/~barneymg/mg01.htm), he found the minimum and maximum resistance of the fuel sender although you could measure your own- if I remember correctly it is zero ohms at empty and 250 ohms at full. I then bought a pack of resistors that were about 1/4 the value of the impedance range- I think they were 50 ohms. We removed the guage and used a battery charger to energize the circuit. We then connected the sender lead to various places on the resistor array (with the other end grounded) to and looked where the needle pointed. The two electromagnets are mounted on screws that extend throught the back of the guage in slots. By loosening the mount and sliding the screw along the slot, you move that magnet closer to or farther from the needle. By trial and error you can get the needle to read full with all resistors in the circuit, empty with no resistors, and at the proper intermediate value for the number of resistors between the guage to sender connector and ground. We had several extra guages from my XK project to confirm our readings but this is not absolutely necessary since the guage is reasonably linear in respect to sender resistance. The resistors cost 49 cents at Radio Shack.
I hope this helps,
Bill
Art DiLello, Albany, NY
I have a replacement fuel sender unit, and it twitches too, for the reasons Bill E. gave. The number of windings around the post in the replacement sender is "scattered" (not tightly wound like the original). So consequently, the needle will jump around more so than the original. When the car is standing still is when I get an accurate reading.
Even with this, the sender was not calibrated with the gauge. You may have to bend/adjust the float wire. With the gas tank empty and the igition "on" I bent the float wire so the gauge read "E." I then filled the tank with 3 gallons of fuel, and had to rebend the wire so it read "1/4." At this point, I put the sender in the tank, and filled it with 3 more gallons (a point which gasoline would have poured out from the sender hole), and the gauge read about "1/2." And 3 more gallons in to 3/4. etc.
Also, for some reason, the replacement gauge was too large for the tank hole, and I had to file some of the aluminum off the back of the sender unit.
If I knew how to fix the original sender, I would put it back in the car.
Bill E, Maple Grove, MN
Wes,
As others have mentioned, your guage is acting normally. The MGA uses a different kind of movement as the B or most other cars. The A is a galvanometer- the guage changes position with small changes in electrical current through two electromagnets. In order to function, the needle needs to be free swinging. So bumps will make it move in two ways- first the fuel level effects the magnetic pull and second the undamped needle can swing from inertia forces.]
Since the needle position is set by the difference in force between two electromagnets, it should be self compensating for source voltage- both magnets will change similar amounts. Another unique aspect of the guage's design is that it reacts in reverse compared to typical guages- the guage reads full with the sender wire disconnected and empty with the wire grounded.
The MGB uses a heated bimetallic spring to move the guage. More current heats the spring which bends and moves the guage. This is typical of most fuel guages. The guage is fairly insensitive to short term changes in voltage due to sloshing gas or the on/off behavior of the voltage stabilizer. The bimetallic spring also holds fast the needle so it can't be bounced around. On the other hand, the guage is sensitive to long term changes in voltage since there is no internal compensation device, hence the voltage stabilizer.
How do I know this? Two weekends ago, my neighbor and I spent some time working on his A's fuel guage. We had a B bimetallic guage and a few extra galvanometer type guages to play with. In the end, we were successful in getting his guage working although I am not sure how.
Concerning aftermarket vs original senders, I have heard that the originals have two wipers to help stabilize the resistance while the new ones have just one wiper. The wiper is the contact that moves up and down a wound resistance wire. This could have some effect on the stability, I guess but they bounce arond so much I can't imagine that you would notice.
Regards,
Bill
