Fuel Sending unit Question

[bugedd]

My sending unit is original with a rebuilt gauge, and teh gauge is twitching and going all over the place. That leads me to believe the sending unit is the issue. The wiring harness is new also. Now, am I better off trying to fix the original or get a Moss new part. I understand some of the repro stuff is crap which is why I ask.

 

[DrEntropy]

Check to insure you don't have a flinky connection along the green/black (I ~think~ that's the right color/tracer) wire to ground someplace. Mebbe go so far as to pull it off the gauge and drive around a bit with an ohm meter attached between it and ground, watch the meter to check for the "all over the place" reading. If you see it, disconnect the sender end and drive some more. If it quits jumping all over the place you have a sender issue. If not, there's a wiring problem. If it doesn't dance around at all in either case, question the gauge.

Moss for the sender so you'll have recourse if it goes goofy, since it isn't going to sit on a shelf for a year before it gets placed and checked.

 

[dklawson]

The early gauges were "twitchy" by nature. They are also highly dependant on having good ground connections. If the sending unit looses its path to ground, the gauge needle will move quickly to "full". If the gauge housing looses its ground connection the needle will jump (sorry... I don't remember if it goes to full or empty) but the needle will quickl move to an extreme when the gauge ground is lost. Check and clean the ground connections before buying any new parts.

You can hook a meter up to the sending unit like the DR suggests. However, the sending unit reading will bounce all over the place as you drive. You can see this on a properly working early gauge. On the later gauge systems with the voltage stabilizer and bimetallic gauges... the system responds so slowly to changes that it is inherently dampened.

 

[spritenut]

Usually a bad ground on the gauge and/or sending unit.
Just painted gas tank with new rubber pads? That compromises the ground.
It is best to run a separate ground from the sending unit to the chassis.
Put a ground wire on one of the sending unit screws.

 

[bugedd]

Actually this is interesting, don't really know what to make of it. When the ignition is on, it reads the tank, and has a steady needle. When the motor is running, it twitches all over the place.
Again, new Moss wiring harness, rebuilt generator, new adjusted regulator. Rebuilt and calibrated gauge. Thoughts?

 

[dklawson]

Grounds.

Google for "MGA Guru" "fuel gauge". Barney Gaylord who runs that site has pages devoted to the early fuel gauge and provides an excellent explanation of how the gauge works. Once you see what's going on, you'll better understand what problems to look for and why we all suggested ground related connections.

Even with your new harness, the gauge and sending unit can have bad grounds that may be subject to intermittent connections due to engine vibration.

 

[Jim_Gruber]

I recently put in a new sending unit in my '68. Design is totally changed from original and is now just about 100% plastic. Not sure if good or bad. Also wonder if any other instruments are jumping if voltage stabilizer might be having an issue.

In my case I pulled tank to check sender and when I reassembled with new fuel pump, my original problem I was trying to diagnose, I put the sender in wrong orientation so I believe the arm was hanging up on fuel pickup. I ordered a new sender as I thought it might have gotten dropped or otherwise banged around when I tripped over the box it was in. Gauge did not read 100% full when full. After I got it new sender reinstalled and all working fine the light bulb went on. Did not pull tank to reinstall old nit but I bet I have a good one sitting on my shelf.