Wondering if anyone else has experienced this.

When I bought my 74 the guy told me that the gas gauge hadn't worked the entire time they owned the truck (8 years or so). So I crawled under and found the sending wire which 'appeared' to still be intact but it was a little melted and flat on one section. Turns out it had separated the internal conductor but managed to still be attached by the insulation. So I stripped, soldered, and repaired it. Gas gauge now works but it shows exactly half of what it should. When the tank is full its at half full and proceeds down from there until its empty. Any ideas?

 

My guess since the gauge was inoperable for the 8 years or so with the previous owner that the gauge may be stuck/rusted @ 1/2 full. The float inside the tank is suppose to rise to the top which messages over to the gauge to show a full tank. The float is suppose to swing freely. The only way to confirm this is to drop the tank and pull out the gauge. Or, you can fill up the tank a few times to see if this knocks the float free.

Keep us posted!!!

Rick

 

MercMan,
I went through hell with my fuel sending unit on my 78. I would guess that there could be some issues in the gage itself and what I know for sure is that most of them work off of heat sensative wire and based on how many ohms are present will controll the heat. The more full the tank, the less the ohms, which allows for the coiled wires in the gage to heat and move the gage. The less fuel, the more the ohms and of course less heat. Thats how I know them to work from my 78 woes and factory manual. I ended up pulling my fuel sending unit and replacing with aftermarket. However, the after market ones are set up to 200+ ohms when empty and 20 some when full... These numbers dont work for the gage. The factory specs show somwhere around 124 ohms as the max (empty) and 18 ohms as the minumum (full).

I ultimately, after about pulling that tank 100 times for a fuel delivery issue, sent the sender off to SURR and they refurbished the whole thing for about 140$, slapped it in, and voila, no more issues with quantity and delivery! Here is the website address:

S.U.R.&R. - Fuel Sending Units

I would get you the exact ohm numbers but all my stuff is packed up for may last move... Hope this helps and certainly keep us posted!!!

Cheers!!!!

OBTW, the issue with my sender was a broken winding that sets up the correct ohms for the gage and I believe that is what you are experiing!!!!

 

variable resistance
if there is a lack of connectivity and earth, nothing one can not nut out with a multimeter.
with a multimeter a test can be done between the active/positive of the battery and ground/earth upon the frame/chassis. these readings should be around the same at the sender wire at the tank and nearest decent earth/ground, as it is only a gauged ignition active (pending there is not an inline resistor).
then comes testing the sending unit in the tank as a full tank is that of a solid earth/ground and the lesser amount of fuel the lesser the reading.

as i am a australian the whole gas/petrol, miles/kilometres thing, but a fuel gauge is only a heating coil and the more power the coil gets the hotter and higher the reading, no different to temperature, expansion and contraction

 

Thanks for the feeback all. I had originally thought the sender was broken when the gauge didn't work at all because it looked like the wire was intact and sadly my tank looks a little like it got wrestled by a concrete gorilla (several small prominent dents on the front side). So now after your input I am thinking the sending unit is just feeling a little old and beat up and is just about half right! Luckily the tank doesn't leak and fuel delivery is fine.I will replace the tank when I can and check out that dealer for a new sending unit. Half right is better than no idea when it comes to fuel.

 

My gauge worked from full to 1/2 then dropped to empty, turned out the wire on the sending unit had a bad spot right in the middle as the float moves up and down.

 

Just like Oldschool, my sender's winding was broke right in the middle. It is an interesting set up because you can tell just from looking at it with the cover off, that there is no way those little wires are going to hold up to the wiper moving back and forth. It is simple technology and SURR did a great job fixing mine. I just took the sender unit itself out for repair. Left the assembly in the tank so I could continue to drive it... Good luck and let us know what you found. It COULD be some mystery discrepancy that none of us have experienced! Law of averages says those gripes are still out there yet to be found!

CHeers!

 

Thats funny, I had the same issue with my 71 also, I pumped a full tank, next day it only read half, i thought someone jacked my fuel, but then I notice the temp wasn't reading correctly also, I then twiggle the harness behind the cluster around and it read correctly again, I guess sometimes these old cars have bad connectivities

 

Are you saying you adjusted the wires behind the dash and it fixed both issues? Unfortunately the PO seriously jacked up my wiring by adding TONS of extra lights and wires and generally doing things that make me scratch my head and go "Whaddaf&%! is this?" I have removed about 200 yards of added wires, 9 extra brake lights, and I'm not quite done trying to remove this BS. When I hit the brake the running lights will glow, even when switched off! The guys son-in-law that I bought the truck from called it a bunch of 'gonner' as in it needed to be gone. ;-)

 

I love when "shade tree" mechanics feel they have some special skills! Skills alright, because I have no idea how to install 200 yards of wire just to get the lights to work or added lights to work! Funny though, keep us posted about what you find with that fuel quantity issue though. I am curious!

CHEERS!