I have a 78 cab on an 83 chassis so I'm assuming the gas tank is 83 as well. 30 gallon plastic tank. The fuel gauge was acting wonky so we dropped the tank, needed to replace fill pipe grommet also, and put in a new sending unit from LMC Truck. We tested the new one before we put it in and here's what's happening. The ohm meter was pulsing! So we tested the wire coming to the sending unit with the unit NOT attached, and it was pulsing. Now I figure this is why I'm getting weird gauge action. The needle goes up, then back, then up, then back, but the extent of travel is not always the same. The highest the needle gets is when I first turn on the key. After it settles back it never goes that high again. What's happening here and how do I solve it? Why are the ohms pulsing?

 

so when you tested the sending unit did it test fine? the pulsing you are seeing may be the mechanical voltage regulator going bad.
https://www.allpar.com/threads/get-e...ost-1085245432

 

The sending unit seemed to check out ok. The pulsing is originating in the front of the tuck somewhere because we we’re getting that on the wire going to the sending unit without the unit plugged into it.

 

Trace it back to where it gets power from. Something ain't quite right there.

 

Ya we were talking about that. I think we’ll test on each side of the gauge and try to figure out where the pulse is originating. Could it possibly be caused by the gauge itself?

 

as i already said it is likely the mechanical voltage regulator which is why you are seeing the pulse. it could even possibly be failing.

 

@crazzywolfie At the risk of sounding naive what are you referring to as the “mechanical” voltage regulator?

 

well if you read the article i linked to you would know

 

Great article. Thank you.