I'll be installing a new fuel sending unit in my '49 GMC 150 this weekend. Researching the archives I found that several folks, including Nate, recommend... strongly recommend... that the fuel tank be full to the very top when installing a new sending unit. That's got me puzzled... I don't understand why that would be important.
Hi Mike, I would not want to second guess Nate...But my guess is to use a full tank to make it easy to adjust the arm if the gage is not showing full. Hope this helps and if I am wrong Nate please let us know. Enjoy, ric :7
Hi Ric - Thanks for responding. I thought on that some too, but if I were to be concerned about which end of the gauge/sender was accurate it wouldn't be the full end. My attention to the info provided by the sending unit increases the closer the gauge needle gets to the big "E". My "Owners and Drivers Manual" says my behind the seat tank capacity is 18 gallons. If I hadn't read the earlier recommendations to ensure the tank was full when installing a new sending unit I would have started with an empty tank. With no power to the gauge or sending unit the gauge needle rests right on the edge of the lower leg of the "E" symbol. That's where I'd be looking to see the gauge needle point when the gauge and sender are powered in an empty tank. If I installed the new sending unit in an enmpty tank and the gauge read 1/4 full I'd want to adjust the arm on the sending unit so the gauge read empty. Then I'd start fueling with a known fuel amount... say from a one gallon container. With gauge and sender powered, I'd put someone in the drivers seat watching the gauge needle and then start pouring measured amounts of fuel looking for the first movement of the gas gauge needle. As soon as the needle made it's first move off the "E". Then I'd know exactly how much fuel was remaining in the tank when, in normal operation, the needle moved downward and first touched the "E" symbol. Then continuing to fuel in measured amounts I'd know exactly how much fuel was in the tank when the gauge needle registered 1/4 full, 1/2 full, 3/4 full, and 4/4 full. So, even if the upper range of the gas gauge needle was inaccurate I would know, for example, that when the gauge needle first dropped off the "F" symbol there was 17 gallons remaining in the tank... and when the needle said 1/2 there was 8½ gallons remaining... and when the needle first touched the "E" I had one gallon remaining. So, if I'm so smart, why don't I just do it that way and not waste folks time here. Well, I don't know... I do know lots of folks here are lots smarter than me... Nate has proved that many times over. Maybe if I put a new, but shorted, sending unit in a full fuel tank I'd just get a bogus gauge reading... but if I put the same new, but shorted unit in a tank with a nice rich fuel/air mixture (i.e., empty)I'd get an get an explosion. Just looking for the "how come?" behind the recommendation. Safety?... equipment damage?... gauge/sender accuracy? Mike M '27 Chevy '41 John Deere B '49 GMC FC-150
Mike, I'm sure Nate will drop in sooner or later to answer. I am pretty sure the reason is for safety. An empty fuel tank is very dangerous to play around with, especially with a live wire that can spark if touched to the wrong surface. With a full tank, there are no stored vapors, and the likeyhood of an explosion is reduced to a minimum. As for adjusting the sender, i agree that the low end is more critical to know than the full side. I need to adjust my sender, as it reads 1/4 when empty!? Just what I need, more projects........Mike
up , for safety and accuracy too ~ if the gauge is accurate @ full , it should be accurate @ empty too but you've given me food for thought . BTW : my fuel gauge died again , I think it's the harness but too busy working the old truck to take a look so I just fill it every 250 miles . -Nate
