need help with fuel sender/gauge

[pinheadandy1]

Im new here, been lurking for a few weeks though so I guess this is my intro. So here's my situation my gas gauge never reads above half tank. when its full it reads half and goes down from there as the gas does too. now i've replaced the sender, tank, ground, and sending unit lead to the cab. As far as from the cab forward (sender lead and gauge) i've done the whole ground open test and the gauge works fine. The gauge reads full when the lead is not hooked up to anything and the ignition is on and empty grounded (i even tried an extra gauge that i have and it also works fine. It also did this exact same thing with the old tank and sender. My question now is my new sender bad or possibly the wrong sender or something else im totally over looking? Did the heavy half's have their own special sender? Also, if say the trucks gauge cluster was replaced with one out of a different truck could that cause this and what can I do to rectify any of this? The truck is a 78' heavy half single cab short bed 350/350. Thanx in advance for any info you guys can help me out with.

my names Andy by the way..

[Captkaos]

Fuel senders are all the same.  Am I reading that you replaced everything and it still doesn't work?  Was everything new?

[DnStClr]

Let us know if any other gauges are working weird too.Bad grounds can be the cause of errant readings.

[pinheadandy1]

I replaced everything from where the sender wire comes out of the cab too the tank (including the tank) I tried a spare gauge that came out of another truck and everything was the same. Water temp, clock, speedo, and all the lights in the cluster work fine. The oil pressure however does not work, so it could very well be something in the cluster causing this. if thats the case though why would the gauge seem to work fine when I ground and UN ground the sender wire to the tank with the ignition on? Could the printed circuit be going bad would that cause this?

thanx,
Andy

[VileZambonie]

Pull the cluster out and check it. Resistance will cause it to rise so if you have unwanted resistance in the circuit kiss your accuracy of the gauge good bye. If the sending unit is good and you have good continuity to the cluster, all that's left is the printed circuit or the gauge provided the fuse is not open.

[pinheadandy1]

Thanx Vile Im pretty sure your right about it possibly being a problem within the cluster. So im gonna jump on that the first chance I get.

Andy

[autorepr]

I think I'd lean towards something maybe keeping the float on the sender from physically moving through it's full range of motion. Did you try hooking everything up electrically and with the sender out of the tank move the float up and down while watching the gauge?( have somone watch it) Did you feel how smooth it moved up and down?  I agree bad grounds do strange things but they usually don't create the same problems exactly, the same over and over, meaning everytime and no matter the weather/ moisture or electric load on the effected circuit. Like 1 time it will be affected at 1/2 tank then 3/4 tank another and sometimes just move all the way. Just an idea, let us know.

[pinheadandy1]

Yeah tried moving the float manually with the sender out ot the tank (both new and old sender). plus I also tried moving the float up and down manually inside the tank with a wire through the filler. Everytime with the same result. So I think its got to be something in the cab. I just havent had the time in the last two days to tear into it but will no later than this weekend. heck either that or if this truck is going to give me that many electrical problems ill just start over with a whole painless & dakota digital setup. Which is my eventual plan anyway I was just planning it further down the line.

[pinheadandy1]

Well I got all working. Basically I just got tired of fooling around with it so I went out and bought a whole set of moon gauges and they work beautifully. Thanx everyone for the helpful info..

Andy

[DnStClr]

Great gauges there pinheadguy! Glad you're up and running.Where'd ya get your gauges from?

[SLUG]

why dont you post some pics of your new guages?

[pinheadandy1]

I got them from moon. I will post some pics when I get a chance I have yet to finish up a couple details and I've been working alot of O/T so it could take a few days.

Andy

[Redneckchevy]

Yea  i was going to ask the same thing...Im looking to install aftermarket gauges...What all did u have to do to get them fit? did u cut up the old bezal?

[pinheadandy1]

Yeah you have to "massage" your dash/bezel a little bit. I don't think its too big a deal though your not messing with anything that you couldn't replace. Their are some guys on here that have done basically what i am doing with autometer gauges. Do a quick search and it shouldn't be too hard too find a couple tech articles on those. Nevertheless Ill post up some shots of mine with moon gauges as soon as I get it all completed.

Andy