replaced sending unit, still not working...please help!!

[1972BlueC20]

Hey guys,

Ever since I bought my 72 C20, the gas gauge has had a mind of it's own. It sorta works, but its not at all accurate. I've been just keeping track of the miles driven which is inconvenient as hell. I figured that it needed a new sending unit to correct the problem. I ordered one from LMC and installed it today. The one that came out looked just fine and looked just like the new one that I installed in it's place.

After getting everything put back together, the gas gauge still isn't working any differently than before.

What am I missing?? What else could be causing the problem??

Please help!!!


Thanks,

[Oldtruckfanatic]

My 70 did that. I had to replace the gauge. I have also seen corrosion on the spot the printed circuit board plugs into the harness. It was a greeninsh color. In that case the gauge wasnt working at all.

[1972BlueC20]

sweet thanks, can I buy just the gas gauge??

Also I just realized I posted this in the wrong sub forum....oooppsss!

[lowrollin70gmc]

Another thing to check is that the tank has a good ground. I like to run a separate ground from clean tin on the tank/sending unit to a bare spot on the body or frame.

[1972BlueC20]

I don't think there is any ground currently. mine is a behind the seat tank....I saw no ground wire or strap coming from the sending unit housing or the tank....do you know where the ground should be? or if there should be one from the factory?

[Oldtruckfanatic]

Yes you can buy just the gauge thats how I fixed mine. Our tanks usually ground through the bolts to the cab. Usually without a ground the gauge just pegs past full but its always a good idea to check it.

[GuyO]

The tank is "grounded through the sending unit". Do not put 12v current into the sending unit! Boom! The guage is powered by the harness and the sending unit is a variable resistance ground with a brown wire connection at the fuse panel. Clean contacts help a lot, also I added an additional ground wire from the back of my cluster to the firewall (Can't have enough grounds). Ask Vettevet for assistance if still stumped, he is a genius. Guy

[Oldtruckfanatic]

Ok lets get real here. The gauge is grounded through the sending unit. The sending unit is grounded to the tank. The tank is grounded to the cab and the cab is grounded to the frame through straps on the firewall and the passenger side front cab mount on the frame. The tank is always grounded. The sending unit only alters the signal to the gauge. I was merely telling him that If improperly grounded most fuel gauges just peg one way or the other and that his gauge was likely the fault. Nobody suggested he put 12 volts straight to the sending unit without a ground. Before you try to discredit someone trying to help please please know what youre talking about. "tank grounded through the sending unit" gees.

[GuyO]

Not my intention, sorry that you took it that way. In the end, it sounds like he has either a bad ground or dirty contacts. Just trying to help him.

[1972BlueC20]

It's all good dudes, I appreciate all your help. I think the problem is possibly a worn out gauge, considering it works but just isn't accurate or instant to show changes in fuel level.

This is what happens, when I fill up the needle stays where it was prior to filling up until I drive like 3 miles down the road. Sometimes it even takes till the next day. It then shoots just barely past full. From there it seems to work fairly accurately. It goes down a little everyday as I drive and put more miles on her. Since I'm not sure of the gauge, I always keep track of the miles driven so I don't run out of gas and get stranded on the side of the road. Once I have only about 1-2 estimated gallons left it only goes as far towards empty as a 1/4 of a tank. It may go further down towards empty if I drove somemore, but I'm too scared to run out of gas lol

Does this sound like a bad gauge or a ground issue? I just put a brand new sending unit with a brand new float so I don't think it's likely either of those, but what do yall think?

[TBONE1964]

Originally Posted by 1972BlueC20

It's all good dudes, I appreciate all your help. I think the problem is possibly a worn out gauge, considering it works but just isn't accurate or instant to show changes in fuel level.

This is what happens, when I fill up the needle stays where it was prior to filling up until I drive like 3 miles down the road. Sometimes it even takes till the next day. It then shoots just barely past full. From there it seems to work fairly accurately. It goes down a little everyday as I drive and put more miles on her. Since I'm not sure of the gauge, I always keep track of the miles driven so I don't run out of gas and get stranded on the side of the road. Once I have only about 1-2 estimated gallons left it only goes as far towards empty as a 1/4 of a tank. It may go further down towards empty if I drove somemore, but I'm too scared to run out of gas lol

Does this sound like a bad gauge or a ground issue? I just put a brand new sending unit with a brand new float so I don't think it's likely either of those, but what do yall think?

From what you describe, it sounds like the sending unit and the fuel gauge are both working as they should but the gauge is just slow to react. These are 40 + yr old gauges and sometimes they do get slow on movement. I test everyone of my gauges for my restorations and some of them are slower than others.

I have several fuel gauges in my inventory if you are interested. PM me and I will give you more information.

Hope this helps,
Tom

[franken]

The gauge is probably the most reliable part of the system.
Electricity goes from source to load and back to source. Round trip. Then we have the gauge to measure.
Source is the battery and alternator (when running).
Load is the resistance in the tank changing as the float moves with gas level. There's probably another load in series with this but ignore this bit.
Finally the gauge, a Voltmeter.

The less reliable parts are the float and variable resistance in the tank, the voltage applied, and the ground.

After 42 years clean the grounds, replace the missing ones, clean up the dirty positive contacts in the harnesses, and if that's not it, I'm wondering...

Start researching ground problems (same with wiring harness, electrical, etc.)... The body grounds to the frame. The frame grounds to... Educate yourself by reading lots, not by asking for a solution. For every 10 replies 9 are completely wrong.

[franken]

Finally, get a DVM and learn Ohm's Law. It only has 3 variables.