One Millionth Post on Fuel Gauge Not Reading Correctly - 1974 stock

[Chief Brody]

So it's either the sending unit or one of the wires...someone remind me...if the resistance is high the needle goes ______________?

I may have a wire that got too close to the exhaust pipe...that would increase resistance...no?

 

[Chief Brody]

Clock works now....

 

[dyeager535]

I'm somewhat confused. The new one(??) appears to be the 80's vintage. Someone correct me, but that design AFAIK is late 70's at the earliest.

You can always simply test the resistance on the terminal on the tank to ground. If it correlates to what you see on the gauge, the sender is the issue.

There is no load on the sender wire, I doubt even a few cut strands would affect how the gauge reads. Since the gauge still moves, I really dont see wiring or connections being the issue.

When I took my old sender out, the wrapped wires were "varnished". I could somewhat clean it, but it still didnt read properly.

 

[rampage]

So it's either the sending unit or one of the wires...someone remind me...if the resistance is high the needle goes ______________?

I may have a wire that got too close to the exhaust pipe...that would increase resistance...no?

I just posted this ...

Just did a little research, 3 O’clock is with it open and shorted to bare metal on the frame is Empty. If you have some resistors or potentiometer in the 0-100 range you could connect that to the plug and see what the gauge reads. Also check the resistance reading of the sending unit to see what it reads.

So, disconnected (open) resistance is high and will peg to 3 O’clock. Shorted to frame (zero ohms) pegs the other way.

 

[Chief Brody]

I'm somewhat confused. The new one(??) appears to be the 80's vintage. Someone correct me, but that design AFAIK is late 70's at the earliest.

You can always simply test the resistance on the terminal on the tank to ground. If it correlates to what you see on the gauge, the sender is the issue.

There is no load on the sender wire, I doubt even a few cut strands would affect how the gauge reads. Since the gauge still moves, I really dont see wiring or connections being the issue.

When I took my old sender out, the wrapped wires were "varnished". I could somewhat clean it, but it still didnt read properly.

and now that I look, I realize that these things came with a 25 gallon or a 31 gallon tank...I don't know which the sending unit I got is for, not am I completely sure what size my Blazer has...I suspect 25 gallon though....

 

[Chief Brody]

I may have a 31 gallon sending unit and a 25 gallon tank...hmmm GM # 6429392

 

[Wes Harden]

Use a incandescent test light between ground and the disconnected wire at tank. If wiring and guage good will read 5/8 of tank.

 

[wasted wages]

Correct me if I'm wrong, but GM fuel gauges all work on 0-90 ohm. So I would think it wouldn't make a difference as both tanks would still have a 0-90 ohm range..

most aftermarket gauges come available with 0-90 ohm ranges to work with the stock sending units..

 

[rampage]

and now that I look, I realize that these things came with a 25 gallon or a 31 gallon tank...I don't know which the sending unit I got is for, not am I completely sure what size my Blazer has...I suspect 25 gallon though....

31 gal sending unit is too long for 25 gal tank. Easy way to tell the difference between the two size tanks - 25 gal tank is flat on the bottom while 31 gal is angle upwards for the last half of the tank for better departure angle.

 

[Chief Brody]

31 gal sending unit is too long for 25 gal tank. Easy way to tell the difference between the two size tanks - 25 gal tank is flat on the bottom while 31 gal is angle upwards for the last half of the tank for better departure angle.

Good to know...pretty sure mine is flat on bottom

 

[dyeager535]

All the gauges are 0-90 ohm. Only thing that changes is the depth of the sender.

Now, if you put a 25g sender in a 31g tank...but obviously it would still read full when full, and empty when you had significant fuel left.

 

[Chief Brody]

Update: so I found a bottle of Techron Fuel Additive on my garage shelf that I had bought for the Honda. I put it in the Blazer tank a couple days ago and shook it side to side vigorously every day I got home from work.

Put the key in it today and it read between 1/2 and 3/4 tank, so it is definitely varnish in the sending unit. I bought a huge jug of Lucas Fuel cleaner and poured it in and shook the truck. I will check it again on Saturday...I might get lucky and the gum and varnish may not be so bad that it can't be cleaned off with fuel cleaners???

 

[Chief Brody]

It's cleaning up a little more....wonder if it will stay that way?

 

[wasted wages]

That's crazy....who would have thought that the sender could have been varnished up to the point of not reading anymore..

Today's gas is such crap.. adding the fuel cleaner is a good trick to remember...I never have put much faith in snake oils...but it seems to have worked out for your issue..

 

[Chief Brody]

That's crazy....who would have thought that the sender could have been varnished up to the point of not reading anymore..

Today's gas is such crap.. adding the fuel cleaner is a good trick to remember...I never have put much faith in snake oils...but it seems to have worked out for your issue..

I still need to keep an eye on it for a while. Gonna continue to put cleaner in it for a while and shake it up. I'm not sure that it would clean it if I weren't leaving it in for days and shaking the truck to slosh the gas.

Also, remember that I have been buying 100% gasoline... It may be a bad idea...not everybody buys 100% gasoline and it may set in the tank at the store a long time.

I am tempted to try half a tank of ...oooooooooooooohhhhh!!!!! 15% Ethanol....

 

[diesel4me]

The rheostat on the sending unit is lust a coil of wire with a "wiper" contact that slides up & down with the float..
It's not unheard of for them to get a coating of varnish on them when the vehicle sits a long time--when your driving it the wiper is constantly moving across the coils of the rheostat and keeps it clean..

The "gas" we have today is far from what these trucks were designed to run on ,thats for sure..the alcohol makes more problems than it ever solved..When ethanol & water mix it eats away at carbs and other die cast fuel system parts,and most carb cleaners wont cut the grain based white pasty crap it leaves behind..
I have boiled small engine carbs in lemon juice to remove all that crud,it works pretty well,a biker taught me that trick..I have resurected some carbs I thought no way would ever work again that way..the citric acid really cleans the metal good..

 

[Chief Brody]

The rheostat on the sending unit is lust a coil of wire with a "wiper" contact that slides up & down with the float..
It's not unheard of for them to get a coating of varnish on them when the vehicle sits a long time--when your driving it the wiper is constantly moving across the coils of the rheostat and keeps it clean..

The "gas" we have today is far from what these trucks were designed to run on ,thats for sure..the alcohol makes more problems than it ever solved..When ethanol & water mix it eats away at carbs and other die cast fuel system parts,and most carb cleaners wont cut the grain based white pasty crap it leaves behind..
I have boiled small engine carbs in lemon juice to remove all that crud,it works pretty well,a biker taught me that trick..I have resurected some carbs I thought no way would ever work again that way..the citric acid really cleans the metal good..

well so much for the ethanol idea

 

[Chief Brody]

That's crazy....who would have thought that the sender could have been varnished up to the point of not reading anymore..

Today's gas is such crap.. adding the fuel cleaner is a good trick to remember...I never have put much faith in snake oils...but it seems to have worked out for your issue..

Honestly I believe the Techron did the most good