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Working on the fuel system today. Hooked up gas gauge today ( new instrument gauge, new sending unit, new wire, ground wire to frame, 12V) and gauge shows nearly empty (tank full) ... I took out the resistor and the gauge reads the same ... Nearly empty... Any ideas on next place to look?

Good day to be working in garage... Cold out there today

Sorry I can't offer any clues,i'm suffering a similar problem so will be following this....

which truck is this in? what was/is the 'resistor' and where did you have it installed? "new" gauge as in aftermarket, rebuilt, or ...?
"ground wire to frame" from what? is the sending unit grounded [mount screw to cab]? is the gauge housing grounded to the dash? did you resistance check the sender function before installing it? did you check gauge/sender function with the sender outside the tank?

Bill

1950 chevy 1 ton with '62 261. New rebuilt gauge cluster.. For 12V system. New sending unit from lmc . Resistor from lmc for 6V to 12V on sending unit... Resistor in power supply to center post on sending unit. New ground wire from sending unit mounting screw to frame (clean connection). Didnt check any of gauges or sending unit before install.. Rest of gauges and instruments work fine.. Just having trouble with this one circuit.

if the gauges were "rebuilt for 12V", there is no need for any extra resistors added ... there is never a need for a resistor on the sending unit, it should be a 0-30 ohm sender regardless of the voltage .... there is no "power supply" to the sender, it completes the gauge circuit to ground - power to one post of gauge, other post of gauge to sender terminal to supply ground via the variable resistance of the sender

I'd pull the sender out and [with a ground wire from sender flange to dash], manually move the float arm and see if the gauge responds, if not then check the resistance of the sender with an ohmmeter [terminal to flange] as you move the float - it isn't unusual to find factory defective senders .... if the sender is good and the gauge doesn't respond to it, either the gauge is bad, the gauge case isn't grounded to the dash, or you have something miswired

Bill

red58- Now THAT"S what I call a helpful answer(s)!!

Real simply. The center post on the sender is not a power supply. It's a ground supply. The sender just supplies more or less ground to your gauge, according to how much gas is in the tank.

Did you figure out why it showed nearly empty on a full tank? Thanks HUGH

Leaky float maybe?

Do you check for voltage going into the gas gauge?
Carl

If I remember correctly, doesn't the sending unit have a ground attachment to frame to complete the circuit? If the sender/tank is grounding correctly, that would throw off the operation of the gauge. Am I all ate up on this???

With no ground to the sender, a functional gauge supplied with voltage will read full.
Carl

Seems like a question about the fuel gauge comes up about once a week.
I believe even a fifth grade student could understand the workings from this
drawing that is circulating on the internet:
http://www.pbase.com/dennygraham/image/159356931
You’re working with a basic series/parallel circuit in which the current runs thru
the first winding (load) then splits into two legs for the current to flow to ground.
One is thru the second winding in the dash gauge to ground.
The other path is to the Sender winding to ground.
When the wiper at the sender is in the empty position it offers no opposition to the
current flow and since current follows the path of least resistance all of it will seek
ground through this path and none will pass through the gauge.
When the wiper at the sender is in the full position it offers a very high resistance to
current flow in relation to the meter and the vast majority of the current flows through
the gauge sensing coil and the needle show full.
If the circuit is open between the gauge to the sender or if the sender is stuck in the
full position then the gauge will read full.
If the circuit is shorted between the gauge and the sender then no or very little current
will flow through the Gauge sensing winding and it will read empty.
That’s basically all there is to it.
Denny Graham
All gassed up in Sandwich, IL

Very nice, Denny

Where did you find that illustration (I'd like to reference the source)?

My browsers show your pbase photos in small size, and even smaller when I click on the image. Here is the large size image, if anyone else wants to see it.

Nice


But I find most the issues come from either

1: converting to 12 volt (what post to put the voltage reducer)

2: Using a new aftermarket gauge and issues with the new needle sitting higher and pinching bezel ( probably number 1)

3: Members refurbishing thier housing : painting the inside white thus loosing ground points.

4: painting area where ground wire for sender attaches ( under body) thus loosing good ground point for sender.

Troubleshooting tips are most helpful

Thanks, Craig. I have not been to Jolly's site in a long while.

I also like the added red-annotations on the image Denny linked-to.

Don't understand that Tim, from the link the image should fill
the screen, does with my Google Chrome browser.

Thanks Craig, I'll credit that in the album. I just typed in a
search for fuel gauges schematic and it was the most explicit
one out of the bunch, no reference to where it came from.

1.You would want a voltage reducer in series, not in one of
the parallel legs. That would be the B+ terminal Gus.
2. That's common sense, if the needle drags, do something to
get it to clear.
3. Use of external star washers should solve the ground
problem, gotta scratch some paint if you want to ground
through the body or frame.
4. Again, external star washers and not being afraid of
scratching the paint.

Never had a need to go to the later GMC forum, I'll have to
look it over one day when I've got a spare moment.

Denny G

3. Star washers are not used on the inside of the housing. This area need good unpainted contact. The back of the new replacement gauge has a metal plate used for the ground path. This plate contacts the INSIDE of the housing.

Was experiencing this same indication with a brand new 12v gas gauge. My gas gauge is reading less than half when tank is full. Go over to the "interior forum" and read all about it with step by step assistance. The topic is gas tank reading half when full, in short it was as simple as a bad brand new gas gauge (bad coil), thanks HUGH

Dirtball hasn't posted since February 15.
I hope he comes back after he figures out what was wrong, and that he didn't come by his nickname honest, and just leaves us hanging.
Carl

Sorry for not getting back. Took the sending unit out and tested as suggested. Looks like my problem is the gauge... Its such a pain to get to I have not gone any further with this. In general just frustrated at present... I bought rebuilt gauges and speedometer and both dont function correctly.. The gas gauge doesnt work and the odometer jumps ahead .. At 6/10 on gauge the 1 appeared.. At 3 miles the 1 in the 10 column rotated forward... At 60 miles the 1 in the hundreds column rotated forward... Waiting to see when the 1 for the 1000 column will show up?
Hasnt stopped me from driving but with no accurate odometer and a malfunctioning gas gauge im back to guessing when to fill up and carrying an extra 5 gal gas can.

Were the numbers in the odometer resurfaced?

Sounds like someone just restickered it without indexing it properly

Should be an easy fix

Getting the gauge cluster out is a process. Getting a problem after you put them back, is a PITA. I don't fit too well on the floor with the pedals and all so I found a way to get to the dash while still on the seat. How to reach and see what you are doing is just part of the problem. You have to drain the antifreeze below the sender. You have to mark the lights. You will have to deal with the rubber grommet, and you still have not put a wrench on the gauge yet. So be patient, take a deep breath and when the water is out of the radiator, and you have decided what to do with that darn grommet, then get ready to take the dash apart. I start with the battery cables(safety) and then the glove box. Just the liner, funky cardboard part. My truck, 52 1/2 ton with a recirculating type heater has enough room to push the liner towards the firewall and tip it out from under the dash. Pull the ashtray out and reach through the glove box to the fasteners on the trim. A nut driver type of screw driver will fit. You will get enough room to work and see from here. There are good tips on the gauges in the tech tips. And you can fix most things on the bench. The odometer bits can be a challenge, but they can be sorted. If the grommet in the firewall is soft, you can peel it out and shove it through the firewall. But to get the temp sender out I just split it. If it is hard, well it needs to be replaced anyway. You won't be able to save it.

Contacted the seller about the rebuilt instrument gauges he supplied... said he couldn't help - 30 day warranty period... even though odometer is not a malfunction... the odometer is off because of poor rebuild... if the problem could be somehow related to my installation I would accept part of blame... but not indexing odometer cylinders has nothing to do with my install or from sitting in his packing in a heated garage for over 30 days... just plain rebuilt wrong and he doesn't seem to care.

FYI - the vendors name on ebay is "rpenaloza909" - BEWARE - poor rebuild and workmanship... and basically no warranty or effort to stand behind his work... will proceed with ebay for deadbeat vendors but don't hold much chance of resolving... either way I would have to remove and reinstall all wiring and gauges

unfortunate

many of us have run into the same situation when we buy parts in advance and run out of the warranty

the seller did offer a warranty, but the problem was not discovered until after the warranty period. Looking at some of his past sales some of the descriptions are questionable.



Best to put this behind and make the gauges work. You are the best mechanic you know. Members here will help you.

get the knuckles bloody and get them out.

I don't have a radio or speaker so I like to pull the speaker grill, get the speedometer out first.

ask questions maybe start your own thread on the odometer fix, post pictures if possible.