Gas Gauge

[Blue Tank]

So i tackled mine today. Break connection at tank connector and it goes to empty and to ground it reads full.
Hookup a new sender that ohms out correctly outside the tank and it will only swing from empty to 1/4 like the original sender in the tank.
I popped the brown wire from the connector where it goes into the printed board and ran leads from the gauge to the new sender on the floor. It will still only read to 1/4 so that tells me it's in the gauge or the resistor like Longhorn mentioned.
Any idea what the resistor value is? Are they available?
I'm reading about 40 ohms on the brown line back at the tank with the switch off so i think something is causing a parallel resistance which would explain the low reading of 1/4 tank?

[AzDon]

I like your idea of using a new sender at the end of the wire to simulate final ops before dropping tank. When hooking it to the end of the wire, did you also run a ground wire from the chassis to the new sender's mounting plate to complete a circuit? (sorry if this seems like a dumas Q)
My tank is showing 1/4 when connected, so I'll be interested to know what fixes the problem.....

[Blue Tank]

Yes it still requires the ground to chassis. In my hi tech world the brake pedal pad was off and nice and shiny. Of course check for continuity from the grnd. point (brake pedal) to a ground point on the dash to be sure. I used the pedal because my leads were short and i was in lazy mode.
The resistor on mine wouldn't give a reading where the nuts bear down on it. If i lightly pushed the points of my leads thru the blue plastic where the carbon trace comes out on both sides i got 85 ohms. I guess this makes sense since the sender is 90 ohms and they must be wired in parallel. With resistors in parallel the effective resistance is always lower than the lowest value in the circuit. So as the sender float drops it's resistance value goes down and the total circuit R value drops while the current changes giving you a gauge reading. If you do the formula using the 40 ohms i read across the brown line near the tank it comes out to about 28 ohms total which is roughly 1/3 of 90 or a reading of 1/4 tank on the gauge. High school is slowing coming back to me.
When i was searching for the resistor another thread on this board came up and they gave a source. I'll try to include the link below. I'm going to try the "cell phone" store.... oops i mean the former Radio Shack today to see what they have in 85-90 ohm resistors. The place that has the original type is on the other coast and a generic one should do the trick with some rings soldered on. I'll let you know what i find.
http://67-72chevytrucks.com/vboard/s....php?p=3223041

[Blue Tank]

I should clarify one thing about checking with the sender at the dash. You need to remove the big connector right above fuel gauge and pull pin 4 which is the wire from the tank. With the pin end facing you lightly push a real small flat screwdriver in the space below the pin and pull back on the wire. The big connector need to be plugged back in for the gauge to function. Connect your jumper to the passenger side terminal sticking out of the fuel gauge.
The good news is that i don't think you need to go thru all that crap, i believe it's safe to assume it's the resistor at that point.

[Blue Tank]

Well it seems to work but there are a couple of things to mention.
My local Radio Shack had 68 and 22 ohm resistors in stock for .99 cents per pack of 5. The 68 p/n is 271-1106 and the 22 is 271-1103.
Take one end of each and twist them together with a couple of turns to hold them and solder the connection, clip off the excess lead. On their other ends put a small crimp on ring terminal and solder that also. Crimps are a mechanical connection, solder is an electrical connection (beat into my head years ago). With the 2 in series you will have about 90 ohms.
Make sure the key is off and disconnecting the battery is a good idea.
You must put the old resistor on the gauge first like it was before, it will provide insulation and has a recess to clear the raised metal part. The blue side goes out if you forget. Next is the printed ribbon and then your resistor lugs followed by the flat washers and a nut. I didn't use the wave washer because i though it might hurt the ribbon.
Without the old resistor or something for insulation the fuse will blow from the short.
I took a pic of the resistor assy. if guys have trouble visualizing it. I think one of you guys would have to post it, I don't have a snapfish etc. acct.

[AzDon]

I've replaced that resistor with one from a donor gauge cluster and still it does not work... At least now I've got a working odometer...

[Blue Tank]

Did you confirm with a meter that you have 90 ohms where the carbon trace goes around the lugs? I wouldn't be too surprised if it worked fine on the old gauge and doesn't on the new one. Removing and installing could break the trace and still look ok to the eye. If you need help with the resistor pm me.

[72ArizonaBurb]

Dubds10: Did you ever solve this problem? Mine is doing the same thing. Thanks!

Originally Posted by dubds10

Mine moves from empty to 1/4 full and never more, even on a full tank lol Damn old chevy gauges. For how much gas this truck sucks back though, I'd be afraid to see how fast it'd move if it actually worked properly

[AzDon]

Originally Posted by 1971BB427

Ok I'm really confused now. Troubleshooting the wire from gauge to the tank and found several things.
1-With no power to the gauge (fuse pulled) the gauge reads full. Connect power and it goes to empty with a full tank of gas.
2-The wire at the tank doesn't go from the Pass. side, it goes through the driver's side rear tailight area.
3-When I pulled the wire the gauge goes to empty.
4-When I ground the wire the guage goes to full.

Read the sending unit with a full tank of gas and I get 8 ohms resistance to ground.
Not sure if the sending unit reads 90 ohms full or empty. Anyone know for sure?

If your gauge shows full with the ignition on, the gauge head is being supplied with electricity and the sender wire on the negative side is fully grounded! The wire to the tank is the negative side of the circuit and if the wire is fully grounded (as the sender is when the tank is full) then the gauge will show full. If the gauge shows empty, then either your tank is empty or the wire or sender has a full disconnect (broken wire) somewhere!
It's really that simple! If you can demonstrate full and empty by disconnecting (empty) or grounding (full) the sender wire "back there", then the problem is with the sender itself or the grounding of the sender housing to the chassis! If you disconnect the wire from the sender and it shows full, then you probably have the sender wire grounded somewhere and you can confirm this by pulling this wire out of the plug on the back of the gauge cluster and observing that it no longer says full. If it still says full the gauge head is broken!

[67 Burb]

Originally Posted by AzDon

If your gauge shows full with the ignition on, the gauge head is being supplied with electricity and the sender wire on the negative side is fully grounded! The wire to the tank is the negative side of the circuit and if the wire is fully grounded (as the sender is when the tank is full) then the gauge will show full. If the gauge shows empty, then either your tank is empty or the wire or sender has a full disconnect (broken wire) somewhere!
It's really that simple! If you can demonstrate full and empty by disconnecting (empty) or grounding (full) the sender wire "back there", then the problem is with the sender itself or the grounding of the sender housing to the chassis! If you disconnect the wire from the sender and it shows full, then you probably have the sender wire grounded somewhere and you can confirm this by pulling this wire out of the plug on the back of the gauge cluster and observing that it no longer says full. If it still says full the gauge head is broken!

I always thought the sender read 90 ohms at full and near 0 ohms at empty. Therefore a grounded sender wire would show empty.

[AzDon]

Originally Posted by 67 Burb

I always thought the sender read 90 ohms at full and near 0 ohms at empty. Therefore a grounded sender wire would show empty.

My testing indicates "grounded=full" and "disconnected=empty"

Yes- you are making a logical assumption IF the 90 ohms reading indicates full(?) Maybe I've got bigger problems?

[Blue Tank]

You need to remember this is a parallel resistance circuit. So grounding or unplugging the sender wire is only going to tell you part of the story. I feel safe that 80-90% of the time it's the gauge resistor. If you make one up like i described you should be good.
Thinking of this in terms of neg or pos side isn't really correct.
The gauge reads current so it's an inverse relationship to resistance. Current = voltage/ resistance. So current= 12v/total res of gauge circuit.
The total resistance is
R1*R2/ R1+R2
So assuming a good gauge Resistor and a full tank
90*90/90+90 =8100/180=45 ohms
Now the current
I = 12v/45ohms=.267 amps
half tank is 45 ohms so
90*45/90+45=4050/135=30 ohms
so
I= 12v/30ohms=.4amps

[Blue Tank]

now assuming a good gauge resistor and unplugged sender. Using 100k for the open wire to make the point, it's obviously infinite.
90*100000/90+100000=9,000,000/100090=89.92 ohms
I=12v/89.92=.133a
Grounding sender wire
90*0/90+0=0
I=12v/0=max current.
They typically have a shunt resistor in the gauge for this to prevent frying it.
To troubleshoot it you need to break the system into 2 parts.
1 does anything happen when you turn the key on? NO check fuse Yes goto 2
2 unplug the connector i previously described from the gauge assy.
3 measure resistance across the 2 threaded terminals on back of gauge, it should be about 90 ohms. if not that resistor is bad. If yes go to 4
4 measure resistance from tan wire in connector you removed to ground. It should relate to the sender position. If it's infinite you have an open wire or the sender is bad. You should get a value from 0-90 ohms
5 If you get an infinite or open reading break the connection to the sender near the bumper. Take a reading from the wire to the sender to ground, it should be 0-90 ohms. If it's open track it to the connector on the top of the sender etc.
If you get a reading on the sender wire that means you have a broken wire between the dash connector and the bumper junction.

[chevy84life]

This is the resistor created by Blue Tank.

[orange72sub]

Ok I have been working on my fuel gauge, its not worked properly for yrs the previous owner said the last owner 7yrs ago replaced the sending unit and it still didn't fix it. I went ahead and replaced it again to confirm that it was the right one. When I disconnect the brown wire to the sending unit with the ignition switch on the gauge reads full. When I ground out the brown wire from the gauge at the tank, it reads empty. when I plug it back in to the gauge with a full tank it reads 1/4 tank. I tried a different gauge cluster out of a custom package truck oem the wiring harness plugged in but the turn signal indicators stayed lit up and the gauge went form empty to 6:00 empty. I need help I checked the ohms on the gauge read 85ohms from terminal to terminal the gas sending unit is reading 85-90 ohms with the full tank. what am i missing here I have checked connections and wiring harness at the back left corner. Is it possible that the leads out of the sending unit are different orientation than the original? Any help would be great. I have read the other post and there seems to be some inconsistencies with what the gauge is supposed to read versus what I am reading. Any help would be great!

[orange72sub]

Well it appears you were right all along "Blue Tank" thanks for the help I swapped out the other gas gauge from my other cluster and it had a good resistor in it and the gauge now works. I will go ahead and get the resistors from the shack and fix the old one so that the other cluster has a working gas gauge. Thanks to everyone for their input and help. So if your gauge is reading 1/4 tank at full and empty at empty it is 95% of the time going to be your resistor on your gauge. Which by the way is a whole lot easier and cheaper to fix!