Fuel Gauge - The 1947 - Present Chevrolet & GMC Truck Message Board Network

GMC_Aaron · 07-08-2002, 07:50 PM

I recently bought a 1972 GMC Super custom truck and the fuel gauge has a problem. It seems to work slightly, but when I fill it up, it only reads 1/3 tank. I pulled the sending unit and saw that it needed to be replaced. I thought that maybe that would do the trick, it didn't. I checked the electrical and everything checked out ok. I did notice that when I pulled the wire connected to the sending unit, the needle went past full. I'm confused now. is there a way to calibrate these gauges? Or is there more to it? Can the needle be taken off and adjusted? I haven't attempted to take the gauge out yet, but I have checked everything else to this point and I have only the gauge left. It seems like it has its' range of motion, but I was thinking that maybe the needle was messed with at some point in time before I got the truck. I don't want to tear the dash apart until I get some more insight. Thanks.

Bill72BB · 07-08-2002, 08:25 PM

Arron;
When you pull the wire off the sending unit it should put the gauge past full . That is nornal.
Sounds like the problem is ;

The Sending unit . Should read 90 to 0 ohm's as you raise and lower the float arm.
The float make sure it doesn't have a pin hole and has some gas in it .( Shake it )
The float is getting stuck as it rises in the tank to indicate a full tank.

Check these 3 things and they will fix 99 % of what you describe

Good luck Bill

Longhorn Man · 07-08-2002, 08:28 PM

First of all...welcome to the board.
Your sending unit is probably trashed.
When you disconnect the power, and it goes all the way to full, that means that everything north of the wire connection is good. Your getting power to the guage and the guage knows how to read the resistance reading from the brown wire. To verify this, remove that brown wire from the tank, and ground it out on the body 9good clean paint free ground). Your fuel guage should bottom out at or past empty.
Did that make any sence? Hope i didn't lose anyone there.

GMC_Aaron · 07-08-2002, 08:34 PM

but I just installed a new sending unit. When you buy new, you expect that to work perfectly. Is it hit and miss with the gauge? Either it works or it doesn't? I bought the sending unit from Brother's and installed it right away. I also bought a new gauge just for kicks. Everything seems to check out up to the gauge.

GMC_Aaron · 07-08-2002, 11:15 PM

Ok, I'm starting to take different angles to this. I just bought a new sending unit, which should be alright, but I'm still confused, even after the insight you guys have given me. The gas tank might even be different if the float is catching on something. Where might the part number be located on the tank, so that I can verify that it is the stock tank? Like I said earlier, I replaced the sending unit and checked all of the electrical between the sending unit and the gauge. I think there is something wrong with the gauge, but it seems to move through its' full range of motion when I mess with the wire taking it off and on.

1968 C-10Cowboy · 07-08-2002, 11:35 PM

Very nice looking truck there Aaron. Im having the same problem. I took some crome backed 5" guages off an 80's model Buick centry or somethin...anyway I had to re-wire my dash I went from the fuse panel to the + post on the guage..the other + post to the float and the 3rd post to ground. I either get a VERY full tank or if I switch the 2 +'s around I get a VERY empty tank..no woking here either.

GMC_Aaron · 07-08-2002, 11:46 PM

Do you have a posi rear end? That is my next endeavor along with a tranny with overdrive. Do you have any recommendations for either?

bigvinnie · 07-09-2002, 02:29 AM

Aaron, sounds like you're still having a problem at the sender.... if the gauge goes way past full when you ground the sender wire then the gauge is acting normally.... remove the sender and move the float thru a full range of motion and see if the gauge responds normally.... (you may have to ground the sender)

if you can get normal readings this way, then the float is getting hung up on something in the tank, or the float is taking on gas and not floating.... if you don't get normal readings, my guess is you got a bad sender... stuff like that happens once in a while and can drive you crazy trying to figure out what else could be the problem... good luck.

GMC_Aaron · 07-09-2002, 02:38 AM

Thanks bigvinnie. How's the 700R4? I have a TH400, but I want something w/overdrive. Can I do anything or is it worth doing anything to the TH400 to accomplish my overdrive aspirations? I also want to put a posi rear end in it as well, but I'm still in the early stages. Any suggestions?

bigvinnie · 07-09-2002, 03:33 AM

I'm very happy with the 700.... I had a th350 but wanted the overdrive also... it's a straight forward install and you get the lower first gear combined with decreased rpms on the hiway, so it's a win/win situation...

only thing you could do with a th400 is add a gearvendors overdrive behind it, very spendy but would be cool to hear from someone using that combo...

I actually blew up my first 700 pretty quickly because I was towing in overdrive without a trans cooler, not too smart.... had the clutch pak replaced, put in a cooler and haven't had a problem since... the key is running a trans cooler and properly setting up the TV cable...

can't help on the posi unit, but there's lots of good aftermarket drop-ins out there, maybe someone else has a recommendation...???

bmg · 07-09-2002, 09:00 AM

I had the same problem and replaced the sender, it sounds like you may have gotten a bad sender or like the others said its hanging up.

Now my ? i put a new sender in and the gage when full goes equivlent to 1/2 tank past the full marker and when it says "E" I'm pushing. Any way to calibrate this? Luckily my brother was in the truck when it went dry, now i carry a 5 gallon gas can.

1ToBjimmyk5 · 07-09-2002, 10:55 AM

Just a guess, but how is the tank installed? If the tank is not grounded would that make the gauge not work?

ANYONE????

lukecp · 07-09-2002, 01:30 PM

The guage would still work if the tank is not grounded. I know this, because i took the sending unit out of my tank, hooked the wire up to it, and held it in the air moving the float arm up and down. The movement of the float arm showed up in the guage, showing me that the sending unit was OK. Now the sending unit isn't OK, and i need to buy another one. The first time, when the guage wasn't working, all i had to do was take the sending unit out, and put it back in and that somehow magicaly fixed my gas guage

.

TERRYP · 07-09-2002, 04:47 PM

Bad science on the grounding of the sending unit. The whole idea is sending power thru the guage then out to the veriable resistance of the sending unit to ground. You must have a good ground on the sending unit. The resistor is a wound resistance wire with a wiper that rubs on it and in effect measures the length of the wound wire. You can take toe cover off the unit and clean the wound wire and the wiper contact. Lots of times the wound wire breaks, or the wiper contact is crudded up. If it won't work out of the tank, it sure won't work after you install it!

misfits · 07-09-2002, 04:55 PM

GMC_Aaron - I have the exact same problem. I disconnected my guage and now it reads about a 1/2 tank past full. I know its not the sending unit, the guage is fine, and I put new connections on the guage. Still nothing. I just make regular trips to the gas station now.

07-08-2002, 07:50 PM	#1
GMC_Aaron Registered User Join Date: Jul 2002 Location: Ventura, Ca. Posts: 87	Fuel Gauge I recently bought a 1972 GMC Super custom truck and the fuel gauge has a problem. It seems to work slightly, but when I fill it up, it only reads 1/3 tank. I pulled the sending unit and saw that it needed to be replaced. I thought that maybe that would do the trick, it didn't. I checked the electrical and everything checked out ok. I did notice that when I pulled the wire connected to the sending unit, the needle went past full. I'm confused now. is there a way to calibrate these gauges? Or is there more to it? Can the needle be taken off and adjusted? I haven't attempted to take the gauge out yet, but I have checked everything else to this point and I have only the gauge left. It seems like it has its' range of motion, but I was thinking that maybe the needle was messed with at some point in time before I got the truck. I don't want to tear the dash apart until I get some more insight. Thanks. __________________ Aaron Ventura, Ca. 1972 GMC Super Custom SB350/TH400

07-08-2002, 08:25 PM	#2
Bill72BB Thats the Hillbilly way Join Date: Jul 2001 Location: Torrance,Ca Posts: 5,980	Arron; When you pull the wire off the sending unit it should put the gauge past full . That is nornal. Sounds like the problem is ; The Sending unit . Should read 90 to 0 ohm's as you raise and lower the float arm. The float make sure it doesn't have a pin hole and has some gas in it .( Shake it ) The float is getting stuck as it rises in the tank to indicate a full tank. Check these 3 things and they will fix 99 % of what you describe Good luck Bill __________________ BILL Torrance Calif 90504 Member DUF Club Member Old Farts Club 72 3/4ton Custom Camper BB402/400 trans Dynomax Cerama-coat headers , Hei distributor ,Edelbrock 1411 air,ps,pb,leafsprings ,98 chevy 60/40 seat,S10 shoulder belts tilt,3.54 Dana 60 rear. 16" Eagle wheels with LT265/75 16 tires,Gaylord Lid,H4 headlights with 100W relay conversion. " If I knew I was going to live this long I would have taken better care of myself " Marriage changes passion...suddenly you're in bed with a relative. A GUY'S PRAYER.... Lord, I pray for a nympho with huge boobs who owns a liquor store and likes to fish and hunt and has a Harley .Amen ,

07-08-2002, 08:34 PM	#4
GMC_Aaron Registered User Join Date: Jul 2002 Location: Ventura, Ca. Posts: 87	but I just installed a new sending unit. When you buy new, you expect that to work perfectly. Is it hit and miss with the gauge? Either it works or it doesn't? I bought the sending unit from Brother's and installed it right away. I also bought a new gauge just for kicks. Everything seems to check out up to the gauge. __________________ Aaron Ventura, Ca. 1972 GMC Super Custom SB350/TH400

07-08-2002, 11:15 PM	#5
GMC_Aaron Registered User Join Date: Jul 2002 Location: Ventura, Ca. Posts: 87	Ok, I'm starting to take different angles to this. I just bought a new sending unit, which should be alright, but I'm still confused, even after the insight you guys have given me. The gas tank might even be different if the float is catching on something. Where might the part number be located on the tank, so that I can verify that it is the stock tank? Like I said earlier, I replaced the sending unit and checked all of the electrical between the sending unit and the gauge. I think there is something wrong with the gauge, but it seems to move through its' full range of motion when I mess with the wire taking it off and on. Attached Images __________________ Aaron Ventura, Ca. 1972 GMC Super Custom SB350/TH400

07-08-2002, 11:35 PM	#6
1968 C-10Cowboy Registered User Join Date: Jul 2002 Location: Midland,Texas Posts: 46	Very nice looking truck there Aaron. Im having the same problem. I took some crome backed 5" guages off an 80's model Buick centry or somethin...anyway I had to re-wire my dash I went from the fuse panel to the + post on the guage..the other + post to the float and the 3rd post to ground. I either get a VERY full tank or if I switch the 2 +'s around I get a VERY empty tank..no woking here either. __________________ 350/.30 400 Turbo Trans 3.73 Rear end

07-08-2002, 08:28 PM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	First of all...welcome to the board. Your sending unit is probably trashed. When you disconnect the power, and it goes all the way to full, that means that everything north of the wire connection is good. Your getting power to the guage and the guage knows how to read the resistance reading from the brown wire. To verify this, remove that brown wire from the tank, and ground it out on the body 9good clean paint free ground). Your fuel guage should bottom out at or past empty. Did that make any sence? Hope i didn't lose anyone there.

07-08-2002, 11:46 PM	#7
GMC_Aaron Registered User Join Date: Jul 2002 Location: Ventura, Ca. Posts: 87	Do you have a posi rear end? That is my next endeavor along with a tranny with overdrive. Do you have any recommendations for either? __________________ Aaron Ventura, Ca. 1972 GMC Super Custom SB350/TH400

07-09-2002, 02:29 AM	#8
bigvinnie Registered User Join Date: Aug 2001 Location: escondido, ca 92026 Posts: 1,222	Aaron, sounds like you're still having a problem at the sender.... if the gauge goes way past full when you ground the sender wire then the gauge is acting normally.... remove the sender and move the float thru a full range of motion and see if the gauge responds normally.... (you may have to ground the sender) if you can get normal readings this way, then the float is getting hung up on something in the tank, or the float is taking on gas and not floating.... if you don't get normal readings, my guess is you got a bad sender... stuff like that happens once in a while and can drive you crazy trying to figure out what else could be the problem... good luck. __________________ '72 short step, 350, 700R4, tilt, ps, pdb, a/c, lowered coils, etc., other work in progress... San Diego, CA 72 Stepside Project

07-09-2002, 02:38 AM	#9
GMC_Aaron Registered User Join Date: Jul 2002 Location: Ventura, Ca. Posts: 87	Thanks bigvinnie. How's the 700R4? I have a TH400, but I want something w/overdrive. Can I do anything or is it worth doing anything to the TH400 to accomplish my overdrive aspirations? I also want to put a posi rear end in it as well, but I'm still in the early stages. Any suggestions? __________________ Aaron Ventura, Ca. 1972 GMC Super Custom SB350/TH400

07-09-2002, 03:33 AM	#10
bigvinnie Registered User Join Date: Aug 2001 Location: escondido, ca 92026 Posts: 1,222	I'm very happy with the 700.... I had a th350 but wanted the overdrive also... it's a straight forward install and you get the lower first gear combined with decreased rpms on the hiway, so it's a win/win situation... only thing you could do with a th400 is add a gearvendors overdrive behind it, very spendy but would be cool to hear from someone using that combo... I actually blew up my first 700 pretty quickly because I was towing in overdrive without a trans cooler, not too smart.... had the clutch pak replaced, put in a cooler and haven't had a problem since... the key is running a trans cooler and properly setting up the TV cable... can't help on the posi unit, but there's lots of good aftermarket drop-ins out there, maybe someone else has a recommendation...??? __________________ '72 short step, 350, 700R4, tilt, ps, pdb, a/c, lowered coils, etc., other work in progress... San Diego, CA 72 Stepside Project

07-09-2002, 09:00 AM	#11
bmg Registered User Join Date: Nov 2001 Location: Indianapolis, IN USA Posts: 157	I had the same problem and replaced the sender, it sounds like you may have gotten a bad sender or like the others said its hanging up. Now my ? i put a new sender in and the gage when full goes equivlent to 1/2 tank past the full marker and when it says "E" I'm pushing. Any way to calibrate this? Luckily my brother was in the truck when it went dry, now i carry a 5 gallon gas can.

07-09-2002, 10:55 AM	#12
1ToBjimmyk5 Registered User Join Date: Apr 2002 Location: Centreville,VA & Millsboro, DE Posts: 47	ground? Just a guess, but how is the tank installed? If the tank is not grounded would that make the gauge not work? ANYONE???? __________________ 1971 Chevy Blazer(looks like Jimmy),1988 Ford E350 Van,2001 Nissan Sentra(Wife), 1993 Nissan Sentra (to/from work),1985 Dixie (boat) with a big Johnson,And I want my 1970 Buick Conv. back!!!! Centreville,VA

07-09-2002, 01:30 PM	#13
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	The guage would still work if the tank is not grounded. I know this, because i took the sending unit out of my tank, hooked the wire up to it, and held it in the air moving the float arm up and down. The movement of the float arm showed up in the guage, showing me that the sending unit was OK. Now the sending unit isn't OK, and i need to buy another one. The first time, when the guage wasn't working, all i had to do was take the sending unit out, and put it back in and that somehow magicaly fixed my gas guage . __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

07-09-2002, 04:47 PM	#14
TERRYP GYRO GEARLOOSE Join Date: Jun 2000 Location: Nevada City, CAEverett, WA Posts: 361	Bad science on the grounding of the sending unit. The whole idea is sending power thru the guage then out to the veriable resistance of the sending unit to ground. You must have a good ground on the sending unit. The resistor is a wound resistance wire with a wiper that rubs on it and in effect measures the length of the wound wire. You can take toe cover off the unit and clean the wound wire and the wiper contact. Lots of times the wound wire breaks, or the wiper contact is crudded up. If it won't work out of the tank, it sure won't work after you install it! __________________ Nevada City, CA Everett, Washington 57 3/4 rat, 235, 3spd 66 C-10 Panel, SWB. 350 from '68, 200R4 tranny, 3.07 rear ratio, restoring 68 C10 LWB 350 4spd, 4.56 rear dump runner 82 Camaro, 305, THM-200-R4 84 Monte Carlo 305 TH350, land tuna 85 Tercel SW, snow runner DISGRACED OUTLAW MEMBER OF THE DRUM BRAKE CLUB

07-09-2002, 04:55 PM	#15
misfits Registered User Join Date: Feb 2001 Location: Sturgeon Bay , WI USA Posts: 375	GMC_Aaron - I have the exact same problem. I disconnected my guage and now it reads about a 1/2 tank past full. I know its not the sending unit, the guage is fine, and I put new connections on the guage. Still nothing. I just make regular trips to the gas station now.



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