Gas Guage Prob's - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**hatzie** · 01-13-2013, 07:37 AM

Bouncing is normal in an air core meter. It can be mitigated somewhat with an injection of some special silicone damping gel. Gauge repair guys can do this for you pretty quick.

Not dropping below 1/2 tank indicates increased resistance somewhere in the circuit. Is the ground wire from the sender to the frame making good contact?

You can bench test a fuel gauge with a charged 12v battery, 4 alligator clip test leads, and a couple 1/2W resistors (100ohm and 50ohm 90 & 45 would be better but 100 & 50 are close enough).

It's worth noting that the resistors can get VERY HOT so be careful of where you place them... not on the kitchen table or those greasy rags.

If your gauge passes these tests it's not the problem.

Lay the gauge face down with the 1/2 tank position pointing straight toward you and the indicator lamp window straight away from you.
There are 3 studs on the back of the canister arranged at the points of a triangle.
The (-) GND stud is at the apex of the triangle away from you and closest to the indicator light window. Clip your battery (-) to this stud.
The (+) IGN stud is on the Left hand side of the gauge closest to you. Clip your battery (+) to this stud.
With just the + & - studs hooked up the needle should go almost horizontal @ 3:00 on the face past full.
The Sender stud is on the Right Hand side of the gauge closest to you. Hook a jumper from the ground to the sender stud. The gauge should drop to Empty.
Remove the ground to sender jumper.
Clip the 50ohm resistor between two test leads. Hook the loose ends of this from the ground stud to the sender stud. This should put the needle at just over 1/2.
Un-clip the battery (+) lead and wait till the resistor cools enough to handle.
Swap the 50 ohm to the 100 ohm resistor and hook the battry (+) lead back up. This should put the needle past Full but not 3:00.

motornut · 01-13-2013, 03:59 PM

my 78 Jimmy bounced
when I changed from idiots to tach and cluster
i had to add a seperate ground for the fuel gauge ground
it stopped

my 79
I was worried changing to a spectra unit this summer it would bounce
nope

if it's working right, grounding the sender wire is E,off is over F
I'd check the tank ground for sure

01-13-2013, 07:37 AM	#1
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,927	Re: Gas Guage Prob's Bouncing is normal in an air core meter. It can be mitigated somewhat with an injection of some special silicone damping gel. Gauge repair guys can do this for you pretty quick. Not dropping below 1/2 tank indicates increased resistance somewhere in the circuit. Is the ground wire from the sender to the frame making good contact? You can bench test a fuel gauge with a charged 12v battery, 4 alligator clip test leads, and a couple 1/2W resistors (100ohm and 50ohm 90 & 45 would be better but 100 & 50 are close enough). *It's worth noting that the resistors can get VERY HOT so be careful of where you place them... not on the kitchen table or those greasy rags.* If your gauge passes these tests it's not the problem. Lay the gauge face down with the 1/2 tank position pointing straight toward you and the indicator lamp window straight away from you. There are 3 studs on the back of the canister arranged at the points of a triangle. The (-) GND stud is at the apex of the triangle away from you and closest to the indicator light window. Clip your battery (-) to this stud. The (+) IGN stud is on the Left hand side of the gauge closest to you. Clip your battery (+) to this stud. With just the + & - studs hooked up the needle should go almost horizontal @ 3:00 on the face past full. The Sender stud is on the Right Hand side of the gauge closest to you. Hook a jumper from the ground to the sender stud. The gauge should drop to Empty. Remove the ground to sender jumper. Clip the 50ohm resistor between two test leads. Hook the loose ends of this from the ground stud to the sender stud. This should put the needle at just over 1/2. Un-clip the battery (+) lead and wait till the resistor cools enough to handle. Swap the 50 ohm to the 100 ohm resistor and hook the battry (+) lead back up. This should put the needle past Full but not 3:00. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.

01-13-2013, 03:59 PM	#2
motornut 78K & 79C Jimmys Join Date: Nov 2009 Location: Ottawa Ont CANADA Posts: 7,901	Re: Gas Guage Prob's my 78 Jimmy bounced when I changed from idiots to tach and cluster i had to add a seperate ground for the fuel gauge ground it stopped my 79 I was worried changing to a spectra unit this summer it would bounce nope if it's working right, grounding the sender wire is E,off is over F I'd check the tank ground for sure __________________ John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350



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