 Electrical Question, Fuel Gauge reading jumps with lights on... |
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| jim_hoyland |
 Sep 1 2014, 09:03 AM
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#1
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Get that VIN ?  Group: Members Posts: 9,262 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California 
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This has been bugging me for several years; every time the light switch is "ON", the fuel gauge level jumps up almost 1/4 tank.
I have a Getty 5 gauge dash: Clock, Speedo, Tach, Oil Press/Temp, and Fuel/Voltmeter. All the gauges share a common ground and common voltage source for the gauge lights; I'm wondering whether one or both need to be isolated to prevent the interference? Need some electriccal input here... (IMG:style_emoticons/default/smile.gif) |
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| 904svo |
Sep 1 2014, 12:36 PM
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#2
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904SVO Group: Members Posts: 1,118 Joined: 17-November 05 From: Woodstock,Georgia Member No.: 5,146 |
When I put a Subby in my kit car I ran inti the same problem, it turn out to be the
battery lead from the fuse to the battery was dirty. I fix my problem by running another battery lead to the fuse block and supplying battery to the gas gauge. The real problem is the voltage drop from the battery to the fuse block when the headlights are turn on. |
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| Tom |
Sep 1 2014, 12:55 PM
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#3
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Advanced Member Group: Members Posts: 2,139 Joined: 21-August 05 From: Port Orchard, WA 98367 Member No.: 4,626 Region Association: None |
904svo,
you hit the nail on the head! With these cars getting older, the problem will become worse. Tom |
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| Tom |
Sep 1 2014, 12:55 PM
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#4
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Advanced Member Group: Members Posts: 2,139 Joined: 21-August 05 From: Port Orchard, WA 98367 Member No.: 4,626 Region Association: None |
904svo,
you hit the nail on the head! With these cars getting older, the problem will become worse. Tom |
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| stugray |
Sep 1 2014, 01:01 PM
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#5
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Advanced Member Group: Members Posts: 3,824 Joined: 17-September 09 From: Longmont, CO Member No.: 10,819 Region Association: None |
You have a bad ground.
When the lights are on, more current flows through the ground path from the light switch to the battery. A high resistance in the ground path means that the extra current causes a bigger voltage drop in the return path. This means that the gauge sees a smaller voltage from the sensor because the voltage drop in the ground path is shifting the reference voltage up. |
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| jim_hoyland |
Sep 1 2014, 03:05 PM
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#6
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Get that VIN ? Group: Members Posts: 9,262 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California
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Thanks for the inputs: should the Fuel/Voltmeter gauge have a separate ground and separate 12v, instead of the existing series of grounds and power wires ?
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| MartyYeoman |
Sep 2 2014, 01:28 PM
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#7
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Senior Member Group: Members Posts: 1,517 Joined: 19-June 03 From: San Ramon, CA Member No.: 839 Region Association: Northern California
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My car exhibits the same phenomena. I'm just always glad to see that extra 1/4 tank show up, even if it's not real.
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