gas gauge rising and falling - Impala Tech
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post #1 of 10 (permalink) Old 11-13-2008, 06:20 PM Thread Starter
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gas gauge rising and falling

noticed the other nite that when I turn on the lights on my 63 Impala wagon, my gas gauge falls about one quarter of a tank, when I turn off the lights, you guessed it, the gauge returns to normal level. This happens everytime the lightswitch is used.
Help!

Nodak
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post #2 of 10 (permalink) Old 11-13-2008, 06:37 PM
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The 90 Ohm resistor wire that measures the level in the tank is voltage sensitive (another way of saying sensitive to excess resistance. If you have a bad ground the sending unit will not see the full voltage it needs to measure the level of fuel. So the bulb's added resistance to the rear wiring harness makes the gauge move.

Big Dave
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post #3 of 10 (permalink) Old 11-13-2008, 10:43 PM
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Hey Richard,

Check ALL ground connections... One positive aspect of this is ...You'll never run out of Gas at night!!!!!!
Gary
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post #4 of 10 (permalink) Old 11-16-2008, 10:02 PM Thread Starter
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Thanks for the tips and ideas, it's gonna give me something to do. Thanks for the good thoughts, Gary.

Nodak
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post #5 of 10 (permalink) Old 11-27-2008, 02:27 PM
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three for ground
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post #6 of 10 (permalink) Old 12-09-2008, 05:58 PM
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Quote:
Originally Posted by Big Dave View Post
The 90 Ohm resistor wire that measures the level in the tank is voltage sensitive (another way of saying sensitive to excess resistance. If you have a bad ground the sending unit will not see the full voltage it needs to measure the level of fuel. So the bulb's added resistance to the rear wiring harness makes the gauge move.

Big Dave
I was thinking the ohm reading for 64 down was 30?

Durg
1962 Impala SS
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post #7 of 10 (permalink) Old 06-11-2009, 01:24 AM
 
 
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Hey guys, I have a '78 Impala that jumps around too...I have a new tank, new sender, tonight just put a jumper from the tank ground to another good ground, no luck. With key off, tank shows 1/2 full, turn on the key or start the car and it'll jump to full. While driving, will usually be pegged or show around 1/2 until tank is about empty (fumes!), then jump from empty to 1/2...HELP! - RK.
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post #8 of 10 (permalink) Old 06-11-2009, 11:58 AM
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Sounds like a bad rheostat in the sending unit. You will have to drop the tank and check out the sending unit (replace it if needed)

Big Dave
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post #9 of 10 (permalink) Old 07-11-2009, 04:37 PM
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you can make a tester out of a headlight switch with the rheostat for the cluster dimming built in it. most GM gas gauges work on 90 ohms being full and close to zero equals empty. just measure the resistance on the switch, mark 90 and mark the low point. then you can connect in series between the sender wire and ground, operate the rheostat and see if the dash gauge follows accordingly. if so, the sender is the issue.
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post #10 of 10 (permalink) Old 07-21-2009, 03:13 AM
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On the 63, you have a bad grounding Dash, I know it sound like something from the X-files but I encounter this a few times before. The dash is supposed to ground via the metal to metal contact. When you turn on the lights it see's a semi ground through the Gray wires(dash lights). Anyhow Sometime condensation or rust will mess up the grounding. One thing you can do is to wire a ground wire to your dash so this will not happen again. This is a 46 year old classic. Also There's another way to check if the sending unit is at fault without dropping the tank. Pull apart the plug that connects the Dash harness to the Rear lamp harness just left of the head light switch, you'll find a brown wire in that connector, that's the wire that goes to the fuel sending unit in the tank. Ground the side(brown wire)that goes to the DASH, it should go to empty, take off the ground and it should return to the half point. If the Gauge responds it's ether the rear lamp harness or the sender. One last thing I'm pretty sure that 63's came stock with 30 ohm rheostat assembly and changed to 90 ohm in 66 or 67. Hope this helps
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