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Fuel guage ups with headlights??

Any thoughts on this? My fuel guage goes up about 1/8 tank when I switch on the headlights. I cleaned the grounds under the dash.

Jay
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Old 10-07-2003, 04:55 AM
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mine does the same thing, and I cleaned those grounds as well. didn't clean the ground near the sending unit or near the headlights, though.
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Old 10-07-2003, 06:51 AM
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Mine too.
It could be in the instrument light connections since they share that in common.
If I solve it before you I will let you know

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Old 10-07-2003, 06:59 AM
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Could be a problem on the power supply end of things. Still likely a dirty connection. I wouldn't worry, myself...

--DD
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Old 10-07-2003, 08:04 AM
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Sounds like a bad mass connection somewhere... IIRC there are four places where masses of the wiring harness are conected to the body... check those out first.
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Old 10-07-2003, 09:20 AM
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Jay,

I think the voltage regulator over compensates a little when kicking up the alternator output as a result of the significant draw down of current when the headlights are turned on. My thought, anyway.

Phil
Old 10-07-2003, 09:58 AM
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Why complain about free gas in todays world?
Old 10-07-2003, 07:49 PM
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I played with it more this morning. It is directly related to the instrument lights. I brighten up the lights, fuel goes up, Dim them, it goes down.

So I will dive in and see if there is a flaky ground in there somewhere.
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Old 10-08-2003, 05:37 AM
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I had a similar problem with my 71 Buick Skylark. Found out that the Voltage Regulator was wearing out. When I relplaced the VR, the problem went away. Well, I also replaced the Altenator too.

Again, my 2 cents.
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Old 10-08-2003, 09:00 AM
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It's the voltage regulator but, it's not a big problem, drive it as it is or replace the VR when you get tired of the ups and downs.

Phil
Old 10-08-2003, 09:42 AM
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I have the same problem with my Oil Temp gauge in the console. It doesn't jump when the needle is near the bottom, but the closer to straight up it gets, the bigger the jump. I figured it had to be a grounding problem so I added a new supplemental ground wire, but it made no difference. I never thought of the voltage regulator - would the vr only affect 1 gauge out of all the gauges in the car?
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Old 10-23-2003, 08:56 PM
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A fluctuating system voltage would, proportionately, affect readings of the Oil Temp gauge, the Voltmeter, the Fuel Level gauge and the Oil Pressure gauge. I think that's it.

Phil
Old 10-24-2003, 02:53 AM
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I haven't checked this out but it's an Ohm's law problem. Turning the headlights on increases the current from the headlight's gnd through the chassis to battery ground. As the current increases, the voltage drop (E) across the headlight gnd path to the negative post of the battery increases (E=I*R). The gas gauge is really a voltmeter. 12 volts or what ever the charging system is putting out is felt across the gauge, fuel sending unit and then the resistance between the ground at the front of the car and the negative post of the battery. Turn on the headlights causes the current to increase along the gnd path, the voltage drop across the gnd bath increases likewise. The voltage drops across the gas gauge circuit change when the gnd path voltage increase so the fuel gauge reading drops. This can be easily checked by connecting your DC voltmeter between the headlight gnd (connect positive meter lead) and the negative post (connect negative meter lead) of the battery. Measure the voltage drop and then turn on the headlights. With the headlights on the voltage measurement will increase.

To fix this problem decrease the chassis resistance by cleaning all of the connections in the circuit or run a separate gnd wire from the instrument cluster to the negtative battery post (Note: If you fix the chassis connectors your headlights should be brighter also).

--John
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Old 10-25-2003, 04:27 AM
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John,
I couldn't have said it better myself but, your statement is an answer to a problem not part of this thread. No matter that all of the positive and negative (chassis ground) connections are perfect, if the VR does not adequately respond to to fluctuations in system current, there will be corresponding fluctuations in system voltage, i.e., fuel level, oil temp., etc., go up and down.
Phil
Old 10-25-2003, 04:51 AM
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philinjax--

I think it does answer the original question "My fuel gauge goes up about 1/8 tank when I switch on the headlights.". If the voltage regulator is functioning normally the system voltage will remain the same within the tolerance of the charging system. By the way, if the voltage regulator is maintaining a higher then normal voltage the fuel gauge will not be actuate with the headlights on or off (reads high), but the needle shouldn't move as the system load changes (when they are switch of or on as soon as the system settles down to the new load).

--John
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Old 10-25-2003, 06:46 AM
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Damn, That's what I thought I was saying. If the system voltage goes up and down when you turn on the headlights, then the VR is not regulating as good as it might/should. It has nothing to do with bad grounds.
Phil
Old 10-25-2003, 09:32 AM
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philinjax--

I understand that if the VR is bad the reading will be erroneous. And that if the VR is fluctuating the gauge will fluctuate too. However, the fuel gauge fluctuation with the headlight operation occurs on 914's with normally operating VR's. I've read about this problem several times in the past and my 914 fuel gauge fluctuates just as sasquatch described at the start of this thread. My 914's VR and charging system have been working without problems for several years. The fuel gauge increase when the headlights are switched on just never bother me enough to fix with.

--John
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Old 10-25-2003, 12:35 PM
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John,
I refer to my responce to this thread of 10/8, above. Nothing has changed since.
Phil
Old 10-25-2003, 12:52 PM
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There's one way to settle this disagreement between the two 'theories'. If Jay has a voltmeter and connects it from the ground point under the dash and the voltage at fuse #8 and then looks for a voltage change with the headlights on and off. If there's no change then there's some kind of ground problem or somehow stray headlight current is getting on the fuel gauge. But if the voltage changes a little then it's the regulator or maybe the battery is getting old, or the battery terminals need a good cleaning. I kind of think it's the regulator or corrosion on the terminals myself, so really you could be both right. Resistive battery terminals cause IxR drops with headlight current, and confuses the regulator.

Last edited by 914GT; 10-25-2003 at 04:38 PM..
Old 10-25-2003, 04:34 PM
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Whatever,
Old 10-25-2003, 04:51 PM
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