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Old 05-26-2005, 12:21 PM
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Gas Gauge dies when heater fan runs

Any ideas why my gas gauge goes dead when I have my heater running? I have checked the ground connections on both items and both work fine except when the fan is on. The fan runs fine. It is not on a reostat or stepped speed switch. A standard on/off toggle is what is used.

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Old 05-26-2005, 06:11 PM
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Loaded Ground

Doc here,

Sounds like you have a bad ground on one or the other Circuits, "borrowing" a ground from the other, thus depriving it of ground while the other is in operation..

The reverse can be true of a loaded ground borrowing power to series run the other circuit...When the fuse blows on one circuit (no power provided from the buss to that circuit) and the grounds are series tied and the ground goes bad..It can pull power through the first item to operate the second...or neither..(like Christmas tree lights..with a bad bulb)

The way to test it, is just isolate the ground on both, and put a meter on it from hard ground to the component ground, then turn it on...if it's bad it will rise to 12 volts, (meaning it's drawing ground from the other device..)

Also check the instrument fuse, (most common) and the fan motor fuse..before you go too far troubleshooting, usually easiest is often overlooked.

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Old 05-31-2005, 12:12 PM
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Thanks for the advice. I'll look into it now that I have something to go on.
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Old 06-07-2005, 11:58 AM
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OK. I've checked the grounds. Here is what I have. The switch for the heater fan is grounded to the body. There is only one wire going to the fan motor.

The gas gauge only has one wire that I can find.

Should I add a ground wire from the tank to the body? I don't want to do anything that could cause an explosion or fire.
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Old 06-07-2005, 12:11 PM
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Check the ground on the dash panel.........a bad ground will cause havoc on guages.

There should be a ground from the sending unit on the tank to the frame. Usually, when it gets disconnected, the guage will go to way past full.
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Old 06-07-2005, 04:29 PM
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Gauge

Doc here,

On the Instrument Cluster, You should have on the reverse side of the Gas gauge, If It's Stock, a laminate card with three terminals marked "S", "G", and "I"....

While you want to inspect all three, for Corrosion, or loose or burned connections or laminate card Trace opens, the "G" is the ground and pay extra attention to that..The "S" is sender, and the "I" is 12 volt switched Ignition..Check also the cluster plug for corrosion, or ill fitting pins.

While you have the cluster plug out, Using an ohm meter to the sender wire going to the tank, with The sender at the tank unhooked, you SHOULD read infinite readings at R x 1 scale or auto range on the meter to ground, If you show any readings at all, you need to inspect that wire for shorting or chafing..

To include or exclude the sender as a suspect, get a couple of 1/2 watt resistors from radio shack, 10 to 15 ohm, 50 ohm, 150 ohm, hook up the cluster and apply power with The sender hooked up...

what does it read? Full or Empty?

Unhook the sender, It should peg the opposite way..

then using alligator clips, jump your resistors from the sender line to ground..one at a time..

depending on the actual resistance value of your circuit, you should get (on at least one resistor) a half tank reading..or ANYTHING besides pegged full or empty..If that happens on ANY one of the resistors, your Circuit and Gauge are Both Good, replace the sender..

If it doesn't happen, and the wire , cluster connections and terminals have all checked good, Replace the gauge..The Wheatstone bridge inside the gauge has been damaged and beyond the range of resistance provided by the sender..or may just be flat open.

The Tank SHOULD ALWAYS have a ground wire on it..There should be a ground tap somewhere in the area of the sender/Fuel pickup bezel, and that wire should go to a hard frame ground..The tank sits on rubber mounts...and depending on aftermarket add~ons may or may not have rubber hose on the fuel lines, isolating the tank from hard ground.

Doc
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