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Old 03-07-2011, 08:04 PM
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Broken Fuel Gauge

I have a 74 GMC truck and the fuel gauge seems to be broken. It sits at about a quarter tank but I know it has more. I started ripping apart the dash and stopped when I realized I might have to take out the speedo cable.

Can anyone give me some tips on how to fix the gauge. Do I need to pull the speedo cable out to gain access to the back of the gauges.

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Old 03-08-2011, 04:03 AM
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Gage works off the sending unit in the tank, which uses resistance levels (via the float) to ground. That is.... the higher the fuel level the higher the resistance and the gage reads full..... as the fuel level drops the resistance drops etc..... (I may the that reversed) any way you should be able to temp ground the wire from the sending unit and see if the fuel level goes to full, if it does you may have a faulty sending unit.
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Old 03-08-2011, 05:08 AM
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The gauge is fairly easy to remove without ripping out very much from the dash. The speedometer cable is easy, but you may also have a mechanical oil pressure gauge with a hard line running to the engine. However, the problem is more likely to be the sending unit or wiring.

Sending unit wire is tan color. It runs out of the dash cluster, through the firewall and across behind the engine in a wiring bundle. From there it breaks out of the bundle and runs down the passenger side frame rail to the fuel tank. Inside the frame rail near the tank there should be a junction you can pop apart to get access to the sending unit wire.

If you turn the truck on and ground that wire, you should get full scale on the tank. If you check voltage on that wire with the truck on it should show 12 volts. If either of these two tests fail, you need to follow the wire back and make sure its connected. The area where it runs behind the engine and near the passenger side exhaust manifold is very vulnerable.

For the dash removal
- Remove radio knobs and the screws that hold the trim on.
- Work the trim off around the wiper switch and the A/C vents - don't remove the switch or vents.
- Remove the plastic gauge face (one big piece of plastic)
- Remove screws that hold the speedometer/fuel gauge into the cluster
- Reach behind the dash and squeeze the clip that holds the speedometer cable on; then pull the cable out of the speedo
- Pull the gauge out of the cluster

Let me know if you have questions. I think I have the cluster apart on my '75 every couple of years to fix or check something.

Bruce
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Old 03-08-2011, 05:48 AM
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hi,
since the GM sender is 0 ohms empty and 90 ohms full, shouldn't grounding the sending unit wire deflect the gauge to empty? likewise, an open wire would swing the pointer to full, or past full?
just going over my notes.
mark
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Old 03-08-2011, 07:16 AM
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Quote:
Originally Posted by 74GMC
I have a 74 GMC truck and the fuel gauge seems to be broken. It sits at about a quarter tank but I know it has more. I started ripping apart the dash and stopped when I realized I might have to take out the speedo cable.

Can anyone give me some tips on how to fix the gauge. Do I need to pull the speedo cable out to gain access to the back of the gauges.
Does the gauge needle change position when the key is turned off and on? Does the needle drop any as the tank is emptied, or does it just stay in the same place?
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Old 03-08-2011, 09:34 AM
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The gauge needle changes just a little bit. Not much though this is why I figured it was broken. Normally it moves doesn't it? I havn't been driving it so I'm not sure if it will drop as it is emptied. I just run my truck once a week or so.

Thanks for all the reply's I'll try out your suggestions. Appreciate it.
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