Fuel gauge issues

[jpatters13]

I have an 84 K10 Silverado I recently purchased for my 16 year old daughter.  It has dual fuel tanks.  I am slowly going throughout he truck and fixing what seems to be broken or not functioning properly. Here's my issue: I have filled both tanks and regardless of which one is selected, the fuel gauge will only read about 2/5 full when the tank is completely full.  Then, whenever the selected tank gets near empty, the gauge will begin to come down and function properly.  I believe both sending units are original equipment.  Should I replace the gauge first or start with the units...or do you have another suggestion?  Any advice would be highly appreciated. Thanks!

[VileZambonie]

It reads 2/5? So are you saying below a certain point it reads normally just won't climb about wherever 2/5 is? Can you pop the wire off of the switching valve and observe the gauge? It should go past full

[rich weyand]

It's probably the resistor on the back of the fuel gauge.  If they break open or lose contact, you will see that.  As VileZambonie says, if you pull the plug off the tank selector valve (on the passenger frame rail in front of the rear tire), then the gauge should go to Full.  If not, it's at the gauge, and simplest is to replace the resistor.

This is a pain in the butt to do, because you have to peel the instrument panel back until you get to the instrument basket so you can pull the gauge out.  The resistor is bolted across the terminals on the back of the gauge, the gauge just plugs into the basket.

Here's the resistor you need.  At least this is the one on my 78; I think 1984 is the same.
http://www.classicmuscle.com/p-6237-fuel-gauge-resistor.aspx

[jpatters13]

Yes, the tank reads about 2/5 full and no more regardless of how much more fuel is in the tank.  I will start with the plug on the frame rail.  I think I know where it is.  Do I just pull the three-plug adapter out which should then send zero resistance to the gauge and it should go to full?

[rich weyand]

I don't know what you mean by "the three-plug adapter".

You should have a widget like this on the inside of the frame rail in front of the rear tire.  Pull the plug off and see what the gauge reads.  This will put infinite resistance (not zero) to the gauge and it should go to FULL.

[jpatters13]

Okay, sorry, I meant three-wire.  Yes, that's what I found. I pulled the plug and the gauge went to full.  That means the gauge is good, right?  Does this also mean that both sending units are in disrepair by having poor contact or having floats that are damaged which means I need to replace them?

If I have to replace them, is it easier to pull the truck bed off the frame and gain access to both units without dropping the tanks?

[rich weyand]

Yes, that means the gauge is good.  Not sure about the resistor.  If both the resistor is open and the sender wire is open, I'm not sure you won't see FULL anyway.

Can you put a 90-ohm resistor (from Radio Shack; a 1W resistor will be fine for this test) between the sender wire and ground?  This should also result in a full reading, but it will read 1/4 or 2/5 if the resistor is broken.  Should be PNK wire, I think, between a PNK/WHT and a PNK/BLK, on one end of the connector.

[jpatters13]

When I get the resistor from RS where exactly do I put it when I am putting it between the sender wire and the ground? At the back of the gauge or at the widget or somewhere else?  Sorry for the need for further explanation but I am not as well versed in the technical stuff as I wish I was.

[rich weyand]

For ground, the frame rail will do.  By sender wire, I mean the wire back to the gauge.  This should be the pink wire on the connector you pulled off the valve.  It should be between a pink wire with a black stripe and a pink wire with a white strip.  These three wires should be clustered on one end of the connector.  So you put one end of the resistor into the connector position with the pink wire, and jumper the other end to ground, which is the frame rail, the sheet metal, pretty much any place you can find some bare metal under there.

If the resistor is OK, the gauge will stay at full.  Then it must be the sender units.  If it drops to 1/4 to 2/5, then the resistor is bad.

Pulling the box off to get at the sender wires is a PITA.  So is stripping the instrument panel back to get to the resistor, though not as bad.  Knowing which you need to do, so you don't guess wrong and end up doing both, is worth some experimentation.

I doubt the senders, because there are two of them and they would both have to fail in the exact same way to get what you are seeing.

[rich weyand]

BTW, how many wires are on your connector, five or six?

It's probably a six-position with five wires.  The one empty position is on one end.  If you call that empty position "1", and count across the connector, then the gauge wire is "5".

[Boone83K10]

I had this problem already. Mine read 3/4 all the time in both tanks. Senders were new, gauge was good. Mine had failed inside the selector valve. It was switching mechanically but not electrically with the soleniid inside of it. New selector valve and everything reads correct.

[VileZambonie]

If you ground the wire does it go to empty? Even better if you have a fixed 45 ohm resistor you can see if it swings to the middle. So it's odd that both tanks do exactly the same thing. I would check the wiring and am assuming everything else is stock and functional. What switch do you have in the dash?

[jpatters13]

I'll try the 45 ohm. Didn't try grounding it yet.  All equipment is stock including the switch in the dash.

[jpatters13]

Okay, so the gauge goes to full when I unplug the widget completely.  I picked up the 90 ohm, 1 watt resistor suggested from Radio Shack and tested it.  When I plugged into the 5th wire recommended above and grounded the other end to the frame, the gauge goes to 2/5 full just like it does when the tanks have that much or more in them.  From what you all have told me, it appears the fuel resistor is bad on the back of the gauge.  I found this one on ebay and I would love for someone to confirm it is the correct one before I purchase it. Thanks for all the assistance!

http://www.ebay.com/itm/Camaro-Nova-Ventura-Resistor-Console-Fuel-Gauge-/131058467349?pt=Motors_Car_Truck_Parts_Accessories&fits=Make%3AChevrolet&hash=item1e83b18215&vxp=mtr

[rich weyand]

That'll work.  The same resistor/gauge setup was used from 1965-1997 on almost every GM car and truck.

Here is another, a bit cheaper, and it's a reputable supplier and a reliably new part.  This is where I got mine and it was correct and fixed my gauge.  eBay can be hit or miss.
http://www.classicmuscle.com/p-6237-fuel-gauge-resistor.aspx