gas gauge headache

[skinu]

I got a 78 1/2 ton 4x4 k10 and all but tried everything I could do to get the passenger tank to register.I've given up on the drivers side. when I got the truck no gauges worked now it's just the fuel, I done about everything I could find on the forum and others, I put a new sender unit in and now got another used one that tested good in, got a good ground and power to the selector valve ,it's got the one wire ,6 port, that I just by passed today. my gauge is at 3/4 with key off and 3 o'clock when on or running. at the selector valve theres 4 wires 2 tan lime green and a blue tank ground. I've read many schematics and traced them to a 4 wire plug on the fire wall,I checked and got ohm's on 2 terminals yesterday one was 70 and the other was 103 ohm's,i ran about 6-7 gallons out today knowing only 1 ohm reading there should change but both did... 1  to 72 ohms and the other to 45.  I ran a wire from the sending unit to the selector switch on the dash as per schematics and now some that match my wiring say the pass tank is the aux. other say the drivers tank is so I don't know as I just put a known good gauge in and still same prob.does anyone know if one of the 2 tan wires at selector valve goes from drivers tank across to the ? at the selector valve and back across to drivers frame rail up to fire wall plug ,mine does ,can I read tank ohms at the 2 tan wires or plug ? I know the ohm reads for full and empty. whats the values at the dash switch ,I got power to gauges at fusebox and cluster plug and printed plate looks OK and has continuity ...HELP A GAUGE IS IMPORTANT WHERE I LIVE!!!

[hatzie]

Get the 1978 CK 10-30 Factory GM wiring diagrams I scanned from the tech section.  They should have the correct AUX / Production wiring relationship.  http://forum.73-87chevytrucks.com/smforum/index.php/topic,25639

You can test the gauges with a variable resistance test tool, or just plain old resistors, as outlined in the tech section.  http://forum.73-87chevytrucks.com/smforum/index.php?topic=25339.0

70Ω should be somewhere near 3/4 tank 103Ω should be Full.  If the sender ohmage changes randomly you should check the ground strap from the sender ring to the frame.  You can hook up a temporary tank ground from the tank itself to the frame and see if it stabilizes.

[skinu]

Thanks Hatzie !!! this is driving me crazy. I forgot to ask what the 4 wires in the plug at the firewall are I know 2 are tank sending wires and the green is for the selector valve but am lost as to the 4th. I have had the tank out 3x's and regrounded it ,(pass side), tried all I could read on it ,made a tester out of a light switch, do you know if 2 of the 4 wires in the plug would both change ohm readings ,I only use the passenger side and both wires changed readings. I'm outta options here or totally missing something, I've changed parts to known good ones and retested ,I don't got no shorts as far as I can tell by putting test light between the neg. post and cable so I'm stumped.

[rich weyand]

Here's a good discussion of GM fuel gauges, how they work, how to troubleshoot them, etc.
http://www.chevyclassicsclub.com/the-gm-fuel-gauge-mystery-1965-newer/

[skinu]

lotsa great info guys THANX! I have dual tanks are both brown or tan wires over by pass. tank coming from sending units, does the drivers tank wire(tan) go over to pass. side then back and up drivers frame rail mine do. both tanks are at 3o'clock when on and 3/4 when off. where do I test for ohms starting at tank and how do I test the circuit board on the cluster. I'm gonna pull the pass. tank and start there if I got a good ohm read can I just run a new wire straight to the gauge?

[78BIG-TEN]

If memory serves,the fuel gauge is switched through the selector valve on the frame if its a 6 port.Switch bezel on dash says depress fully.Had lots of problems with those valves when switching tanks.gauge would go way past full.3 port valves gauge was switched at the dash switch,only 3 wires going through firewall to tanks.1 to left 1 to right and 1 to selector valve

[rich weyand]

Sounds like your sender wires both go through the firewall to the selector switch.  If the selector switch has five lugs on the back, then the side with two lugs powers the solenoid selector valve, and the side with three wires on it is the two sender wires from the tanks (upper and lower) and the wire to the gauge (middle). 

Let's test where the wires are easy to get at without dropping tanks and all.  With the ignition off, disconnect the center wire, the wire to the gauge.  Connect the ohmmeter to either sender wire and measure ohms to ground.  Repeat on the other one.  What are your measurements?

[skinu]

ok i'll test there ,also I do got four wires at the fire wall plug. I'm trying to post pics and having a lil trouble. is there a way you can test tank grounds at the tanks? I think my drivers tank sender wire goes over to selector valve then doubles back ,does that seem right?

[rich weyand]

The driver side sender wire goes across to the passenger side sender wire, then they both head forward.

[skinu]

Thanx Rich and Hatzie, what I believe is going on is that the po used a circuit board for idiots not gauges. I still don't know what the fourth wire at fire wall plug is for but it has .03 volts with key on. I'm looking for a whole new harness for the tanks. do I need to hook the lime green wire to selector valve if I bypass the valve ? should I run the new single sender wire to the dash switch or to cluster plug ? whats the best solder and flux to use on electrical? I feel so needy,sorry boys

[hatzie]

If memory serves,the fuel gauge is switched through the selector valve on the frame if its a 6 port.Switch bezel on dash says depress fully.Had lots of problems with those valves when switching tanks.gauge would go way past full.3 port valves gauge was switched at the dash switch,only 3 wires going through firewall to tanks.1 to left 1 to right and 1 to selector valve

Only on the 81-91 trucks.  73-80 is handled by the tank switch.

[hatzie]

Don't just throw parts at it.  You'll end up frustrated and out cash. 

If you page down to PDF page 16 of the 1978 wiring manual you will find in the bottom Left-Center pane a diagram for RPO NL2 (dual tanks) that's specific to 1978.

All 4 wires at the firewall come from the DPDT tank switch on the dash.
-(LT GRN) Light Green. Power to the solenoid when AUX is selected.
-(TAN)Tan. Out to the fuel gauge.  Should run with the harness to a disconnect on the frame rail.  GM used the same harness regardless of single or dual tank.  The disconnect can be plugged into a single sender wire or the TAN sender output from dual tank harness.
-(TAN/WHT) Tan with White tracer stripe. Production tank sender.
-(LT BLU) Light Blue. AUX tank sender.

With both gauges reading at the 3 O'Clock position you have a break in the sender circuit.  Since you get some sort of ohmage from the sender wires the tank grounds may be OK but that's where you look first.

Pull out the dash switch and unplug it without dropping the plug back through the dash.  You can test the system right there.  The replacement switch is pretty cheap...  73-80 uses Standard Motor Products DS455 or the obsolete AC Delco 329854.  If you are going to do a Hail Mary swap replace the knee panel switch.  It's a cheap, easy to replace, part and replacement may nail it.
You can do a quick sanity check by grounding the 'Out To Fuel Gauge" terminal in the plug with the ignition switched on.  The gauge should drop to E.  Don't ground the valve side of the plug.  If it doesn't drop to E it's a simple matter of finding the break in the tan wires or the miswiring of the gauge conversion at the instrument cluster plug.

If you suspect the printed circuit connector at the instrument cluster is incorrect remove the fuel gauge from the cluster housing leaving the housing in the dash and measure the ohmage from the sender clip in the cluster bucket to the center fuel gauge lug of the tank switch plug you pulled out of the knee panel.  It should be 0Ω - 1Ω.

This is a pretty good representation of the plug you'll be looking at after you pull the switch out of the knee panel and how the switch works internally. 


Your last resort would be to ohm out the actual dual tank sub-harness but you are showing some ohms at the switch firewall plug so that's unlikely to be your problem.

This is the NL2 wiring diagram from Page 16 of the 1978 wiring manual...

[hatzie]

Some pretty good instrument cluster plug pinout charts are posted on another board.  The chart at the bottom is the best one...
http://67-72chevytrucks.com/vboard/showthread.php?t=456877

[rich weyand]

Thanx Rich and Hatzie, what I believe is going on is that the po used a circuit board for idiots not gauges. I still don't know what the fourth wire at fire wall plug is for but it has .03 volts with key on. I'm looking for a whole new harness for the tanks. do I need to hook the lime green wire to selector valve if I bypass the valve ? should I run the new single sender wire to the dash switch or to cluster plug ? whats the best solder and flux to use on electrical? I feel so needy,sorry boys

I repeat: What is the resistance in ohms of each sender wire at the switch in the dash?

Really, we can't help you much if you won't do some simple testing.  You could replace everything, rewire everything, and still have the same problem, which may be trivially easy and no cost to fix.  But we can't figure it out without you doing some measurements.

[skinu]

they are all over the place and I have not moved the dash switch or using the drivers tank ,the ohms read 101 and was 125 earlier from the drivers tank I am not using ,my pass tank is at 40 and started the day at 27, both senders bench test 27 to 118 ohms, no shorts wiring is all ok no chapped ,brittle or melted wires . got different board on cluster and now got all but fuel .