gauge cluster lights and fuel gauge

[hatzie]

Originally Posted by Hammered 77

If your fuel gauge is stuck way past full. Check the wire coming from the tank, it may be grouning at some point between tank and gauges. Mine got against the header pipe and ground out causing the gauge to read way past full and not move.

Grounding the sender wire will set the gauge at E.
Breaking the sender wire will send the needle to 3:00.

I assume the other gauges work..???
Is the (P) PANEL LPS fuse good?
Is the (I) CLUSTER FEED fuse good?
Turn signal and HI-Beam indicators work?
Volt, OIL, & TEMP gauges work?

Checking the instrument lights.

1. Does the heater control light up when you pull out the light-switch and crank it round till the dome position almost clicks?
2. NO?? Make sure the heater lamp isn't burned out.... Leave the light switch in the same position and test that the GRAY cluster light power wire shows battery voltage (10-14.5 VDC) at the instrument panel plug.
   No power? Fix the Grey wire.
3. If the heater control lamp lights and the Gray wire is hot check the ground points on the instrument cluster disconnect with a Digital Multimeter.
4. All three Ground wires are black. All three are spliced together and to a single wire in the harness. (see Motornuts picture)
5. Test continuity from ground to ground and then check from one ground wire to the steering column or other known good ground.
6. If you have one or two bad grounds, but not all three, that splice is your problem child.
7. If there's no continuity from a good ground to the instrument cluster and all 3 BLACK wires have good continuity check the connection to the cab ground BUSS BAR.
   NOTE: if all 3 grounds are bad then nothing will work in the instrument cluster but the speedo.

Fuel Gauge
(see the 1978 NL2 dual tank wiring diagram below or download the whole 1978 GM wiring diagram book in PDF from the thread in my sig)
Way past full is usually a bad tank ground or broken sender wire. With dual tanks it could be as simple as bad dash switch.
You can test the switch and gauge mostly from inside the cab as follows.

You'll need;

1. A handful of alligator clip test leads.
2. A digital Multimeter
3. A handful of cheapie Male 1/4 Disconnect Terminals
4. A handful of cheapie Female 1/4 Disconnect Terminals
5. Some string.

Testing the switch. The switch has 3 terminals on one side and two on the other. The fuel gauge is hooked to the common terminal (#4 on the GM NL2 wiring diagram) senders are on terminals #3 & #5. The valve is powered by Terminal #2 from terminal #1.
If the switch fails any of these tests replace it.

1. Plug female terminals onto positions 1, 2, 3, 4, & 5.
2. Set switch to AUX and meter to OHMS or continuity buzzer.
3. Insert probes in #1 & #2.The meter should read 0.00 ohms or close to it and buzz or scream if the meter is set to continuity.
4. Insert probes in #4 & #3. The meter should read 0.00 ohms or close to it and buzz or scream if the meter is set to continuity.
5. Move one probe from #3 to #5 and set the switch to Main. The meter should read 0.00 ohms or close to it and buzz or scream if the meter is set to continuity.

Testing the gauge and sender wires from the switch plug on 73-80 systems.

1. Reach under the dash and pinch the latches (top and bottom) that hold the switch in the rectangular hole in the dash. It'll slide out the front of the knee panel.
2. Tie a loose slip knot around the wires behind the plug so you don't drop it back behind the panel.
3. Insert male disconnects in the gauge wire holes #3, 4, & 5 of the switch socket.
4. Clip a test lead to the ashtray, steering column, or some other good ground. Turn the ignition on and test your ground with the meter set to DC 20V scale and the test leads attached from your ground to the STOP LPS TRAFFIC HAZ fuse in the fuse panel.
5. Clip the other end to the disconnect in hole #4 (TAN wire) and switch the ignition on. The fuel gauge should drop to E at a kinda lazy pace.
6. If the gauge stays at 3:00 then check the 2 cluster power wires (PINK). Test continuity between them with the ignition OFF. Then check for battery voltage with the ignition ON. The ground wires, if damaged should, already be fixed...
7. Check the TAN gauge wire, with the meter set to OHMS or continuity buzzer, from switch plug #4 to the tan wire on the instrument panel plug. If it reads high resistance or OPEN then check the tan wire for breaks between the switch and the instrument panel including the disconnect near the tank valve on the RH frame rail.
8. If the TAN wire is good and the power is good swap in good fuel gauge.
9. Still not moving to E??? Check the printed circuit on the back of the cluster.

Gauge is working checking the senders...
NOTE RH & LH tanks... This procedure assumes 1978 AUX tank position. Not all 73-80 AUX tanks are on the LH side.

1. Clip a test lead from #4 TAN to #5 TAN/WHITE (white stripe). With the ignition ON the gauge should read the fuel level in the RH tank.
2. Move the test lead from #5 TAN/WHITE to #3 LT BLUE. With the ignition ON the gauge should read the level in the LH tank.
3. If the gauge goes to 3:00 with the test lead on either or both #5 or #3 then check the sender wire(s) for breaks and check the sender ring to frame ground(s).