gauge cluster lights and fuel gauge

[jerry73]

I'm new to the forum and looking for help....I recently just traded for a 78 lwb 2wd that was a diesel and was converted to a 454,the truck is in great shape and ready to drive other than the gauge lights don't work and the fuel gauge does not work for either tank....any suggestions to what I need to try to check,how to diagnose?
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[motornut]

there is a spot where the grounds go to one behind the cluster
bout a foot from the plug in
it's the grey wire supplying the juice for dimming
make sure the copper strips @ the plug are straight
gauge up or down?
was the cluster changed?
up can mean the sender is unhooked

[jerry73]

Originally Posted by motornut

there is a spot where the grounds go to one behind the cluster
bout a foot from the plug in
it's the grey wire supplying the juice for dimming
make sure the copper strips @ the plug are straight
gauge up or down?
was the cluster changed?
up can mean the sender is unhooked

What plug are you talking about? If its a ground wire then it shouldnt have any juice to it at all
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[motornut]

Originally Posted by jerry73

What plug are you talking about? If its a ground wire then it shouldnt have any juice to it at all
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the cluster plug
nope no power, but if other people added extras in the dash it may have hacked/burned the ground

[jerry73]

Gauge is stuck all the way passed full and hasn't moved for either tank,Im pretty sure it's the stock cluster from when it was diesel....those wires could be the problem for my gauge lights as well? (Also my lights in my radio and the one by the windshield wiper switch work)
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[motornut]

if those ones work pull the cluster check the paths
you can see how they bend (i use crazy glue)
meter the grey wire for power,should vary when dial turned
I'd check the clips in the cluster ,corrosion etc is possible too

could do a temp wire to test/bypass the switch and sender

[jerry73]

Would it just be easier to buy the new cluster?
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[1967gm4x4]

I always check fuses with stuff like that first.
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[jerry73]

Originally Posted by 1967gm4x4

I always check fuses with stuff like that first.
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Yea I replaced as many as I could but they are so hard to read and hard to tell if they are burnt out,and they break so easily. Anyone have a diagram of the fuse box?
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[rollin thunder]

I`d say check fuses and if they end up being good then check your your bulbs and printed circuit on the back i had to replace mine and they can burn through in spots easily.if that all seems to be fine then check grounds and wire leads fixing or replacing any that needs to be. If the fuel gauge is reading past full and not moving its most likely a bad ground mine did that and i found a few open spots on the ground and taped em up and it worked like a dream afterwards. if its not a ground problem then check sending unit, make sure its the right for the tank ( I ran into that problem on my dads scottsdale with the float hitting a baffle) you can also check to see if the sending unit is good all you need is a volt meter one with the needle gauge... attach it to where the wires go in and get a good ground and with the float at the bottom it should read o. when you move the sending unit float the needle on the voltmeter will move accordingly. Just set it on the normal 12v reading setting. i imagine a digital one would work but i have never tried with a digital one so i do not know for sure.

[1967gm4x4]

Here's a link to a diagram. http://static.cargurus.com/images/si...2934815889.gif
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[rollin thunder]

Is this what you are asking for????

A - Fuse - Traffic Hazard

B - Fuse - Tail, Stop Courtesy Lamps

C - Receptacle - TP2 /M40

D - Fuse - Radio & TCS

F - Receptacle - C91/UF2 (Cargo Area Lamp)/U35/U37

G - Fuse - Heater / Air Cond.

H - Receptacle - C62

I - Fuse - Accessory

J - Fuse - Panel Lights
K - Receptacle - NL2/A33/U16 (Tachometer)

M - Receptacle - Accessory Lamps

N - Not Used

O - Fuse - Reversing Lamps

P - Fuse - Instrument Cluster Feed

[jerry73]

Originally Posted by rollin thunder

Is this what you are asking for????

A - Fuse - Traffic Hazard

B - Fuse - Tail, Stop Courtesy Lamps

C - Receptacle - TP2 /M40

D - Fuse - Radio & TCS

F - Receptacle - C91/UF2 (Cargo Area Lamp)/U35/U37

G - Fuse - Heater / Air Cond.

H - Receptacle - C62

I - Fuse - Accessory

J - Fuse - Panel Lights
K - Receptacle - NL2/A33/U16 (Tachometer)

M - Receptacle - Accessory Lamps

N - Not Used

O - Fuse - Reversing Lamps

P - Fuse - Instrument Cluster Feed

Exactly what I was looking for.....what is the panel fuse? Instrument panel?
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[ron350]

Fuse ratings.

A-15A- Directional Signal Indicator Lamp, Stop Lamp, Traffic Hazard

B-20A- Courtesy Lamp, Roof Marker Lamp, License Plate Lamp, Parking Lamp, Side Marker Lamp, Tail Lamp, Clearance Lamp

D-15A- Idle Stop Solenoid, Aux. Battery, Radio, Time Delay Relay, Emission Control Solenoid, Transmission Downshift (M40)

E-25A- Windshield Wiper/Washer

G-20A- Heater, Front A/C, Generator Warning Lamp

I-20A- Cigarette Lighter, Clock, Dome Lamp, Cargo Lamp

J-4A- Instrument Cluster Lamp, Heater Dial Lamp, Radio Dial Lamp, Cruise Control Lamp, Windshield Wiper Switch Lamp

O-15A- Cruise Control. Rear Window Aux., Fuel tank, Tachometer, Back-up Lamp, Directional Signal Indicator Lamp, Directional Signal Lamp, Headlamp Buzzer

P-4A- Fuel gauge, Brake Warning Lamp, Temperature Warning Lamp, Oil Pressure Warning Lamp

[rollin thunder]

Originally Posted by jerry73

Exactly what I was looking for.....what is the panel fuse? Instrument panel?
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im not sure what the panel fuse is, that is just a diagram from a 77 i had saved on my computer from a while back.

[jerry73]

Yea I know for sure my gauge cluster fuse is good because I changed it......as for the fuel gauge where is the ground wire because the PO told me that its probably a ground wire
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[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.

[jerry73]

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.

That's exactly what mine is doin...I think I got it narrowed down to the ground but I want to narrow down where the ground is
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[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).

[jerry73]

Originally Posted by hatzie

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).

Is this the same for a dual tank truck? Mine is a long bed also so its got the bigger tanks
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[hatzie]

Originally Posted by jerry73

Is this the same for a dual tank truck? Mine is a long bed also so its got the bigger tanks
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This is specific to the 78 dual tank truck. A single tank would not have a switch in the knee panel of the dash.

[Hammered 77]

From experience, burning thru to the wire and grounding on the header will also cause the needle to the three o'clock position. This has happened twice since I bought the truck new in '77.

[hatzie]

Originally Posted by Hammered 77

From experience, burning thru to the wire and grounding on the header will also cause the needle to the three o'clock position. This has happened twice since I bought the truck new in '77.

The wire was burned through but the sender wire was open. It was not grounding enough to drive the gauge. The gauge needs a firm ground. Casual contact usually won't do it.

Firmly grounding one of the 73-91 GM fuel gauge sender wires will always drive the needle to E. Unplugging the sender wire or the tank ground (open circuit) will always drive the needle to @ 3:00.
Sender wire shorted to ground = E, 90 ohms to ground = F, and Open circuit = 3:00. Been there done that with over 20 73-91 GM trucks.

[Hammered 77]

Sorry for using the wrong terminology, the wire had melted thru the casing and stuck to the header. So would this have been an open short?

[hatzie]

Originally Posted by Hammered 77

Sorry for using the wrong terminology, the wire had melted thru the casing and stuck to the header. So would this have been an open short?

The Jacket melted. The wire was badly damaged if not severed (Open). Something insulated the wire from the headers enough to keep it from making a solid grounding contact (Short).

The fuel gauge sender runs from 0Ω E to @90Ω F + or - 3Ω. If the resistance on the sender side rises higher than 90Ω the needle will continue to rise corresponding to the resistance till the needle is pegged at roughly 3:00. I haven't tested the resistance that actually pegs the needle. Probably around 120Ω give or take 2-10Ω . From there to completely open ( ∞ Ω ) the needle will sit pegged at roughly 3:00.

If you don't believe me. Get some liquid electrical tape to repair the pinhole you are about to make. Then. Stick a pin through the sender wire and firmly ground it. The gauge will drop to E.