gauge cluster lights and fuel gauge

[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.

[school boy]

listen to hatzie, this man is giving you some great info i wish i had when chasing my fuel guage issue. my gauge sat at 3 past full on both tanks so i figured bad wire, nope. replaced gauge, still didnt work. checked connections through firewall, fuse box, etc. mine ended up being both senders were bad on the floats. REPEAT! listen to this guy, i spent 10+ hours figuring out my problem

[jerry73]

Originally Posted by hatzie

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.

What's the easiest way to get to the sender? The truck is 2wd so there isn't alotta room under truck.....also I checked the wires that run by the headers n they are fine(so they appear)
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[Skunksmash]

Get an Ohm meter and hook it up to the fuel sender wire right there at the gauge. Verify that its somewhere between 0 and 90 ohms. 0 ohms is an empty tank, 90 is full. If you're getting somewhere between 0 and 90, then your sender is working and you're getting signal.

[jerry73]

Does anyone have a diagram or preferably a picture of the fuel sender with the wires that come off of it labeled? I'd like to know what color,which are grounded and which are hot....I plan to free up the bed and slide it back to check out the fuel sender tomorrow
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[hatzie]

Originally Posted by jerry73

Does anyone have a diagram or preferably a picture of the fuel sender with the wires that come off of it labeled? I'd like to know what color,which are grounded and which are hot....I plan to free up the bed and slide it back to check out the fuel sender tomorrow
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LH & RH are always the seated drivers left and right hands.

See the 1978 dual tank (NL2) wiring diagram in post #18 for the colors.

Inside the Cab Circuit 139, PINK with a BLACK tracer stripe, is power from the fuse panel to the tank switch for the valve. It should be hot (battery voltage) with the ignition switched on.

Circuit 53 LIGHT GREEN is power from the switch, in the cab, to the tank valve, inside the RH frame rail. It should be hot (battery voltage) in the AUX switch position.

Senders;
LH sender "AUX" = LIGHT BLUE
RH sender "Production" = TAN with WHITE tracer stripe

Circuit #30 ,30A, 30B, 30C is the sender circuit. They are variable resistance to ground based on sender level. Empty=0Ω Full=90Ω to a good ground. You can test this from inside the cab, as I outlined in post 18, assuming the rest of the wiring is intact.

The sender has two connections.
The sender wire color depends on which tank, LH or RH, you're looking at. It attaches to the sender with the black rubber connector with a female socket (connector is pictured below the sender picture). The male terminal on the sender itself looks like the threaded end of a small brass machine screw protruding through a plastic and rubber isolator.
There should be a BLACK wire with a 90° "flag" disconnect close to the sender retaining ring. That BLACK wire should go to a ring terminal bolted to the frame. This is the sender ground that usually causes this kind of grief.

Sender

Sender connector

[jerry73]

Thanks for the info hatzie I appreciate the details....so far its helped a lot
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[jerry73]

Is ther a ground wire on bothe senders? The passenger side tank has the "flag" ground connector but the driver side doesn't...I'm thinking about ordering new senders and dropping the tanks to make sure the senders and grounds are all good
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[motornut]

Originally Posted by jerry73

Is ther a ground wire on bothe senders? The passenger side tank has the "flag" ground connector but the driver side doesn't...I'm thinking about ordering new senders and dropping the tanks to make sure the senders and grounds are all good
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ground on both senders ,yes
checkin is always good

[jerry73]

The grounds are the same on both sides correct? As in they both have the "flag" ground on the sender?.....if so then my drivers side is broke off and I need a new sender
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[motornut]

Originally Posted by jerry73

The grounds are the same on both sides correct? As in they both have the "flag" ground on the sender?.....if so then my drivers side is broke off and I need a new sender
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You can replace it
Clean tab and drill a new hole
I've let the unit air dry after blowing/ cleaning out the sender
And soldered a new wire in
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[jerry73]

I will try your method first and hope they work
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[hatzie]

A #6-32 PEM stud through the sender top would do the job.
http://www.amazon.com/Press-In-Captive-Stainless-T...
Drill with the exact bit required no larger. Be sure to seal the knurled portion with a very small amount of fuel resistant RTV (think wiping onto the knurling with a toothpick) to keep it from leaking.

[jerry73]

Ok so here is my new problem...now the truck won't get fuel...I have to pressurize the tank to prime the ELECTRIC fuel pump mounted on the right frame rail....are these things supposed to suck the fuel from the tank because I prime it and let it idle and ones it burns off the fuel in the lines it won't run until I prime it...bad fuel pump maybe?
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[hatzie]

That sounds like a fuel pump problem... Or maybe a rusted out pickup tube.

[jerry73]

Pickup tube could be the problem because the pump runs....I had interesting thing happen today that leads me to believe that the gauge problem is definitely a ground problem....when I got in the truck before I turned the key on the gauge read just below full and then as soon as I turned the key it went passed full to 3 o'clock...thoughts?
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