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73 Gas gauge fun

A

aceroth

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So ever since I got my 73 the gas gauge has read 1/8 full. Tank full, tank empty, battery connected or disconnected, ignition on or off, sending unit wire attached or cut. Even, as I learned tonight with the f'ing gauge out of the dash. I figured sending unit. Replaced it. No change. Figured it was the gauge. Replaced the gauge and the new one sits at 3 o 'clock (like way over full) again whether ignition is on or off. 30 gallon tank has at most 4 gallons in it.

WTF? I have read a bunch on here and the internet posted by guys who understand electricity testing all manner of things with their multimeters. I am a C- wrench and a solid F electrician. I am pretty sure that ohms are what I want to measure but not sure which horseshoe symbol on my multimeter I need to use or what I need to touch my red and black leads to. Can anyone tell me what to test in words a slow 4th grader can understand?

FWIW, I think my sending unit ground is good. When I put the new tank and sending unit in I poly wheeled a spot on the crossmember near an empty hole till it shone like the sun and bolted the black terminal to it.

I have never hated my truck before but this is getting me close. I am not one of the proud souls here looking for perfection from a gas gauge on a truck older than I am but when I drained the tank to change it, I thought based on odometer it had 3 or 4 gallons in it and it had 20. Need better than that.
 
Reading 3:00 indicates no input from the sender unit.
Tan wire runs from the sender up the right frame rail then crosses over to the engine harness by the starter. That wire is more than likely broken or cut somewhere from the sender to the gauge.
The tan wire joins the engine harness at the starter from the frame which is routed to the block. And Up the front of the block to the back of the block along the right valve cover.
Crosses over the firewall to the firewall junction on the drivers side that leads into the fuse box inside the cab and then to the gauge.
The ground wire at the sender goes from the sender to the frame.
 
2nd the bad wire idea. On my 73 there was a junction basically inboard from the rear wheel, check there too. It should be the only wire on the passenger side of the frame. Heat and age = broken wires or bad connections.

If you decide to run a whole new wire: run it down the driver side frame rail with the rear light harness. Avoids having the wire go through the starter area (high heat/limited access) completely.
 
Not looking at a multimeter, cant recall the setting symbol but if you've had the cluster out, and hopefully still have easy access, test the sender wire from the connector near the tank to the connector at the back of the instrument panel for continuity.

If an electronic multimeter, that's the setting that beeps when you put the two leads together. The problem can be a connection or track on the cluster flex circuit, continuity test will tell you where to focus, assuming the sender and ground are good. Wouldn't hurt to check continuity from the top of the sender to ground either.
 
Good stuff here, guys. Thanks. I followed the wire up the right frame rail and down around the starter. There are half a dozen places where it looks bad and could be cooked/broken inside the insulation. I ran a new wire up the left frame rail like Mechted said. Now I need to tie the new wire in, probably at the firewall junction block. But I can't tell which wire to splice it into. 46 years of dirt and heat have them all looking sort of tan. Any ideas as to how to tell which wire is right?
 
A wiring diagram for the year your truck is would show what color wire and the location on the junction block would be..some years had different color gas gauge wire,most were tan or light brown,but some ,like my 82 GMC,turns to pink somewhere between the harness and junction block (perhaps because it had dual tanks ?)...

If you can get an online wiring diagram of the dash area to the junction block from the gauges you can identify the wire and get the color code.
Also wiping the wires off with a rag soaked in solvent like brake cleaner may reveal the colors,they are often sprayed with some factory undercoating where they go in the junction block and make them all look the same..
 
Based on the wiring diagram found here:
http://www.mediafire.com/file/6w92z...ruck_Wiring_Manual_CK_10_30_Pgs_Only.pdf/file
On page 2, you can find the firewall junction block in the lower center of the page. Fuel sender goes through pin #30. Identified as 18T(CXL)-30.

How accurate is that? Not a clue, mine has been modified. A lot. And I dont want to pull all the loom off again.
I would recommend just start slowly taking loom apart and chasing the wire "hand over hand" from the starter area to the firewall junction.
 
IMO, forget all that. Pull the cluster loose, run the wire directly to the gauge terminal for the sending unit. If it works, the stock wiring is bad. You do not want to mess with the firewall connector if you don't have to, as mentioned they are coated in some sort of tar(?) based mess, and it's not a lot of fun to deal with, if you don't have to. It coats the actual terminals, not just the connector. You need to verify that the gauge and sender work, before you worry about the wiring. Yes, wire could be an issue, but if you can't get the gauge to work in the first place, then good wiring won't "fix" anything. Once you establish the gauge and sender work, THEN worry about why they don't work.

If the gauge works when you do this, you have not eliminated the various connections as the cause.

After making sure the sender and gauge are capable of working properly, move on to continuity for the whole setup, and work from there. Worst case you have an issue between the gauge and the wire to the tank, but then you check continuity from the instrument panel (IP) connector and if that shows none, then you have to pull the firewall connector and check from there back to the tank, which by process of elimination, would have to be the issue at that point.

It would probably behoove you to remove the cluster as well, and before you disturb anything, check that the gas gauge terminals (the copper threaded pieces) have continuity to the IP connector, and that one wire is 12V, and one ground, and with key in run, you've got 12V and ground at the gauge.
 
Finally got this done Junction block on the firewall gave me fits. Pretty sure some PO messed with it. I ended up having to run a wire into the tub and tie into the cluster harness. It's the tan wire that is by itself on the corner, at least on my '73.

Also for anyone reading this in the future, remember that out of your sending unit, wire and gauge, you may not just have one problem. Both my gauge and my wire were bad. Assuming that 2 of those 3 "must" be good was responsible for 3 hours of work and a lot of swearing.
 
Simple test for wiring harness and gauge.
Disconnect wire at sender if possible, place a test light between gauge wire and ground. Turn key on, if gauge and wiring are good, gauge will read 5/8ths of tank. If it doesn't you have a gauge wiring issue. If you have 5/8ths reading, then sender or sender ground is bad
 
Wes Harden said:
Simple test for wiring harness and gauge.
Disconnect wire at sender if possible, place a test light between gauge wire and ground. Turn key on, if gauge and wiring are good, gauge will read 5/8ths of tank. If it doesn't you have a gauge wiring issue. If you have 5/8ths reading, then sender or sender ground is bad
Click to expand...

Even easier, unplug sending unit at the tank and gauge should peg to the 3 O’clock position. Short to the purple wire (sending unit) to the frame (aka ground) and it’ll peg to empty.
 
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