sending unit or fuel guage

[sgreene]

Guy, there is a post on the under dash fuse block that reads Fuel Ga. Am I correct when I assume this is the post wher the sending unit plugs in. If so, am i correct when I assume that a change in state on this post froom open to grounded should result in a change on the fuel guage. Trying to determine which is bad. So is it basically power flows through the gauge looking for a path to ground which is regulated byt the potentiometer in the sending unit. Therefore this post is there for convenience but actually is fed from the guage so removal of guage is not needed when changing the sending unit. This post has voltage to it. Or am I totally wrong and this post is guage power and the sending unit plugs in somewhere else like in the back of the guage? Any hints on how to trouble shoot would be most appreciated. Its a 71 C30 1 ton

[JimKshortstep4x4]

The brown wire from the tank sending unit as you surmised does plug into the location described in your post. I believe that you have the system figured out!

If you ground the terminal where the sending unit plugs in then the gauge should read empty.

With the wire disconnected the gauge should read full. The sending unit sends a signal to the dash gauge between 0 to 90 ohms (0=empty, 90=full).

Jim

[sgreene]

sweet..thanks, I then have a bad guage or no power to the guage as the state does not change. I have not looked yet, how does the guage plug in, with a quick disconnect or harness of sorts? just trying to determine how I verify power to guage. Suppose unplug it and put a meter on the contacts to the harness. IF voltage present at guage then I should probably order a guage. I think I can assume the pass-though to the fuse block is good since I can read voltage at this post...so all signs point to guage....so far anyway. Is my thinking sound?

[sgreene]

Also the sending unit pictured in the LMC has a ground lug, mine does not and looks to be grounded through the sending unit locking ring itself, it this correct as well?

[VetteVet]

There are minor differences in the larger truck dash clusters, but the main wiring is the same. Your gauges are powered by a pink wire to the plug in the back. This wire goes through the plug at terminal 3 and goes to the hot side of the fuel gauge and then feeds the rest of the gauges in the cluster. See the diagram below.
There is a fuse on the fuse panel that reads "cluster feed" and it fuses this wire. Make sure it's good. If the tan sender wire is disconnected from the sender or the fuse panel the gauge should go past full to the 3:00 o'clock position. If you ground the sender or the terminal on the fuse panel the gauge should go to empty just like Jim said.

If you don't get anything on the gauge then it's either getting no power or the resistor on the gauge is bad or the gauge isn't grounded to the dash and to the cab.


The fuse is in the bottom middle it's 3 amp the fuel terminal is up on the right as you know.




this is the back side of the cluster. The fuel gauge is just below the plug-in and the red wire going to the fuel gauge is a power feed for an accessory tach. Ignore it except that it goes to the power side of the gauge.




This is a drawing of the cluster gauge panel and it shows the no. three wire on the plug and shows how the gauges are wired.
all the wiring is via the circuit board but it's the same except the colors of course.






Most important is to have the cluster grounded to the dash and the cab. The factory had a short grounding wire on the cluster but most of them disappear so you may have to make one and put eyelet terminal on it then connect it to the dash at a good ground spot.

[sgreene]

thanks much for the info, all signs point to the guage I think at this point, cheap enough to replace, all the other indicators seem to function fine, fuses look good but did not reseat them but based on the diagram a few other things would not work if the fuse was bad, they all work as far as I can tell. Gonna start with a guage after verifying power for sure on back of guage. Thanks so much guys.

[JimKshortstep4x4]

If you don't have power to the gauge in the dash then check the fuse closely and also make sure that power is there and actually goes through the connectors. I would not say it cannot happen but I have yet to run into a bad dash gauge.

Jim

[sgreene]

What is that 3rd on the lug below the pair on the fuel Guage..does not look like it hooks to the cicuit board...also where is the ground location you speak of, on the board somewhere..and lastly I don't not have no. 5 wire it is dashed in the diagram so I assume was an option I do not have? Also no 4 feeds the fuel GA. On the fuse panel (for sender)and both will have voltage?
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[sgreene]

Got it..sending unit is bad..still curious about the this post on the fuel guage though..can't thank you guys enough..these trucks are so simple but so timeless...love it can actually work on it
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[aventur1]

I would like to know also where to run ground wire on the back of the cluster?

[Turkey nest]

I have a 1971 K5 the truck is near mint except for one lagging issue the fuel gauge. I have read every post related to this and Im still having the same issue. I replaced the tank complete with a new sending unit the gauge has been pegged it 3 o'clock since I have owned the truck and my grandfather bought it when it was 4 years old. Here is what I have done so far replaced the resistor that didn't work, replaced the gauge that didn't work, ran new brown wire from the sender to the fuse block that didn't work, Ran new ground wire that didn't work, dropped the tank pulled the sender tested it on the work bench with the old gauge it worked, put everything back together wired everything to the gauge direct (bypassed the printed circuit) that didn't work.... the gauge is still at 3 o'clock I'm going crazy!!! any help would be much appreciated.

[sgreene]

3:00 is full or past and is what mine was doing so sounds as if the curcuit is open ...grounding the sending wire and giving it a sec for needle to react should change state of needle to opposite...sound like you covered that tho..let one of these more experienced guys chime in
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[cdowns]

Originally Posted by aventur1

I would like to know also where to run ground wire on the back of the cluster?

ground wire from the gauge cluster was a fricton sissor type push on connector that was usually pushed onto the pinch weld of the firewallsort of inline with righthand side of cluster

a very piss poor way that it was done i always snipped of that pushy thingy and installed an eyelet and drilled a hole thru pinchweld and used a nut n bolt