Author Topic: Fuel Gauge Wire  (Read 282 times)

Offline Chevy Guy

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Fuel Gauge Wire
« on: May 09, 2019, 10:06:58 PM »
Hello every one, would appreciate help as to where the wire from the fuel tank sending unit connects to the fuel gauge.

Those of you that have been here for a while are probably tired of seeing this question asked, Iíve seen tons of info on fuel sending units and fuel gauges but I have not found any info that helps solve my situation.

Not understanding electrical that well doesn't help.

Working on a 1978 C10 V8 2 wheel drive; instrument cluster has gauges for oil, temp, amp and fuel, no tach, no clock.

I have studied the diagram for pin location, and in researching back posts on the forum have found back on Aug 3 2012 a reply from BD informing what pin # went to what circuit and what that circuit was for.

Have studied the 77 to 80 wiring diagrams found here on the forum, it shows of the 3 posts on the fuel gauge, 1 goes to ignition, 1 goes to ground, the third pin is for fuel sending unit.

On the pin diagram and the 2012 reply, it informs that pin 7, 18 are for the fuel sender,  pins 4, 6, and 16 are ignition fused 12 volts, pins 3, 8 and 10 are ground.

Now, on the connector that plugs into the back of the instrument cluster on the truck I'm working on, pin 18 is constant 12 volts, pin 18 has a jumper that goes from pin 18 to pin 7, which naturally makes pin 7 constant hot.

On the printed circuit board:
Pin 7 goes to the clock (constant hot)
Pin 18 goes to the fuel gauge (constant hot)
Pin 16 goes to the fuel gauge (ignition hot)
Pin 10 goes to numerous places and to the fuel gauge (ground)

So, where or how do I connect the wire from the sending unit to the gauge?

Thanks for Any and All Help.

Offline Irish_Alley

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Re: Fuel Gauge Wire
« Reply #1 on: May 09, 2019, 10:20:52 PM »
ok lets start with the basics, do you have the pink wire (maybe with black stripe) going from the fuel tank to the bulkhead connector?

EDIT: i was thinking of the newer trucks. BD listed the right color for the wire
« Last Edit: May 10, 2019, 11:50:54 AM by Irish_Alley »
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Offline bd

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Re: Fuel Gauge Wire
« Reply #2 on: May 09, 2019, 11:13:27 PM »
On a 1978 C-chassis the fuel gauge circuit wire colors should be as follows:  black = ground (ckt 150), pink = ignition (ckt 39), and tan = fuel gauge sender (ckt 30).  With this data on factory wire colors in mind, take a fresh look at the wires entering the I/P cluster connector.  I/P connector pins 7 & 18 are the fuel tank sender circuit, which can have measurable voltage present depending on the position of the in-tank float, but only when the ignition is switched on.  If the sender wires are constantly "hot" regardless of ignition switch position then the wiring has been compromised in some fashion. 

BTW - If your truck has a factory clock, it should also have a dedicated 2-wire plug that connects fused battery positive (orange) and ground (black) directly to the back of the clock, bypassing the PC board entirely.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 75gmck25

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Re: Fuel Gauge Wire
« Reply #3 on: May 10, 2019, 05:44:39 AM »
First question - do you have dual tanks?  Since you don't mention a switch, I assume its a single tank on the driver's side (I think this was the primary tank in '78).

The sending unit wire has a rubber plug that is pushed on a post on the sending unit, and then routed through a large hole in the frame (from outside to inside, with a grommet) so that it is inside the frame rail.  From there it runs up the frame rail and is routed to the bulkhead connector where it passes through into the cab.  Is all that wiring present and in good condition?

IIRC, the sending unit wire should have a pop-apart connection inside the frame rail, which was used if they added a dealer-installed aux tank. If you take that connection apart you should be able to ground the wire (gauge max), or put various resistors in the circuit to see that the gauge moves from low to high.

You can also check continuity from the sending unit wire near the tank to the connector on the cluster, and make sure you have a good wire with no breaks.  Then plug it into the cluster and check continuity to the post on the back of the gauge (you can see the post spring clip from the back of the cluster).

Bruce

Bruce

Offline Chevy Guy

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Re: Fuel Gauge Wire
« Reply #4 on: May 10, 2019, 11:11:34 PM »
Irish_Alley, bd and Bruce, Thanks for the reply.

It seems I have left out some need to know information, there is only one fuel tank on the truck, driver side outside frame, in restoration of the truck, after removing the bed, I removed the fuel tank, being its prone to side impact explosion, I relocated the tank, in removing the tank it had 1 wire going to the frame, and another as Bruce mentioned that pushed onto a post for the fuel sender, the push on wire was cut off, I did not nor have I seen any wire going thru the frame in a grommet or otherwise that would indicate it was for the fuel gauge.

 I got a late start today, didnít get as far as I would like to have to give you feedback to your reply.

Since I relocated the tank, I have a ground wire to the frame and a wire for the fuel sending unit ran to the front of the truck waiting to be connected to the fuel gauge when it gets figured out.

Irish_Alley Ė

I didnít see your edit in time, but the pink wires you asked about, all go to ignition, pins 4, 6 & 16

bd Ė

First, in your comment to constant voltage going to pin 7 & 18, ďmeasurable voltage present depending on the position of the in-tank floatĒ, as stated above the wire to the sending unit is not connected to anything, which at the time you didnít know, because I didnít inform of this, but with it not being connected, it at least eliminates the float position creating the constant voltage.

I do not have a clock in the cluster, in finding pin 7 constant hot and going over to where the clock goes, I figured this was the way it was supposed to be, since the clock needs constant voltage so it runs all the time, but now I know better.

In tracing out the tan fuel gauge wires, as stated, pin 7 goes over to pin 18, I traced the wire from pin 18 down going behind the fuse block, thru the bulkhead connector, in tracing this wire back, I didnít see any other wires connected to it or that may have shorted out and melted to it that would create the constant voltage.

I ran out of time and didnít get to trace the tan wire from the bulkhead connector back thru the engine compartment wiring harness, but I will.
 

Bruce Ė

As I stated at the top of this post, there is only one fuel tank on the truck, driver side outside frame.

The push on wire was cut off, I did not nor have I seen any wire going thru the frame in a grommet or otherwise that would indicate it was for the fuel gauge.

bd has an article on how to make a tester to test the function of gauges, I made one of these testers trying to figure out what was going on, now I canít do the test you mentioned, but I have done the test on the fuel gauge using the tester and the fuel gauge responds to different ohm settings.

Now I hate to say this, but I have to put this on the back burner for a few days, hit a deer while driving the wifeís truck last week, the parts came in so I need to get her truck back on the road.

Offline bd

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Re: Fuel Gauge Wire
« Reply #5 on: May 10, 2019, 11:55:27 PM »
bd Ė

First, in your comment to constant voltage going to pin 7 & 18, ďmeasurable voltage present depending on the position of the in-tank floatĒ, as stated above the wire to the sending unit is not connected to anything, which at the time you didnít know, because I didnít inform of this, but with it not being connected, it at least eliminates the float position creating the constant voltage.

I do not have a clock in the cluster, in finding pin 7 constant hot and going over to where the clock goes, I figured this was the way it was supposed to be, since the clock needs constant voltage so it runs all the time, but now I know better.

In tracing out the tan fuel gauge wires, as stated, pin 7 goes over to pin 18, I traced the wire from pin 18 down going behind the fuse block, thru the bulkhead connector, in tracing this wire back, I didnít see any other wires connected to it or that may have shorted out and melted to it that would create the constant voltage.

I ran out of time and didnít get to trace the tan wire from the bulkhead connector back thru the engine compartment wiring harness, but I will.

It sounds as though you are on the cusp of your solution.  The "constant voltage" on pin 18 most likely sources from the fuel gauge, not the tank sender.  Without going into unnecessary details, ignition power is fed to the fuel gauge via a pink wire.  Ignition power then flows through the fuel gauge to the fuel tank sending unit via the tan wire previously discussed.  The sending unit is a simple rheostat inserted between the fuel gauge and ground.  As the sending unit tracks the fuel level in the tank, the tank sender resistance varies accordingly and controls current flow through the gauge, which directly affects needle position.  With the ignition switched ON, if you ground the tan wire coming off of pin 18, the fuel gauge needle should sweep to Empty.  If so, your solution should be as simple as connecting the relocated fuel tank sender to the tan wire from pin 18 of the I/P cluster connector.  By all means, find that wire along the frame.  Of course, proper gauge function depends on the 'new' sender bearing zero ohms with the tank Empty changing linearly to 90 ohms with the tank Full.

The only variable that needs to be reexamined is the 'constant voltage' that you claim is detectable on pins 18 and 7.  Since gauge power is supplied via the ignition switch, ALL of the fuel gauge pins should indicate zero with the ignition switched OFF.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Chevy Guy

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Re: Fuel Gauge Wire
« Reply #6 on: May 11, 2019, 09:06:55 PM »

Hey bd

  The constant voltage on pin 18 was at the plug for the cluster, cluster and gauges were on floor board with ignition off.

Anyway, being pin 18 is not supposed to have voltage and since Iím going to have a splice on the tan wire somewhere, I cut the tan wire bout 6Ē from the bulkhead, checked and had no voltage at pin 18.

Temporally attached the wire from sending unit to the tan wire at the bulkhead, fuel gauge responded, but it showed right at 3/4 full, I know this was not right since I only put 5 gallons of gas in the tank.

Long story short on what I did, but hitting the fuel tank with the heel of my hand a couple of times the gauge responded showing right at 1/4  tank, so guess the float was stuck in the full tank position and the vibration let it fall, not sure why it was in the full position, the new tank as never been filled completely up.

Now all I need to do is solder my splice and seal it up, trace out the tan wire and find out why it had voltage on it, then I can move on to figure out why the oil gauge isn't working.

So once again Thanks a lot for the help you Irish_Alley and Bruce gave.