cluster wiring plug 1977 gmc

[4x4k30]

I have been trying to get my fuel gauge to work after doin a ls swap I need pinout of the back plug the goes in the back of the cluster my truck is a 77 gmck3500 crew it came with gauges and manual oil gauge. I put in a 79 tach cluster it worked fine with the orgnail 400 smallblock but I did a ls swap 6.0 and bought 2 new 87 tanks with efi pumps and installed a 6 way Pollack switching valve and 87 fuel selector switch it has 4 poles on it the 77 switch has 5 where does the 5th wire need to go?

[4x4k30]

more pics diagrams wiring the tan wires are at 7 and 18 for the fuel gauge.............. shows 6 13 tan wires that go to the brake light wires are tan too...

[ray_mcavoy]

Originally Posted by 4x4k30

more pics diagrams wiring the tan wires are at 7 and 18 for the fuel gauge.............. shows 6 13 tan wires that go to the brake light wires are tan too...

Yes, the tan wires going to pins 7 and 18 are for the fuel gauge. If you have a gauge cluster without a factory tachometer it only makes use of the wire at pin 18 and leaves the one at pin 7 unused. Similarly, if you have the factory tach cluster with the small fuel gauge down in the lower left corner, that only makes use of the wire at pin 7 and leaves the one at 18 unused. I believe they were connected like this to make the wiring compatible with both styles of gauge clusters (with & without tach).

If you look closely, the tan wires for the brake warning light should have a white stripe on them. These wires were also routed to 2 different pins on the connector to make the wiring compatible with all gauge clusters (with or without tach or clock) as well.

And just so you are aware, the 2nd diagram you posted for the dual tank wiring sort of abbreviates the "production fuel gauge" wiring. That section of tan wire runs from the cluster plug, out through the firewall bulkhead connector (located behind the fuse box). Then it runs along side the wires going down to the starter. From there, it runs back along the frame and on a single tank truck it would have plugged directly into the sending unit on the passenger side fuel tank. But with dual tanks, it instead plugs into the separate dual tank harness that is detailed in that diagram. That dual tank harness actually routes the tan wire back up inside the cab again to connect it to the tank selector switch.

[4x4k30]

Originally Posted by ray_mcavoy

Yes, the tan wires going to pins 7 and 18 are for the fuel gauge. If you have a gauge cluster without a factory tachometer it only makes use of the wire at pin 18 and leaves the one at pin 7 unused. Similarly, if you have the factory tach cluster with the small fuel gauge down in the lower left corner, that only makes use of the wire at pin 7 and leaves the one at 18 unused. I believe they were connected like this to make the wiring compatible with both styles of gauge clusters (with & without tach).

If you look closely, the tan wires for the brake warning light should have a white stripe on them. These wires were also routed to 2 different pins on the connector to make the wiring compatible with all gauge clusters (with or without tach or clock) as well.

And just so you are aware, the 2nd diagram you posted for the dual tank wiring sort of abbreviates the "production fuel gauge" wiring. That section of tan wire runs from the cluster plug, out through the firewall bulkhead connector (located behind the fuse box). Then it runs along side the wires going down to the starter. From there, it runs back along the frame and on a single tank truck it would have plugged directly into the sending unit on the passenger side fuel tank. But with dual tanks, it instead plugs into the separate dual tank harness that is detailed in that diagram. That dual tank harness actually routes the tan wire back up inside the cab again to connect it to the tank selector switch.

I changed the cluster with a tach from a 79k20 the 77 dash fuel switch has 5 wires to it the new dash switch has only 4 I am running dual tanks with efi fuel pumps all the wires on my 77 are tan and white no solid tan also look at last pic there is a tan and white wire that goes to 17 and nowhere else ??

[ray_mcavoy]

Originally Posted by 4x4k30

I changed the cluster with a tach from a 79k20

Okay, the 77 vs. 79 cluster pin-out is different as I'm sure you already know. However, the fuel gauge wiring still goes to the same pins (7 & 18) for both years so that shouldn't be an issue.

Originally Posted by 4x4k30

the 77 dash fuel switch has 5 wires to it the new dash switch has only 4 I am running dual tanks with efi fuel pumps

That sounds like your new dash switch is the newer style that is used with the motorized tank selector valve (as opposed to the solenoid style valve). Am I correct in assuming that your truck has been changed over to the newer style motorized valve? Likely the 87 style due to the EFI with in-tank pumps? If that's the case, the switching of the sending unit wire to the selected tank is done at the valve (instead of at the dash switch). And you would want to refer to the newer 80's dual tank wiring diagrams instead of the original 77 diagram if yours has been converted to this style.

Originally Posted by 4x4k30

all the wires on my 77 are tan and white no solid tan also look at last pic there is a tan and white wire that goes to 17 and nowhere else ??

That's kind of unusual to have white stripes on all of the tan wires. I wonder if someone has been in there before and swapped/spliced in some wires out of another truck? Probably not a big deal so long as they connected everything correctly but having different colored wires does make troubleshooting more difficult (especially if they splice to a different color wire somewhere along the way).

The tan/white wire going to pin 17 (and nowhere else) would be the correct setup for the brake warning light on the 79 cluster. It was only in the earlier years (like 77 & earlier) that had the tan/white wire going to 2 different pins to accommodate the different cluster styles. The 78+ gauge clusters (with or without tach or clock) all have the brake warning light in the same location so that eliminated the need to run that wire to 2 different pins.

[4x4k30]

I did install a Pollack switching vavle with 6 prongs but only 5 were used one is blank . I have tried to just hook up the 4 wires for the 87 switch. but I do not know where the 5th one needs to go. I put in new tanks and sending units and pumps when I put in the 6.0 I have just been running on the right tank with no fuel gauge at all..i have done run out of gas several times and have to keep a 2 gallon can full in the bed ..I thank you for the help I will keep trying to figure it out

[hatzie]

The 1981 and later motorized valve system only uses five of the six positions on the valve Weatherpack.

This is an electrical schematic of just the Pollack Motorized valve.


You can see terminals D & E on the Weatherpak plug go to the motor to switch the valve from tank to tank.
Terminal B is the common of a SPDT (Single Pole Double Throw) switch. GM calls this the METER SWITCH. Terminal B should run to the gauge. In 1977 this is a TAN wire running from the dash switch to the RH frame rail disconnect. You can probably find the original disconnect plug or at least this wire next to the starter on the RH frame rail.
Terminals A & C should be your sender wires from the tanks. Originally these ran to the dash switch on a 1977. With the motorized valve they should run from each sender and each sender wire should terminate at one of the Weatherpak valve plug sender positions.

How much of the original 1973-1980 NL2 wiring is still present and unmolested?
If, and only if, the original valve wiring is still present and accounted for... the minimalist solution would be to keep the original wiring in place and use a pair of SPDT Bosch Automotive relays to control the valve motor. This method only requires that you interrupt the solenoid valve power wire from the switch and add two new wires from the cab to the motorized valve D & E plug positions and the relays can be hidden inside the cab. The 1973-1980 dash switch would still control the gauge and the relays would trip when the switch is set to AUX.


If the tank sub-harness wiring is already chopped up... Here is the the 1989 Diesel tank valve wiring schematic you can use to roll your own wiring. Wire colors will change from year to year but the basic from-to will not.