Fuel Gauge wiring location with factory single tank

[cerial]

Bought a 85 GMC 2500 4x4. 5.7, sm465, 208, SF14, 10 3.42 ratio standard cab 8'bed Yesterday. The truck was used as a airport truck then it was sold to a family who had it for 20+ years.

It runs drives and stops great. It has rust of course. But I really just want it for a deep snow winter ride. It has a few issues besides the rust that I will be addressing. One of those is that the fuel gauge does not currently work.


Now I will no doubt eventually do a 5.3 swap and run a back seat out of a 2017ish truck. But this thing runs and drives currently. This year I really just want to fix the floors so its not throwing snow inside and refresh things so it's not going to leave me stranded during a blizzard.

The sending unit on the drivers side is right between the cab and the bed and I can get my hand on the green and orange wires to see that they have been cut. I should be able to connect these to new wiring without dropping the tank.

Green is the ground and the orange is the sending unit from the looks of it. But looking around I can't seem to find what I am assuming is going to be green and orange or orange and white wires going to the gauge. Looks like they ran a new taillight wiring when a newer bed was installed and the harness from the cab back was removed.

Now I have a 91 cab and chassis R30 square and a 87 1/2 ton V10 square so I simply went up to the selector switch on the dash when going to single tanks on those. But this 85 does not have a selector switch.

So where will I find the wires coming off the gauge preferably without cutting into the plug behind the cluster. (worst case I do this)

Should I start behind the dash near the center of the dash? Would the gauge wiring be there?

I am guessing it would run from the dash down the passenger frame by where the old selector would be. Or with a single cab single tank it could be ran down the center of the cab. Even skipping the dash all together and running next to the speedo cable.

Before I spend hours searching and testing wires I thought one of you guys might know where to look maybe even if the colors change behind the dash etc.

Thanks for reading and any help in advance.

[MikeB]

Originally Posted by cerial

But looking around I can't seem to find what I am assuming is going to be green and orange or orange and white wires going to the gauge.

According to the chart below, the fuel tank sender wire running to the instrument panel printed circuit connector is TAN. It is connected to terminal #18.

The document that I copied the chart from also shows both terminals #7 and #18 are used for trucks with factory gauges, but I have no idea why.

If the image is too small to read, try opening it here.

http://forum.73-87chevytrucks.com/sm...ch=29746;image

[ray_mcavoy]

Originally Posted by MikeB

According to the chart below, the fuel tank sender wire running to the instrument panel printed circuit connector is TAN. It is connected to terminal #18.

The TAN color listed on that chart is correct for the earlier years. However, they changed over to a PINK wire for the fuel gauge sending unit sometime around 1978.

Originally Posted by MikeB

The document that I copied the chart from also shows both terminals #7 and #18 are used for trucks with factory gauges, but I have no idea why.

That was done to accommodate the factory tachometer clusters. The small fuel gauge in the lower/left corner of the tach clusters used pin #7. While all other clusters with the large fuel gauge off to the right of the speedometer used pin #18.

From the back of the cluster, the fuel gauge sending unit wire should run down to the firewall bulkhead connector that is located behind the fuse box. The under-hood side of that connector is divided into 3 sections (engine harness, front light harness, and rear light harness). The fuel gauge sending unit wire comes through as part of the engine harness. From there, it should simply run down to the frame and back to the tank.

[cerial]

Trucks dripping dry in the garage right now. I gave it a bath to wash off years of "sitting" grime as well as chase off spiders etc.

I am just going to pull the cluster then will splice off of 18(No factory tach). Run a new wire back and avoid any chance of a T or other cut connection.

Thanks for the great help.

[MikeB]

Originally Posted by ray_mcavoy

The TAN color listed on that chart is correct for the earlier years. However, they changed over to a PINK wire for the fuel gauge sending unit sometime around 1978.

Just found my 1982's OE connector with all the original wires attached. You're right, a PINK wire connects to pin #18. And a dark brown wire is connected to #7. The connector was wired for an instrument panel with warning lights.

Someone put a lot of time into those charts, but got it wrong. They also left out several pins and wires.

[hatzie]

Originally Posted by MikeB

According to the chart below, the fuel tank sender wire running to the instrument panel printed circuit connector is TAN. It is connected to terminal #18.

The document that I copied the chart from also shows both terminals #7 and #18 are used for trucks with factory gauges, but I have no idea why.

If the image is too small to read, try opening it here.

http://forum.73-87chevytrucks.com/sm...ch=29746;image

18 is the big fuel gauge, 7 is the small gauge with the tach IIRC