Fuel switch relay
 

I’ve 1987 GMC Sierra R3500, that I just restored, I had some issues with fuel electrical wires.
Nothing worked, but I fixed most of it, I installed new tanks, new sending units, wire harness is good.
But: I need your help in finding what’s missing here (in the pictures)?
I need to know the relay/or whatever missing (if any) that’s located behind the instrument cluster, for the fuel switch selector in the dash.
I checked and chase all wires, from the fuel selector to the fuel selector valve (underneath the passenger side), to the fuel sender units, they all ok. I ended up with the selector fuel switch’s 4 wires, 2 go to the fuel selector valve (gray & tan), and 1 ground black and white, and 1 tan that enters the bulk head behind the fuse box, then it end up with 3 wire plug (black and white, red and black, tan and white).
If I excited it with direct 12v from the battrey, the fuel switch will work on both fuel pumps.
So it’s missing a part, does anyone know the relay type or whatever missing name?
Pictures attached.
Sorry for the description, not an expert in terminology.

Re: Fuel switch relay
 

Download the 1987 wiring diagrams. This is a FULL LINE manual so there are other vehicles wiring diagrams. You want the RV diagrams.

They are a pain in the neck to read but once you figure it out it's possible.
Each page has letters and numbers around the perimeter. The Letters are the Y axis and the numbers are X axis. You'll see comments next to some connectors with a Section number followed by a letter and number. The section numbers are at the bottom of each page. Then you use the grid letter and number to locate the continuation of the circuit.

Your pictured eight position plug has Tan/White, Pink/Black, and Black White wires. It's not directly part of the factory NL2 Aux tank wiring. It is tied into the fuel pump wiring but it can be left unhooked without killing power to the pump.

Look on PDF page 176 Grid point M55. That looks like your pictured connector. It's connected to the Hot Fuel Handling controller pigtail. If you don't have a Hot Fuel Handling controller ̶n̶e̶x̶t̶ ̶t̶o̶ ̶t̶h̶e̶ ̶f̶u̶e̶l̶ ̶p̶u̶m̶p̶ ̶r̶e̶l̶a̶y̶ ̶i̶n̶ ̶t̶h̶e̶ ̶e̶n̶g̶i̶n̶e̶ ̶b̶a̶y̶,̶ ̶o̶r̶ behind the dash in at least 1987, it's just an extra plug. The Tan/White circuit 120 wire does connect to the fuel pump relay wire downstream from the relay.

Open PDF page 203 for the NL2 Aux tank sub-harness diagram. The hookup points between the NL2 harness and the rear-lamp/fuel-tank harness are shown on PDF Page 212 Grid points H7 & K7.

The Fuel pump relay is on PDF page 172 Grid point K55.
You'll usually find this relay on the engine bay side of the firewall between the heater or AC box and the transmission tunnel.
This relay should get power from the ECM on the Green/White wire and it's coil switch power from the ECM on the Orange wire. The ECM connectors are shown between E44 & M44. You can follow the wires using the circuit numbers.
You'll find a red wire with a Packard plug that will feed test volts through the normally closed contacts of the relay to the fuel tank wiring in the rear lamp harness and then through the NL2 harness to the selected fuel pump.

The Fuel Pump Oil Pressure Switch is shown on PDF page 171 at Grid point S7


If you can print 11x17 Tabloid that's the native page size. I have access to an Okidata 830 that prints 12x18 (Architectural-B) so I sometimes crop and scale tabloid pages up for a lot more readable detail.

Re: Fuel switch relay
 

Thanks very much for everything, I did mention my truck in the top (1987 GMC Sierra R3500 with 7.4L and auto tranx), I’m no electrician, but I’ll try to figure it out, however to add to your comments, I’ve a Hot Fuel Handling controller next to the fuel pump relay in the engine bay, I heard it clicking and fuel pumps are working for few seconds.
The issue is that with all wires connected, I can’t hear the fuel valve, or the fuel pumps are working. But when I took a wire from + of the battrey directly to fuel selector switch, I can get it all working.
I’m not in the USA to get a professional to fixed it.
But again thanks for all the help.

Re: Fuel switch relay
 

Here is a simplified diagram which shows the fuel module

Re: Fuel switch relay
 

The ECM commands the Fuel Pump Relay. It should run for around 5-10 seconds or so when you first turn the ignition on.
The Hot Fuel Pump Controller is completely solid state... If you hear clicking it's the Fuel pump relay.

I need to correct something I said earlier. Apparently the hot fuel pump controller is under the dash not a sealed unit in the engine bay next to the Fuel Pump Relay.

You probably should have one in Qatar. It's to keep gasoline from boiling in the lines and vapor locking the pump.

The attached picture is what a 10052973 Hot Fuel Module for a 1987 R V series 5.7L & 7.4L with TBI looks like. You can get them on Amazon here in the states.
It's from a post by BIGJWEAVER. You can see the pigtail as well as the circuit board inside the module. That TO220 FET controls the fuel pump power. FETs don't make noise.
http://67-72chevytrucks.com/vboard/a...1&d=1289096084


The fuel pump relay and pump is running as long as the ECM is commanding the pump on...

If the ECM is not getting pulses from the ignition module in the distributor it will not continue to feed the fuel pump relay coil power after the initial pulse when you turn on the key.

If the fuel pump is not running after the oil pressure exceeds 5psi you need a fuel pump oil pressure switch too.

Re: Fuel switch relay
 

Excellent, I’ve this one disconnected because it was fried somehow, and I bought new one but never connected it.
Never thought of it at all

Now; how to test that the fuel relay in the engine bay is working fine?

Thanks again

Re: Fuel switch relay
 

The easiest way to test the fuel pump relay (underhood) working is:

-Turn key on. You should hear the fuel pump run for 3 seconds, a click at the relay, and the pump turns off.

If the pump doesn't turn on or off, the relay isn't working (might not be bad, but something isn't right).

Re: Fuel switch relay
 

Thanks, that’s another issue, it will do that for the first attempt, the subsequent test nothing.
The fuel sender units are new (sending unit&fuel pump), i tested all of them before installation, excellent ground, new tanks, wires from the fuel switch to both sending units are excellent, I tested the continuity. The fuel valve selector is excellent, the fuel switch is working fine.
I removed the bed to test all of that.
Right now no fuel in the tanks.
Currently I’m missing the hot fuel pump controler, as it was fried before.
All I need is to be able to select which tank without any issues, and they all work properly.

Re: Fuel switch relay
 

This an update
Found the hot fuel module and I installed it (picture attached), connected the battery, turned the key on, heard a click at the fuel relay (picture), connected the wire to the hot fuel module by the bulk head near the wiper (attached), and I selected the left fuel pump it worked, turned it to the right fuel pump and worked too, all worked for few seconds and it turned off.
Which means success👏🏻, all thanks to 👋🏻Hatzie and 👋🏻Wwes.

Re: Fuel switch relay
 

Excellent, glad you got it working.

Re: Fuel switch relay
 

This is an update
All work is done, started the truck today, fuel selector switch works, fuel selector valve works, both fuel tanks are working perfectly.
BUT the fuel gage reads past full regardless of tank selected. It was working before but now it’s not.
Thanks to everyone who helped me to get my issues fixed, especially Hatzie & Wwes.