Fuel selector

[NCali]

87 2wd 1/2 ton 305 tbi

First off I have copied the schematic in the tech section and downloaded the fuel and emissions manual and I am studying them for an answer

I drove 65 miles to work on the right tank , parked the truck and the pump would not work at the end of the day on that tank. I drove home on the left tank and looked for and found a loose connection on the right tank
  I have had problems before with the selector switch so I popped it out and found that I have power at the switch left tank side for 2 sec and when it runs. I have no power on the right side tank  side of the switch
 with the switch removed   , I jumped the hot wire for the left tank to the fuel pump wire at the connector in the cab and it started and ran for a minute like the selector valve didnt switch tanks.

    Every picture that I have seen of components in the engine compartment show the fuel pump relay on the passenger side of the firewall. I can remove that relay and the truck will still run.
That throws me , I dont understand how it will run with the relay removed

[bd]

87 2wd 1/2 ton 305 tbi

    Every picture that I have seen of components in the engine compartment show the fuel pump relay on the passenger side of the firewall. I can remove that relay and the truck will still run.
That throws me , I dont understand how it will run with the relay removed

The TBI fuel pump feed is a redundant system.  The fuel pump relay is backed up by a parallel wired oil pressure switch.  If the relay fails, the oil pressure switch will still power the fuel pump once the engine oil pressure builds sufficiently while cranking.

[NCali]

ok , I understand that , so where does the power to the fuel selector switch come from? or through I should say. like I said, I only have power on one side of the switch. or 1 pair of wires but nothing on the other set

[NCali]

  Ok looking at  the schematic it looks like power is supplied to both sides of the switch through contacts on the switch. so I need to find another switch, I think?

[bd]

The fuel tank selector switch in the dash has four terminals.  Power in (from the fuel pump relay) and ground connect to two terminals of the selector switch.  The remaining two terminals simultaneously alternate between power and ground as the switch is flipped between the left and right tanks.  In other words, the gray and tan wires that exit the switch reverse polarity each time the switch is thrown.  So at any given time, one wire is ground and the other is hot.  Flip the switch and the two wires reverse - what was hot before is now ground and what was ground before is now hot - and so on, back and forth.

The gray and tan wires both connect to the tank selector valve and together actuate the valve.  The relative polarity on the gray and tan wires determines whether the selector valve draws from the left tank or the right tank.  To switch the fuel gauge between the left and right tanks, a non-serviceable switch is incorporated into the selector valve that connects the gauge to either the left or right tank sender in conjunction with dash switch position and tank selection.

In addition, the gray wire connects directly to the right tank fuel pump and the tan wire connects directly to the left tank fuel pump.  When the gray wire is powered, the tan wire is grounded, the selector valve actuates to the right tank, and the right fuel pump energizes.  Conversely, when the tan wire is powered, the gray wire is grounded, the selector valve actuates to the left tank, and the left fuel pump energizes.

In summary, the tank selector switch in the dash directly controls the selector valve under the bed and the in-tank fuel pumps, while the selector valve under the bed switches the dash gauge between the left and right tanks.

With the foregoing concept in mind, recheck the operation of the dash switch and selector valve.  And, be sure to verify a good connection between the switch and cab ground, via the black wire.

[NCali]

bd: I thank you for that answer. In all the years that I have owned this truck and all that I have read about it I have never heard  that this is how that system works   Even  by looking at the schematic I didnt catch that, how interesting.

Thanks again

[bd]

You're welcome. 

The above description applies to TBI with factory dual tanks.  Carbureted dual tank systems are wired and function differently, having undergone several design iterations through the years.

[NCali]

 and mine is a TBI  with factory tanks

[Iowa Fox]

The fuel tank selector switch in the dash has four terminals.  Power in (from the fuel pump relay) and ground connect to two terminals of the selector switch.  The remaining two terminals simultaneously alternate between power and ground as the switch is flipped between the left and right tanks.  In other words, the gray and tan wires that exit the switch reverse polarity each time the switch is thrown.  So at any given time, one wire is ground and the other is hot.  Flip the switch and the two wires reverse - what was hot before is now ground and what was ground before is now hot - and so on, back and forth.

The gray and tan wires both connect to the tank selector valve and together actuate the valve.  The relative polarity on the gray and tan wires determines whether the selector valve draws from the left tank or the right tank.  To switch the fuel gauge between the left and right tanks, a non-serviceable switch is incorporated into the selector valve that connects the gauge to either the left or right tank sender in conjunction with dash switch position and tank selection.

In addition, the gray wire connects directly to the right tank fuel pump and the tan wire connects directly to the left tank fuel pump.  When the gray wire is powered, the tan wire is grounded, the selector valve actuates to the right tank, and the right fuel pump energizes.  Conversely, when the tan wire is powered, the gray wire is grounded, the selector valve actuates to the left tank, and the left fuel pump energizes.

In summary, the tank selector switch in the dash directly controls the selector valve under the bed and the in-tank fuel pumps, while the selector valve under the bed switches the dash gauge between the left and right tanks.

With the foregoing concept in mind, recheck the operation of the dash switch and selector valve.  And, be sure to verify a good connection between the switch and cab ground, via the black wire.

I know this is a follow on an old post. Where does the black ground wire usually ground under the dash for the selector switch? Its impossible to see under the dash of my 87 or do you have to pull the switch out of the dash to trace it?

Thanks

[hatzie]

You're welcome. 

The above description applies to TBI with factory dual tanks.  Carbureted dual tank systems are wired and function differently, having undergone several design iterations through the years.

There are only three differences from the NL2 81-86 Carb tank valve system to the TBI NL2 valve system... they use the same motorized Pollack valve.
1 81-86 don't have fuel pump wires wyed to the polarity reversing valve actuator wires.
2 On 81-86 trucks GM used a darn poor excuse of a three position momentary on-off-on switch that I always immediately swap to the 87-91 on-on version they should've used in the earlier rigs.
3 The TBI sender uses a weatherpak disconnect 81-86 use molded pin connectors on the sender.

The 73-80 systems are very different.

Look at the NL2 diagram in the 88 or 89 wiring book... 87 is harder to follow and unlike some wiring this option didn't change.

[Iowa Fox]

I know I'm going blind in my old age but for the life of me I can't find the black ground at the cab end and yes I have a wiring diagram for a 87 TB.  Even taking the ash tray out you can not see the back of the selector switch to get a visual on where the black wire leads to. I'm thinking it should be by the parking brake mechanism or it might be hidden behind the dash someplace. I'm going to see if I can see anything with a flashlight and mirror.

[bd]

It should connect to the bus bar that is fastened directly to the kick panel sheetmetal, adjacent to the e-brake pedal assembly.

[Iowa Fox]

Found it. Its redish pink with a black stripe. I've owned it since it was almost new so I know this is the way it was built at the factory. There are absolutely no black wires on the buss bar. I tried pulling the switch out of the dash and it didn't want to come and I didn't want to break it. I found a old paper service manual from the same time as the truck was new. In the carbureted section for dual tanks I found a foot note that states the ground wire from the switch to the buss bar should be black but could be black & pink. I can say for certain if you are having issues don't waste you time with diagnostics until you clean the ground. Cleaned every connection to the buss bar while I was there. Thanks for the help guys.