87 Engine dies switching to R tank

[Dieselfume]

Howdy. Purchased an 87 R-10 LWB Silverado 305tbi earlier this year. Been running it off the L tank because soon as you select the R tank, she dies.

I'm not really familiar with where all the electrical gizmos and wires that make the tank switching work. The wiring diagram in my manual leaves some question marks with this part of the system. Anyway, did some troubleshooting today, and the results were puzzling.

First, found the fuel valve and disconnected the connector. Found that the enigne would still start/run with it disconnected, but the gauge reading pegs past full.   So I presume this connector at the valve does not supply power to the fuel pump.
 
Also, when I have the engine running, and I switch to R (which kills the engine) when I switch back to L I could hear a click from under the truck.

So I found two more connectors right above the frame rail where the cab and bed meet, two wires, one going to each tank. A pink wire and tan or grey couldn't see too good. Anyway, I disconnected the one to the left tank, and she still starts/runs, but again, it pegged the gauge past full. With the key on, disconnecting the same two wire connector for the right tank, pegs the gauge.

So, looks like I found everything related to the tank sender and switching, but nothing for power to the fuel pumps at each tank. I looked all over under that truck and can't spot where any other wires are going to the tanks.  I presume there must be some wires that are hidden that supply the fuel pumps. Since I can't find the wires to the fuel pump, I can test em to see if the R fuel pump is getting voltage.

So, right now I suspect the R fuel pump is bad, but just can't find the proper wiring to test and confirm. The switching seems to be working fine, so I think the tank switch in the cab, and the valve under the cab to be good. I don't want to drop a fuel tank to find I didn't have to (and if I have to, I'll be asking about swapping a 20 gal tank in place of the 16).
Is there a fuel pump relay somewhere?
The diagram I have shows a "hot fuel module". What the heck is that and where is it located?
Earlier I was going to test some wires out of the cab switch, but too much plastic ducting in the way for access, and the switching seems to work.

Any Ideas? Thanks in advance.

[VileZambonie]

Take a test light to the R tank connector for the pump. Have an assistant switch to the R tank and crank the engine. If the test light lights up you probably have a bad pump.

[Dieselfume]

So where do I find that connector for the R tank fuel pump? Or do I have to drop the tank to get access to it?

[DnStClr]

Tom, there are 2 elect. connectors at the L/h tank.They are located at the L/h frame rail near the tank. One feeds the R/H tank with a signal for the pump and the sending unit. It's a two wire connector. One wire is pink/black tracer and one wire is grey. The grey wire feeds the pump and the pink/black wire feeds the sender. here's a pic of the two elect. connectors taken apart..



Here's a pic of a pump/ sending unit removed from the tank..



There is a fuel pump relay. It's located on the passenger side of the firewall in the engine bay. I'm sorry I don't have a pic- I've lost a flash drive with most of my pics and can't find it.

Hot fuel module is only on the heavier (one ton) trucks.

I don't think a 20 gallon tank will fit under the side of the trucks. Some guys have installed them in the rear after modifying the cross member under the bed.
Hope  this has helped a little-

[joesgarage71]

Shouldn't the blue, purple or lt. green wire be the ground for the gauge? And is there a ground for the pump? Just thinking out loud. don't want to see you buy a pump when it could be a bad ground.
 Steve

[Dieselfume]

Thanks for the help. So those two wires were it, it is just that it comes out on the left instead of going to the left tank (and over to the right).

So that grey wire tests good, 12v briefly, then 12v again while you crank, and stays 12v until the oil pressure bleeds off. I recently did the fuel pump on my Olds, so this should be easier to drop (and siphon from). Ratchet straps rock.

I suppose it could be a bad ground too, but I bet the ground strap isn't exposed until you drop the tank. Probably the pump considering how infrequently it was used. I didn't find any blue, purple, or lt green wire going to the tank.

As far as the 20 gal tank, I imagine there must be a few little differences. I'll try to find a LWB with 20gal tanks to compare to. (Neighbor has a SWB so can't use it for comparison). I bet the fill & vent hoses are different (shorter).
Thanks again!

[DnStClr]

Steve, the wiring schematic from GM does show that there are 2 grounds- one for the sending unit and one for the pump. I'll bet you're right- the sender ground must be used for the gauge.
The black ground in my pic is the one for the pump. On my truck it used to be blue, and some po cut it, left it hanging, and added the black wire to replace it.
Those grounds cause a lot of problems.