jake_smith777

I posted a while back but still having a problem. when I switch to the rear tank it says empty even tho I have gas in it and it still uses the fuel out of the front tank I have tried a new switch but still nothing. was wondering if I need to take the box off and do some investigating on the selector value or other electrical lines.


thanks jake

Chris_1

iTrader: (1)

You don't have to take the box off - but it does make it much easier to look at everything and try to figure out what's going on.
Before you do you might just have a good look at things on your frame rail under the cab.
Also check things under the dash - Don't remember for sure if the 88 has an inertia switch, but I think it should - the dash switch should power the inertia switch on both settings.
I believe you have two low pressure tank pumps and a high pressure frame rail pump. The rear tank pump should run and the front tank pump shut off when you select the rear tank, if it's switching properly.

If you can't tell for sure, take the gas tank cap off to listen.

fltdriver

I would venture to guess that it may actually be pumping from the rear but the front check valve may be shot, allowing fuel to return to the front tank while using the rear. Like Chris said it is easy enough to verify it the rear pump is running, switch to rear, have someone turn the key to on while you are standing/laying in between the two tanks and listen for which tank is priming, if the rear runs then you need a new pump assembly for the front.

Chris_1

iTrader: (1)

That's an 88 - should be a 3 pump system w/ a switching valve (I believe).

fltdriver

But don't they still have the return (check valve) from the FPR in the tank pump assembly?

Chris_1

iTrader: (1)

I'm sure they probably have a check valve but going back to his post, he switches to the rear tank but it still uses fuel out of the front tank which would lead me to believe that the front tank lift pump is still running.
If that's the case, you would think that the power is not switching from the front pump to the rear pump ?
The reading empty on the rear when it has gas could be the rear sending unit, but if there's an electrical problem in the switching over (which seems to be a possibility), the sender problem could be related to that.
That's all guesswork though, I'm more familiar with the next generation fuel system, but it seems to me there is a canister on the frame rail under the cab that handles the 2 tanks delivery to the hi pressure pump, but I'm not real familiar with what elecrical bits are included there. It's where I would start looking though.

Chris_1

iTrader: (1)

We'll call this one a learning experience. Here's what I was able to find out :
Power from the F/P relay thru the inertia switch. 1 wire goes to the hi pressure pump, the other goes to the dash switch.
From the switch, the red wire goes to the front pump. The dark blue/yellow wire goes to the front sender. That part seems to be working ok ?
The brown/white wire goes to the rear pump and the yellow/light blue wire goes to the rear sender. The problem may well be in those 2 wires - or all 4 of them.
I'd start following that harness looking for a break/short. Be especially suspicious of electrical tape.
There is sometimes another wire on the switch - a short jumper to the gauge. Those connections can cause problems with the gauge, but that doesn't seem to be your issue.
I would suspect an incorrect wiring repair in the harness if you're sure the switch on the dash is working correctly.
Let us know how you make out.

Just call me Sean

iTrader: (1)

It doesn't have check valves. No electrical to the DFR (switch valve), it works from the pressure of the selected tank pump. If the front tank pump runs always then someone had to bypass the switch.

Devildog1

Replaced rear tank sending unit on my 89. Tank is full, reads empty. Truck will barely start and won't idol when on rear tank, but runs fine if I switch to the rear tank when going down the road. Driving me and my mechanic crazy

Chris_1

iTrader: (1)

Could be a wiring issue considering the sender gauge being out of whack.
The wiring colours are in a post above and I believe orange and black should be your grounds.
The not running unless you switch while driving sounds like low fuel pressure on that pump. When you switch to that tank while driving, will it still climb a steep hill ? If not, will it do it in reverse ? Those are a couple of symptoms I've noticed when pressure was too low.
Do a fuel pressure test with a gauge at the schrader valve on the fuel rail. Test both tanks KOEO & KOER. For a V8 you need 30 to 45 psi (low to mid 40's is normal)
If you have low or no pressure at the rear tank some possibles would be:
Dying or dead pump, I've seen brand new (cheap) ones drop to 20 psi after 2 weeks.
Wiring or tank selector switch.
Bad ground.
Restriction in line or at switching valve.
Fuel pressure gauge and a voltmeter are a must have to sort that one out.