Ok, so I recently purchased an '88 F150 for a work truck. Originally a 5 speed, converted to a C6. Truck was running (though not well, I'm told) prior to setting for several months. At this point, the truck will crank great, cranks hard, but no ignition.

Spraying ether into the intake has the truck running as long as the ether is applied. The fuel is extremely old, and smells like varnish, so I decided to clean the fuel system up to the rails.

Removed the plug for the schrader test / pressure point on the fuel rail. Nothing. Stuck a hose onto this outlet, with the hopes of using the fuel pump to pump the tank dry. Not all at once (burn up the pump), but several bursts.

Turn the key to on (but not to "start"): Nothing. No fuel from hose. No fuel pump buzz, anywhere. Turn key to start...as expected, truck cranks, but no fuel out of the hose.

Track down the relay, and it -appears- to be ok (no corroded, melted, hot, etc). The shutoff switch in the footwell appears to be ok. I clicked it up to the "off" position, then clicked it back down as well. Held relay block in my hand, while reaching in to rotate the key. 2 distinct "clicks". One happens (it seems) right after the key is rotated out of "off", the other happens a fraction of a second (or slight rotation later). The relay seems to be working, at least enough to warrant a check at the pump.

Crawled under the truck, and pulled the connector from the front fuel pump. Hooked a voltmeter to the two wires. Had a helper turn the key to the "on", but not "start" position. Upon turning the key, the gauge reads 12v momentarily, then drops back to 0. It's my understand that it is supposed to do this, with a 1 second cycle. No noise / movement from the pump, though, when the harness is reconnected.

It sure seems to me that the fuel pump is toast. This is the high-pressure, framerail mounted pump under the driver seat. Does this diagnosis seem reasonable? Anything I'm missing, or input as to what it may be?

Should I be hearing a "buzz" from the two tanks (dual tank truck), even if the high-pressure pump isn't working? I'm curious if all 3 pumps are out (I was told the truck wouldn't switch tanks), or if the two in-tank pumps won't activate if the high-pressure pump isn't running.

Ideas or input? Any links or info on a compatible, or otherwise inexpensive, fuel pump? I'd hate to sink $70 - $120 into a fuel pump, only to find it was something else...

 

The hi-pressure and the selected tank pump should run for one second when you turn on the key. If you want them to run continuously just ground pen #6 of the test connector (DLC) with the key on.
The frame pump does not have to run for the in tank pumps to run but they get their power at the same time.
Make sure the ground return is good or none of the pumps will run.

Here are some diagrams and photos to look at.

Fuel System Diagrams for the 88 F series:

Electrical:
http://i54.photobucket.com/albums/g9...-89FSeries.gif

Mechanical
http://i54.photobucket.com/albums/g9...Fuelsystem.gif

Selector Valve 1:
http://i54.photobucket.com/albums/g9...Fuel/dfres.gif

Selector Valve 2:
http://i54.photobucket.com/albums/g9...l/FuelRes1.gif

Selector Valve 3:
http://i54.photobucket.com/albums/g9...l/FUELRES2.gif

Selector Valve 4:
http://i54.photobucket.com/albums/g9...ectorvalve.jpg

In Tank Fuel Pump:
http://i54.photobucket.com/albums/g9...9FuelPump2.jpg

High Pressure Fuel Pump + Filter:
http://i54.photobucket.com/albums/g9...mpFilter88.jpg

Fuel Pressure Regulator:
http://i54.photobucket.com/albums/g9...Fuel/fpr02.gif

Fuel Manifold:
http://i54.photobucket.com/albums/g9...elmanifold.jpg

Fuel Pumps test ground at the Test Connector:
http://i54.photobucket.com/albums/g9...taLinkConn.jpg

Inertia switch location 87-89:
http://i54.photobucket.com/albums/g9...SwLocation.gif

 

Thanks! Where would I locate this connector?

Sounds like I definitely want to check the ground, then. I know the owner replaced a fair amount of the starter / battery / ground wiring recently, so maybe a ground was missed or not connected properly.

Grounding the pump at the connector, and feeding it 12V should help determine if it's alive or not, correct?

Thank for the help and info!

 

I am not 100% sure on an 88 but I think it is over by the drivers hood hinge.
There is a diagram of it above.
Yes if you put 12V and ground on the wires to the pump(s) they should run.

 

My 89 had nothing working either. Had to replace the relay, both tank pumps, and the frame pump. Worked til today, then had to rob the front tank pump to use on the 88.

 

Well I guess we're getting somewhere. I dragged a motorcycle battery under the truck with me, and hooked it directly to the high pressure pump. Nothing. I pulled the pump, and decided to test it again, off the truck. Still nothing, so WTH...when it doubt, beat it out. After a few raps on the concrete, I noticed it started to work, but not very well. Then...POOF...there was a little burst of air on the output side, and it started humming like a champ. What the heck...clogged maybe? Stuck from sitting for nearly a year? Who knows. I put it back in the truck.

Once that was done, I grounded the #6 pin on the test block, turned on the key, and crawled under / felt under the truck. Sure enough, the pump was pumping away. The problem was, I wasn't getting any fuel. I've taken the core out of the schrader valve test-point on the fuel rail, in hopes of using the fuel pumps to drain the old fuel out of the tanks. No fuel out the hose I've attached to the test point means no fuel pressure, means something -still- isn't working.

I figured that the in-tank pumps aren't working, and aren't getting the low-pressure fuel to the frame pump, which bumps it up to rail pressure. I toggled the in-cab switch back an forth, but didn't get any fuel out of my hose, and didn't hear / feel any pumps in the tanks.

Do the in-tank pumps make as much noise as the frame mounted one? Granted, it's pretty quiet, but if the in-tank pumps generate similar noise / vibration, I should be able to hear / feel it in the tanks.

I pulled the connector from the front tank, and ran a voltmeter to it. With the front test block still grounded (but the high-pressure pump unplugged, so I didn't burn it up), I was getting about 12.5v (I'm guessing battery power) through the two left side leads (looking down onto the tank, both red). The wires aren't the same colors as the picture posted here, but I figured the position was the same. I read about 8.13 volts between the other terminals, which I believe would be the gauge and sender ground. So, if my terminal guesses are correct, I'm getting 12.5v at on the power side, and ~8.13v on the gauge / sender side. Do these #'s sound correct?

At this point...I'm thinking both in-tank pumps are bad. I couldn't get to the rear pump (or connector) to measure it, but I didn't hear anything in that tank, either. This is while lying under the truck, with my ear only a few inches from the tank.

What do you think? In-tank pumps are toast? If so, any leads on the least expensive compatible replacements?

 

Yes Yes or jammed.

Also the selector valve is more than likely full of non-pumpable stuff.
I think I would take this selector valve off and replace it or take it apart and clean it up. The in tank pumps might not be able to pump through the valve.

 

That sounds reasonable as well, given the amount of time the truck has sat, and the (seeming) blockage of the high-pressure pump. I'll pull the selector valve and try to clean it tomorrow.

One thing...I see where the selector valve has multiple lines in, and multiple lines out...but there doesn't appear to be any sort of electrical to it. How, exactly, does it function? The only think I can think of (with the selector having no electrical) is that the in-cab switch controls power to the in-tank pump. The switch controls which tank pump gets power, and the pressure caused by that manually moves the selector plunger / thingie.

Or maybe there's an electrical line to it that I couldn't see...but I sure didn't see one...

 

You have it right, the two things that the switch on the dash does is swap the fuel gauge source and select the in tank pump that gets power.

The in tank pumps put out less than 5 psi and that is enough to lock the selector valve to the tank that has its in tank pump running.

This is all in the links I posted above.