Got a bit of a head scratcher and wanted to consult you guys. Vehicle in question is a 1990 F-150 4.9/auto. Bit of backstory: this is the truck I got and am fixing up for my youngest brother as a first car. He got his license last week and, of course, the truck is all finished but quit running! So I'd like to get it fixed fairly quickly for him.

Here is what happens: you'll be driving along and when you let off the gas to brake for a turn or for a stop, the truck will cut off. Won't fire back up again. This has happened about 3 times so far. When you attempt to fire it up again, you DO NOT hear the fuel pump prime. That led me to believe it's a wiring issue in the fuel pump circuit. Got it drug home after the last time it acted up and proceeded with my troubleshooting.

First, I jumped the OBDI self-test plug to ensure we would have constant power to the fuel pump relay. Checked for power coming in to the relay with a test light - it was there. HOWEVER - had NOTHING coming out of the relay, despite the fact that I could hear the relay "clicking" on when the key was turned to RUN. Tried a few spare (known good) relays we had sitting around, same thing. Power coming in, I can hear the relay click on to activate, but NO power out.

My next step: jump the two pins in the relay plug to send power directly out. After I installed the jumper wire and turned the key, I had nothing UNTIL I touched the exposed end of the jumper wire with my test light. Then, I had a perfectly running fuel pump. I repeat: the fuel pump did not kick on (despite having a jumper wire that sent power from the RED wire directly to the Green/Yellow wire) until I touched the jumper wire connection with the test light. After a few cycles of the key, the test light was no longer required to activate the pumps.

Which brings us to where we are at now. With the relay out and the jumper wire attached, I have constant fuel pumps and the truck WILL run. However, re-installing the relay will kill the system again. I've never been good with electricals, but am learning. I'm looking towards you all who have much greater electrical knowledge to point me in the right direction as to what to try next. I know the computer has a role to play in switching the fuel pump relay ON and OFF. Is it not providing the electrical signal? I don't know. I'm wondering if it's a grounding issue, as introducing a ground in the form of the test light kicked on the pump.

Before anyone asks, no unfortunately I didn't pull codes, I can do that this afternoon if I get the requisite "Pull codes" replies .

I'm just kind of stumped, due to never having to deal with a problem like this before on any of our Fords. As mentioned, I'd like to get the truck fixed soon as I can - I know my brother's bummed that he's finally got his license, insurance, and the truck's finally road-ready and then decided to take a dump!

 

Inertia switch ?
https://www.ford-trucks.com/forums/1...confusion.html
https://www.f150forum.com/f10/1990-f...switch-235730/

 

Thanks for the links, Hobo. I don't think it's the inertia switch. If it were bad we wouldn't get any power to the pump, right? We do, as long as I have the relay pulled and a jumper wire installed..

What's got me confused is with the relay in I get NO power on the green/yellow wire that runs to the inertia switch and down to the FP. I DO get power on that wire (obviously) with the red wire jumped to it.

 

Perhaps you are testing the wrong contacts on the relay. Have you opened a relay up and visually followed the circuit ?

 

On my 91 I was thinking the main pump power wire was yellow, and the red wire was hot to the sense side of the relay. You seem to know better, so hope this isn’t taken as an insult to intelligence. Got a few pics of your relay plug?

 

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I sure hope he does not take it as an insult. Sometimes when I unplug a connector and flip it up where I can get these bi-focals on it... I go dyslectic.

Hobo

 

That's very possible! I haven't taken the actual relay itself apart - I was using a Haynes manual diagram as well as googling. There was a printed diagram on top of the relay that seemed to confirm my findings - red power in, switched out to green/yellow.

Trust me packager - I don't know jack SQUAT I will get a photo for you today when I get home after work. I suppose jumping red to the relay output would inadvertently make the fuel pumps work, since technically power is going to it....... I was just going off what the wiring diagram in the Haynes manual and the internet were telling me. The concerning thing is I do NOT have power at any other wire at the current time!

I'm the king of unplugging something and then completely forgetting how it goes back together, that's for sure!

 

You were right!! And I was wrong! Cheers!

Here's a snapshot of the wiring diagram, showing that indeed the YELLOW wire is the 20ga fusible link power from the starter solenoid:



But this raises another question: why don't I have power here (as I believe I should) and why is the RED wire energized? Are they both always energized?? It's a little too cold to do anything today (and my brother isn't even home) so I'll dig a little deeper tomorrow I think. Until then, I'd love to have more brainstorming going on here.

 

Perhaps the fusible link is blown. I think the only thing on that FL is the pump but don’t hold me to that.

The red wire is always hot IIRC. Computer switches the ground side when it wants to run the fuel pump. Again, IIRC.

Disconnect the FL from its power source and check continuity from lug to harness. I salvaged a ‘91 harness for my 460 swap and about the only place it was ate up was the fuel pump fusible link.


Good luck!

 

Fuseable links don't always burn 100% and will show continuity sometimes.

Also, the spade connectors that the relays actually plug into inside the fuse/relay box sometimes melt, corrode or just fall out.

The box is @#$% to open & inspect.

The no prime when voltage restored would have me opening up the ECU to look for leaking capacitors.

 

That's the next step! Thanks for setting me straight!

The pins looked half decent when I pulled the relay out. I was afraid of a potential computer issue - we will have to restore continuity at the fuseable link first and see what we've got then. I'm not ready to condemn the computer quite yet as it was working just fine before we lost power to the relay. Either way what we're gonna do (my brother that is) is check continuity at the fuseable link and replace if necessary, then we're gonna clean up the grounds and go from there. Hoping that will lick the problem and we won't be pulling the computer quite yet.

 

Yellow wire should have 12v at all times.
Red wire should have 12v when key is on.
Dark Green/yellow wire should have 12v when relay is engaged.
Light blue wire goes to the ECU. The ECU supplies a ground to this wire to engage the relay. You can also use the EEC test connector to force the relay and pump on any time the ignition switch is in the "run" position.

 

Might as well loose your virginity with it now under semi-controlled conditions, verify the ID of the box, make sure it matches the calibration code & start making a plan for a spare.

You can never have too many known spares with these things. Good luck with y'alls project.

 

Thanks, Prototype! I believe that's our problem there - no 12V to the yellow wire. Red has 12V with the key on. Have not verified with a multimeter yet what's going on with the light blue wire, but as I stated I can hear and feel the relay "activating" when the key is turned. I know it's bad to assume but I'm thinking the red - light blue circuit is likely working. I did have the EEC test connector jumped at one point - but turns out I was focused on the WRONG wire in the first place. Need to get power restored to that yellow wire and then we can troubleshoot all over again!

Well, that's very true. I agree - I've got all kinds of small spare parts lying around for our old Fords. No computers, though...... Might have to hijack some from the local pick a part.

 

Any chance you could try another relay? I have modified the brown relay(EEC) to work when it was all I had.
That is I only had a spare brown relay to work with. I didn't have a spare green relay.