Updated previous post from 2 months ago but not getting any response. Posting full details in this so don't have to bounce around the other post.




Truck sat for 10 years. Wouldn't start. Replaced the fuel pump (fuel pump was bad), and still wouldn't start. Noticed now that the fuel pump was not cycling off. If I primed the intake with a little fuel, it would start. Pulled the EEC and replaced the caps. Then she started to fire without any assistance. After a couple days, she wouldn't start again. I sent the EEC off to have it rebuilt. Got it back and the truck fired. Next day the truck wouldn't start. I cleaned the grounds and that made no difference. After much advice from this forum, I sent the EEC back and gave them very descriptive details of what the truck was doing. They took it, re-rebuilt it, and tested for about 5 weeks. They could not get the EEC to fail in any way. Got the EEC back, put it in, truck fired. Next day it would not start. After looking at a wiring diagram I saw there is a fuel pump relay as well as an EEC relay. I bought a new EEC relay and put it in. Truck fired and I started her every day for about a week. Last night I moved her to clean her. Then after a couple hours I went to start her and she did the same thing. Turn the key on, fuel pump kicks on - fuel pump DOES NOT cycle off, and truck will just crank and crank and crank. So in order to get her moved I put a splash of fuel in the intake and she fired. This time though it was a little different...she fired and ran odd...like not all there...then after about a minute (I was standing in front) she revved way up and it was like whatever was causing it to run off cleared out and she ran just fine again. I thought the new relay and rebuilt EEC had taken care of it, but now I am completely baffled...What else can it be? It's like anytime I start unplugging things and plugging them back in...she decides to fire a few times...please help! I need this truck running!
Thank you in advance!!

 

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Since you already stated in the other thread you checked/cleaned ground G100 & G101 the problem is back to a bad computer. Something more than soldering in new capacitors is going to fix.

I would search out a replacement (F1TA-AFA) listed from your first thread. Not a fan of Cardone even though they are the one most places carry these days.

Try looking for Standard Motor Products EM556

 

Is there anything else in line that could cause this? Because they tested the computer for 5 weeks and could not get it to fail...it fails on me in 1 day. A wire, or relay, or anything else?

 

Have you checked the PIP in the Distributor? I had a similar experience and replacing the distributor made it go away...after changing the computer too. Having sat that long, did you check all your spark plug wires and various connections all around the engine bay?

 

Hey RLA2005...is there any chance that I'm not getting power to the EEC relay or not getting power to the EEC so it is not "waking up"?


Binkowski, my only thought with this is that I still have spark when this occurs. Because if it won't start, I can squirt some fuel in the intake and it fires. Also, I never have hesitation or stalling issues once it fires.

 

No. In this scenario the fuel pump would not turn on since the EEC relay is not energized. The load side contact supplies power to the Fuel Pump relay coil.



Take a really good look at the PCM (computer) pins. An intermittent ground connection could be your root cause instead of another failed computer.

 

So it might not make a difference, but this is the exact wiring diagram for my truck. Mine is a 4.9L...again, might not make a difference, but here it is:

 

Have you checked your fuel pressure? I know the computer primes the pumps for three seconds or so when you turn the key, but will the pumps keep running if they don't actually get to the recommended pressure? Have you gone over your lines and connections or been able to check pressure when you do get it running?

 

The answer is no. The computer is programmed to energize the fuel pump relay for 1-2 seconds when cycling the key from Off to Run. There is no pressure feedback in the design. It's strictly time based.

 

So if this is the case...and there is absolutely no chance of anything else wrong...What other computer can I use? I can't seem to find one. Also...is there something in my truck that could keep causing the computer to fail? If this computer has been sent in twice and tested extensively with no failure...what else could be wrong with it?

 

In your previous thread I believe I mentioned we had a similar scenario of multiple computers and continuously running fuel pump. Turned out a "helper" did not attach all the grounds to G100. This is a ground located on the driver side of the radiator support for 1992 and later trucks. I have never figured out why a missing ground could cause this, but it did happen. Where G100 is located on your earlier model is not known to me.

Scanning through my EVTM diagrams I do not see any connection between the computer and G100 but the fuel pumps are grounded to G100. All I am stating is I have seen one other cause of the continuous running pump, but it is very rare. In 99.9999% of the cases it's a bad computer.

You can use a computer from ~1988 to 1993 F150/Bronco with the same 4.9L engine and manual transmission. In a pinch if you found one with a C6 or AOD you could probably use that.

For that matter you could find any computer to swap in just to see if the pump runs continuously. Even a V8 computer would work for this purpose. Hopefully you can find one cheap just as a test mule.

Another thought on the short life: If the alternator output voltage is too high it can damage the computer.

 

So I could order any of the EEC's from rock auto that are for a 1991 F-150 4.9L with standard transmission? what's going to be the difference or issue if I order a different one?
Also, looking at the wiring diagram I posted, can you tell me where the "clock" is that wakes up the EEC? or how the wiring works for that?
I really appreciate all your help...you are the only thing keeping me SANE through this GodAweful process...

 

You could order a computer for a 1988 through 1993 for that matter. The main difference is the programming for different emission standards (Federal - Cali.) and the transmission type. Manual transmission/C6/AOD equipped trucks have no computer control for the transmission, only the engine. Trucks with the E4OD have a computer to control both. But for testing purposes, a junkyard computer from just about anything in that era would work.

Your diagram shows all the external connections to the computer. The clock (timer) is inside the computer (PCM). Like any digital device it has a timer inside of it.

 

that's awesome, that will make it much easier to find one for testing purposes. Thank you.
So, if I open up the computer, there is a physical timer inside of the computer? Which one of the pins, or wires, activates the timer?

 

The timer is part of the computer. The typical failure is the clock is no longer running so the computer has no way to "count" the oscillator pulses. Therefore the fuel pump relay is always energized.

I believe the oscillator is the silver component laying on it's side on the left side of the photo below:


Without some high level test equipment your hands are tied.

The CPU is the surface mount square black piece in the center. The EPROM has the white label on it. As many device labels state "No user serviceable parts inside".