Electrical SUDDENLY FUEL PUMP SHUTTING OFF RANDOMLY

General karthief said:

One of those ecms is for a stick (A9L) the other (A9P) is for an auto trans.
I feel you are risking damage to the ecu if not matched for your trans.
Send the ecm in to be checked out and be done, you may be trading problems. They are 30 years old and prolly tired.
JMO

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Thank you for sharing your valuable opinion.
This is maybe one of those things that makes you scratch your head.
A’m almost certain that the A9L computer has lost pin 1 (KAM) code 15.
When I bought the other computer I asked specifically for my application 90 mustang GT 5 spd. And they gave me this one that has no labels and no way to ID.

Trying to identify it by the look of the internal components and only by assumption I “called” it “A9P” but not 100% sure.

Anyways, I have already ran this “called” A9P computer before for a while and works awesome. The damage usually occurs when running an auto computer in a manual car is due to the O2 wiring jumper and the damage happens on pin 46

I opened this A9P computer and there is no damage whatsoever. In fact this computer has a tiny bit better acceleration, more stable idle, and more pleasant smell out of the exhaust pipes.
So I’m not sure if this computer is a truly A9P or another A9L.

I appreciate your comments

Mcmahst said:

Ok, deep breath.

You‘re not at the “swap random parts in and see the downstream results” stage yet. I would concentrate on the potential loss of power issue coming to the ECU over looking for a downstream issue. As I remember, the pin 1 power comes from a fusible link starting on the positive post of the starter solenoid. It’s like Fusable link L. On my TBird, it was a dedicated wire directly to the ECU power relay, then bridged from that connector to pin 1. It’s a black wire with an orange tracer. That relay could be generating a random failure. I don’t suspect the fuel pump relay; doesn’t fit the facts as you‘ve provided them. Also, if the “run” signal was interupted to to ECU, it could demonstrate simular issues.

Swapping the ECU relay is an easy test. LMR has them, probably auto part stores as well, it’s like a $15-$20 part:

https://lmr.com/item/LRS-RY71/86-93-Mustang-Pcm-Power-Relay?utm_source=google&utm_medium=cpc&adpos=&scid=scplp&sc_intid=&gclid=Cj0KCQiAr8eqBhD3ARIsAIe-buOg283K-wmpmI-ohAGNK1OENnjRQjioPR9_H38wIOciWD4qh6j_GGYaAsF3EALw_wcB

The loss of “run” signal is a bit more complicated. You could replace the ignition switch as others have recommended. The run signal, if also failing randomly, would cycle the ECU relay on and off. I believe that would generate the code 15. But so would a wonky ECU relay.

Also, something to consider, as some of us have to, based on how and where are cars are parked: any chance you might have anything chewing on wires? I chased a problem on a 2017 Honda Pilot for 2 months before I realized a mouse had chewed a segment just enough to create a random loss of power. Just another thought to consider.

Hang in there. As I said, I wouldn’t attack the results of the power loss until I’ve run to ground that power into the ECU isn't the problem.

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You’ve convinced me to proceed with next test on the A9L.
I have the other computer in place but have not energized the system yet. So I will remove this comp. And mount the suspected bad one.
I already removed the AC fan to run more tests on the EEC power relay.
All the wires from the starter solenoid were moved inside the fender so will open all of that area to verify that fuse link and measure all voltages at the components. Also now that switch makes me very suspicious. It is probably the most likely point of a false contact. If the problem is there it should be a clear evidence of false contact like flamed contacts caused by the rapid ON/OFF of the relays and fuel pump.

My car is stored in a close garage where I never saw a rodent or any other creatures that could cause damage to wires.

AeroCoupe said:

Ignition switch is a known issue in these cars so if you do not know the integrity of it then its not a bad idea to change it out.

You need to verify your wiring. Once you can say the wiring is good then it comes down to relays, ECU's, etc. Start at the starter solenoid at the fusible link and work your way to the ECU relay, over to to the ECU, then work towards the fuel pump relay. Once you know all the wiring it good and have verified the relays then its time to look at the ECU(s). Make sure the ground for the ECU behind the battery is in good shape as this bit me once.

Scroll down to Post #10 in this thread:

engine ground locations

well i broke one ground that goes from the tranny to driverside firewall. putting my engine back together and just trying to make sure i have all grounds hooked up. and are there alternate places to put my grounds? this is my first engine build. need to know where ALL grounds go

stangnet.com

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You’re right I will go thru everything in more detail. And since the starter solenoid is tminside the fender I will dive inside of it and very everything is ok.

Yeah I will look into the ignition switch and check it out for any possible false contact.

Long time ago I was concern about my ground connections so I upgraded them with three 1/0 cables; two from the engine to the chasis, one from the battery to the chasis (Battery is in the trunk) and the O2 sensors ground to the firewall.

The computer ground is bolted solid to the chasis too.

I will continue with this not easy task but must be resolved.

Thank you

AeroCoupe said:

Ignition switch is a known issue in these cars so if you do not know the integrity of it then its not a bad idea to change it out.

You need to verify your wiring. Once you can say the wiring is good then it comes down to relays, ECU's, etc. Start at the starter solenoid at the fusible link and work your way to the ECU relay, over to to the ECU, then work towards the fuel pump relay. Once you know all the wiring it good and have verified the relays then its time to look at the ECU(s). Make sure the ground for the ECU behind the battery is in good shape as this bit me once.

Scroll down to Post #10 in this thread:

engine ground locations

well i broke one ground that goes from the tranny to driverside firewall. putting my engine back together and just trying to make sure i have all grounds hooked up. and are there alternate places to put my grounds? this is my first engine build. need to know where ALL grounds go

stangnet.com

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I went down under the dash and removed the ignition switch. There was only a ninuscule gap in between the plastic part and the metal bracket, to tighten this little play I pressed on the four fingers and right now is solid I tested the spade terminals with the DMM for resistance/continuity and didn't see and miss contact so I called it good.
The ground I already explained it on the other reply.

Good duggestions

Ladies and gentlemen,
I'm happy to announce you all that I believe I found my problem. Or at least that's the way I feel.
I followed Mcmahst suggestion. I isolated pin No 1and connected it directly to the battery + post.
What I noticed after I removed the red retaining piece from the EEC connector and as I was removing the pin connector No1 the little plastic tab that holds the pin inside the connecto was broken so I just pulled the wire out of the connecto and it just came out very much on its own. I then pushed the connector I fixed to be connected directly to the battery, re-connected the battery and took off for a test drive. I noticed the difference right away, when I came back I ran a scan to retrieve the codes. to tell you the truth I was concerned and I was expecting the firs two usual codes 81 and 82. to my surprose The scanner gave me code 11 then 10 and then 11... WHAT???? was this fixed???...

I proceeded to diconnect the battery again, removed the temporary test wire and pushed and pressed the original wire No 1 from the harness all the way in. Re connected the battery and took off, well at least up until then I knew my A9L is not faulty!!!! I went driving for a bit and came home and immediately ran a scan it gave me 11 - 10 - 11... THAT WAS THE PROBLEM!!! (! HOPE SO!!!)

What happened was that those little pins fit quite tight and as I was pressing the connector with the 10mm bolt pin No1 was pushing the pin connector out of the big connector because there was not retained by the little plastic tab..... I couldn't believe it!!!

I don't know if the stalling will happen again but for now I'm very happy! I could try to replace the connector transfering one by one all the wires to a new connector housing but for now I will put a drop or two to that wire in the connector to fix it permanently. Now to put everything back together

Well I call this repair a success. In case the stalling happens again I will continue this thread, but in the mean time I want to thank everyone for helping me. I will try to help others whenever that is within my knowledge and expertise.

Thank you all!!!

Mcmahst said:

You da man. Congratulations on smartly discovering the problem without becoming the guy who just replaces parts until he replaces the bad one. I’m sure with the knowledge you gained on this fix, you’ll be able to help lots of folks. Again, congratulations!

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Thanks man! I have to admit that I was so frustrated but I always take my challenges very seriously. I took the computer apart some five times trying to find out a single evidence of a failure but non of the components were visually bad. Took few measurements and tried to find out wher the KAM is stored. The entire EEC board smell nice with no burnt smell.
As you might noticed I’m located in México, down here is really hard and costly to find components so I’m very pleased resolving this issue.

And yes the worst thing is when we start replacing parts going thru a guessing game.

All the guys that contributed many thanks!!!