A9P circuit board issues - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 7 Old 10-30-2014, 02:44 PM Thread Starter
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A9P circuit board issues

I need to repair my A9P ecu. Everything works on it except the fuel pump wont kick on with it. I have tried it in 3 different working cars and each time I have the same outcome. My question is can someone direct me to a link or something to show me which part of the board controls the fuel pump? Ive looked it over and everything appears to be in good conditon under I magnify glass and everything appears to be well fastened to the board. Im thinking there has to be some sort of diagram or something for it, but I just cant seem to find it. Any help would be appreciated! Thanks!

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post #2 of 7 Old 10-30-2014, 08:14 PM
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pin-22 grounds coil of fuel pump relay. Follow that trace.




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post #3 of 7 Old 10-31-2014, 05:59 AM Thread Starter
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Thanks. Thats exactly what I needed!
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post #4 of 7 Old 10-31-2014, 11:41 AM
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Originally Posted by projectstang92 View Post
Thanks. Thats exactly what I needed!
That is not what you need. That is the car wiring.

The fuel pump transistor is out in the middle of the board on the A9 style computer. It is a three wire plastic case NPN transistor. A MJF3055 will work as a sub. (edit from MJE to MJF3055)

The open collector output of that transistor pulls the fuel pump relay "ground" line to ground. The NORMAL way that transistor circuit fails is from someone dumping 12 volts on the test pin, or dumps 12V on the relay coil "ground" line by messing around with the relay or the wiring.

When that transistor gets too much current because someone shorts the fuel pump relay coil, or applies 12V without series resistance to the relay pull down line, it blows a small low resistance "fuse" resistor by that transistor. That resistor goes from the transistor emitter to ground, and acts like a fuse.

It is not a critical value. Anything from 1 ohm to perhaps 10 ohms will work, but it should be low wattage thin film type resistor. This is so the resistor opens if someone crews up the wiring outside the computer.

The normal transistor failure, since it is a semiconductor, is shorted. If the transistor shorts the pump would run all the time.

The normal resistor failure is open. If you overload the transistor the resistor fails open and protects the transistor and the foil traces.

You also could have an open foil trace or via (the plated through connection between traces that swap board sides), but usually it is the resistor if dead pump relay or a bad transistor if the pump relay locks on.

You could have another issue, but this is probably nearly 100% of things that go wrong in the EEC.


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post #5 of 7 Old 10-31-2014, 04:58 PM
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Originally Posted by TomR View Post
That is not what you need. That is the car wiring.

The fuel pump transistor is out in the middle of the board on the A9 style computer. It is a three wire plastic case NPN transistor. A MJF3055 will work as a sub. (edit from MJE to MJF3055)

The open collector output of that transistor pulls the fuel pump relay "ground" line to ground. The NORMAL way that transistor circuit fails is from someone dumping 12 volts on the test pin, or dumps 12V on the relay coil "ground" line by messing around with the relay or the wiring.

When that transistor gets too much current because someone shorts the fuel pump relay coil, or applies 12V without series resistance to the relay pull down line, it blows a small low resistance "fuse" resistor by that transistor. That resistor goes from the transistor emitter to ground, and acts like a fuse.

It is not a critical value. Anything from 1 ohm to perhaps 10 ohms will work, but it should be low wattage thin film type resistor. This is so the resistor opens if someone crews up the wiring outside the computer.

The normal transistor failure, since it is a semiconductor, is shorted. If the transistor shorts the pump would run all the time.

The normal resistor failure is open. If you overload the transistor the resistor fails open and protects the transistor and the foil traces.

You also could have an open foil trace or via (the plated through connection between traces that swap board sides), but usually it is the resistor if dead pump relay or a bad transistor if the pump relay locks on.

You could have another issue, but this is probably nearly 100% of things that go wrong in the EEC.
Nice information.

So if the resistor blows, the way to find it (if not visually obvious) will be that there is no continuity between transistor emitter and the ground trace.
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post #6 of 7 Old 10-31-2014, 09:36 PM
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Nice information.

So if the resistor blows, the way to find it (if not visually obvious) will be that there is no continuity between transistor emitter and the ground trace.
Yes. Just use a sharp probe tip to poke through the insulated board coating.

Warning: Understand I'm old and going by memory and I'm distracted at the moment. If I had time to pull a cover and take a photo and verify my memory I would, but I have to take a quick trip to Detroit/Cleveland area so I'm packing now.

As I recall the pin out, facing from the top away from the flat mounting side with leads down, is base left, collector center, emitter right. You can get a standard transistor pin layout off the web. You won't hurt any parts around the power transistors with an ohmmeter, so you can ohm the emitter to ground on a dead EEC. It is actually safer than measuring with power. The fuel pump relay transistor is somewhere in from the board edges. It does not dissipate much heat, because the load it sinks is only a few hundred mA tops.

There are some driver parts there that drive the transistor, but the normal failure mechanism for any overloaded semi is a short. That would make the pump run all the time. The normal failure mode for a metal or thin film carbon resistor is open. That's why my guess would be someone shorted the relay and applied 12V from battery direct to the collector, which should normally open the emitter resistor.

Of course it could be something else, but that is what people outside the case can usually damage by applying 12V to the FP test pin on the test port by the master cylinder or by connecting 12V battery to the EEC side of the FP relay.


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post #7 of 7 Old 11-10-2014, 09:27 PM Thread Starter
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Ok. So can someone point out the fuel pump transistor for me. I believe it is right next to the red pill looking guy in the middle of the board. Am i right? Please dont make fun of my technical terminology lmao!
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