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5.0L (1979-1995) Mustang Technical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.

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Old 11-17-2011, 09:20 AM   #1
projectresto83
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Default Electrical going haywire....

91 5.0 vert GT with auto trans.

It will start and run just fine for a few mins then you will start hearing the fuel pump relay and WOT relay clicking on and off and kinda slow at first while the car still runs and then it gets kinda fast causing the car to die and not want to start. If you let it st for a couple mins it will start right up but start doing it again.....The ECU relay DOES NOT go haywire though....just he fuel pump and the WOT cutout relay...and if you watch the check engine light also flickers with it.....

only KOEO codes are 81 and 82, havn't done a KOER test...

changed out the computer and it ran perfectly fine for 2 months then came back with the exact same issue.......

Replaced leaking capacitors in the ECU that cause similar issues as stated by Joel at sbftech.....still does it....

http://www.youtube.com/watch?v=RsupA5EkmEc
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Old 11-17-2011, 05:11 PM   #2
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As a general automotive rule, any time the electrical stuff starts gettin wonky, first check to be sure that the wonky components are receiving their intended voltage. Then go after the grounds. Inline ground testing, under 100mA. Put your multimeter on DC volts, attach one lead to the ground on the battery, attach the other lead to the ground you're testing.

The stuff I've seen from electrical systems that are just a smidgen under where they're supposed to be... unreal.
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Old 11-17-2011, 05:55 PM   #3
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Quote:
Originally Posted by mattdel View Post
As a general automotive rule, any time the electrical stuff starts gettin wonky, first check to be sure that the wonky components are receiving their intended voltage. Then go after the grounds. Inline ground testing, under 100mA. Put your multimeter on DC volts, attach one lead to the ground on the battery, attach the other lead to the ground you're testing.

The stuff I've seen from electrical systems that are just a smidgen under where they're supposed to be... unreal.

Wiring has been looked at, traced, tested, relays replaced... Everything I see says something in the ECU is screwed but the fact that messed up got looked at, changed the ECU, worked perfectly fine for 2 months then did the exact same thing....

Its just the power from pin 36 that is acting up....
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Old 11-17-2011, 06:32 PM   #4
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K i just watched your video. My very first culprit of suspicion would be the ignition switch. The flickering you're experiencing is the exact same thing that happens when you justttttt barely have a solid ground. You'll eventually start popping things in the ECM if it continues. If all your wiring is ok, check the switch (electrical portion, not the tumbler).
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Old 11-17-2011, 06:56 PM   #5
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Makes sense....I'm just helping my brother at the moment diagnose this one so I don't have a whole lot of face to face with it....that video is the first time I have actually heard it in person...


I just talked to my brother about it and he said he thought about the looking at the ignition switch but with some of the issues he had found he never did....

I am going to go out and take a peak at it tomorrow and look at wire 16 and see if there is anything obvious....

Click the image to open in full size.
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Old 11-17-2011, 07:00 PM   #6
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Yah with the whole system clicking and flashing like that, you have to start looking at the power sources. Battery, solenoid, switch. One of them is bad(or has bad wiring), and its probably not the battery.
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Old 11-17-2011, 07:10 PM   #7
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Battery is good, it has a new alt, all the fuse links are good....it just acts like something gets hot and acts up....let it sit for a min or 2 and it will start right up then do it again.....you can just leave the key on and it will start doing it, it doesn't even have to run....
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Old 11-17-2011, 07:46 PM   #8
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Yah I'd be willing to bet there's a problem with the switch then. These cars have an ignition switch recall for melted wiring at the connector. You don't have to look very far, and you can find a new plug-in replacement harness. It's very common.
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Old 11-17-2011, 08:52 PM   #9
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Yes, I know.....

As I was discussing with my brother it also has a new alternator plug as well...the other connector that likes to melt over the years.....
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Old 11-18-2011, 03:43 PM   #10
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Alright, the computer is definitely what is causing everything to go haywire. I put the vert's computer in my car and it ends with the same result as above....SO the next part is figuring out why the body does this to the computer....

So now my question is, what is the resistance supposed to be when testing the ground quick disconnect by the battery for the ECU for the harness end to a body/main ground (with the quick disconnect disconnected, testing the end that goes into the rest of the harness).... It gets 88.5 ohms...being for the ECU I wasn't sure if it goes through any components before it grounds to the body causing the resistance.
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Old 11-18-2011, 03:43 PM
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