Crank no start, the Anti-theft is engaged but we cannot get through the data link connector?

GAILEY2006

MEMBER
2004 FORD MUSTANG
3.9L
6 CYL
2WD
AUTOMATIC
1,600,000 MILES

I have the car listed above 40th anniversary, I need the wiring diagram for the BCM/dash harness for the 2004 and a 2003 GT dash harness. It won't read the PCM which is (used/new to the car). My engine control fuses are not even getting an entire volt reading. (Just.02MA). PCM has power. The data link connector will read the other parts just not the PCM. Also, can you help me with ground locations? I've only found PCM ground. One behind the radio. Chassis to body and one's located around battery. I feel like there should be one in the back somewhere.

Thank you for your help.

6 Replies

GAILEY2006

MEMBER
8 POSTS

Sorry, I know that's a lot and half information. If you need voltage readings for anything, let me know. Last thing, I'm not getting a 5-volt reference. Just 0.01 or 0.02. Don't know if that helps with your diagnoses.

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Monday, September 19th, 2022 AT 3:10 PM

KEN L

MASTER CERTIFIED MECHANIC
47,368 POSTS

From everything you have said it sounds like the PCM is out. I would check the PCM grounds and if they are okay, swap out the PCM with a rebuilt or used unit. Here is a guide to help you check the wiring and the PCM wiring diagrams with ground locations as well in the images below.

https://www.2carpros.com/articles/how-to-check-wiring

Here is how to change out the PCM as well. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem.

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Tuesday, September 20th, 2022 AT 10:12 AM

GAILEY2006

MEMBER
8 POSTS

***Sorry been super busy with work

I bought a second PCM, BCM, key transponder, gauge cluster and the dash harness, still no luck. Anti Theft won't turn off. I've had a professional programmer/ Locksmith (mobile mechanic who was certified, not very professional though) He couldn't get his $7k computer to read the PCM via Data link connector. We can manually turn it off but when you go to crank it, it turns back on immediately. However, I did find out through talking with other Mustang enthusiasts, a 03 GT/Cobra Dash Harness will not work with the 04 3.9L model. So, I'm going to repair the old harness and re-install it today.
How would I test the Key Transponder on its own? On the wiring harnesses BCM and PCM. Which wires do I probe to test them? Did I mention I have zero voltage to the turn off switch?
Last question, do you know where all the grounds are located and or what should be grounded to the body? (I know everything positive has to have a ground, that's not what I'm asking. Lol) The main ones like starter, engine ground, the battery and grounds that are around it. Forums keep mentioning one by the radiator? Where would or should that ground be?

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Tuesday, October 11th, 2022 AT 3:27 AM

KEN L

MASTER CERTIFIED MECHANIC
47,368 POSTS

No problem,

Here are the ground locations, also I have included the data link connector wiring so you can check that out as well. Check fuse #31 in the interior fuse panel. Check out the images (below). Please let us know what you find.

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