Battery light on, new alternator new battery?

STEFFOR 67

MEMBER
2011 FORD FIESTA
1.6L
4 CYL
2WD
MANUAL
90,000 MILES

Battery light on. New OEM alternator new calcium battery. Lin bus wire is communicating with alternator. When I start the vehicle and turn on rear demister on within 15 seconds, I can hear the alternator is working and revs drop then battery light comes on and rear demister light turns off at the same time. I'm wondering if this car was jump started the wrong way and stuffed the ECM?

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Saturday, October 14th, 2023 AT 10:02 PM

12 Replies

KEN L

MASTER CERTIFIED MECHANIC
48,396 POSTS

Yep, jumping a car backwards can hurt the electronics. We should be able to do a CAN scan to confirm the issue, you can get a CAN scanner (Controller Area Network) which will work on most cars from Amazon here is a video to show you how. The PCM controls the alternator so it could be bad, but we should run the codes.

https://youtu.be/u-4syLc-ifQ

and

https://www.2carpros.com/articles/resolving-reverse-battery-connection-in-cars-a-step-by-step-repair-guide

Here are the wiring diagrams for the charging system. You can see there is not too much to it. Here is how to change the PCM out in case a code comes up for it, you can get a preprogrammed unit by searching Google or Ebay.

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Sunday, October 15th, 2023 AT 12:04 PM

STEFFOR 67

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7 POSTS

Thanks for that Ken. I'm using a d7 xtool scanner. Is that different to a can scanned? Only code I get is in the pic I've sent. When I clear this code. Battery light goes off demister light comes on but after 10 seconds it comes back on again and demister light goes off. Generator is charging at 13.85. And 13.70 under load. Ie lights fan radio.

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Sunday, October 15th, 2023 AT 1:10 PM

KEN L

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Yep, that is internal to the PCM, so it looks like the PCM is bad. I would remove it and send it in for testing.

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Monday, October 16th, 2023 AT 9:25 AM

STEFFOR 67

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Okay, thanks Ken. Will do. Can a junkyard PCM fix the problem? Can I programmed it myself with the xtool scanner or does it need a Ford dealer to do it?

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Monday, October 16th, 2023 AT 3:35 PM

KEN L

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48,396 POSTS

You should be able to program it with your scanner because it is bidirectional. If not, you can send it in for programming, just have your VIN number ready.

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Tuesday, October 17th, 2023 AT 9:37 AM

STEFFOR 67

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7 POSTS

Okay, thank you so much for your help Ken.

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Tuesday, October 17th, 2023 AT 12:57 PM

KEN L

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You are welcome, please let me know what happens.

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Thursday, October 19th, 2023 AT 10:38 AM

STEFFOR 67

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Will do. It might be a while before I get round to it. Thanks again

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Thursday, October 19th, 2023 AT 6:33 PM

JACOBANDNICKOLAS

MECHANIC
109,896 POSTS

Hi,

I noticed we haven't been in contact with you for a couple of days. Have you been able to make any progress? Is there anything I can do to help?

Let me know.

Take care,

Joe

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Saturday, October 21st, 2023 AT 8:51 PM

STEFFOR 67