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Ask Chris (aka- Moose) Your Own Question
Chris (aka- Moose)
Chris (aka- Moose), Technician
Category: Ford
Satisfied Customers: 44433
Experience:  16 years experience
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Fuse 102 blowing when starter should engage on 2004 f150

Customer Question

Fuse 102 blowing when starter should engage on 2004 f150
Submitted: 2 months ago.
Category: Ford
Expert:  Juan Crespo replied 2 months ago.

Hi there.

The rule is that every time a fuse blows that was put there to protect a power/feed circuit, then that circuit is shorting to ground before getting to the intended load. Looking at the starting circuit diagram, we see that fuse F102 protects several electrical loads including the starter relay (which yo have replaced, so it should be good) and the ignition coils - one or more of these probably being the culprit.

The least complicated way to diagnose this issue would be to first put in a new fuse, then disconnect all 8 ignition coils. Next, turn the ignition switch very briefly to START. If the fuse does not blow, repeat the procedure connecting the coils one by one until you find the one(s) causing the fuse to blow. Once it is identified, make sure its connection and wiring are not chafed or rubbing against anything. Also make sure it's replacement meets the factory specs.

Please let me know what you find.

Expert:  Juan Crespo replied 2 months ago.

Almost forgot... Make sure to also check that condensation from the A/C plumbing is not falling on the coil connector.

Customer: replied 2 months ago.
I will let you know asap , since you mentioned this I think there's a good chance it could be a coil , I have a misfire from a coil triggering a check engine light
Expert:  Juan Crespo replied 2 months ago.

There you go... that's probably where the whole issue started. Hope you find it soon.

Best Regards.

Customer: replied 2 months ago.
Cylinders 2 3 4 have broken tabs on the coils where the connector connects to the coil body , going to get 3 coils and go from there
Expert:  Juan Crespo replied 2 months ago.

That could very well be it - just test them as I suggested to make sure that is indeed the cause of the issue.

Please let me know what you find.

Best Regards.

Customer: replied 2 months ago.
All coils are unplugged and fuse still blows but is not instant , about a 2 second delay while the key is in the starter engage position then the fuse blows , did find those 3 coils defective regardless needed to be replaced , sorry I know it's late , will await your reply in the morning
Expert:  Juan Crespo replied 2 months ago.

I won't be able to log back in until late tomorrow afternoon or evening. I'll opt out to let other experts pick this up. Please don't rate or reply until another expert responds or the site will cycle this chat back to me.

Best Regards.

Expert:  Chris (aka- Moose) replied 2 months ago.

Welcome, I'm Chris (aka Moose) I prefer to work when others don't, I am here around lunch and after dinner till after midnight.

Would you like me to try and figure this out for you?

Customer: replied 2 months ago.
please
Expert:  Chris (aka- Moose) replied 2 months ago.

I am going to start over fresh so everything is on my mind what you and I have written. So I do not have to read what another wrote and make sense of it.

Does this fuse fail with the key in the accessory position, run position, or only while trying to start?

Customer: replied 2 months ago.
Good idea , only while trying to start , last night with all the coils disconnected the starter rolled over , if that matters
Expert:  Chris (aka- Moose) replied 2 months ago.

Since it only blows the fuse with the key held in the start position, that narrows it down to just one wire.

Do the next step using these 2 files.

https://dl.dropboxusercontent.com/u/90220402/110.png

https://dl.dropboxusercontent.com/u/90220402/1101.png

Now hold the key in the start position and let me know if the fuse fails. If it does not fail, then reconnect 110 and unplug the TR sensor on the drivers side of the transmission https://dl.dropboxusercontent.com/u/90220402/DTR.png Now try to start it, does the fuse blow or not blow?

Customer: replied 2 months ago.
Ok I haven't had a chance to trace out the diagrams you sent , I finished installing all my new coils and the truck started , after a test drive and reinspecting all of the coil connections truck failed to start again , checked the connections over a 2nd time then the truck started again , and also failed to start the following time , so now it is intermittent , should I continue on with the diagrams you have sent or focus on the coil wiring harness