98 GT ALT Fuse blowing

Jules98GT

New Member
Apr 4, 2021
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I posted this over in general discussion and realized it should be here my apologies.

Here is the run down.

I drove the car to Taco Bell, around 3 miles from my house, in December. While sitting in the drive through I noticed my battery light came on.
Drove it home and didn't really look into it. That day was unseasonably warm.

So when I started working on the problem this is how it went.

Silly me - I exchanged my alternator that I had just replaced not 200 miles ago without checking the fuse. Put the new one in, battery light still on.
Checked the fuse and was elated. Cool. I found the issue. NO.

Replaced the fuse, took the car for a drive and battery light again.

Popped the fuse.

Note: The fuse will only pop once the car is warmed up and rev'ed to above 3000RPM. I can leave the car sit at idle for as long as I want and the fuse will not pop and the regulator charges the battery.

I went back to Autzone and they upgraded me to Duralast Gold instead of the cheapo I started with.
Alas that didn't work either.
At this point I got the meter out and started checking.
Alternator body to negative terminal - 0 ohms
Alternator charging post to Positive terminal - 0 ohms
Regulator Pig tail wire# 36 from connector to fuse box - 0 ohms

Got looking into the print and there is a connector BEFORE the fuse box named "c100" in the print. I took wire#36 and bypassed that connector just to be safe.

I have checked the seating of the pin inside the fuse box itself. The main "lug" on the fuse box is a little wonky. It looks like someone spun the nut at one point but I have good connection non the less.

So in summary.
I have replaced the pigtail to the regulator.
I have removed the fusable link and replaced it with a new fuse of the correct rating.


I have yet to replace the charging cable itself. The ohm reading on the cable is 0.00. I did check for voltage drop in that cable when I first started this adventure a couple months ago.

I ordered the black and white regulator connector today as well as a new charging cable. I'll update once those are installed.

I don't really want to have to purchase a motorcraft alternator but I am wondering how many bad ones from AutoZone I can get.

How likely is it that it is still the alternator?

Thanks!
 

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wmburns

SN Certified Technician
Aug 14, 2009
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How likely is it that it is still the alternator?
IMO it's highly unlikely. If this were my car I would look for a cause outside of the alternator.

IMO the possibility of excessive voltage drop should be ruled out. For example voltage drop in:
  • alternator case ground to battery negative
  • alternator main B+ terminal to battery positive.
  • Alternator field control power back to battery positive.
Howto perform charging system voltage drop test

Please review the above document. The important take away is the method for directly measuring voltage drop. Many guys will blow it off because they don't understand HOW it works. It confusing them because it seems so odd to measure for example the voltage between the alternator case and the battery negative.

Suggestion. Don't ignore the grounding issue. I'm serious enough about it that when I change an alternator I CLEAN with a wire brush the motor where the alternator mounts to improve the ground. Also make sure the case of the alternator itself is clean. It only takes a few minutes to wire brush the mounting services and this will ensure a good grounding path.

I would also ensure that the battery is in tip top shape. It's possible for an internal fault inside the battery could cause excessive charging current.

Finally perform a through visual inspection of the whole wiring harness. Look for any areas where the wiring harness is touching any metal parts.
 
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