Piston Slap: Why You So Stupid, Smart Junction Box?!

TTAC commentator claytori writes:

Hello Sajeev,

This email is about my 80-year-old mother-in-law Shirley, who is a sweetie and thinks I can fix anything. I hit the MIL jackpot.

Shirley owns a 2010 Lincoln MKZ 3.5 V6 with about 35,000 miles on it. About a year ago, the battery died on the Lincoln. CAA replaced it with another battery with a 13-month replacement warranty, on which less than 1 week remains.

Two weeks ago, the car wouldn’t start again, so I boosted it from my Saab 9-3, which sits beside the Lincoln in a heated garage. It started right away. As she doesn’t drive it more than about 3 miles at a time, I drove the car for a day to charge up the battery.

The battery was great for two days — then dead again.

I drove the Lincoln to work after another boost and did some parasitic drain diagnosis. There’s a 2A drain from a 120A fuse on the battery post feeding a wire that goes to the “smart” fuse panel. That panel is thoroughly buried behind the instrument panel. I subscribed to Alldata for the electrical schematics and found out the panel has some microprocessor parts, MOSFETs, and other miscellaneous bits.

Since it must be divorced and married to the car’s computer network for proper replacement, I condemned the Lincoln to the dealer. The techs said they couldn’t find a problem, and called Shirley to pick up the car. When she went to start the car to head home from the dealer, the MKZ was dead. She’s understandably very upset.

The dealer wants to replace a perfectly good battery because the techs are too lazy or incompetent to do a proper investigation. The battery is as fine as you’d expect after being drawn down a few times. The alternator is also fine. I don’t know what to tell her after this.

I’d fix the issue myself, but one of the four cables connected to the panel won’t allow it to come down, as it’s held with a plastic clip. Do I just pull hard and break the clip? Do I tell her to leave the car at the dealer until the mechanics get tired of looking at it and actually fix it?

Shirley won’t take cabs as she thinks the drivers are all robbers, rapists, terrorists, or a combination of all three, nor will she drive our spare car.

Sajeev answers:

Your mother-in-law is right about you: I admire your masterful diagnostic work.

From what I see on the Ford Fusion forums, either break the clip or remove the driver’s seat (mind the airbag!) for access without turning your back into a pretzel. I’d break the clip and be done with it, but that’s your call.

After you complete the diagnosis, let’s hope you don’t have to replace the smart junction box. I reckon you’re pulling out the driver’s seat for better access on that job.

[Image: Lincoln]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.