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

Sajeev Mehta
by Sajeev Mehta
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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.

Sajeev Mehta
Sajeev Mehta

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5 of 43 comments
  • DC Bruce DC Bruce on May 19, 2016

    Lead/acid batteries self discharge, so if you put one on the shelf, it will eventually die even if nothing is connected to it. Every car today uses some battery juice when the car is parked--even for something as mundane as the alarm system. It may be that the OP's measured 2 amp draw is normal. A 30-minute drive will not fully charge a discharged battery, because as the battery voltage rises back to normal, the amount of current delivered to the battery falls off. Cycling the battery between fully discharged and partially charged is a great way to kill it. So before ripping out black boxes and so on, the OP's M-I-L may have to change her driving habits or invest in a low-amp charger and be willing to connect/disconnect it when using the vehicle. Perhaps her clever son in law can figure out a simple plug/unplug way to do it, like the engine block heater pigtail lurking behind the grill of many frozen north vehicles. As an aside, this usage profile would be ideal for an EV.

    • See 2 previous
    • NeilM NeilM on May 20, 2016

      There's no way that a constant 2A drain is normal. Period.

  • Olddavid Olddavid on May 19, 2016

    My 450SL has the same problem. I have gone through every damn possible drain scenario to no avail. We only drive it about 2000 miles per year. But , after yet another dead battery, trust my dear wife to sum it up succinctly. "What does a battery cost-$100? $100 divided by 18 months equals $6 per month - what are you complaining about"? I shut up.

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  • Carsofchaos The problem with congestion, dear friends, is not the cars per se. I drive into the city daily and the problem is this:Your average street in the area used to be 4 lanes. Now it is a bus lane, a bike lane (now you're down to two lanes), then you have delivery trucks double parking, along with the Uber and Lyft drivers also double parking. So your 4 lane avenue is now a 1.5 lane avenue. Do you now see the problem? Congestion pricing will fix none of these things....what it WILL do is fund persion plans.
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