Shifty Battery Drain

    Had an interesting problem the other day, a 2012 Ford F150 
with a battery drain and no remote start. (If you tried the 
remote start all it would do is beep the horn) Also, the instrument 
cluster backlit feature would not go off. This I believed, was the 
cause of the slow drain problem, which usually took 4 to 5 days 
to completely drain the battery. I did at least narrow it down to 
the BCM, but I was sure it wasn't the fault. The backlit portion 
of the IPC was my main concern. I'm not talking about the 
illumination lights just those quirky backlit lights 
that make the hash marks glow to a brilliant white color. The 
lights normally time out after you walk away from the vehicle. 
Lock the doors and they immediately go out. But now, they’re staying on all the time.


    Typically when you're doing a parasitic draw test you'll watch your meter and wait for the amperage load to drop and the various systems to go to sleep, but this time something never would. Time for a different approach to the problem. One thing to keep in mind with these newer interconnected systems, every component in a particular system has to function exactly as engineered or the entire system will not respond properly. Not to make things any more complicated than it already is, the last thing you want to do is start disconnecting things and then try to solve the problem. Because anything disconnected is just as bad as anything shorted. You’ve got to approach these repairs from the outside looking in. No swapping parts, and definitely no disconnecting anything.

    I decided to use my IDS and start checking things that don’t seem to be related but may, in some odd way, be indirectly related. On the IPC page I found the section for the PRNDL. Since I’m there why not run the floor shifter through all the gears and watch the IDS response. No matter what I did it would never display the P on the IPC or indicate the P position on the IDS. Could this also be the reason why the remote start wouldn't work? (My “spider senses are tingling” and those internal diagnostic alarms in my head are be going off right about now.) Something was amiss, and if it's no surprise, I didn't find one write up about the lights staying on and draining the battery with a combination of the remote start not working.

Back to the wiring diagrams

    Searching the wiring diagrams was my best option. The hunt was to find why the P wouldn't register on the dash or on the IDS. Time to dig through the wiring diagrams and find some commonality. What I found was a micro switch on the PRNDL that never changed potential . . . it was stuck... On the wiring diagram it is listed as the park detect signal which a basic open closed switched circuit that is ground enabled. (Wire CET52 green, Pin #3 on the floor shift and pin #7 on the IPC) To confirm the switch was the entire problem I could simply ground the switch with the ignition in the off position and see if the lights would go out. Sure enough they did. The remote start didn’t work right away, I had to cycle the key one more time but this time ground the switch prior to turning it off. (Gotta get the sequence right too.)

The repair

      A new switch is NOT available... only if you buy a complete shift assembly. But, the little micro switch is a dead match for one at the electronic store for a whole 15 bucks...seriously. I popped the passenger side panel off and could get to the micro switch fairly easy. After replacing the switch I gave it a more dead battery, backlit lights go out, and the remote start is back to normal.

    Apparently this little switch is the final "YES I'M IN PARK" signal to the BCM so that it knows that it's really parked. Without knowing the position it thinks you've walked away leaving it in a gear, and if it is in gear obviously that would be a good reason not to start the vehicle without the driver sitting in the seat. 

The results

    The big issue with this case study is the fact that I didn’t disconnect anything. The battery drain was obvious and for the most part was directed to the BCM by the draw found on the fuse. The dash didn’t need to come apart because you could clearly see that the backlit portion of the IPC was on, and it doesn’t take a genius to figure out the remote start doesn’t work when you hit the button. So, the whole process was accomplished by reading and understanding the wiring diagram.  

    Using the scanners for more than code reading, while paying attention to the various PID’s and reading the appropriate wiring diagrams was the real key into solving this problem. The answers are there in front of you. All you have to do is sort through the maze of information until you find what doesn’t seem correct. Prove your guess by simulating the results and then confirm your hypothesis. Even more today than a few years ago a mechanic is far more than a job turning nuts and bolts. The real job is in the diagnosing and understanding side.

    As I’ve stated many times, “Codes don’t fix cars”. In this case study not a code was present, but a problem still existed. I hope this helps you out when a 2012 F150 with a floor shift comes in to your service bay with the backlit lights stuck on and no remote start.