Shifty Battery Drain

Shifty Battery Drain

Typically, when you're doing a parasitic draw test, you watch the meter and wait for the amperage load to drop and the various systems to go to sleep. But on this truck, something was staying awake. Time for a different approach to the problem.

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 was 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 four to five days to drain the battery.

I did at least narrow it down to the BCM, but I was sure it wasn’t its fault. The backlit portion of the IPC was my main concern. I’m not talking about the illumination lights, but those quirky backlit lights that make the hash marks glow to a brilliant white color. The lights usually “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 watch the meter and wait for the amperage load to drop and the various systems to go to sleep. But on this truck, something was staying awake. Time for a different approach to the problem.

One thing to keep in mind with these newer interconnected systems is that 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 they already are, the last thing you want to do is start disconnecting things and then try solving the problem. Because anything being disconnected is just as bad as a short or an open. You’ve got to approach these repairs from the outside looking in. No swapping parts, and no disconnecting anything or pulling fuses.

I decided to use the factory scan tool and started checking things that didn’t seem to be related, but, in some odd way, may be indirectly related. On the screen that shows the PIDs for the transmission and the gear selector, I decided to run the floor shifter through the gears and watch the scan tool. No matter what I did, it would never display the park.

Could this also be the reason why the remote start wouldn’t work? Something was amiss, and if it’s no surprise, I didn’t find one tip or TSB about the lights staying on and draining the battery, with a combination of the remote start not working.

Wiring Diagrams
Searching the wiring diagrams was my best option. The hunt was to find why park wouldn’t register on the dash or on the scan tool. It was now time to dig through the wiring diagrams and find some commonality.

What I found was a microswitch on the gear selector that had never changed position; 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. 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 I had to ground the switch before turning it off.

The Repair
A new switch is NOT available … only if you buy a complete shift assembly. But, the little microswitch is a dead match for one at the electronic store for a whole 15 bucks.

After replacing the switch, I gave it a try and it worked great. No more dead battery, the backlit lights go out and the remote start is back to normal. Apparently, this little switch is the final park signal to the BCM, so that it knows that it’s parked. Without knowing the position, it thinks you’ve walked away, leaving it in 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 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 PIDs and reading the appropriate wiring diagrams, was the real key to 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 confirming your hypothesis. More true today than a few years ago, a mechanic is far more than someone turning nuts and bolts, whose real job in tackling a problem requires diagnostic acumen and ingenuity.

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 into your service bay with the backlit lights stuck on and no remote start.

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