Silly compatibility problem

[kobresia]

I had a ridiculous problem on the Ecoboost PI Sedan I bought as a donor car, but started driving as a cheap beater car because it runs & drives very well despite having 209k miles on the clock (12k idle hours!!!) and having been rear-ended by a light rail train or something.

So, ever since I got it, the left button bank on the steering wheel has been really flaky. I replaced the button bank with one from one of the wrecked PI Utilities, it was still acting-up. Basically, the arrow pad worked, but not a single button matched what it was supposed to do, and the cruise control bank was basically non-functional.

I decided to take the full SCCM assembly (module/turn signal stalk/clockspring) from the demolished Utility, figuring everything in it should be much better, being low miles, and why wouldn't it be compatible, being identical from a mechanical standpoint? I connected it to make sure, and the buttons all worked flawlessly, everything on the stalk worked as it should, all seemed to be perfect, so I disconnected the battery again & put everything back together. After getting the airbag back in place, I reconnected the battery, and noticed I'd left-off the left headlight dust cover from last week when they both decided to stop working due to poor connections.

When I put the dust cover on and turned-on the headlights to make sure they still worked (the left one was out again), I found that I couldn't turn them off anymore. If the battery was connected, the headlights & markers all came on as if the switch was turned fully on. The switch did nothing and they wouldn't go off even when it was disconnected completely, but the high-beam control on the stalk worked as one would expect. I figured I may have disturbed the harness and some of the police function rewiring when I had the column apart, so I started messing around there, when that didn't work, I took the column back apart.

Long story short, the only thing that didn't work with the Utility SCCM assembly, when installed in the Sedan, was the ability to control the headlights & marker lights. The BCM was clearly interpreting a signal it got from that assembly as the signal which the headlight switch would transmit when fully on. Reinstalling the original SCCM assembly got headlight control back, and replacing only the clockspring fixed the crazy button issue.

It seems I'll have to be careful when sourcing parts that seem like they should be compatible, at least if they contain programmable modules...which is basically everything that has any electronic function.

 

[thefranchise713]

I believe the switch is failsafe--if the switch isn't detected, the BCM will default the headlights on.

Next time I can get to my service manual I will confirm this.

 

[kobresia]

That does make sense-- what doesn't make much sense is that the switch wouldn't be detected due to a different SCCM, unless the switch is a daughter module of the SCCM one, and being different than what the SCCM expected made it behave the same as if it was unplugged altogether. I guess the way to test that would be to see if the Utility switch will physically plug-in to the Sedan's wiring harness, and if it actually works.

I'm pretty sure the headlight switch isn't interchangable, since the instrument cluster dimmer is part of the Utility headlight switch, but it's a completely separate button several inches away in the Sedan.