Update - I haven't been able to drive the car very much since I had the work done, but everything has been fine. Until friggin yesterday.
I was sitting in a drive through car wash (the paint is already pretty damaged on this car), and I messaged the guy that has helped me with these issues. I thanked him for finally getting this fixed after nearly 3 years of chasing the problem. ... ... And just a few minutes later, literally within a few hundred yards after leaving the car wash, it all happened again. Voltage shot up beyond 16v, gauge cluster shut down, radio+aircon, everything. It happened a couple of times.

I drove the car for another 90ish minutes after that, and never had another problem. Voltage was rock solid at 13.8-14.1v.

I am so disheartened. I have now replaced the pcm, the battery, the entire engine wiring harness, and the alternator x2, and multiple thousands of dollars chasing this issue. I have not changed the bcm. What else could it possibly be? I don't want to get rid of the car, but I can't trust driving it.

t-ray said:

Update - I haven't been able to drive the car very much since I had the work done, but everything has been fine. Until friggin yesterday.
I was sitting in a drive through car wash (the paint is already pretty damaged on this car), and I messaged the guy that has helped me with these issues. I thanked him for finally getting this fixed after nearly 3 years of chasing the problem. ... ... And just a few minutes later, literally within a few hundred yards after leaving the car wash, it all happened again. Voltage shot up beyond 16v, gauge cluster shut down, radio+aircon, everything. It happened a couple of times.

I drove the car for another 90ish minutes after that, and never had another problem. Voltage was rock solid at 13.8-14.1v.

I am so disheartened. I have now replaced the pcm, the battery, the entire engine wiring harness, and the alternator x2, and multiple thousands of dollars chasing this issue. I have not changed the bcm. What else could it possibly be? I don't want to get rid of the car, but I can't trust driving it.

Click to expand...

car wash could be a clue. feel the passenger side footwell. is it wet?

Yeah the carwash was my first hunch as well. I didn't think to feel the passenger footwell at the time, and by the time I posted this, it was the next day so everything was dry.
The car has been fine since then, except for one day last week. The car sits outside, uncovered. I cranked it, and took it to the grocery store. While I was still in my neighborhood I saw voltage spike between 15 and 16v, and it bounced around in that range for maybe 90 seconds, before it stabilized back down at 14v. After that it was fine. At that point, we had not had any rain in weeks, so I don't think it's necessarily a water/moisture issue.

Not directly related - I have decided to purchase a dark horse. I'm still keeping the boss; it will just be a long term project car.

To reinforce what BackinBlack was leading in with, did you check the cowl drains under the cabin air filter? For S197s parked outside, its extremely common for detritus to build up on the rubber drain grommet and end up flooding the passenger footwell since it can no longer drain water - which is conveniently right where the Smart Junction Box is. If it stays outside, it should become a spring and fall routine to clean them out.

I did make sure both cowl drains are empty and free of obstruction.

Also for clarification - it has not always lived outside. I moved a few months ago and as a consequence the boss has to live outside. For the previous two years, it was parked in my garage. The electrical issues predate either, however since the most recent changes (pcm, alternator, wiring harness replacement), it *has* lived outside.

I guess I'll try the bcm next. But regardless, I think the boss is now being relegated to project car / toy, and will receive much less daily use (which means I'll be more tolerant of the voltage issue)