BMW L6 into limp mode with no tach

A couple of nights ago, I drove into work and left on the lights. I got a jump in the dark (I work 2 to 2) and we momentarily had the positives and negatives crossed. We VERY quickly changed and jumped
the car. As I was leaving the parking lot, the car went into limp-home and the tach stopped registering. The warning light came on as well.
I got home and charged the battery. The next day, everything seemed fine and after driving two block (heading back to work), the car went into limp-home again.
Do I have a voltage problem caused by the bad jump? Is something in the tach causing the car to go into limp-home? HELP?
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I don't think the tach is causing the limp mode, I think the tach is a result of the limp mode. That is, the issue that shuts the tach off is causing the limp mode. What came first, the chicken or the egg?
You need to connect your car to the diagnostic computer.

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