Joe, Art Benstein knows this stuff. Yea!

Me, I just know that my 1987 745 survived this mistake last winter, when I tried a good Samaritan deal, and let the OTHER driver connect the cables at his end. A quick spark and &^%&^%*^ exclamation, but no untoward effects. Lucky.

As has been said, access to economical and good (reliable) used parts make all the difference in this case. No way you could pay dealer prices... or even NAPA (etc.) if you tried swapping en mass.

Good luck!
--
Gregg; Mpls, MN; 1987 745, 1967 Amazon 4-dr.

Art and Steve, Thanks for good information. The owner is asking $900
for the car. The car's at a garage where the owner apparently got
a repair estimate that's more than he wants to spend on it. I haven't
looked at the car yet, but $900 seems high for a car that's not
running.

All of the vulnerable electronics (ignition and fuel injection ECUs, air mass meter, and alternator regulator) can be replaced with used units for around $250. Buy the car for zilch, replace these sequentially until it works, and have a nice car.

Joe, we'd only be guessing how much was damaged. So much depends on how hard the son tried-- anywhere from a quick pow (oops) to clamping it full on the battery.

I'd expect the diodes in the alternator to be gone, because there's absolutely nothing to protect them. The ICU, ECU, if LH2.4 the AMM, radio and other sensitive items may actually survive if the built-in reverse polarity protection diodes blow the fuses fast enough. Not so sure about the -016 AMM though. I'm afraid the only way to learn is to begin repairing the car. Can you get it for $100? I'm sure he could sell it easier if it ran. Might make a big difference to you if you have parts available cheaply from salvage.
--
Art Benstein near Baltimore