Voltage and over-voltage problems
Maybe I can shed some light in the dark corners of 928 electronics, having
things for too many years.
The problem is that many ECU's (Electronic Control Units) can't tolerate voltages above maybe 20 and reverse voltage at all. Not even for a little bit. A totally discharged lead-acid battery doesn't have any polarity at all - it can be charge the wrong way just as well as the right way. Along comes your good samaritan and he momentarily hooks the jumpers up backwards and then says "oops." Too late.
Another scenario: You call your local tow service to give you a jump. They have this gas engine on the back of the truck and what they don't tell you is that it has no voltage regulator at all - they just pump as much voltage in as they can to spin then engine faster so they can charge you the 25 bucks and get out. Only trouble is these gizmos can put more than 30 volts on the system. Too late again.
One other thing to worry about is the dreaded "load dump." This is when you have a loosed connection at the battery. The alternator is pumping out max current - remember, the loose connection is probably why it wouldn't start in the first place - and then the battery connection opens. The current has no place to go and the alternator can shut off fast enough, creating a voltage spike often over 100 volts. Not good.
Rather than explain all that the books just say "don't jump start." Actually you can do it in perfect safety (I have several times) IF you follow the logical precautions - don't reverse the polarity and don't over-voltage the system. Always FIRST verify that all the battery connections are good.
Newer cars without Bosch ECU's are protected against all those terrible things.
(If Lucas was knighted the "Prince of Darkness" Robert Bosch must have been....)