1990 740 Turbo - Charged battery outside car, now car won't start

Are the cables neat and clean, especially where they hook on to the battery terminals?

Is the battery in fact "good?" Perhaps it has a bad cell, and can't put out enough power to turn over the engine, although there is enough to run the accessories.

The fact that you hear a "click" would imply you are getting power to the solenoid, so the starting circuit is OK.

If the cables are clean, the battery is good, the problem is most likely with the starter / starter solenoid or its connections.

The starter circuit in the 740 is as simple and classic as they come - no sensors or other mystery stuff. The battery should have lasted longer than a few hours of the interior lights being on, so I think it's very likely to be bad. One of the failure modes I've seen a lot in the last few decades is what I believe to be broken connections inside the battery. When that happens the car won't start and the headlights go out when you try the starter. A shadetree diagnosis (and temporary repair) is done with a hammer, around a 12 or 16 ounce is good. A few modest whacks, like driving a nail, on each post often jams things back together and will get the car started.

Alternatively, just swap in a battery from another car if you can borrrow one. After all, what are friends for?

Mike

Something I should mention, as you don't know how good the alternator is. Believe the voltmeter more than the warning light. As long as you are noticably above the red you are good. The most common alternator failure by far in these is worn brushes, and I dunno for sure about the 1990 but in our

1985 the warning light does not come on when the brushes wear out. The current to light the warning has to go through the brushes.... That condition is also heralded by a real non-event; the "battery" warning light does not come on in the "run" position of the ignition switch before you start the engine.

When the brushes wear out (expect approx 150K mile life) they can be replaced by replacing the regulator assembly they are built into and without removing the alternator. The brushes themselves may also be available in the Specialty Hardware section of Ace hardware stores (bring 'em in and match 'em up), but I'm not sure of that. Check back when the time comes.

Mike