jcman

I think the OP covered that on his post#5.

The clue is most obvious when he said that during a jump it starts.
Here is my answer, the load reduction relay directs as much battery current to the starter by opening the main bus contact to other current loads when the key switch is in the start position and when released the power is back to the other loads, this is why the car will not start with one battery, now it is possible that the one or more of the 5 ignition switch contacts are weak too like the one going to the relay or the relay is bad.

We can't go on a park position safety switch or a clutch engagement switch, that wouldn't be logical if it starts with battery jump, not that you or anyone else mentioned this.

Just drawing down conclusions.

Airbag

yeah i see that now . I think you are right if OP can start it repeatedly that way .

jcman

At this point I doubt that the OP will even report back, that's a shame because others might need to know about this happening to them and what remedy cures the problem.

Airbag

i just had a though about this ...

Op did no say HOW he jumpered it in the engine bay. did he do both posts of the battery , or one post of the battery and a ground some place.

could it still not be the ground starp from the battery to the car ? if he puts positive jumper cable on the battery and negative to a solid ground point , he has bypassed the original car connection from battery to the body on the negative side .

just musing..

Airbag

thats ok .. if we think about it long enough we will come up with hypothetical solution others can use

jcman

My thoughts are that he knows what he is doing at this point, most seasoned technician's are logical minded but still need to know all the pieces to make the picture clearer.

jcman

This is where Columbo would reenact the scenario to prove the point, we really don't want to loose members in a becoming obsolete forum of passer bye's.