1986 928S2 16V

An odd one. I've just completed an engine rebuild and lots of other (while I'm there) jobs including making all new engine looms. So there's plenty I could have got wrong.

Anyway, much to my amazement, it started first go. But it's not quite right. It starts if I crank for a few seconds and then let the key spring back from the cranking position. It's at that point it bursts into life. NOT while it's cranking. Which is odd.

I've done no investigation whatsoever (yet). But it appears something isn't happening that should during the crank. To save me some time, has anybody seen this or have any ideas before I get digging?

Cheers

 

Sounds as though the ignition switch may not be powering up the 15 bus when in the cranking position then when you let go of the key the switch instantaneously springs back to the "ignition on" position and then makes contact once more. Needless to say the 15 bus should be live in both positions but when something is knackered there is always a reason for such.

You should be able to test to see if this is what is happening with a good multi meter relatively easily.

 

I've not been near the ignition switch but the fuseboard was out to remove the injection loom and (it turned out) to fix some previous aftermarket alarm bodges. However, sounds like a logical explanation.

Out with the Power Probe...

 

If anybody is interested....

There was no spark when cranking. After a bit of digging, I found/remembered an earth strap next to the coil (UK driver side) that wasn't connected. It had been connected to one of the ground points on the adjacent cam tower.

I'm rather surprised it worked at all. Some odd earth path...