I used to have a European 740 Turbo 89 model, that was occasionally
sensitive to poor signal across the flywheel motion sensor connectors (two
of them). I think the engine used to turn over on the starter but wouldn't
fire (and had no spark). Reason's probably that the fuel injection is
shut off for safety if the lack of signal from the flywheel sensors suggests
that the engine isn't seen as turning over. Fix was to locate the two
thick black cables going towards the top of the back of the engine around
the distributor area, disconnect the connectors to the leads going down to
the top of the flywheel housing (one cable at a time, they look identical
and apparently don't work if crossed over), spray with switch cleaner and
re-connect. I've heard that if connector blades and contact fingers are
corroded, it may be enough to break and remake the connection, the force of
reconnecting wipes off the corrosion. Apart from corrosion, oil blowback
from the engine oil filler cap or dirt/damp blowback from the road, can
cause these leads and connectors to get dirty. Checking them would be
worth a try, doesn't cost much unless a connector comes to bits in your hand
! If not, there are other sensor leads that can give trouble. Does your
engine turn over at all? I think the air mass sensor device, the
intercooler air temperature sensor, and the air intake temperature sensor,
were all places where I tweaked connectors occasionally to get it to start,
though generally the car was very reliable and fired at the first turn of
the key 99% or so of the time.
