[W126 Coupe] No spark - 500SEC

[Ian Gatenby]

My 1983 500SEC is in trouble.

It has been a bit troublesome to start on our rare wet mornings - and a few 
weeks ago refused to do so.

So - new spark plugs - no go, but had spark - checked the fuel filter - 
clogged (I have had problems with this recently - either a local petrol 
station has dodgy fuel, or a hose on the car is deteriorating).  Replaced 
fuel filter - car started and ran for a couple of minutes - next time I went 
to start it - no go.

Checked spark - no spark.  The water pump on the firewall for the climate 
control is running when the ignition is on (haven't noticed this before) and 
there is about 8 volts across the ignition coil terminals with the ignition 
on.  My other car, a 1981 500SE has negligible voltage here - so, swapped 
coils and the SE still runs and the SEC doesn't.

Can anyone help?  Which computer module(s) or sensors should I start looking 
at?

Ian