Porsche Repair: 944 computer, 944 porsche, sensers

Question
Hey, I'm in a rough spot.  I just swapped a used motor out with my 86' 944 Porsche.  Now I can't seem to get the thing to run.  After hooking up all the wiring and everything, the engine turns over but doesn't get a spark at the coil.  I had some testing done, I bought 2 new reference sensers for the flywheel, and still no spark.  Everything seems to check out ok, the coil, the distributer, sensers....Since I happen to have an extra 944 in my yard that I purchased for parts, I took the computer out and swapped it with mine. Bam, it starts up.  I run it for a while, cranked it a couple of times, then I drive it 5, 10 miles up the road to work.  I get back in it to leave about an hour later and it won't start.  It has no spark again.  Do you think you could have any idea what's going on here?  Computers are not cheap, I just got lucky and had one laying around, if I replace it again I'm worried that the new one would mess up like the other two.  I haven't confirmed yet if my computer is messed up but I'm pretty sure it is since it was the problem the first time.  Any suggestions or advice would be appreciated.  Thanks, Scott

Answer
Hi Scott.  Sounds like you've already done a lot of investigative work.  As you know, the culprit could be any number of things.  I'd suggest we begin by eliminating potential problems one at a time.

The COIL.

Verify you have three wires at the coil ...a green wire to the computer and a small black wire (these two are the ones under the nuts)and the coil wire itself to the distribuitor.   Remove all three and check the coil with your ohm meter.  Reading from the low scale test the resistance between terminal 1 and 15 (where the nuts screw on) and you should get 0.4 to 0.6 ohms.  Now remove the tiny rubber plug on the top surface of the coil to expose terminal #4 hidden  inside the coil.  Reading from the high scale, check the resistance between terminal #4 and the coil wire terminal...your reading should be 5 to 7.2 K ohms.

These specs are critcical to the ignition system.  If you don't get these readings, the coil has to be replaced...with the right one.

Let me know what you find.  Write back using the follow-up feature of All-experts.

Dave