#3 cylinder dead. HELP!!!

[Dreed76]

Ok I have spark, compression, and fuel.  I just got finished putting a new set of fuel injectors in thinking I had a faulty injector.  I have the cop conversion so I swapped the coils around and still the same cylinder is dead.  I've taken the plug and coil out of the head and turned it over and it sparked.  All four cylinders are at 150psi.  I was just driving it home a few days ago and it backfired a little bit then a couple miles down the road it lost power.  I could really use some help.  thanks.

[DesktopDave]

Might have a damaged coil driver in the DME.  I've seen it happen to maybe four M42'ers on here.  I have a cooked DME in the basement right now.  If you take the DME case apart you'll see that one is usually roasted - they're four large, identical plastic squares on the right side under a tin clip/heat sink.

I was attempting to salvage used transistors from other units to rebuild them.

If you can find a good used one, send the dead one to me, I'll give you a few bucks for it.

[Dreed76]

Thanks!  I'll go ahead and pull it tomorrow and take a look.  Any other ideas if that isn't it?

[DesktopDave]

Can't be much - could also be a weak battery, loose plug, stripped plug threads, plug insulated from the head, spark plug gap that's too wide, slightly damaged wiring harness, loose connector pin or mixed-up timing on the coil packs...

I'm just guessing here based on what I know about electrical theory.  Even though the plug sparked outside of the head, when it's in the high compression of the motor the low voltage from a damaged coil driver transistor can't be stepped up enough to jump the gap.

[Dreed76]

I finally got a chance to pull out the dme.  1 of the transistors definitely look fried.  Will I need to buy a new dme or is there a way to save this one?

[DesktopDave]

I haven't done it myself on a DME yet, but if you're careful with a good soldering iron it's definitely possible.  I'd use a 20 watt iron at a minimum.  The biggest problem is that those transistors can't be ordered from what I've seen.  They're a Bosch proprietary design with no published datasheet that I've been able to find. 

I think they're just a medium-sized darlington transistor.  It's close in size to a TO-220, but not the same.  They did use those in about a million other DMEs though, so spares shouldn't be a problem if you're good with a desoldering iron or braid.

I bought a few EWS DMEs to experiment on but never got around to it.