Chevrolet Repair: 1985 chevy p/u will not start, 1985 chevy c10, wire loom

Question
My 1985 Chevy C10 P/U will not start, no spark. It has a 305 V8 Vin code H, automatic, 2WD.  It has just over 61,000 actual miles.  It has had this intermittent problem for several years. Sometimes it will start, sometimes it won't. When it won't start, there is no spark at any of the spark plugs. I have voltage at the B+ terminal and ground at the coil. I've changed the entire distributor.  Had it towed for repair, but after getting it to the shop, it started OK and they couldn't find anything wrong.  I've followed the troubleshooting tree in the typical repair manuals.  There is a 4 wire harness coming from the distributor going into a wire loom.  Can't find any information about these wires.  Any ideas?  Thanks!

Answer
Hi Terry, I'm not sure if this truck has one but there is a spark control unit that is mounted to the firewall. This unit controls spark advance and can fail causing the no spark problem your having. It can be intermittent also. I was thinking that you should replace the complete dist but you've done that. Then I thought of this spark control. Yes 1985 was a while ago for me to remember, these cars and truck change too much every year.

Hope this helps

Have a Great Day          Mark