Very low spark

My son has a 91 F150 6cyl and I believe a 4.0 ? The problem is that a any given time, the truck just dies due to no spark at the plugs or or at the coil. Swaped out the coil and it restarted again, only to do the same thing. We put in a new coil a new pickup that goes inside the distributor and the electronic module that mounts to the side of the distributro. Put all the above in and it started and ran for 15 seconds and then did it again.

I check the primary side of the coil and it lights a little 12 volt test light s with the key turned on and the coil seems to be to be getting switched. While crancking their is very little spark off the coil, just enough to see it.

I don't have a schematic yet, seems to me that the 12 volts that feeds the system must be droping very low somewhere. Any Idea's

Try a new distributor cap. Check the coil ground make sure it's grounded good and tight. Could be the rotar bug inside the distributor too. Try replacing it. If all else fails, on the driver's side wheel fender there's a silver brain box held on by 2 3/8 blot's. Try replacing that. Bone yard should have one pretty cheap. I'm not for certain but I think a 91" has that brain box.

OK I had to look this problem of your's up. The "brain box" as I was calling it is the "IGNITION MODULE".
I believe you have the "TFI-IV" ignition system due to the fact their the only style that has the module on the side of the distributor as you discribed. The "TFI-IV" is also refered to as the "DI" as well.
A "91" does not have the ignition module on the fender as I thought, your's is the one on the distributor that you already changed. Try changing the starter relay, it should be located on the side of the fender close to the battery or before changing it make sure it has a very good ground being that is the first place that power is sent from the battery to the coil and distributor.