84 308 GTSi - No Spark

I would venture a guess and say it is a faulty ignition box. Gets too hot, stops working. Cools down, works

 

I'd be less suspicious of the DigiPlex Boxes -- because they'd both have to go bad at the same time to take out both banks (and they'd both have to come back to life together to restore normal operation). Has to be something common to both banks like:

1. the RPM sensor
2. the +12V power from the fuseblock to the DigiPlexes
3. the +12V power from the fuseblock to the Coils

Quitting after 20 minutes (and rehealing after cooling) matches well with the fuseblock plastic softening and the riveted connections having a problem (and then rehardening after cooling) IMO so I'd start there. One simple thing you can try is adding jumper wires to better connect:

the A (light blue) wire on the top of fuse B (2nd from the left) -- this line goes +12V when the key is "on"

to

the G (yellow) wire on top of fuse B -- this goes to the Digiplexes

to

the G (yellow) wire on top of fuse A (the leftmost fuse) -- this line goes to the coils

(which is what the fuseblock is supposed to do if healthy)

Alternatively, with a voltmeter you can confirm/deny if you have +12V on those two G (yellow) wires when you are having the problem. Do you have the stock fuseblocks or have you upgraded to something else?

 

That would definitely decrease the probabilty that it is a problem at the fuseblock, but can't really add anything new -- if the two G (yellow) wires (leaving the fuseblock and going to the DigiPlexes and the Coils) are at +12V, but you still don't get spark during starter motor cranking, then the next step would be to check the RPM sensor's function.

 

Its not likely that there is a problem with the connector, if you need a new one anyway, check an electronics store and ask for a Molex MLX connector.

I would follow Steve's recommendation and check the RPM sensor, i have seen my share of dead or unreliable RPM sensors while fixing such issues, and a new RPM sensor is relatively cheap. You might also want to check that the ground connection to the Digiplex units is good.

 

Would still suspect the crank sensors, either the crank sensor wiring or the attached cable or connector. A distorted or erraneous signal from the sensor will impact the ignition timing, as the Digiplex is not so advanced to either correct the timing or issue an error code, it just maps the incoming signal to the map and creates the advance timing.