5 pin ignition module question (need diagram)

[jseabolt]

I read on another Forum that some GM V8 vehicles (e.g. 1986 GMC 350ci pickup) had 5 pin ignition modules instead of 4 pin modules. He says the reason for the 5th pin is some high compression engines had trouble starting and when this pin is grounded through a vaccum switch retards the timing to help the engine turn over easier. Is this true?

 

This is going on another application. Not an HEI distributor.

 

He says you can use a 5 pin control module in place of a four pin and when the "fifth" pin is grounded will retard the timing 10 degrees. Supposably this pin is marked "R".

 

So I got one of these in my hand. The mounting holes are the same as the 4 pin module. The side with two pins are marked "B and C". One goes to the + side of the coil and the other one goes to the - side of the coil.

 

The side with three pins are marked H, L and D. The H pin has a notch cut into the module above the pin. Don't know if that means anything or not.

 

My question is which pins could goto the magnetic trigger and which pin when grounded retard the timing 10 degrees?

 

Here is a diagram for the four pin module but they are marked W & G. I suppose I could experiment and see if the pin across from the mounting hole is the one that retards the timing.

 

 

Thanks!