I have a ‘69 442, and I’m gaining a better understanding of the points ignition system. I realize most folks hate points, but I think it’s pretty cool and its worked well for me thus far.

My question is...there is a black lead that connects from a screw under the distributor vacuum advance to a screw under the points condenser assembly - what does this wire do? It’s contained within the distributor. Curious engineer I guess...

 

Its an internal ground strap for the points connection. The points don't work off of B+, they supply the ground side of the coil.

 

 

Yes, when the points close it supplies a ground to the coil.

 

 

To expand your knowledge of ignition systems, the coil is a transformer. There is a primary winding that is connected to the points and a secondary winding that is connected to the coil wire. The spark is generated in the coil when the magnetic field collapses in the primary winding, inducing a current (and thus a high voltage) in the secondary winding and sending that voltage to the plugs. +12V is applied to one side of the primary winding. The other side is connected to ground though the points.



When the points are closed, current flows in the coil and the system is static. As soon as the points are opened, the magnetic field collapses and the coil generates the high voltage pulse. In reality, the coil neither knows nor cares how that ground wire is opened - it could be a mechanical switch (the points) or an electronic switch in an electronic distributor. The electromagnetic process in the coil is exactly the same. If the points are brand new, the ignition system will function exactly the same as if you had an electronic system. The reason why people prefer electronic is that the points deteriorate over time - the arcing that occurs when the points open cause them to pit, which affects dwell and timing. Electronic systems obviously don't deteriorate like that.



And lastly, since people are enamored with HEI distributors, we need to remember that HEI was NOT developed as a performance part. It was developed by GM to meet EPA requirements that new cars meet emissions standards for 50,000 miles without a tune up.

 

 

I should have added one more thing. The plate that the points sit on is moved by the vacuum advance diaphragm to alter timing advance. It sits on a greased bushing in the distributor body. A jumper wire is used from this point plate to the distributor body to endure a good electrical connection to ground. If this wire were missing, the current would flow through the bushing, which would pit from the arcing and eventually seize up.

 

 

My experience with that wire is when its open, car quits running or won't stay running.