Ignition drops voltage while cranking/starting

[HaZmatt]

12 hours ago, Chickenman said:

MSD boxes are infamous for out right failing and having intermittent problems. The older model 6AL's and 7Al's  are particularly bad. Might be an idea to disconnect the MSD from the Ignition switch power though and temporarily wire the small red wire ( +12v switched) directly to the Battery positive terminal. Use a small switch and a fused line. Large red wire should of course be connected directly to the Positive battery cable with a 20 amp fuse or Circuit breaker. 

It's best to run the switched +12v wire from it's own separate relay. They can be sensitive to voltage drop from loads on the ignition switch. 

MSD shorting internally could cause the voltage drop to 0. 

I will try that this weekend. Thank you very much!

[HaZmatt]

11 hours ago, Chickenman said:

That's not a good way to do it.  The ignition switch cannot handle a lot of amperage. Fuel pumps can draw significant amperage, especially when used in a returnless system and when they get older. The factory wiring also uses too small a gauge of wiring to the F/Pump and that also increases current draw and Voltage drop. 40 year old wires don't help either if they are still stock. The addition of the extra Fuel Pump amperage draw, plus all the normal devices connected to switched ignition, could be enough to burn the contact points in the ignition switch. 

Connect the Fuel pump through a proper relay.  A Bosch 30 or 40 amp relay is better than the factory EFI F/Pump relay  Use 12 gauge wire minimum to the Fuel pump from the relay. Both on power and ground side

The MSD is also sensitive to voltage drop on the small +12V ignition feed. That's why I recommended isolating the small +12v switched power lead and running it though a Relay to the battery. 

Yeah this was temporary. I am planning to rewire and install an oil pressure switch and wire the fuel pump to that. But I figured I would try and fix this problem first.

[HaZmatt]

Update:

Apparently the new ignition switch I bought was faulty.  Replaced and it seems to be fine now, I can't get it to replicate the issue anymore. Also, the new switch has seemed to help tremendously with the hot restart issue I was having. Hmmm...two birds with one stone!