1971 350/270.  Since I installed a HEI ignition system the engine will only start when the key is released from the start position.  I have 12v to the distributor from the IGN post on the fuse block. The battery spins the engine well buts drops to about 10 volts when cranking. Is this normal? I suspect I'm getting a voltage drop during cranking.  How do I troubleshoot this.  Thanks for your help.

10 volts while cranking the starter isn't too bad, might be just a touch on the weak side, but it should still be enough voltage to work the ignition coil.  Voltage drop below 10.5 can be a sign of a battery starting to go... if you have access to another battery, or maybe a jump box/or jumper cables,  it might be worth a try to see if that helps the situation... As for the engine not wanting to start untill the key is released, that's kinda odd.... just for the fun of it, take the hot wire off the coil and put a volt meter on it and then turn the key to turn over the engine and see if there is voltage there, if the voltage there is considerably lower(under 9 volts) it may not have enough to fire the coil.... hopefully, someone else will have an idea of whats going on here.... of course the common answer to most electrical issues on vette's is a grounding problem...but not sure here...

Stingrayjim, Thanks for the comeback. Tried the jumper box with no change so the battery is not the problem. Looks like there is a power drop, (0 volts) at the primary ign wire to the distributor when the key is in the start position. I also note that I don't have any power to the radio etc when the switch is in the ACC position so I'm starting to think ign switch.  What do you all think?  What should I be checking next?

ok, trying to look at some wiring diagrams for your year model, and not sure how you ran the hot wire for the HEI, however, the original wire was tapped off the starter solenoid, so i would assume(yeah i know, not the greatest thing to do) that the hot wire to the coil is a direct feed from the battery.  As most of us notice whenever we start a vehicle, when turning over the engine all the other electronics seem to go dead, or nearly dead until the engine starts.  This is usually a result of the power draw the starter pulls,  and seems that is happening here if you have zero volts at that wire when turning the starter....    so, logic would dictate that we need to find a wire that is hot while cranking the motor..  :)  

OK.  I'm getting my 12v to the coil from the fuse block.  With the hot wire disconnected from the distributor I've got 12v with the key in the run position and zero v in the start position with no draw on the circuit.  It appears either the key lock switch or main switch at the base of column has failed.  I'm not sure how to check them without just changing them out.  My radio works when the key is in the run position so it looks like the ACC circuit has failed as well.  How do I narrow it down to the fail.ed switch?  I also just read on another forum that a bad starter solenoid can cause the same symptoms. Thanks again.

|UPDATED|6/19/2017 2:27:52 PM (AZT)|/UPDATED|

ok, didn't think about the solenoid itself...it could be the gremlin here.... it should have a wire or wires that feed off there that stay 'hot' when cranking the engine, providing the voltage to the coil... it appears that its spliced(looking at a wiring diagram) with another wire that is probably 'hot' with key on, and then 'cold' with key off killing the ignition in order to stop the engine...  you might duck under the car and check the wiring off the solenoid(sounds easy i know, i cant fit my butt underneath a vette without jacking it up)... 

|UPDATED|6/26/2017 7:01:50 PM (AZT)|/UPDATED|

I'll test the solenoid tomorrow, if its not that then its either a broken wire,  faulty key interlock or ignition switch.  Thanks for the diagram.  I'll let you know what I find.

The starter solenoid checks out.  Going to replace the ignition switch next.

Ummm, before getting into the ignition switch,  double check the wires I circled in red on the schematic.... you'll see one coming off the solenoid, the other splicing into it just at the coil....  the one splicing in from the firewall(key switch) is hot when the key is on, the other wire coming off the solenoid is hot when you turn the motor over....so if the solenoid is good, there will be 12 volts when you activate the solenoid(turning the engine over) then the other wire is hot when the switch is in RUN position....  if the wire is hot(12 volts) with the key on, then more than likely the key switch is ok, in this instance....    might check the wire at the solenoid with key in Run and see if 12 volts is read at the wire there.... basically checking to see if its a broken wire....  




|UPDATED|6/26/2017 7:04:53 PM (AZT)|/UPDATED|

Thanks.  I have 12 volts on the wire to the coil in the run position and 12v at the solenoid "s" terminal when the key is in the start position.  I'm pretty sure it's a bad ignition switch. I've got a new switch but changing it will require dropping the steering column so I'm waiting for a rainy day to take this on.