Hi,

Have an original 71 Z28 (LA). Recently replaced the engine wiring harness and removed a crappy security system for a few reasons namely because there wasn't any originally. No start at the moment that can be from the removal of the security system but with the pink/yellow wire connected to the coil "+" I'm getting 12v with the key turned on. With all other wires removed (has a module replacing the points plus the standard tach wire), I'm also getting 12v to the negative side. No joy on starting the car but thought that was weird getting 12v to the negative side with simple having the key turned on and not cranking. Tried two different coils (original working one, replacement).

Car cranks, doesn't start.

Any thoughts would be appreciated.

MD

 

No, with the way you tested it (disconnected from the distributor) that's not weird having a constant 12 volts on the - side. What wires do you have connected to the - terminal on the coil? Remember, that terminal is not a ground, it's an output to the (originally) points, so make sure you don't have a wire connected to it that's grounded out. Was your ignition working and engine running before you removed the security system? If everything is wired as it should be, your module might be toast. What type of module is it? Some of them don't want to have a full 12 volts to the coil, your original coil supply wire was a resistance wire supplying in the 10 volt range.

 

Everything was the same for the battery cables, everything to the alternator and to the coil and distributor. The difference is a new engine harness which has a pink/yellow combo to the +, same as the previous harness (part of it fried), radio thingy to the positive, tach wire to the negative with one of the two distributor wires to the -, other to the positive. The points were replaced for a module. Not sure if that it fried but it wasn't before. Hmmm.

 

"I'm getting 12v with the key turned on".
But did you check voltage at the coil when trying to start the engine? That's a different position on the ignition switch, and may have been a circuit that was bypassed through the security system.

 

I'm still curious what the distributor module brand is! Might help us to help you if we knew. A new harness should have eliminated the security system, and anything in it that might have cut out the ignition. Depending on the module brand, there are differences in how each is wired. Also, some don't like the tach wired to them, but if yours was wired the same previously with the tach, it's probably OK.

 

He replaced the engine wiring harness. If the security system was tied into the ignition switch, that's under the dash (part of the harness he didn't replace).

 

No idea what the brand is. Might be getting a Crane Cams setup this weekend that comes with the coil. Anyway, the voltage drops to 9.5v when cranking. Both wires are connected the starter solenoid and both go back to the + of the coil (pink/yellow).

 

Thinking the module may be bad as well (someone mentioned it). Is there a way to test the module? Doing a resistance test it went down to zero like both coils I have (assuming they're both working).

 

Missed that! And that could definitely be the issue, as security systems often route the ignition power through the system, so if removed it will leave an open in the start circuit, or power to the ignition.

As for the module resistance. Without knowing what the brand is, I'd only be guessing what you should read with an ohmmeter. Generally I would expect it to have extremely low resistance, as they are really just an electro-magnetic pickup. If I knew the specs I'd probably check it by pulling the distributor, applying the required voltage (if known) to the leads, and turn the distributor to check if the module sends the proper signal to open and close. You could try a simple test of the module without power by putting a ohmmeter across the two leads and then watch the meter as you turn the rotor slowly. It may work without power to open and close, and this test wont hurt the module regardless.
Many electronic ignition systems use less than 12 volts, so having a reduced voltage to the ignition is not that unusual. The original distributor on a '71 was a points distributor, and should have had less than 12 v. to power it. But most aftermarket setups use full 12v. power, so if there's a resistor of any kind they should be found and omitted if the system requires full 12v.