No spark at plugs

[Harleyq]

Hi folks, am beyond stumped here. Have 12 volts to distributor, but nothing to plugs. Have replaced dist cap and icm, coil ohms out perfectly. Normally have this thing up and going in 15 minutes. What am i missing? Rotor is good, distributor spins just fine but no spark. Any suggestions? It seems w8th 12 to the distributor, coil is good and new icm, i should have spark at plugs. This is an hei distributor on a gm 350 that was dropped 8n before I got her. Thank you in advance for any suggetions.

[James]

Is the carbon button on the inside of the cap there? If it is missing, the spark will not reach the distributor rotor or spark plugs. This can cause the coil to burn itself into.

Just a side note and it might not be a problem. With the ignition on and with 12 volts feeding the distributor, measure the voltage between the distributor housing and the engine ground. Should be zero volts (if a voltage {any amount of voltage} is present it is not grounded). If the distributor housing is not grounded, no sparks. The ICM is grounded through the base plate to the vacuum advance plate and then the housing. Only apply a thin layer of heat sinking compound to the ICM base plate, nothing should be insulating the ICM base plate to ground. Then secure it to the distributor vacuum advance plate.

Another way this can be check by removing the cap (and all of the wire connectors connecting to the cap and set it aside on the bench or toolbox.) and place the meter on 200Ω scale, measure between the ICM base plate and to the vacuum advance plate, then to distributor housing, and to the engine. Should be less than .5Ω resistance (don't forget to short the leads together first to determine what leads resistance is and substract that from your measurements).

If everything check good, the the ICM is at fault. I don't know if the auto parts store can still check the ICM (with a ICM tester) like they used to do years ago. This might also be worth a try.

[James]

Another though. If you are not using a tach output from the distributor, make sure you plug the power into the correct distributor B+ terminal. Connecting it to the tach output will provide no sparks.

[Harleyq]

Thank you for the advice, I think that is the one thing I haven't checked ground from engine. Will check that forthwith and see what we have. It was running fine, started fine and had just left when it died and thos is where we are at, no start, no soark.

[Harleyq]

The button is therenand free, I made sure it was there and free and making good contact. Replaced the icm already. Not sure about the testing of...

[Harleyq]

Still have no spark, pulled distributor gear is good turns freely and wires to icm are good. What can I be missing?

[AZKen]

What year is the motor

[Prowbar]

Are you using the stock resistor wire or a separate wire from the ignition switch to power the HEI? And did you measure the voltage with a test light or a multimeter?

Temporarily jump the ground from the distributor housing to the negative terninal of the battery to confirm a working ground I'd suggest.

Otherwise pull the dist and test it on the bench for spark.

[Harleyq]

How would I benc test, haven't had the opportunity to do so yet. ...

It is a 78 gm 350 height dist. Have also changed the magnetic pickup with no change...

[Harleyq]

We have done both, ran wire from battery to dist no change. Tested v going in at 12.9 dropped to -9 when cranking. At a loss for what it could be, the dist is practically rebuilt...