what is causing me not 2 have fire at the plugs on my 74 dodge pu 318 motor replaced the e.c.u. box, (brain) ballast ressistor,and the ignition coil. Truck has electronic distributor

Bad Rotor, distributor cap, bad plug wires, improperly gapped or fouled plugs, incorrect order of plug wires comes to mind as some options. If this has no distributor cap or rotor please disregard those 2.

It’s a '74 model. Dodge wasn’t using DIS yet.

A crack or carbon trail on the distributor cap can cause the spark from the coil to get lost in the distributor. Same with the rotor button. If the coil is firing, that and the coil wire are prime suspects. The coil wire can be ohm-tested. It should give 1000 ohm resistance for every foot of wire.

If the coil is not firing, check to make sure power is getting the the positive side of the coil. You can also ohm-test the coil to make sure it is in spec. Some parts stores can even test the ignition module for proper operation.

Test, Test, Test. Throwing parts at it only makes your wallet lighter.

Bad ballast resistor ( ceramic bar ) on the firewall. These used to be really cheap, I used to carry a spare in the glovebox. There is one bolt that holds it on and the wires just unplug .

OP said he replaced the ballast resistoer. I am not sure what DIS means but a 74 did have electronic ignition.

DIS - Distributorless Ignition System.

I’m at a loss. Did Dodge still use a ballast resistor with electronic ignition in '74? Everyone else stopped using ballast resistors once electronic ignition systems became a standard. And by e.c.u, did the OP mean the ignition module? This truck had a carburetor and no other electronic modules on the engine.

Or maybe the OP has a more modern engine fitted into the old truck. Let us know?

my ballist ressistor shows to have continnuity on both sides across the top and bottom.Getting 12v at the pos side of ign coil,However when attempting to start no spark at plugs,going to test the coil wire today.Purchased new cap and button tested both for continnuty prior to instullation check out fine.(however checked old ones and they check out as well)Check entire wiring under hood and no shorts or bad wiring.I have not hade this trouble before on any vehicle. thanks for the helpful advise from everone

Disconnect the starter so the it won’t crank the engine. Have someone to hold the ignition key to START while you check for voltage at the ballast resistor and the + side of the ignition coil.

I would change the distributor cap, rotor, plugs and plug wires. That will cover everything you have not yet changed. Rocketman

If you’re reading 12VDC constant on the coil primary I’d go to the hall effect transducer (or sensor) in the distributor. I have to admit, however, that that setup in a '74 says to me that this is probably not the original ignition system. And these systems did not need ballast resistors. So I guess I really can’t envision what’s going on here.

Your coil primary should be putting out a square wave, and if you’re using a multimeter you should be reading some average of the on and off times (depending on the pulsewidths), some voltage below 12 volts. The coil fires only when the voltage going through the windings is stopped and the generated magnetic field collapses into the core and induces the voltage spike. Continuous voltage on the coil primary will not fire the plugs.