Start the truck and runs but as soon as I let.

Replace the ballast resistor. You're a youngster, aren't you? That was a real common problem years ago. That resistor is bypassed by the ignition switch in the "crank" position to create a stronger spark. During cranking battery voltage gets drawn down and that voltage feeds the ignition coil. The lower voltage means lower spark voltage and harder starting. Bypassing the resistor gets the voltage back up.

If the pickup coil was defective the engine wouldn't run at all. That system was introduced by Chrysler on Dodges in '72 and Chryslers and Plymouths in '73. It was the industry's first fully electronic ignition system with no breaker points. It's reliable, simple, effective, and easy to diagnose and repair.

Most common cause of backfiring when nothing else has changed is bad spark plug wires or a weak ignition coil. Another possibility is a burned intake valve but that's not common.

If the problem started after parts were replaced look first at the gap for the pickup coil in the distributor. As I recall that should be.012". I had a problem on my '78 LeBaron when I couldn't get the rotor off. I cracked it in two with a hammer and screwdriver. That resulted in a bent shaft and it pushed the pickup away another.006". After I managed to pound the shaft straight again I had a no-start because the gap was.018".

Disconnect and plug the vacuum hose to the distributor. If the backfiring stops, besides the air gap, look for signs of carbon-tracking on the distributor cap. If you have a V-8 engine, look at the four spark plug wires where they're clipped to the rear of the left valve cover. They should not be in the order of the spark plugs, (1, 3, 5, 7). Number 5 should be on one side, then follow the rest in order so it's 5, 1, 3, 7. You want 5 and 7 as far apart as possible. 7 fires right after 5 in the firing order, and under some conditions the spark for cylinder 5 can electromagnetically couple to the wire for cylinder 7 and cause that spark plug to fire as the piston is on its way up on the compression stroke.

If you see a switch with only two wires that is likely for the key-in buzzer.

If the new resistor didn't solve the problem there has to be a broken wire. Turn the ignition switch on to the "run" position, then measure the voltage on both sides of the resistor. There must be 12 volts on one side. The other side may have 12 volts too or a little less but it can't have 0 volts.

Voltmeter or test light. Ohm meters are for measuring resistance and checking for continuity in circuits that are not powered up.

That proves the resistor is good, and no current is flowing through it at this time. You can compare that to a garden hose. You might have 12 pounds of water pressure at the faucet. That's your 12 volts of electrical pressure coming from the battery. The ballast resistor impedes the free flow of current. You can do the same thing by standing on the garden hose, as long as it is not pinched off completely. The nozzle at the end is turned off so no water is flowing. You would see 12 pounds of pressure right before the nozzle, (and anywhere in the entire length of the hose). You'll have 12 volts everywhere in that circuit, just like you measured.

Once you open up the hose nozzle some water starts to flow but it is limited by the restriction caused by your foot. The result is you'll still have 12 pounds of pressure all the way from the faucet to your foot, and less pressure after your foot. When the engine is running current will flow through the ballast resistor, and just like your foot caused a drop in water pressure, that resistor will cause a drop in electrical pressure, or voltage. You'll see a lower voltage after the resistor.

The only problem with this story is in this electrical circuit the current flow is pulsing on and off very rapidly. That makes the voltage after the resistor pulse up and down rapidly too. Digital voltmeters take a reading, analyze it, then display it for a while as they take the next reading. It will take some readings when the voltage is high and some when it's lower. That makes the readings bounce around. A test light will work better in that type of situation.

One of those wires will be blue or red. Since the early '70s, every blue wire under the hood has 12 volts when the ignition switch is on. They used red wires on a few truck models and years. Go to the ignition coil or the alternator and find that blue wire and measure the voltage there. If it is missing you have a burned contact in the ignition switch or a break in that wire, typically a corroded connection at the bulkhead connector.

If you have 12 volts at the coil and / or the alternator but not at the resistor, there is a break in the wire between the resistor and the splice, most commonly a corroded splice.

If it is missing everywhere you have a burned contact in the ignition switch or a break in that wire, typically a corroded connection at the bulkhead connector.

If you have 12 volts at the coil and / or the alternator but not at the resistor, there is a break in the wire between the resistor and the splice, most commonly a corroded splice.