The distributor dictates when to activate the pulse for each spark plug to fire in the engine. Where exactly is the distributor? It’s that round thing with all the wires coming out of it . . . there it is, behind the air filter.
The distributor contains a rotor. I small arrow piece that spins as the engine rotates. The rotor is attached to a gear on the end of the distributor. Inside the distributor cap are contacts around the peripheral edge. As the rotor spins with the engine it comes into contact with these contacts. When the tip of the rotor touches each contact, in turn, it’s saying, “Fire that spark plug! Fire this spark plug! Fire this spark plug!” Over and over again as it spins. This creats a nice smooth idle as the contacts are spaced evenly apart inside the distributor cap.Distributor
If you can imagine damage to these contacts, or distributor cap, or build-up of any sort, or a cracked cap, could easily displace this precise system of Fire, Fire, Fire. You might have instead, “Fire, …, Fire,…! The contacts that are not fired would cause the engine to shake from the piston that is not fired. The shaking is one of the first clues you will notice when your distributor is going bad.
You may notice stalling, or hesitation. These are caused by the same mis-firing of the distributor because the contacts aren’t being touched correctly by the rotor.
Occasionally you may notice an oil build-up near the distributor. This is the seal not properly sealing and the oil is pumping up and out of a cracked metal washer, or loose bolt, etc.
The important thing to know here is that the distributor can be easily replaced by the home mechanic. It’s not science but you do need to work with care. Let’s go over the process of replacing the distributor. I’ll be working on a 1989 Chevrolet Silverado 7.4 engine:
- Remove the negative battery cable from the battery.
- Number the spark plug wire and it’s corresponding position on the distributor cap. This is fairly important to do. You will transfer the numbers from the old distributor cap to the new distributor cap, so the spark plug wire marked #1 will have the corresponding #1 on the new distributor cap for easy installation. If you forget, or get the numbering mixed up, all is not lost! You can refer to a shop manual for the firing order.
3. After numbering the cap and wires, remove the wires from the cap and set to the side. There’s no need to remove the distributor wires from the spark plug so leave them attached.
4. Now, mark the position of the distributor cap to the engine.
5. Using a Phillips head screw driver remove the screws on the sides of the distributor cap (two screws). They are not captured so be careful the screws don’t fall out. Unscrew them until you can lift the cap and leave them still in the cap has been my preferred method.
6. Inspect the inside of the distributor cap for burns and cracks. These telltale marks can give you insight to what your engine is doing. If one of the prongs inside are burned or scored, then the cap was probably not screwed on straight. If there’s a crack or hairline fracture this could cause misfiring.
7. Unplug the connectors on the back of the distributor and any other connectors your vehicle may have. Mark them, or take a photo, so you remember where they came from.
8. Now your plugs are unplugged, and your distributor cap has been removed. You should be looking at the rotor atop of the distributor. Very precisely mark the position the rotor is pointing. This is your first rotor mark.
9. At the base of the distributor there is one large bolt holding the distributor in place. It will have a forked tang it is holding down as well. This is the same bolt that is used to set the timing. Clean away any dirt and grease in this area.
10. Mark the position of the base of the distributor.
11. Remove the bolt and forked tang.
12. Pay attention to the position of the rotor. It may turn as you remove the distributor from the engine. By holding the base of the distributor, lift it out of the engine. Watch how far off of your original mark the rotor turns. Mark it. This is your second rotor mark. This is the position you will refer to when installing the new rotor.
13. Take the new distributor and dip the gear of the rotor (the side going into the engine) in a bit of engine oil. Hold the base of the distributor in the position you marked on the old distributor base.
14. Line up the rotor with the rotor second rotor mark. As you slide the distributor down into the engine, allow the rotor to turn with the gears it is meshing with. When the distributor is seated the rotor should have turned to the position it was in on your first rotor mark position.*
15. Replace the hold down bolt and forked tang. You don’t need to tighten this bolt as hard as you can if you are planning on checking/setting the timing next, Just make it snug.
16. Holding the distributor cap mark the corresponding number from the old distributor cap to the new cap. Make sure you have it in the same position as the other one was when you mark it.
17. Attach the spark plug wires to the appropriate, corresponding number on the new distributor cap.
If the distributor was NOT disturbed after removing the old distributor then you should not have to set the timing. You may want to check it with a timing light just to make sure.You are done! Re-attach the negative battery cable and start it up!