Distributor Cap and Rotor Ignition

distributor cap and rotor

Distributor Cap and Rotor

This post is for people that still have a distributor cap and rotor ignition like me instead of the more modern ignition systems such as direct ignition or coil on plug type systems. Some vehicles including my 2004 Chevrolet Blazer still use a distributor cap and rotor to fire off their spark plugs.

This type of ignition system can cause car problems like an engine misfire or shudder under load. Caps and rotors have been around for a long time and for the most part they are extremely reliable. There is no shame in having a distributor ignition.

Sure your friends may say your vehicle is old-fashioned as they point and laugh at you while you’re in the parts store buying replacement parts. When this happens remind your friends that your cap and rotor only cost $28 while their individual coil that has failed might cost $200. A direct ignition coil and module assembly could cost around 350 bucks.

Problems with cap and rotors

The reason I’m writing this article is that I continue to see a common problem that develops on vehicles that use a distributor cap and rotor ignition systems. This would be moisture and corrosion that can build up inside. Back in the early 1990s the 4.3 Liter engine was extremely susceptible to this type of corrosion.

Bosch distributor cap

Bosch Distributor Cap

Technical service bulletins were created by General Motors to make technicians aware that the vent on the base plate could become clogged and accelerate the moisture buildup and corrosion inside the distributor. Not only would this damage the cap and rotor but it could also affect the ignition module.

The module connections can turn green and even disintegrated if the problem is not addressed. The technical service bulletin informed technicians that they needed to check that the vent screen and the base of the distributor was clean and allowing moisture to be vented out.

Just about every distributor that I checked in the early 1990s was experiencing this problem. When the cap was removed it looked like it snowed inside the distributor. Often it wasn’t until the vehicle was towed in for a no start condition that the problem was identified.

More problems with rotors and distributor caps

You should also make sure the distributor cap is properly seated on the base plate and all clips and screws are tightened securely to provide a good seal to keep moisture out. It is a good habit to inspect the distributor cap and rotor and check for physical damage and also corrosion build up inside.

Other types of damage that needs to be inspected would be electrical damage from high voltage. You can check for burned metal terminals and also carbon tracking inside the distributor cap. Carbon tracking is the formation of a line of carbonized dust between distributor Terminals or between a terminal and the distributor housing. Carbon tracking indicates that high voltage electricity has found a better path over or through the plastic.

The result can be car problems such as a misfire or shudder under load due to the spark plug firing at the wrong time. You can check the outer Towers and metal terminals for defects. Although distributors are usually well protected sometimes you find cracked plastic that may warrant replacement. You should also inspect the distributor rotor for a buildup of corrosion on the tip that passes voltage to the terminals of the cap.

Although you can take an emery board to the tip of the rotor to clean it I usually recommend replacing it because a distributor rotor is only about six dollars. For more information about solving car problems and the latest posts from this auto repair blog visit the homepage from that last text link. I recently wrote a page on my other website about no starts and no spark. If you have a no start condition and are interested in learning how to test for spark take a visit.

6 thoughts on “Distributor Cap and Rotor Ignition

  1. Mark Post author

    Robert: On a 25 year old car the likely causes of a no start condition would make for 1 giant list. Cap and rotor would be on that long list. For someone in your situation I recommend following a no start diagnostic chart in a factory manual. You can get them online from All Data or a similar product. You can get a 1 month subscription for $12.95 using this code in the coupon field (1MOTR2587). The no start diagnostic chart walks you through a logical process so you don’t waste money replacing good parts.

  2. Robert

    I have a 1987 Nissan 200SX, 2.0 L engine. I went to the car on Wednesday night to get something to eat and the engine turned over but would not stay running. What could the problem be? I checked the fuel pump and it seem to be working properly but while under the hood I noticed that the #1 spark plug wire where it goes into the distributor had a lot and I mean a not of corrosion on the spark plug wire and at the #1 spark plug wire slot on the DC. I am thinking that the distributor cap and rotor are bad and that is why the engine will not run. The battery is only about 2 weeks old. Do anyone know what the problem could be, if so let me know “in the comments below”. Thanks

  3. Mark Post author

    Tommy: Good point and thanks for including the link to the Mercruiser TSB. I liked the part about “explosion arresting ability”. The marine cap is different (I think) then the automotive one. On some years for cars it has a top mounted vent. From what I understand air was originally designed to enter the bottom vent and exit the top. As for the marine version maybe if its sealed real good no moisture gets in? I had a 4.3 mercruiser in a Four Winns and when I pulled the cap off it was corroded badly. I think sealed or vented it’s a problem (My opinion only).

  4. Mark Post author

    Eric: I really don’t know the answer to your specific question. BUT I can tell you that the bulletin I did read was about the vent screen in the base plate of the distributor would get clogged and accelerate the moisture build up problem. I have not seen the replacement/ upgraded distributor but I bet it has larger venting ports to promote better airflow and reduce moisture collection.

  5. Eric Simandl

    Hi, I have a 2000 blazer with a 4.3 that has the moisture in the distibuter cap problem. What I was curious about was that I was told that the Replacement distributers have been redesigned for this truck. So, If I were to replace the distributer with a new delco, the problem would be solved. Is this true? Or am I resigned to replaceing the cap and rotor yearly. Thanks.

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