Broken Distributor Cap

Dec 13, 1996

RAY: A customer came in the the shop with a car that didn't run and he said, "I went out to start my car the other morning. I turned the key and it cranked but it wouldn't start." I said, "Did you open the hood," and he said, "Of course not. I just called the tow truck and the car should be pulling up here any minute."

Sure enough, the tow truck arrives and we turn the key and the car cranks and it sounds good. It sounds like there's compression in the cylinders and we can smell gasoline even. And we open the hood and see that the distributor cap is broken in a million pieces. Smashed to smithereens. So here's the distributor cap with pieces stuck to the wires. The spark plug wires are still attached to it for the most part, but it is broken beyond recognition you might say. I said, "Well, maybe there was a crack in the cap and it finally let go, maybe the rotor hit it, maybe there's something wrong with the rotor and it spun around and hit it."

TOM: Gee, those are all very unlikely things, but possible.

RAY: In any event, we put a new cap and rotor on this thing.

TOM: Let me guess. It blew up.

RAY: It starts right up and runs great. We park the car outside. We make sure everything's okay, drive it around. At the end of the day, the customer comes by to pick up his car, he pays the bill...after a lot of complaining about the $400 we charged him... and he goes out to get his car and comes back 5 minutes later and says," What is this, some kind of joke?" And I go out and look under the hood and notice the new cap is broken in a similar fashion. What was happening here? I'll give you a hint. It would happen to a car of yesteryear much more readily than it would happen to cars of now-a-days, and there are some cars today that it couldn't happen to at all because they don't have distributor caps or rotors.

TOM: Would you care to say what kind of car it was?

RAY: There are a lot of different cars it could be, but I think this one was a Honda -- an older Honda, 80's vintage.


RAY: Here's the answer: This wouldn't have happened to older cars or even a newer car with a distributor cap, because they are missing a piece that older cars have. That is a vacuum advance. The vacuum advance is simply a vacuum operated device which contained a diaphragm and a rod, and when the engine vacuum was high it would pull the plate and advance the timing.

TOM: An elegant little device I might say.

RAY: It's rather crude. It's all done electronically now.

TOM: Well, compared to electronics. But it was elegant in that you could see something happen.

RAY: Well, it's analogous to beating on drums, but crude compared to a telephone call.

TOM: Or email.

RAY: And the vacuum advance diaphragm had a little hole in it. So that it really didn't work, but it also didn't affect the performance of the car all that much.

TOM: Don't forget where it's getting the vacuum from -- the manifold.

RAY: And when the thing was shut off...

TOM: What's in the manifold?

RAY: A mixture of gasoline and air would waft into the vacuum advance, penetrate the diaphragm, and get on the other side of the diaphragm, where it would be in the area of the cap and the rotor. Where there are sparks of high voltage! And when that mixture of gas and air - and you turn the key, that first spark exploded the cap.

TOM: Badabing, badabang!

RAY: So he needs another 800 bucks for the vacuum advance.

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