The Case of The Hungry Buick
While in high school Dabney really liked the Buick Specials of the early 1960s. He liked the styling, ride and handling - which was pretty decent by the standards of those days. He also liked the "forward thinking" on the part of Buick's engineers and the fact that they designed and produced a small block aluminum V8 engine. The little 215 cubic inch V8 was light, efficient and downright clever for the time; attributes that Dabney admired.
He was very happy, therefore, when he took delivery of an original, pristine 1963 Special convertible that he acquired from an estate. The little, white Buick was one of those lucky purchases wherein the buyer just happened to show up at the right time. The car's black interior was perfect and the odometer showed a total mileage of 11,500 — astounding, but verified by service records. Dabney couldn't wait to show the car to the Doctor, so as soon as he could attach the license tags he was off to do so.
"Well, what have we got here, Dabney?" said the Doctor, as Dabney drove up, "Is this as clean an old Buick Special as it looks?" "It sure is, Doc," replied the proud new owner, "let's take it for a shakedown cruise."
About an hour later the pair was cruising down the local parkway on the way back to the Doctor's house, when the engine started missing. "Looks like the points might be a little oxidized, Dabney," said the Doctor, "maybe you'd better do a tune-up on the car when you take it back to the garage."
"You're right, Doc," said Dabney, "but I'd swear that the estate executors told me the car had just been serviced and new points were installed. I'll check the records to make sure."
The next day Dabney came over to the Doctor's house and in his hand was a set of badly pitted ignition points. "Here are the points from the Buick, Doc," said Dabney. "They look pretty bad, but the service records clearly show they were installed new just a couple weeks ago. I called the shop and they verified that they put in new points."
"Look at these," said the Doctor, "they aren't really corroded like you'd see with a set of points with a lot of miles on them. Instead, they have eroded in a funny way. The metal from the stationary contact has pitted away from there and built up on the moving contact. Hmmm."
"Well, I put a new set in and the car runs great, so I guess this set was just bad from the get-go," said Dabney.
"I'm not so sure of that," said the Doctor, "let me know how the car runs now, but don't be surprised if you run in to engine missing problems again."
A couple days later the Doctor was walking past Dabney's garage and noticed him leaning over the engine bay of the Buick. "Whatcha doing," asked the Doctor.
"Doc, you wouldn't believe it," replied Dabney, "but I'm replacing the points again. The new set lasted all of about 150 miles."
"I'm not surprised," said the Doctor, "and I think I told you this might happen."
"Okay, Doc, what am I missing here?"
"What you're missing is what's making the engine miss," quipped the Doctor. "Think hard about the way the points are failing."
"Well, Doc, just as you noticed on the original set, the metal from the stationary contact is depositing on the moving contact," said Dabney. "Aside from how fast this is happening, I don't see what the big deal is."
"The big deal is this, Mr. Unaware," scolded the Doctor. "These points show that the current is flowing in the wrong direction. The polarity is reversed."
"What?" said Dabney. "How could the current flow in the wrong direction? It comes from the ignition system to the coil and then to the points. How could it flow backwards and just why is this causing the points to fail?"
"Well, let me explain a little bit about ignition points," replied the Doctor.
"Points are designed to pass current in one direction in a car's ignition system. The flow is supposed to pass through the coil to the points and then to ground. The points just act as a switch to turn the coil on and off. The condensor is in the circuit to limit the amount of current flowing each time the points come together, but it won't work if the current is flowing in the opposite direction, or reverse polarity.
"Okay, but how can the polarity be reversed," interrupted Dabney.
The Doctor gave Dabney the skunk-eye, then continued. "The polarity would be reversed if the coil weren't hooked up properly. In your ignition system the coil's "+" terminal is supposed to be connected to the wire from the ignition switch and the "-" terminal is connected to the distributor wire. If these wires are reversed the polarity of the internal windings of the coil will be reversed, creating a high current flow across the points and reversing the direction of current flow, thus taking the condensor's beneficial effects out of the circuit. The points heat up, metal vaporizes under the intense current flow and deposits on the other contact."
"So how can we know if this is happening?" asked Dabney.
"Simple," said the Doctor, "let's look at the coil terminals and see how they are wired." They did, of course, and they quickly verified that the wires were reversed on the coil's terminals. A quick rearrangement fixed the problem.
"Oh, I get your point..." said Dabney seriously.