Mar 18, 2023
An automotive puzzler for you this time! Finally. Stick to what I know best, as they say.
Here you go.
Years ago, one of my guys at the garage bought himself a brand new snap on timing light. I had one of these too, by the way. It was a great timing light, even though it was overpriced.
And this timing light, which was new at that time, had all these things built into it. Among other things, this timing light had a tachometer feature. It is very simple the way it works. You hook it up to a spark plug wire. And while it is doing the timing for you, it counts how many pulses there are, and then it counts how many times that happens in a minute. Basically, it figures that for every two revolutions of the crankshaft, the cylinder has to spark and it tells you how fast the engine is going.
So, he hooks this thing up to some GM car, don't remember which one. He hooks up this brand new expensive timing light and the thing reads that the engine RMP is 1900. 1900 RPM! And he says, "This can't be right. It's idling way too fast. It doesn't sound like it is that fast, but that's what it says... So, I guess it is."
So, he turns it down to 750. And at that time, the car stalls. And it won't run anymore.
So he thinks this new timing light is broken. So, he takes the tachometer to another car. Hooks it up, and it reads 900. And another guy uses his tachometer, and it also says 900. So he knows the new timing light is not broken.
He then takes it back to the first car, hooks it up, and it again says 1900 RPM. And he throws his hands in the air and says, "What is going on here?"
And that is the puzzler question today. What is going on here?
Okay, time for the answer to this puzzler about timing lights.
So to recap, one of the guys at our garage, years ago, bought this new timing light that had all these fancy things built into it, including a tachometer feature. So the way it works is you hook it up to one of the spark plugs and it counts how many pulses there are in a certain amount of time. It knows that the crankshaft must go through a certain amount of revolutions for all the cylinders to fire. So it figures out the revolutions per minute. It doesn't matter how many cylinders you have, all of them must fire within two revolutions of the crankshaft. So that is what is supposed to happen.
So, imagine his surprise when he hooks up this new fancy timing light to some GM car we have in the garage, and it reads at 1900. And he says, "That can't be right... that is way too fast..." But he thinks he spent all this money on this timing light, it must be right. So he turns the idle down to like 750, and then the car stalls. So, he put the tachometer on another car, with another tachometer, and they both read 900, which is what they are supposed to read, so he knows the tachometer isn't broken.
The puzzler was, what is happened here? Why does the new device say 1900 when it is obviously going about half that speed?
So, an important factor in this puzzler is the fact that it was a GM car. A very important factor. So, at that time some cars out there had what is called direct fire ignition. This GM car had this. They have no distributor. This car was a 6 cylinder car with three ignition coils. And each coil has two terminals on it. So in this case, each coil is firing twice for every turn of the crankshaft. So it is just shooting out sparks like crazy.
One of the spark plug wires that hooks to one pair of these coils, it sends a spark on the compression stroke, and it wastes a spark on the companion cylinder and then sends it on the exhaust stroke. So that's why the tachometer reads almost exactly twice what the engine is actually running at. Because those sparks are flying at twice the rate in this sort of engine that it would in an engine that does not have direct fire ignition.
Phew... long one.
