The Mysterious Push Start

Feb 12, 2022

It's time for the new puzzler. This is a real life puzzler. A customer came in with an old car.

I don't remember the make or year of the car or the customer's name, or even if it really happened, the names and places have been changed to protect the innocent. This fellow came in with an older BMW something like a '74 or '75 2002.

And he said, "Gee, I have this problem. I go out in the morning to crank the thing to start it up. And I crank it. And it goes RhuhuRhuhRRHHuhrrHuRRHHuh but it doesn't start.

And he says, "However, I sometimes have the advantage of parking on a very slight hill. And I roll the thing three feet down the hill with the key on, and I pop the clutch and it starts right up. He said, "And as long as I do that, it seems to run pretty decently, and but it's getting worse and worse. And I don't know what it is and I want you to figure it out." So he leaves the car and off he goes.

And there are no hills at the garage so we pushed it in. And one of the guys at the garages says to me, "Gee, what do you think it is?" I said "Well, must be cranking too slowly. That's why it doesn't start!"

So he goes and checks it out and turns the key and it cranks over great. He says, "Ah, I know what it is. Must be low compression." Sure. So he takes the plugs out to see if they're wet and does a compression test. No. That's another Mickey Mouse answer. "Low compression my foot," he says!

The compression is great. And it's cranking fast enough. And yet a push start starts it but a crank start won't. So what's going on?



This puzzler puzzled us so we figured we'd puzzle everyone else with it. What was happening here is this guy's points were burnt up. And his points have burned up such that when he had 12 volts available to the points which is what he would have if he pushed started the car with the key on--

It's a little-known fact that when you turn the key to the start position, you don't get 12 volts to the ignition. That's because you get a voltage drop immediately because the starter is drawing some of that voltage.

And so you get lower voltage to the ignition, so you might have 10 volts available. And 10 volts isn't enough to cut the mustard, so to speak.

Whereas when you're rolling, rolling, rolling, rolling, you're not using the starter motor, you're not getting the voltage drop from the battery. You're getting the full 12 volts of the ignition system and boom, the thing starts up. You know what he needed to fix this car? New points.

Even if your old car doesn't start, it can start conversations that will help all of us learn more about the world. Donate your old car to your favorite NPR station. Here's how


Get the Car Talk Newsletter