2004 Toyota Prius Long-Term Road Test


2004 Toyota Prius: ESC Freak Out!

September 25, 2010

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Over the last couple of days our 2004 Toyota Prius has sent out numerous Electronic Skid Control false alarms. Light flash, brakes sometimes grab to "correct" a non-existent slide and, most of all, a piercing piezo beep fills the cabin.

The first time it happened I has sweeping through an on-ramp. Was I going that fast? Not really. Did I hit a bump, or something? Probably. I didn't think much of it.

Next morning, my wife grabbed the keys to take the kids to school, 10 miles distant. She called back in 5 minutes.

"Get the van ready," she said. "I'm bringing this thing back. The skid control is going nuts."

One of the places she said it happened was the road above, not 200 yards from our home. There is nothing in the way of G forces here, just a gentle bend to the right as you coast slightly downhill to the stop sign ahead. It happens every time, exactly where this picture was taken, so long as the speed is 33 mph or higher. The posted limit is 35 mph.

And then I remembered some things that might add up to a theory...

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When I first drove the Prius home from the office, I noticed was off to one side--way off. Ten degrees, at least. Could be alignment, could be excess "conicity" in the well-worn tires, could be lots of things.

Here's another piece of information: the ESC false alarms only occur when turning right. I proved that to myself with some lane change maneuvers on a deserted road. There are no problems turning left.

Here's how these two facts add up to an early ESC warning. Let's say my local bend in the road requires 70 degrees of steering input. The ESC computer actually measures this using a steering angle sensor deep in the steering column. It compares the measured steering angle to the cornering G measured in another sensor to calculate the degree of understeer or oversteer.

But if the wheel is at minus 10 when I'm going strainght, my 70-degree right input will measure only 60 degrees on the steering angle sensor. The computer could be comparing this to the lateral G being produced and say to itself "That's not enough steering input for this amount of lateral G. The car must be oversteering. I'd better DO something."

Beep! Beep! Beep! Beep! <brake grab> Beep! Beep! Beep!

Now, you're probably think about left turns of the same magnitude. The computer would see 80 degrees, and think this was more understeer than usual. Well, oversteer is the more heinous situation, (to an ESC computer, at least). It will be much more sensitive to oversteer.

Beyond that, I'd like to think such systems would be smart enough to "learn" where straight ahead is and compensate for worn tires or bad wheel alignment. Maybe some do and some don't. The technology was still in its infancy when this particular car was originally in development.

For grins, I looked underneath for any signs of damage. Was there a rock strike? Is something bent. Nope. Everything looks just fine under there.

Damage can't explain it. The steering wheel is off center for some other reason. Here's my plan:

First: I'm going to install new tires. One of them is bad anyway, but it's a rear, and rear tires don't cause pull or steering wheel misalignments. Besides, the front ones are worn, too, and worn oddly. There could be excess conicity at work, enough to create sufficient right-hand force to make me have to steer left to go straight.

If these were good tires, I'd simply swap the front tires left-to-right to redistribute the conicity so it acts in the opposite direction. If conicty was the culprit, I'd see a change at the steering wheel. But since these tires are worn, I'm going straight to new ones.

Second: If the steering wheel is still "off" to one side after the new tires, I'll adjust the toe-in at the tie rod ends to recenter the steering wheel.

Third: There is no third. If these two steps don't work, my theory is wrong and I'll have to take this Prius to the dealer.

But I think I'm right. We'll all soon see after I get back from the tire store.

Dan Edmunds, Director of Vehicle Testing @ 84,490 miles

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