Can working on a car increase the odometer?

Dear Car Talk:

Will a car's odometer increase when being driven stationary on a mechanic's lift in a garage? – John

RAY: Let me guess, John. You took your car into a shop and it came back with 75 extra miles on the odometer.

You went to the mechanic and said, “Hey, what’s this?” And he said: “Oh, gee, my assistant, Walter, had it on the lift, listening for a noise, and then the burrito truck showed up. So he went for a burrito, then he got into an argument about carnitas versus tofu. Then, after the burrito, he spent 45 minutes in the bathroom, so an hour later he came back and there were 75 miles on the odometer.”

And your mechanic emphasized that the miles were definitely not put on by his nephew, Horace, who definitely did not drive it to an out-of-state party at Phi Kappa Barfa last night.

Well, the answer to your question is yes, the speedometer and odometer will move if a car is driven on a lift.

When the car is in Drive, the wheels are turning. And when the wheels are turning, the vehicle speed sensor is picking up a signal, and that’s what moves the speedometer and odometer.

And there are often good reasons for running the car in Drive up on the lift. If there’s a noise or vibration that only occurs when you’re driving the car, that can be the best way to figure out where it’s coming from. On the other hand, that kind of diagnostic work should rarely add more than a handful of miles to your odometer.

Think about it. If you put the car on the lift, and put it in Drive, the engine is running at idle speed. The wheels are turning lazily, at the equivalent of maybe 10 mph. At 10 mph, if you run it for six minutes, that’s a mile. More often, we’ll be trying to find a drivetrain noise that only occurs at a certain speed.

Let’s say we heard the noise at 60 mph during our road test. Then we’ll put a guy in the car while it’s up on the lift and tell him to bring it up to 60 mph while another guy is listening underneath. But even that process only takes a minute or two. And even at 60 mph, that’s two miles on the odometer.

So – including the mechanic’s test drive – if you’ve got more than 10 extra miles on the odometer after a trip to the shop, your mechanic owes you an explanation. There may be a legitimate one. It may have required several long test drives to get the problem to occur. But if you see a bunch of empty red Solo beer cups in the back, be skeptical, John.

A problem that’s hard to define

Dear Car Talk:

I have a 2011 Subaru Outback. The “icy road” sensor misreads the road all the time and cuts the power, making the car stutter. It happens most when I turn a corner and slowly accelerate out of the corner.

I got the whole sensor computer box replaced and it stopped happening for four months, but then started again! I hope you can help. – Defne

RAY: Let's start by defining a few terms, Defne.

By “icy road” sensor, you mean the traction control system. That uses the car’s ABS (anti-lock braking system) to figure out when a wheel is spinning. It brakes just that wheel, and if the wheel keeps spinning, it then reduces engine power to stop it from spinning.

By “whole sensor computer box,” we’re going to assume you mean the ABS computer. Now that I’ve correctly defined your question, I still have no idea what the answer is.

I’ll give you two educated guesses, though. The most likely guess is that one of your wheel speed sensors is faulty.

Like the ABS, the traction control system uses the wheel speed sensors to compare how fast each of the four wheels is turning. If one is suddenly turning a lot faster, the “whole sensor computer box” concludes that the wheel must be spinning, and it takes action to stop it from spinning. That should result in your ABS or traction control light coming on. And if a dashboard light is staying on, your car’s computer should be able to tell you exactly which sensor is malfunctioning.

If your warning lights have not come on, I suppose you could try testing each sensor with something called a lab scope. Or you could try replacing one wheel speed sensor at a time and seeing if the problem goes away. But that’ll cost some money.

A more remote possibility is that it’s not the traction control system at all, but a really bad CV joint. The only reason I suggest that is because of when you say the problem occurs. When CV joints go bad, they will often make a clacking noise (not unlike the ABS makes), and it tends to happen when you are accelerating out of a turn. If the CV joint is bad enough, it could even make the car seem like it’s losing power.

So, ask your mechanic to check your CV joints, just in case. And if it turns out it was a CV joint all along, ask him if he wants a good deal on a barely used “whole sensor computer box.” Good luck, Defne.

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