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September 18, 2020
Paul Gipe

Tire Pressure Monitoring System: What is it and What EV Drivers Need to Know


Since 2008 new cars in the US include a Tire Pressure Monitoring System (TPMS). The monitoring system includes a sensor in each wheel that sends a wireless signal to the car's computer. The computer monitors tire pressure and either reports a pressure measurement, a warning of low pressure, or both to the dashboard display.

All EVs have some form of TPMS. Our Bolt and previously our Volt had a TPMS that reported actual tire pressure on the dashboard display. You have to toggle between various screens, but there is a screen devoted to tire pressure that shows a picture of the car in plan view and the pressure for each tire and its location on the car.

 

Our base model 2015 Leaf didn't report tire pressure on the dash directly. It simply told you when tire pressure was low. I set up Leaf Spy to read the diagnostics from the Leaf's computer. Leaf Spy reported actual tire pressure for each tire. This was helpful in keeping the tires properly inflated for hypermiling in the limited-range Leaf. (That is, the car was capable of reporting tire pressure in each tire but it didn't do so in the base model. You had to buy the more expensive models to get this feature.)

This is all well and good when the TPMS works as it should. Unfortunately, that's not always the case.

Why Resetting the TPMS Matters

When you take your car in to have a mechanic rotate the tires, they should reset the TMPS so that the TPMS reports the actual physical location of the tire on the car. Then, if you see that the right rear tire has low pressure, you know to inflate the right rear tire.

I've learned the hard way that not all mechanics do their job the way it is described in the books. They either forget to reset the TPMS, don't have time, or don't know how to do so for your particular vehicle.

This has happened twice in the three years we've leased our Bolt. The first time was with the GM-trained mechanic at our Chevy dealer. After a tire rotation, I found the tire pressure for tires on the front of the car were now reading on the dash as on the back. Ok, I could live with that.

However, toward the end of the lease we replaced the tires. When I picked up the car from our long-time tire dealer I noticed that the tire pressure page was displayed on the dash. Good, I thought. They reset the TPMS as they should.

No they hadn't, but I didn't learn this right away.

A few weeks later the dash display said a front tire was down in pressure. I pumped it up by hand. That didn't seem to do the trick. Hmm. I pumped up the rear tire on that side. That didn't seem to work either. I took out my trusty pencil tire gauge and measured both tires. Pressure was where I wanted it on both so I figured one of the sensors must be bad.

Wrong. And no, I didn't do the common sense thing to do and measure all four tires. Getting a bit lazy I guess with all this technology.

The next day the dash flashed a low tire pressure warning. Ok, I pumped up the left rear tire again. The dash still displayed a warning but the tire pressure gauge said the pressure was fine.

Off we went on one of our Covid-19 escapes from Bakersfield. We were only five miles from home when the handling just didn't feel right and the TPMS display was now on all the time. That should have been a hint.

I pulled over at the next exit and finally did a walk around. Yep, there it was a nearly flat right rear tire.

I nursed the car back home, pumped up the tire with the hand pump. (This time I had Nancy help me as that's a lot of air to put in a tire with hand pump.)

Naturally, I had to drive twenty minutes across the city to get the special Michelin tire fixed by someone who knew how to do it. (See Chevy Bolt Self-Sealing Tires Work But More Difficult to Repair.) But they did and we were back on the road by mid-morning on our adventure to Oak Flat.

By now I was convinced that I couldn't rely on others to reset the TPMS. I had to do it myself.

Resetting the TPMS

Resetting the TPMS isn't difficult, but it does require a special tool and procedure. The Bolt manual describes the TPMS on page 279 and on page 280 describes the "TPMS sensor matching process."

There's a lot on line about how to do it and what kind of tool you need. There's a hokey YouTube video by ACDelco on TPM systems but it provides good background information. (See Understanding the Tire Pressure Monitoring System (TPMS) | ACDelco Garage.) There several threads about TPMS on the forum ChevyBolt.org. Search on Google for site:www.chevybolt.org tpms for links.

Initially, I'd been put off on buying a special tool just to reset the pressure sensors and especially when I saw the prices for the types of tool used by professional mechanics. Now there a lot of cheap Chinese special purpose tools designed just for this one function. I found one online and with express delivery it was about $20.

For our Bolt, and Chevy's in general, you begin the resetting process by opening the tire pressure screen. Then you hold down the check mark on the right side of the steering wheel that selects various functions on the Drivers Information Center (DIC) until the DIC changes screens. The DIC screen now asks you if you want to enter the "relearn" process. Of course you do, so press yes. The horn will beep twice telling you that you've entered the relearn or resetting process. You now have two minutes. The car's marker lights will tell which tire you need to reset.

Beginning left front you hold the relearn tool on the sidewall near the tire valve and press the resetting button on the tool until you hear another beep. The marker light on the right front will light up and you repeat the resetting process until you hear another beep. Then you're off to the right rear and repeat. Finally you reach the left rear and the repeat the procedure for the last time. The car will now beep twice, signaling the end of the process. Each tire pressure sensor should now match what's displayed on the car's tire pressure screen.

TPMS and Altitude

Many drivers here in California note that they receive a low-pressure warning when they drive into the Sierra Nevada. It's happened to us several times and I've come to expect it. I attributed this phenomenon to altitude and the change in air pressure. That's not the case. Instead, it's due to decreased temperature with altitude.

I use a rule of thumb that temperature decreases from 2.5 F° to 3.5 F° with every increase in elevation of 1,000 feet. For us, driving from Bakersfield to Horseshoe Meadows at 10,000 feet is nearly a 10,000-foot increase in elevation or a decrease in temperature from 25 F° to 35 F°.

Air pressure decreases with temperature. A 25 F° to 35 F° drop in temperature will play havoc with your TPMS, dropping tire pressure by as much as 3 psi. So if you're at the recommended tire pressure at sea level and you drive to Horseshoe Meadows you'll receive a low-pressure warning on the dash. The pressure will return to normal when you drive back down to sea level.


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