Low tire pressure light

On “Car Talk” Tom and Ray have explained how the low tire pressure warning light is actually the ABS system, which checks the rotation speeds of the wheels. If one wheel is turning more rapidly than the others, then that indicates the tire pressure on that wheel is low.

We have a 2000 Alero. Last spring, the low tire pressure warning light started coming on fairly regularly. Every time it came on, we got out to check the tire pressures; all were fine, so then my husband reset it, only to have it come on again. In December we were getting our winter tires put on, and the mechanic noticed that one of our front brake calipers was sticking, so he replaced it. He insisted that the stuck caliper had nothing to do with the low tire pressure light. Later that week, we took the car in to a different mechanic for an alignment, and that he noticed that the other front brake caliper was sticking, so he replaced it. Since then, the low tire pressure light has not come back on.

So, was the first mechanic wrong, and the stuck calipers were causing the low tire pressure light to come on? I would think that a stuck caliper might slow the tire rotation at intervals, causing the same response from the ABS system as low tire pressure (creating faster rotations) would. In both cases, the affected tires are moving at a different speed than the rest. Correct?

Correct…But most tire pressure warning systems work off pressure sensors not the ABS …Some do depend on the ABS signals to spot a low tire…

Googling the Oldsmobile Alero, it comes up as having indirect TPMS, the one derived off the abs wheel sensors readings.

In both cases, maybe the mechanic adjusted your tire pressure. When the weather changes, owners should then check tire pressure. What they don’t realize, is that something as simple as parking a car with one side in the sunlight can vary the pressure enough to set them off. When you reset them then, you are asking them to come on when they agree. I would check them and wait till you have driven a while and moderated the temperatures before resetting them. Nothing compares with a tire pressure gauge and frequent checking.

BTW, mechanics have a habit of noticing “sticking” pistons. If I don’t see excessive pad wear, or feel the excessive drag myself, we’ll wait on the brake job.

Re: adjusting the tire pressure, I don’t think so. Hubby is very good about checking the tire pressure on a weekly basis, regardless of the weather. He keeps the tires filled according to the pressure listed on the door panel (unfortunately, we notice that all too often, when a mechanic changes our winter tires to all-season, or vice versa, they inflate the tires too high). Anyway, I have a very hard time believing that that would be the solution, since the tire pressures are regularly maintained.

And yes, RemcoW, our Alero does take the tire pressure readings from the ABS wheel sensors.

I know that the tire pressure warning system on my Toyota is monitored by tire pressure monitors in each of the wheels. It’s combination valve-stem and tire pressure switch. It sends out a signal to the monitor system what the pressure is for the wheel it’s in. The light will go off it the pressure drops more then 5psi from the recommended psi. In my case the recommended psi is 32…so the warning light will go off if one of the switches reads below 37psi.

MikeInNH: I’m puzzled; if your recommended PSI is 32, then why would the light go off if it’s below 37? wouldn’t it go off if it’s below 27? (Maybe that was a typo.)

At any rate, to my knowledge none of our tire pressures have ever gone that low, and certainly not while the low tire pressure light was coming on at such a regular rate. Hubby was really paying attention to the tire pressures then, and they were all at the recommended rate.

Yup…it was a typo…should be 27

Life was once so simple…

Those tire pressure monitors Mike has are actually stems that send telemetry up to the ECM by means of RF. Not positive whether they actually send the PSI as a number or whether they have a trip point and raise an alarm but I do know that the ones used on our Legacy also trip at too high of a pressure.

I think that the tire pressure warning system that works off a tire pressure system is superior to the early ones that worked off the ABS. I had the tire pressure light come on on my 2011 Toyota Sienna. I found one tire that was below 27 psi. I inflated the tire, but did find a nail in the tire, so I had it repaired.
Some years back, I was returning from a convention about 700 miles away in a Ford Windstar that had a tire pressure warning system that worked off the ABS. I was traveling on a section of interstate highway that been milled and had only one lane open in my direction. The pavement set off the tire pressure warning light. This was 2:00 a.m. in the morning and there was no place to get off so I just kept driving until we got out of the road construction. I pulled into a truck stop for some coffee and checked the tires and didn’t see a problem. Fortunately, my institution had kept the manual in the glove compartment (it was the university’s vehicle) and we found out how to reset the light. It didn’t come on for the rest of the trip.

Wrong, Remco. I have the same system Mike has, and there are no pressure sensors. Only wheel speed sensors.

To the OP, it might help to know that many vehicles, especially those of a few years ago (like yours and mine), use the wheel speed sensors and compare the soeeds. Many other vehicles use pressure sensors.

On those that use wheel speed sensors, there should be an “initialization procedure” in the owner’s manual to establish a new “baseline” whenever tires are replaced or pressures intentionally changed.

Wrong, Remco. I have the same system Mike has, and there are no pressure sensors. Only wheel speed sensors.

How does the sensor detect when the spare is low???

Not that it is important to be right or wrong but there are direct and indirect methods. Some cars have indeed pressure sensors (direct) that send the condition up - my legacy is definitely ‘direct’. The stem assembly is different and larger. I know because it had to be replaced two weeks in of owning it. It has a battery that eventually goes dead and the entire assembly needs to be replaced.

The indirect systems derive it off the wheel speed sensors. It measures tiny speed differences between wheels and infers pressure drops from it.

Mike mentions that his uses pressure sensors so I assumed it is direct. If it isn’t, well, then it isn’t.

I reread Mike’s post, and you’re absolutely correct. Not only was I in error, but it was an inexcusable error borne from lack of attention to detail. I’d claim it was a “senior moment”, but it wasn’t…it was just a dumb error.

You have my sincere apologies. Thanks for your understanding.

No apologies necessary. Those moments are getting more and more frequent with me as well.
When I was young, I had all the answers.
Now that I’m old, they’ve changed all the questions.

Mountainbike: I believe that hubby has typically had to reset that sensor whenever we have our tires replaced. Seems like the guys always forget to do it. But then we usually don’t have the tire pressure warning light coming back on.

When we moved to N.S. in March, and had just had our winter tires replaced with new all-season tires for the trip, that’s when the light first came on. Hubby checked the tire pressures, then remembered this happening whenever we get new tires/tires changed, so he figured that’s all it was. But it continued on throughout the summer, and when we took it in to have our winter tires put on, that’s when the mechanic noticed the stuck brake caliper.

This mechanic doesn’t do alignments, and an alignment was necessary after he installed the new brake caliper, so when we took it to another mechanic for an alignment, he noticed that the other brake caliper was stuck. So then we had that one replaced. In the meantime, the low tire pressure light isn’t coming back on like it was before.