My TPMS is toying with me

So yesterday morning, I notice as I park at work that the tire pressure light is on. I get out and none of the tires look visibly low. There was a pretty sharp drop in temperature here yesterday, down to the 50s, and considering the last time I aired the tires was probably July/August, I figure Boyle has been at work lowering the pressure some. No surprise there.

After work, I air up the tires. Not by a lot, a couple psi. Not ordinarily enough to trigger the warning light, but I like using the TPMS for early warning, so when I initialized it, I started with the tires overinflated, then let them down to my usual pressures after the TPMS memorized the higher pressure. So it does end up turning the light on just a couple psi under what I normally run, which is the way I like it.

The light doesn't immediately go out after I add the air. But in the past it never has either, there's always a little delay. So I take a little drive, grab a bite to eat, drive home. Warning light never goes out.

Oh, swell, I figure. Car is seven years old. Probably losing a battery in one or more of the senders. Have to break out the laptop and see which one it is, and schedule with a tire shop to change it.

So this morning I plug in and pull up the TPMS screen, and it looks perfectly happy. All four pressures are just where I set them yesterday, all four temperatures look reasonable, all four battery voltages read "over". All the alarm thresholds are 34.1 psi, which is about where they end up if you push the Set button with the tires pumped to 42. So everything looks fine. But I disconnect, start up the car, and sure enough, the warning light is still on.

So I connect again, choose Utility from the TPMS screen, and try out the Signal Check utility. It asks if I want to check all signals or just one. I figure I'll see what it does if I pick "one". That takes a few seconds and pops up an error dialog (not everything seems to quite work with every mini VCI), but leaves the tire light blinking. So I cancel, exit, and disconnect, the warning light goes out, and when I start up the car, it is all happy again. No tire warning.

Curious, I connect in one last time just to see if that "Signal Check" had changed any settings, but it hadn't; all the pressure/temp/voltage readings and alarm thresholds were just as before. I guess all it did was goose the TPMS to reconsider its inputs sooner than it was otherwise going to.

First time I've ever experienced that....

-Chap

 

I did. I meant to say so in the post.

Well, I topped off the tires, in the cool of yesterday evening, to 38 and 36, using a nice 3-inch gauge that's easy to read in 1 psi markings.

In today's early morning sun, the TPM sensors had them at 38.1 and 36.1. I think the sensors are pretty decent.

That is the funny thing. Of course they don't give you any direct way to set the warning threshold; it's just "fill the tires to what pressure you want, then hold the button, and it will set the warning threshold to something below that." But it seems to set for something like 8 psi below the pressure you filled to. When I set mine, I had filled the tires to 42, just so I could get the warning set for 34. Then I back the tires down to 38 and 36, so I have a bit more sensitive warning. If you just push the button with the tires at the factory pressure, it sets the warning for something in the 20s. :O

If I were in the 42/40 crowd, I'm not sure what I'd do, though you surely can inflate cool, stationary tires above 42, for purposes of pushing the button, as long as you back off the pressure before driving them much.

All speculation for me, as I'm not in the 42/40 crowd. I still have a few old silver fillings that haven't been replaced.

-Chap

 

I can no longer write it off as a fluke; the same thing has happened a couple more times.

I am pretty sure it is because I deliberately set the warning thresholds high, so I will get early warnings. I like early warnings, and I think I'll keep it that way. But I think the ECU is programmed so that, once it turns the light on, it wants to see the pressures topped up several psi higher before it believes you've actually taken action.

Since my early warning thresholds mean I only add a couple psi when the light goes on, it typically doesn't go back out by itself, for me. (One time I thought it did, the following day, but that day was maybe hot enough to raise the pressures above its 'high' threshold.)

But what does seem to work, every time, is to put a couple psi in, plug in Techstream, and run that TPMS "signal check" utility. That seems to make the ECU say "oh, hey, looky here, the pressures are above the threshold after all!" and boink, the light goes out.

A little bit cumbersome. I wouldn't mind figuring out what signals Techstream is sending the TPMS ECU from that utility, and programming them into the ScanGauge or something.

-Chap

 

Still looking. That Techstream utility even says straight out, "the TS pin will be grounded during this procedure", so last night I tried the resurrected TC/TS twiddler, but whatever Techstream is doing, it's more than simply grounding TS, because that didn't make the light go out.



There are some procedures that involve pulses on TS. We know four pulses on TS will clear the ABS linear solenoid calibration (as I, ah, reconfirmed last night, then had to go through that calibration again ), or TS in combination with something else, but I still haven't found the magic twiddle for the TPMS signal check.

Finally hauled out Techstream and ran the TPMS signal check utility again, and boink, the light went out. Now I don't get to try again until the next time it says the tires need air. I'd love to figure it out, because a twiddler is a lot less to carry around than a whole Techstream rig.

-Chap