Chevy trailblazer abs issues.

[fordwheelinman]

Working on a Chevy trailblazer for some friends of the wifey. Abs light comes and goes along with th another light I'm not familiar with. Checked abs codes and found C0035&C0055 stored with C0035 occurring active as well.

It does have AWD. Unfortunately it also has two different size tires on it: new P245/65/17 front and LT245/70/17 out back with different tread wear side to side. Is the Chevrolet system sensitive enough to set speed sensor codes due to tire differences? Ford and jeep don't ordinarily set codes, they just constantly try to activate the AWD. I'm thinking it's all tire related and they need to replace the 4 of them with same size, same brand, same tread type before anything else is tried. Am I looking up the right skirt here or are they close enough it won't care?

 

[Fabrik8]

Yes, it's very possible that the tires sizes have everything to do with the problem. It depends on the ABS speed algorithms and how many teeth the ABS encoder (sometimes called a tone wheel) has. A higher counts-per-revolution encoder will have a more accurate speed detection and cause more problems with tire size differences, mostly because less filtering is needed to reject changes in velocity and acceleration that can cause problems with lower counts-per-revolution encoders.

Tire diameter changes aren't a great thing for AWD systems in the first place, as it can cause a lot of wear from uneven rotational velocity. That depends on the type of AWD system though, whether it is symmetrical or something more FWD biased like a Haldex-type system, whether it has open diffs, etc.

Back of the napkin calculations to compare tire sizes:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
245/65-17 6.3in 14.8in 29.5in 92.8in 683 0.0%
245/70-17 6.8in 15.3in 30.5in 95.8in 661 3.3%

It is very possible that you have a failed ABS sensor (or sensor wire) or a broken tone ring, but I would definitely agree that the tire sizes need to match around the car if it's AWD. If it's a Hall-Effect type ABS sensor, it will have a permanent magnet on the end that can collect metal dust and cause false readings as well, kind of like how a magnetic drain plug collects metal debris from an oil pan. If the sensor can be removed, it can be cleaned. A lot of the GM light truck stuff has the hub and sensor as one integrated assembly, so I don't know how serviceable it is.

Do you know if the hub with the faulty sensor has been replaced because of wheel bearing problems, etc? Cheap aftermarket replacement hubs sometimes have crappy quality encoder rings or sensors. It's another reason not to buy the cheapest part possible, especially if you have a unit bearing assembly with a sensor inside.

Rust buildup can also shift things around and cause sensor problems.

 

[BLarkin]

Those TrailBlazers are notorious for getting corrosion around the ABS sensor and reluctor ring that's built into the wheel bearing. If putting the correct size tires on doesn't fix it, i would look into that. Sometimes you can clean the sensors, the mounting surface on the wheel bearing, and the reuctor ring itself, and that will fix it. If not, you're looking at replacing the wheel bearing assembly (and they come with new sensors, usually).

Shameless plug - if you end up needing front ABS sensors, I have two new ones from Napa. I put sensors on, and that didn't fix it. Ended up having to put wheel bearings on it. They list for $70 each, I'd take $50 for both, and I'll ship them to you if needed.

 

[CThurmond]

I had customers put two new tires same size on them and have problems till they put 2 more new tires on so the tread depths was the same. So Im sure that tire size would cause this .

 

[NC_IslandRunner]

One tire different has made my cruise control obsolete before, I've never ran different size tires before, other than when my spare was smaller. I've had tires with different tread wear but they were the same on each axle just different front to back.

I think someone on offroadtb.com had this problem recently, you could search that forum and likely find the answer.