Tire Pressure And Service Stability

[300sflyer]

Hi guys,

I am getting the service stability system light coming on intermittently, as well as the low tire pressure warning. I checked the tire pressure, and all 4 wheels are exactly where they should be.

Sounds to me like one of the sensors on the wheel bearings has gone south? I just changed both front wheel bearings about a year ago, for the second time in 2 years. The last time I put Moog bearings in it, and thought I might be done with these issues for a while!

Thanks for your help!

[ddalder]

I would have the car scanned to see if there are codes identifying a specific WSS fault. If you don't have a current indicator, there may be a history code. My guess is that you're onto something with a bad hub sensor. There have also been some intermittent problems with the rear wheel speed sensor wiring harness either at the wheels or where the WSS harness plugs into the main body harness. A code may give you a starting point.

[CMNTMXR57]

Search... We've had many threads on the service stability system.

[300sflyer]

I would have the car scanned to see if there are codes identifying a specific WSS fault. If you don't have a current indicator, there may be a history code. My guess is that you're onto something with a bad hub sensor. There have also been some intermittent problems with the rear wheel speed sensor wiring harness either at the wheels or where the WSS harness plugs into the main body harness. A code may give you a starting point.

I just did a scan, and there are no codes showing.

[00Beast]

You can't see those codes with a basic scanner. It needs a more complicated scanner, like the newer Snap-Ons or the GM Dealer TechII or their new system to read. These are body codes, likely whatever scanner you have only reads powertrain codes, MAYBE ABS codes if you dropped a few bucks. The scanners needed to read this cost into the $$Thousands$$, and why most car repairs are so expensive anymore.

[300sflyer]

You can't see those codes with a basic scanner. It needs a more complicated scanner, like the newer Snap-Ons or the GM Dealer TechII or their new system to read. These are body codes, likely whatever scanner you have only reads powertrain codes, MAYBE ABS codes if you dropped a few bucks. The scanners needed to read this cost into the $$Thousands$$, and why most car repairs are so expensive anymore.

Ah... ok. Thanks for the info.

I thought there might be a direct correlation because of the low tire pressure warning too.

[CMNTMXR57]

It has nothing to do with tire pressure. It has nothing to do with WSS unless you have an ABS problem (which would also flag too).

There are three pieces used in the stability system that require replacement when the stability system becomes inoperative and you've troubleshot it correctly. I have listed those above. In addition, the electronic brake control module (EBCM) can be a culprit too.

1) First you perform an ABS system check. If there is a fault in the ABS system, this is where the WSS comes into play. But NOT anything to do with the VSES!

2) you check that the VSES (Vehicle Stability Enhancement System) centers for roughly 30 seconds. If it doesn't, go to step 3, otherwise check all connections and wiring harnesses for intermittent connections.

3) Using your T2 (or equivalent), perform a steering wheel position sensor test. If the SWPS checks out, move on to the next step. If it doesn't, replace the SWPS.

4) Using your T2 (or equivalent), check the lateral accelerometer input voltage in the VSES. It should return about 2.5v. If it does, then the next step will be to test the yaw rate sensor. Otherwise, if it doesn't, replace the lateral accelerometer.

5) Using your T2 (or equivalent), check the yaw rate sensor. It too should return 2.5v. If it does, move on to the EBCM. If your yaw rate sensor doesn't return 2.5v, replace the yaw rate sensor.

6) Replace the EBCM. If it isn't programmed/setup properly, that could be the problem, and you'll be generating a C1248 or C1255M3 code and possible incorrect operation of the system.

No where in ANY of this, other than making sure the ABS system is funtioning correctly, is mention of the wheel speed sensor (WSS), wheel hub, etc. Why you ask?? The WSS is ONLY used for computational needs of the ABS/traction control system. NOT the Vehicle Stability Enhancement System (VSES).

I just went and looked in my Techline manual for 2004. This is EXACTLY, verbatim, from GENERAL MOTORS;

Quote:
The vehicle stability enhancement system (VSES) includes an additional level of vehicle control to the EBCM. The VSES is activated by the EBCM calculating the desired yaw rate and comparing it to the actual yaw rate input. The desired yaw rate is calculated from measured steering wheel position, vehicle speed, and lateral acceleration. The difference between the desired yaw rate and actual yaw rate is the yaw rate error, which is a measurement of oversteer or understeer. If the yaw rate error becomes too large, the EBCM will attempt to correct the vehicle's yaw motion by applying differential braking to the left or right front wheel.

The VSES activations generally occur during aggressive driving, in the turns or bumpy roads without much use of the accelerator pedal. When braking during VSES activation, the brake pedal will feel different than the ABS pedal pulsation. The brake pedal pulsates at a higher frequency during VSES activation.


Steering Wheel Position = Steering Wheel Position Sensor (SWPS) located in your steering column
Vehicle Speed = Vehicle Speed Sensor (VSS) located on your transaxle. This is NOT to be confused with your WSS out at the axles.
Lateral Acceleration = Lateral Accelerometer sensor located under the rear seat near the electrical center.
Yaw Rate = Yaw Rate Sensor located under the reat parcel shelf in the trunk.

[ddalder]

The stability control problem may well be a coincidentally occurring fault, albeit separate. As for the low tire pressure warning, I'd still be looking toward a WSS first.

[00Beast]

The tire pressure light is on because the wheel hubs help calculate that too, and if a wheel hub is reading erroneously, then it can't calculate if the tires are in sync with each other.

[bobc997615]

Have you tried a "Low Tire Pressure" reset from the main DIC? This is different than just a "reset" when the error shows up.

[300sflyer]

Have you tried a "Low Tire Pressure" reset from the main DIC? This is different than just a "reset" when the error shows up.

I reset the tire pressure warning on the DIC. It has not come back thus far.

[300sflyer]

So I finally got around to having a scan done at a local garage. They used a Snap-on scanner. Once complete, they told me that the scan didn't really tell them much, and that the service stability light could be caused by a number things. [They did not say what they were] They want to me to bring the car in to investigate it further... Should I let them do that, or find another garage?

[CMNTMXR57]

See my post above (7th post). It lays everything out that's in the system and what could be causing it.

If you feel comfortable with that garage, that they're competent enough with their MODIS unit to diagnose the things above, then fine. But if their initial scan didn't pick up anything, they probably just looked for faults (DTC's), and didn't investigate further and didn't do a great troubleshooting.

[300sflyer]

I think I need to find a new repair facility I am comfortable with.... Not an easy task these days!

[CMNTMXR57]

I was kinda hoping you'd see my subtlety in my comment. Nothing against the shop you took it too, but it just seems like they plugged in, looked for DTC's, and didn't go any further, requesting you to bring it back to pay them more to investigate further.

[300sflyer]

It took me two weeks and two phone calls, to get what I got out of them, [which amounted to nothing] so my confidence level in their abilities is not very high...

[ddalder]

There are a lot of problems associated with selecting a service provider. I don't think dealerships are (necessarily) bad, certainly not to the level of hate some people have for them. Yes, expensive. Yes, not without some problems. The advantage is that they primarily work on 'their' brand of vehicle which gives them higher exposure (potentially) to your car. They also have access to some resources that a lot of other shops don't have access to, or won't pay the necessary fees to obtain that access.

Many small shops do a lot of general maintenance on any brand/model of car. To become familiar with the complex and unique systems specific to a few makes/models, and their intricacies is difficult and seldom happens (at least in my experience). Some problems I'd simply never take anywhere but a dealership. Having said all this, there is also a reason I try and do as much of my own maintenance as I can. Nobody will ever care as much about your car and fixing it right as you will.

There is always easier money to be made than to learn all about an odd problem/vehicle system for a one-off repair.