Smog Through Brakes?

Hello 93636,
I had an odd smog repair last month so I thought it would be useful in discussing vehicle communications networks and modules. A module is a microprocessor (more than 50 in some modern vehicles) and they communicate via an onboard network to share data. My new customer inherited a 97 BMW 540I and asked us to evaluate its condition and perform a Smog Inspection for transfer of title. The customer’s priorities were correct: SMOG FIRST. My advice is don’t spend money fixing a vehicle that won’t pass a smog inspection; you can’t drive it.

This BMW’s “Check Engine Light” was on. That’s an automatic smog failure. The “Traction Control” light (TC) was also on but since it’s not emissions related, OR SO I THOUGHT, I put it on hold for now. Starting with the “Check Engine Light,” I connected a scanner and retrieved emission code P0345 (Camshaft Sensor Fault Code). I removed the air intake ducting and traced back the camshaft sensor harness to find a good place to connect my lab scope and test. What I found was the TC connector was plugged into the Camshaft Sensor and the Camshaft Sensor connector was plugged into a TC actuator. This will be easy; I’ll just reverse the communication connection. I started the engine and the TC and emissions warning lights turned off. GREAT. I didn’t even get out of the parking lot before the Anti-Lock Brake System (ABS) warning light turned on. Since the “Check Engine Light” was still off I put the ABS issue on hold also, OR SO I THOUGHT, and continued driving the car to allow the onboard emissions self-evaluation testing to complete. During the drive the “Check Engine Light” came back on. A new code was set indicating a Vehicle Speed Sensor (VSS) fault. Now I have an ABS fault, a TC fault and a VSS fault. The more I fix, the more I need to fix.

The reason for this is when a system fails, the onboard self-evaluation testing for that system is suspended. If not, a failed component could adversely affect testing results and set a false trouble code. Complete system testing may not reoccur until the failures are repaired. If the vehicle continues to be driven without repair, over time other faults can accumulate but may not be known until the original failure is corrected and onboard testing restarts. That’s what I think happened with this car and that’s where I find myself. The VSS fault is an emission fault and is standing in the way of my payday. I need it gone. I can’t find my crystal ball right now so I don’t know where this is headed but with some hard work I won’t need it anyway. The customer is hanging in there very nicely for now. After some research, I determined VSS data is directly related to the ABS wheel speed data. Hey, I have an ABS code also. Is the emission fault and ABS fault related? Yes, as it will turn out. The ABS Module receives data from the wheel speed sensors, uses it and then sends it to all the other modules via the communication network. Some modules like the sun roof or headlamps don’t need speed sensor data and pass it along. Door modules on the other hand use wheel speed data to lock the doors above a certain speed. Other modules like ABS, TC and Powertrain Control Module (PCM – engine) use the data as part of their calculation. Now we are getting somewhere. The wheel speed sensors are the common denominator. This BMW has four, one at each wheel. Here’s how the network works:

The wheel speed sensors generate a speed signal. All four should indicate the same speed at all times. If under hard braking, for example, a wheel speed sensor indicated 0 miles per hour, the ABS Module knows that wheel is locked up and skidding. The ABS module will lower the brake pressure to that wheel until it starts spinning slowly and vehicle control is regained. The TC Module also uses wheel speed sensor data but for a different purpose. During acceleration, if the rear wheels (driving wheels) spin faster than the front wheels, the TC Module activates a component that reduces engine power until the rear and front wheel speed sensors match indicating rear wheel traction is regained. This is especially helpful on icy or slippery surfaces. The Tire Pressure Module (TPM) uses wheel speed data to detect an under-inflated tire. An under-inflated tire has a smaller circumference and therefore spins faster. If this condition occurs the “Low Tire” warning lamp may be turned on. Lastly for our conversation, the PCM uses wheel speed as vehicle speed. The vehicle speed is part of the engine load calculation and is used in determining the fuel injection calculation. Wheel speed sensors are important components that impact many systems. If your doors stop locking as you drive away, better check the brakes.

I decided the best way to solve the “Check Engine Light” problem was to solve the ABS problem. Sure enough, the ABS Module had a right rear speed sensor fault code stored. I went for a test drive to verify. After further testing, the speed sensor was communicating and its information was accurate. The ABS Module failed. My cost from the BMW dealer for a new ABS Module was $1500. WOW!!! I had to replace the ABS Module (BRAKES!) in order for this vehicle to pass a Smog Inspection. I was able to find a good used ABS Module and the car passed its smog inspection easily. All the other related systems are also function correctly and no more warning lamps.