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On 1995 Mazda Millenia: Techs Often Misled in Diagnosing "TCS Off" Warning Light, Code 82Posted 6/15/2001By Guy Dibble
Vehicle: Engines: Symptom/Problem: I have experienced a slight glitch in the engine control unit (ECU) programming on the 1995 Mazda Millenia. Technicians can often be misled when diagnosing problems related to the “TCS off” warning light, accompanied by a code 82. The TCS system malfunction code 82 can be set when the problem is really related to an ECU sensor malfunction. Code 82 is defined as malfunction in the TCS-ECU communication line. In later models, it is referred to as the torque reduction request signal. In either case, I have spent too much time trying to test the actual signal and/or the wiring between the ABS/TCS control unit and the ECU, just to come up empty. Everything tests OK and the code still keeps coming back to haunt me - so what do I do now? Going by the factory diagnostic charts and ECU terminal voltages, it is very difficult to actually find a problem. One of the difficulties is trying to test the actual signal itself, and knowing when you should see some voltage changes in the signal. The actual signal is a burst of data that the control units use to communicate with each other. You won't be able to see the signal using conventional test equipment, nor is the information available to tell you when you should see these bursts of data. Now that all that misleading testing is out of the way, let's talk about the common cause of the “TCS off” light and code 82. When the ECU is monitoring an out-of-range engine sensor input, it quits communicating with the TCS/ABS control unit. The problem with this is that the TCS/ABS control unit will set a code 82 if it doesn't get the communication from the ECU for approximately 30 to 45 seconds. Now we can start to understand the glitch with the ECU programming. If the ECU happens to be monitoring a bad sensor for longer than 30 to 45 seconds, the TCS/ABS control unit doesn't like the loss of communication and sets a code 82. In most cases, a lazy O2 sensor is the cause. Example: Driving along at a nice steady cruise, one of the O2 sensors decides to get lazy and quit switching voltage. At that time, the ECU starts to pay attention to this lazy O2 sensor and quits communicating with the TCS/ABS control unit. The problem is that the ECU has to see no activity from the O2 sensor for up to two minutes before it will flag a code for a bad O2 sensor. But it only takes a loss of communication from the ECU for 30 to 45 seconds and the TCS/ABS control unit will turn on the “TCS off” light and flag a code 82. You can see how a bad engine sensor can cause the TSC code 82 but you will get no indication from the ECU that there is a problem with a sensor input. The best way I have found to duplicate an O2 sensor problem is to drive the vehicle on an open road and put the engine into a coasting fuel-cut situation where the O2 sensor voltage will go to zero for a few seconds. Then, as you ease back onto the gas, look for one of the O2 sensors to stay low voltage. If a lazy O2 sensor is the cause of the problem, it will stay low voltage long enough for the “TCS off” light to come on. I have found that installing an updated ECU will then indicate the actual problem with the malfunctioning engine sensor before turning on the “TCS off” light - if you want to spend the money.
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