As a long time technician I have seen the confused look given to me when I tell a customer that their transmission is the cause of their check engine light. They almost always ask ‘are you sure it’s the transmission” How can a transmission problem cause a check engine or a “service engine soon” light. Today’s vehicles have very sophisticated feedback systems. The pcm/ecu is constantly monitoring and adjusting numerous systems at the same time. Some of those systems are related to the engines emission control system, one component of that system is the torque converter clutch, or TCC. A tcc is used to lower fuel consumption, it also aids in lowering transmission temperature. The transmissions torque converter has a clutch built into it, buy locking the converter housing to the transmissions input shaft your ratio is 1 to 1. That is, there is no slippage in the converter. As you are varying the throttle position slightly the engine rpm stays fairly steady. In some vehicles if the throttle is returned to a closed position the pcm/ecu may disengage the tcc, some other strategies include brake switch pedal pressure or wide open throttle driving as well may disengage the tcc. Since the pcm uses the tcc to aid in fuel economy it will monitor its operation and advise the driver that there is an issue with the tcc system. This is done through the check engine light warning system.

   A faulty tcc system can cause a number of issues. A slipping tcc will cause increased fuel consumption, excessive converter temperatures, internal transmission failure due to heat stress and contamination from the effects of slippage. Some drivers may notice an increased engine speed at freeway speeds mainly. A properly functioning tcc system can cause freeway speed engine rpm to be approximately 250 to 450 rpm lower while the tcc is engaged.

   Various other transmission issues can trigger a check engine light to become illuminated. Some are gear ratio codes, or slippage codes. These codes are sometimes accompanied by harsh shifts or harsh engagements. Transmission over temp conditions can also cause a check engine light.

   Because automatic transmissions are the most technically complex systems on an automobile today, it requires transmission specialists to be highly trained in multiple areas. It will be worth your while to bring your vehicle into our transmission shop for a check engine light diagnosis. Many general auto repair shops replace components and transmissions unnecessarily because they don’t have the training or experience to diagnose transmission symptoms properly. So remember next time your check engine light comes on, bring it by our shop we will run a free fault code test to see if the check engine light is transmission related.