Electronic Control Unit

Originally, automobiles depended on basic mechanical and hydraulic systems to operate. However, the demand for better fuel economy in the ’70s and the need for increased emission controls of the ’80s saw the development of onboard computer systems to monitor and control the various functions of the automobile. Where these early systems had one or two units, today’s modern automobiles have hundreds of electronic modules controlling the performance of the vehicle! Increased electronic complexity of transmissions is a trend that is here to stay.

The automatic transmission is the focal point for manufacturers seeking to improve fuel economy. Electronically controlled transmissions and more gears allowed engines to operate at lower RPMs and thus achieve the goal of better gas mileage. But what does it mean that a transmission is electronically controlled? You likely only care when something is going wrong!

When you are experiencing problems with your transmission, you might hear several acronyms relating to these systems. What is the difference between an ECU, ECM, TCM, PCM, or TEHCM? What is a Mechatronic system? It’s confusing but we are going to explore what each of these means, how they perform, and look at examples of vehicles known for having problems with these units. Knowing how your car, truck or SUV functions and what issues you need to pay attention to, can save you a lot of money in the long run!

What Do All Those Letters Mean?

ECU: Electronic Control Unit. This is a generic term for any computer-controlled module that monitors and controls a system in your vehicle.

ECM: Engine Control Module. An ECM is tasked with controlling the functions of the engine. This includes the fuel and air mixture and flow, spark plug timing, and camshaft position.

TCM: Transmission Control Module. A TCM monitors and controls the transmission. It is responsible for controlling the pressure and temperature inside of the transmission and the points where the vehicle shifts, in order to maximize performance, ride quality, and fuel economy.

PCM: Powertrain Control Module. Essentially, a PCM is an ECM and TCM contained inside of a single housing. The engine and transmission are monitored and controlled by separate circuits, but all are contained within a single unit. Most manufacturers have gone to this type of setup to reduce SKU’s in inventory and save space.

TEHCM: Transmission Electro-Hydraulic Control Module. General Motors developed this unit for their 6 speed automatic transmissions. Essentially it combines the TCM, valve body solenoids, and pressure and temperature sensors into one unit.

Mechatronics: Found in many modern European automobiles with automatic transmissions, the Mechatronic unit is essentially (like the GM TEHCM) the transmission control unit, valve body, and all solenoids combined into a single unit.

As vehicles have become more sophisticated and dependent on computer control, manufacturers have increasingly bundled a variety of electronic controls within a single unit. Of course, the cost of these units has risen as a result!

Common Problems of Electronic Control Units

With additional complexity comes the opportunity for things to malfunction. Many times what may seem to be a transmission problem is actually a faulty sensor or control module. In some cases, the repair might involve the replacement of a relatively inexpensive part. In other cases, a small problem with a sensor or solenoid might involve the replacement of an entire unit like the TEHCM or Mechatronic unit, both of which are located inside of the transmission itself and will often require pulling the transmission out in order to replace the unit. Over the last 20 years, some vehicles have become known for having problems with their electronic control systems.

The 2001-2004 Toyota Rav4 is known for having problems with their ECM’s. The failure of the Toyota control unit to communicate proper shift commands causes excessive heat and premature wear, which leads to harsh or erratic shifting, occasional jumping between gears, poor acceleration and even transmission failures if left unchecked for too long. Unfortunately, Toyota transmission repair can get very expensive, especially if both the ECM and transmission itself requires replacement.

The 2005-2006 Jeep Wranglers have well-known problems with their PCM’s. Owners have complained of stalling, engine misfires, and even false error codes as a result of a bad PCM. As these vehicles get older, finding replacement PCM’s has become more difficult. Dealers quit carrying them and aftermarket companies don’t always offer alternatives. Buying a used PCM also presents a challenge, as there is no way to know for sure whether or not the unit works until you put it into your vehicle. Also, many of these units require reprogramming which adds cost to a unit that may or may not work once installed! Jeep transmission repair has added challenges due to electrical wiring issues and ownership changes that have led to some German ZF or Weistec (Mercedes) manufactured transmissions being installed as original equipment in popular makes such as the Grand Cherokee.

The TEHCM unit that GM uses in their 6T70 and 6T75 transmissions for front wheel drive vehicles are also known for having issues. In this case, there are 4 pressure solenoids that are housed within the TEHCM itself. These pressure solenoids can become damaged by debris in the transmission fluid and start to malfunction. In older transmission designs, replacing a solenoid is a relatively inexpensive repair. Unfortunately, with this newer design, it is necessary to replace the entire TEHCM and will result in a repair rapidly exceeding $1,500.

A Great New World

It is evident cars, trucks and SUVs have gone through significant changes over the years. For roughly the first 100 years of automotive technology, basic mechanical and hydraulic gears and switches controlled how things operated. Starting in the late 70’s, the need for better fuel economy, cleaner emissions, and continued evolution of driving features have led to modern automobiles being almost completely controlled by computer. Each individual system in your vehicle is monitored by sensors communicating with an onboard computer, and that computer in turn controls those systems through various control modules. Your transmission, differentials and transfer case (in the case of a 4 wheel or all wheel drive) are all controlled in some fashion by one or several control modules. And when these driveline components start having issues, it is often one of these electronic components that is to blame!

Transmission Rebuild & Repair Experts

The Advanced Transmission Center team consist of technicians who have decades of experience diagnosing, rebuilding and repairing automatic transmissions in domestic and import vehicles. We stay current with changes and updates to electronic control systems and have the ability to test and diagnose any problems you may have. We have years of experience replacing and reprogramming these electronic control units and use only quality parts in our repairs.

If you are having problems with your transmission or drivetrain, contact Advanced Transmission Center at either of our locations and we’d be happy to help! Unlike dealerships or many independent repair shops, we are transmission specialists trained to fix issues related to a vehicle drivetrain. Call our Westminster (Northwest Denver) location at (303) 421-4140, our Lakewood (Southwest Denver) location at (303) 922-4102 or contact us online to get started today! You can reach out to either location that is most convenient for you.

Please give us a call or send us a message ASAP. We look forward to serving your vehicle drivetrain and transmission needs. Over 35 years, our goal remains to be “Geared for Customer Satisfaction!”

Written by Advanced Transmission Center