The secret to controlling transmissions

Jan. 1, 2020
Want to know a secret? Electronically controlled transmissions on newer vehicles actually are similar to their electronically controlled engines, and often are just as easy to diagnose.

You might be surprised to see you know more about newer transmissions than you thought.

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Want to know a secret? Electronically controlled transmissions on newer vehicles actually are similar to their electronically controlled engines, and often are just as easy to diagnose.

Electronically controlled transmissions, like the engines, need accurate inputs from sensors to make decisions based on pre-programmed data, and then command outputs that provide a smooth ride. Often the engine and transmission use the same sensors to receive data, and might share a control module to make decisions.

Much like engine controllers, transmission controllers aren't that complicated to understand or diagnose. Well, that is with a bit of background knowledge and a few good diagnostic tricks. (And always referring to service information and bulletins!)

Diagnosing transmission controllers often is a matter of testing inputs and outputs for correct operation, looking for obvious problems and knowing a few common fixes. If you have spent any time at all diagnosing engine-related drivabilty problems or even just diagnosing electrical concerns on today's vehicles, diagnosing faults in a transmission control system should be easy. Just understand that it comes down to inputs, decisions and outputs.

Inputs

Just like the engine, the transmission controller relies on input from sensors to understand what's going on and what needs to be done. Transmission controller inputs vary between models and vehicles, but most transmission controllers consistently need to see a few critical things to operate correctly, regardless of specific vehicle or model.

Typically, these transmission controller inputs include vehicle speed, temperature, throttle position, selected gear (from the range switch) and the various switches (such as brake pedal input) and transmission component speed sensors inputs that can be viewed on the scan tool.

Looking up how each system operates to safely perform diagnostic tests is absolutely essential, as with any electronic system, because one diagnosis does not fit all where transmission controllers are concerned. But as you read through service information, you'll find that input signals typically are just voltage signals that need to be really accurate — not just kind of accurate. The small tolerances often cause problems.

Anything that interferes with the system voltage can cause big problems, even if it's only a little bit of variation. On some vehicles, a difference of half a volt is enough to affect the system. The source voltage at the battery and at the controller needs to be within specification or the controller can't do its job properly. A 10-second check of battery voltage can prevent hours of headaches.

Vehicle Speed

Vehicle speed is a critical input — the controller really needs to know how fast it's going so it can make a safe and responsible shift selection. This information might come from the wheel speed sensors on each wheel or from a dedicated speed sensor on the transmission itself.
Keep in mind that the old diagnostic methods (from as little as two years ago) might not work anymore with all speed sensors. Because speed sensors aren't always the familiar two-wire AC signal generators, the usual resistance test with an ohmmeter might not work on a given vehicle. Refer to the repair manuals, but you might find that the best way to initially check this input is to road test the vehicle and view the data on a scan tool to see if it's there, if the sensors' data match each other and if the speed shown makes sense. If there's a problem, you might then need to look for problems in the path from the sensor to the controller, and that might be through another module that's relaying the information.
If there's no scan tool available but you've got a bad feeling about a wheel speed sensor, you're not completely out of luck. You still can check the connectors and harnesses for problems. Wheel speed sensor harnesses do tend to be problematic, especially in areas of the country with severe weather conditions. Salt works it's way into wire harnesses and rots them right out, showing up as green or black rot in the wires and often running through the whole harness.

Wiring harnesses and connectors can be checked by performing a voltage drop test. This is done by putting one meter lead on a positive point in the circuit (usually one side of the connector or component) and putting the other lead on a negative point in the circuit (usually the other side of the component or connector being tested). Then with the meter still hooked up, operate the circuit.

The reading on the meter indicates how much voltage was eaten up by resistance between the two leads of the meter. That's the voltage drop across that part of the circuit. Connectors and wires in control module circuits shouldn't be eating up much voltage. In fact any more than 0.1 or 0.2 volt (depending on the wire and circuit) can cause problems (that's why its important to zero or calibrate a meter before using it). Any more than that likely will interfere with the circuit.

Temperature Sensors

Temperature sensors are surprisingly important to smooth transmission operation — especially if there's only one sensor for both the transmission and engine. If the engine thinks its running at the wrong temperature (too hot or too cold), it affects shift requests. Additionally, if the engine knows there's a problem with the coolant sensor, it might use a fail-safe or default mode that can seem like a transmission with control problems, when in fact it's just doing what it's told to do. (Repair manuals often list what a particular vehicle does in fail-safe mode, so if the symptoms match that list you might have solved part of the problem.)

Temperature sensors are tested by checking resistance at various temperatures, looking for changes in the readings and comparing results with specifications.

But on vehicles using separate engine and transmission controllers, also be sure to check the temperature reading on a scan tool in both modules to make sure that both the engine controller and the transmission controller are getting coolant sensor data. Sometimes the data from the sensor is being sent to one controller but not the other, causing problems in the transmission control system. And of course, inspect the sensor for corrosion, damage or other obvious problems.

Range Selection

Transmission gear selection is an important input — especially on vehicles without the usual PRNDL switch. Many vehicles now just use Park, Drive and a range of selectable or sports settings. Unfortunately, the shifter on newer vehicles often is placed next to the drink holders, and drinks then spill all over the sensitive electronics below. The quickest way to check this input is to watch the selection on the scan tool (or combination meter) to make sure they change as they should.

But on vehicles that still use a shift cable, a misadjusted or improperly seated cable can and will cause problems. For example, Cadillac XLR models, using a 5L50 or 6L50 unit, that display a "Shift to Park" message while in Park, and automatic door locks that don't work might have this exact problem. If so, reseating the cable adjuster lock should fix the concern.

Accelerator Position

Throttle or accelerator pedal sensor data is another critical input — the controller needs to know what the driver wants to do. Sensor operation varies among vehicles (some are Hall effect, some are dual-reverse signals, some are variable voltage), so referring to service information is essential. But there are certain common problems to watch for.

Work vehicles see one problem. Salt or crud from work boots builds up on the accelerator pedal sensor. The vehicle goes into default mode when it doesn't get a signal from the sensor and it barely drives.

Another problem is in the connector itself, when the terminals become loose and don't contact each other as well as they used to. Inspect for this by checking that the sensor's connector has good grip in each of its terminals. A grip test means using the correct male terminal to check if the female connector feels like it's gripping well and vice versa, or in other words that there's a light drag between the two terminals. (In my experience, female terminals become oversized and male terminals become damaged or broken off.)

In fact, one GM tech told me that after a GM connector has been disconnected and reconnected three times, it's considered a weak spot in the circuit and the terminals might need to be replaced. Terminals that aren't gripping need to be replaced, using the correct tools (not pliers and side cutters).

Outputs

Transmission controller outputs typically are actuators for the solenoids, the malfunction indicator light and signals to various other components such as the combination meter and bus communication line.

Solenoids usually are tested for resistance at a specified temperature (heat and cold affect the readings), and results are compared to specifications in the repair manual (or service bulletin). They're also tested with a scan tool, commanding them on and off if possible, and observing the results.

However, one important and often overlooked test is checking the supply voltage at the solenoid. Many tough-to-diagnose transmission shift problems are caused by low supply voltage at the connector, and it's usually not off by that much. Ten volts instead of 12 at the connector can and do cause the solenoid to stop working.

And again, wiring harness integrity is essential for correct shifts. Chevy Malibu models with 6T45E transmissions might have P0700 and various U-codes because of rubbed-through wires at the strut tower, resulting in hard shifts, no shifts or the TCM not functioning. Repairing and repositioning the harness repairs the problem.

The Controller Itself

Sometimes reprogramming the module is the only way transmission controller problems can be repaired. Ideally, checking for calibration updates would be one of the first diagnostic steps, but it's easy to forget this step. One such problem is a TCC surge in third or fourth gear on a Chevy Cobolt or Pontiac G6 using a 4T45E unit. The problem is fixed by reprogramming the control module with updated calibration.

Also, check to make sure the connectors on the controller are not just in place, but that they're locked in place too. Quite often that lock is what makes the final contact, so if it's not locked it won't work correctly.

But if you've checked the inputs and the outputs and they're OK, and the module has power and ground, consider that the controller itself is at fault. Someone far smarter than I am recently confided to me that the solder used in many control units is not holding up well and over time can crack or come apart, causing the unit to fail.

Conclusion

Transmission control units can seem complicated, but they don't need to be. Just like engine control units, they need accurate inputs and correct software to make decisions and command outputs. It's worth sitting down with the repair manuals to figure out what the controller is looking for and why it does what it does because quite often the problems are caused by simple things that can be easily checked and fixed.

Vanessa Attwell is a Master Technician for two major manufacturers and has also worked on the bench of an independent shop. She has developed training for both vehicle manufacturers and independents, and helped develop government training and regulation standards.