The automatic transmission strikes a little fear in a lot of technicians. Automatic transmission diagnostics has become both
easier and harder since it has become more integrated with electronics. If you are comfortable with automotive electronics,
you may already have found automatic transmission problems that you can diagnose. But maybe you could use some help in diagnosing
torque converter clutch (TCC) problems. There are plenty of things you can do and learn to make yourself more comfortable
with this diagnosis.
Electronics Pave the Way
Before electronics were a part of the automatic transmission, we weren't scared to replace an external seal or change the
filter. If there were any problems beyond leaks, maintenance or common adjustments, the transmission was pulled and rebuilt,
usually by a transmission specialist.
The automotive repair field has taken two different paths since electronics have been integrated into transmissions. Most
of us will diagnose and replace a few external electronics, like speed sensors and transmission position sensors. But this
is where the road splits.If the transmission appears to be at fault, most techs will check the fluid level and condition, look at or replace the filter,
check for any transmission-related codes and see if there are any external issues that can be repaired. If they aren't able
to find any problems they want to tackle, they may pull the transmission to get rebuilt, just like techs did before.
But now if you think it's an internal or external electrical issue, you may send your customer elsewhere to have it diagnosed
just to protect yourself and your customer from a misdiagnosis. The worst-case scenario would be if you rebuilt a transmission
and got it reinstalled, only to find the original problem still present.
Techs who are electronically fluent might find out that many transmission problems can be fixed using skills and tools they
use every day to diagnose other electronics and electrical systems problems. These technicians have taken the other path and
have found that they can diagnose and repair transmissions easier now than before. This is because there is more transmission
scan data available to help diagnose problems. Also, testing solenoids and electrical circuits in the transmission is no different
than testing electronics on the engine.
Before, a shifting issue was probably an internal failure that required disassembly to repair. Now that same shifting issue
might be diagnosed externally with a scope, scan tool, digital multi-meter (DMM) and some knowledge. It could be a broken
wire, the control module or an internal transmission solenoid that may be just as accessible as the filter.
One of the more common transmission failures is torque converter lock-up control. Most of the time, your customer's only complaint
with a TCC code is that the Check Engine light is on. One of the first common TCC problems we encountered was in GM transmissions
that caused the engine to stall when coming to a stop. I am sure many of us have pulled the transmission side cover off to
replace the torque converter lock-up solenoid and pressure switch. I'm also sure many of us have just unplugged the four-terminal
connector on some older vehicles to save the customer some money and cure their stalling issue. What I have been seeing more
at the present time is torque converter lock-up slippage or no lock-up at all.
Where Do We Start?
Let's say your customer drops off a vehicle because the Check Engine light is illuminated. You scan the vehicle and find a
TCC code in memory. What do you do next? Do you check the fluid level and condition? Maybe you recommend a transmission fluid
flush. Maybe you assume there is an internal failure and recommend transmission replacement. You might not have a clue what
is wrong and suggest that your customer should go visit a transmission specialty repair facility. Is this the path you want
to take?
Let's start again. Your customer drops off his or her vehicle because the Check Engine light is illuminated. You scan the
vehicle and find a TCC code in memory. You check the transmission fluid level, and it is correct. With your scan tool hooked
up, you take the vehicle for a test drive. A graphing scan tool is helpful for this next step. The data Parameter IDs (PIDs)
you will want to watch or graph are:
- TCC enable
- Engine rpm
- TCC slip speed
- TCC solenoid duty cycle (PWM solenoid percentage).
Not all of these PIDs are always available, or they may have different definitions, so you may want to familiarize yourself
with the common vehicles you work on. Also, this procedure may not apply for all makes or models.
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