Overcoming the Fear

TCC Data PID Diagnostics Is Not as Scary as It Seems.

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.

Slip or No Slip?

With scan tool in hand, go for a test drive while watching the previously noted PIDs. One of the most important is the TCC slip speed rpm. This is what the PCM is watching, and if it goes over a predetermined slip rpm, a code will be set. Before this data PID is useful, we have to make sure that the TCC is enabled by the PCM. If the TCC enable PID says "no," that will have to be taken care of first. Things that will cause the TCC-enable to remain as "no" include:

transmission temperature too high or low
a TCC code in memory
brake apply switch ON
TCC open or short circuit detected
other engine or transmission faults.

On some vehicles, like some late 1990s Dodge Dakotas, even your scan tool may cause TCC issues.

Now that your TCC-enable data PID indicates "yes," we can continue with our diagnostic procedure. While driving the vehicle at a steady speed and load, watch the TCC slip speed. This PID should read near zero, but you may see positive and negative TCC slip speeds that change depending on load. Different vehicles allow different TCC slip speed variations before setting a code. I normally see no more than 75 rpm slip, unless there is a problem.

On some vehicles, a TCC slip speed PID is not available, and you will need to do some math. Engine rpm minus transmission input rpm will give you the TCC slip speed. On other vehicles, you may only be able to see engine rpm (as in Figure 1) as an indication of TCC lock-up.

If the vehicle has a pulse width modulated (PWM) TCC solenoid, you may have even more diagnostic information available.

Vehicles use a PWM TCC solenoid for smoother TCC lock-up control. You may be able to use this PID to detect intermittent or on-the-border issues. I normally see this PID at 30 percent or less. The harder the PCM tries to control the TCC slip speed, the higher this percentage will be.

A higher percentage indicates that the PCM may be losing control of TCC slip speed, and there either is, or soon will be, a problem.

From my experience, when TCC pulse width gets up to 60 percent to 80 percent with TCC slip speed close to 0 rpm, this may either be a future problem or an intermittent code set while towing. TCC pulse width of 90 percent to 100 percent with high slip speed indicates the PCM is doing all it can to control the TCC slip speed but is failing (like in Figure 2).

This does not work with all PWM TCC equipped vehicles (like in Figure 3). Some will have normal TCC slip speeds when the TCC is enabled, with a normal pulse width of 90 percent to 100 percent.

Different vehicles and driving conditions may have different "normal" TCC pulse width readings. Experience with these data PIDs on known-good vehicles is what you need to get a good handle on "normal" for the vehicles you service.

The PIDs Have Spoken

Now you can make your next move. If there is no indication of excessive TCC slip, you may need to get more information from the customer or look at freeze frame data. The problem may be intermittent or related to temperature, debris, fluid or vehicle load.

If you verified that there is excessive TCC slip or no TCC operation, make sure the TCC lock-up solenoid and circuit are working correctly. If your scan tool has TCC bi-directional control capabilities, try to operate the solenoid and listen for an audible click. If there is a click, your problem most likely is an internal issue we will get to shortly. Sometimes you may not be able to hear a click from a good solenoid because of the style of solenoid or its location in the transmission. If you do not hear an audible click, you may need to use a low amp probe and scope to see the actual TCC solenoid amperage and pintle bump (as in Figure 5).

If your scan tool does not have TCC control capability, then you will need to see how that solenoid is operated. Is it ground-side or power-side controlled? What color wire operates the solenoid, and where is a good test point? Once you have this information, you can operate the solenoid yourself by either supplying power or a ground to the control side of the TCC solenoid.

If you don't hear an audible click, you will need to test the TCC circuit from your test point to the lock-up solenoid. If you do hear an audible click, verify the circuit from your test point to the Powertrain Control Module (PCM) or Transmission Control Module (TCM). One way to test this is by probing the control-side circuit and making sure the PCM is commanding lock-up while on a test drive. If you are not getting a command signal, you will have to verify the circuit and PCM. Also, be aware of the PCM inputs like brake switch, transmission fluid temperature sensor, etc., that have an intended effect on TCC operation. If you do get a PCM command, you now know the problem is probably internal. We will go over that soon.

Take an Electronic Picture

If any of the above tests point toward a solenoid problem, you may want to use a low amp probe while trying to operate the TCC solenoid (as in Figures 5 and 6). Is the solenoid drawing amperage? Does the waveform indicate a shorted solenoid? Is there a pintle bump that indicates mechanical movement of the solenoid? Normal amperage draws for a lot of TCC solenoids range from ½ amp to 1 amp, and some even draw up to 4 amps.

If the solenoid is drawing high amperage or is shorted, replace it. If you do not show any current flow, you will need to check for a good power supply and ground. You should also test the resistance from your test point through the solenoid and make sure the circuit isn't open. With these tests, you are able to tell if the TCC solenoid is electrically working.

Now you should be able to determine if TCC slippage is an electrical or mechanical TCC solenoid fault. If your testing proves that the solenoid and control circuit are electrically sound and the solenoid "clicks," you know you have either an internal transmission fluid leak, fluid blockage, a binding or leaking valve (as in Figure 7), or a torque converter failure.

Don't Be Afraid of the Unknown

Testing and diagnosing a lot of TCC systems is no different than any other system on a vehicle once you know how it works and what to look for. What helps me get a good handle on any automotive system is looking at a lot of known-good data. Seeing how different systems work without any problems will help you find problems much quicker and more efficiently.

When you get that next TCC-related code, remember to consult your information source for that vehicle's TCC operation characteristics and code-set criteria. There are many different TCC systems out there, but this should get your foot in the door. Don't be afraid of the unknown in the pursuit of knowledge with the tools and knowledge you already possess.

Travis Dewitz works full time for Dewitz Truck and Auto Repair in Eau Claire, Wisc. and also runs his own mobile diagnostics business, Dewitz Diagnostic Solutions. Travis is an ASE Master Automobile Technician and has other ASE certifications including L1. You can contact him through www.dewitzauto.com.