TCC Data PID Diagnostics
As automotive technicians, I am sure the automatic transmission strikes a little fear in a lot of us. I’m sure we have all had our share of automatic transmission problems to diagnose and repair. Automatic transmission diagnostics has gotten easier for some and harder for others since the automatic transmission has become more integrated with electronics. If you are timid of automatic transmissions and electronics, your transmission fears have doubled. If you are comfortable with automotive electronics, you may have already found automatic transmission problems that you can diagnose. I hope this article helps many of you become more comfortable diagnosing automatic transmission torque converter control (TCC) problems.
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 to be rebuilt, usually by an automatic transmission specialist. I think 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. This is were the road splits. Part of the automotive repair field will have a vehicle were the transmission appears to be at fault. They will check the fluid level and condition, look at or replace the filter, check for any transmission related codes, and see if there is any external issues they may be able to repair. If they aren’t able to find any problems they want to tackle, they may pull the transmission to get rebuilt just like we did before. What has changed from before, is now you think it might be an internal or external electrical issue and may send your customer elsewhere to have it diagnosed to protect yourself and your customer from a misdiagnoses. Worst case scenario would be if you got the transmission rebuilt and reinstalled just to find the original problem still present.
Electronics Pave the Way
Automotive techs who are electronically fluent may be finding out that many transmission problems can now be fixed by using their electrical diagnostic skills and tools they use everyday to diagnose other vehicle electronics and electrical systems. 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 the problem they may have. Also, testing solenoids and electrical circuits in the transmission are no different than electronics on the engine. Before, a shifting issue probably was an internal failure which required disassembly to repair. Now that same shifting issue might be able to be diagnosed externally with a scope, scan tool, digital multi-meter, and some knowledge. It could be a broken wire, the PCM, or an internal transmission solenoid that may be just as accessible as the filter.
One of the common transmission failures that I will focus on in this article is torque converter lock up control. Most of the time your customer’s only complaint with a TCC code, is that the amber check engine light is on. Some of the first common electrical TCC problems that 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 go in and replace the torque converter lock up solenoid and pressure switch. I’m also sure many of us have also just unplugged the four terminal connector, on some of the 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?
I want to show you some of my testing techniques I use when diagnosing TCC related codes and/or problems. Lets say your customer drops off their vehicle because the service engine light is illuminated. You scan the vehicle and find a TCC related 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 a replacement unit to be installed. 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?
fig. 1
Let start at the beginning again. Your customer drops off their vehicle because the service engine light is illuminated. You scan the vehicle and find a TCC related 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 comes in very helpful for this next step. The data PIDs you will want to watch or graph are; TCC enable, Engine RPM, TCC slip speed, and TCC PWM solenoid %. Not all of these PIDs are always available and/or may have different definitions and you may want to familiarize yourself with the common vehicles you work on. Also, this procedure does not apply for all makes or models.
Slip or No Slip?
With scan tool in hand, take the vehicle for a test drive while watching the previously stated data PIDS. One of the most important data PIDS is the TCC slip speed rpm. The TCC slip speed rpm is what the PCM is watching and if it goes over a predetermined code set criteria 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, this issue will have to be taken care of first. Here are some things that will cause the TCC enable to remain as NO, transmission temperature too high or low, a TCC code in memory, brake apply switch, TCC open or short fault detected, and certain other engine or transmission faults. On some vehicles, even your scan tool being hooked up may cause TCC issues, like some late 90’s Dodge Dakotas.
fig. 2
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 around 0. You will see positive and negative TCC slip speed rpm variations that change depending on load. Different vehicles allow different TCC slip speed variations depending on code set criteria. I normally see no more then a 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. You will need to take engine rpm minus transmission input rpm which will get you the TCC slip speed and on other vehicles you may only be able to see engine rpm (as in fig. 1) as an indication of TCC lock up.
fig. 3
If the vehicle your working on has a pulse width modulated TCC solenoid, you may have even more diagnostic information at your hands. 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 the TCC PWM % PID around 30% or less. The harder the PCM tries to control the TCC slip speed, the higher this percentage will be. The higher TCC PWM % indicates that the PCM may be losing control of TCC slip speed and that there will be or that there is a problem. From my experience , when TCC PWM % gets up into the 60% – 80% with TCC slip speed close to 0 rpm, this may be a problem in the future or a intermittent code set when towing. TCC PWM percentages of 90% – 100% with high slip speed rpm indicates that the PCM is doing all that it can do to control the TCC slip speed and is failing (like in fig. 2). This does not work with all PWM TCC equipped vehicles (like in fig. 3). These vehicles will have normal TCC slip speeds and when the TCC is enabled the TCC PWM % PID will quickly rise to 90% – 100%. Different vehicles and driving conditions may have different TCC PWM solenoid normal percentages. Experience with these data PIDs on known good vehicles is what you will need to get a good handle on normal conditions on the vehicles you service.
fig. 4
The PIDs Have Spoken
Now we can make our next move since we have verified that there is, or isn’t a TCC slippage issue or no TCC operation at the present time. If there is no indication of a TCC slippage condition you may need to get more information from the customer or look at freeze frame data if available. The problem may be intermittent or temperature, debris, fluid, or vehicle load related.
If you verified that there is a TCC slippage issue or no TCC operation, lets find out why. Lets make sure the TCC lock-up solenoid and circuit is working correctly. If your scan tool has TCC bi-directional controls capabilities, try to operate the solenoid and listen for an audible click. If there is an audible click, your problem is most likely an internal issue we will get to shortly. Sometimes you may not be able to hear an audible 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 fig. 5).
fig. 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 are armed with this information you can operate the solenoid your self 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, you will need to verify the circuit from your test point to the powertrain control module or transmission control module. One way to test this, is by probing the control side circuit and make sure the PCM is commanding lock-up while on a test drive. If you are not getting a command signal, either power or ground depending on the system, 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 a direct effect on TCC operation. If you do get a PCM command, you now know the problem will probably be internal, that we will go over soon.
Take an Electronic Picture
In any of the above tests that point towards a solenoid problem, you may want to use a low amp probe while trying to operate the TCC solenoid (as in fig. 5 & 6). Is the solenoid passing amperage? Does the waveform indicate a shorted solenoid? Is there a pintle bump that indicates mechanical movement of the solenoid? Normal amperage draw for a lot of TCC solenoids range from around 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 thru the solenoid and make sure the circuit isn‘t open. With these tests, you are able to diagnose if the TCC solenoid is electrically working or not.
fig. 6
Now you should be able to determine if your TCC slippage issue is an electrical or mechanical TCC solenoid fault, by all the test you have now completed. If your testing has concluded that the solenoid and control circuit is electrically sound and the solenoid had an audible click, you can proceed to this conclusion. You now know that the TCC solenoid electrical circuit is operating correctly and you either have an internal transmission fluid leak, fluid blockage, binding or leaking valve (as in fig. 7) , or a torque converter failure. Now you can be confident when you tell your customer what needs to be done to correctly repair their torque converter control problem.
fig. 7
Don’t Be Afraid of the Unknown
I hope you have found that testing TCC operation is easier than you previously thought. Testing and diagnosing a lot of TCC systems is no different then any other systems on a vehicle after 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 good data. Seeing how different systems work without any problems will help you find problems much quicker and more efficiently, when it counts.
When you get that next TCC related code, remember to consult your information source for that vehicles 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 runs the mobile diagnostics business, Dewitz Diagnostic Solutions in Eau Claire, Wisconsin. Travis is an ASE Master Automobile Technician and has other ASE certifications including L1.
