Overcoming the Fear - TCC Data PID Diagnostics Is Not as Scary as It Seems. - Motor Age - Automotive training, certification & parts info

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Overcoming the FearTCC Data PID Diagnostics Is Not as Scary as It Seems.

Publish date: Jun 1, 2008
By: Travis Dewitz
Source: Motor Age

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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.

Figure 1 There was no input rpm PID or slip speed PID on this GMC truck, but even without them you can see that the PCM has control of the TCC. The engine rpm drops with a TCC duty cycle of only 50 percent.

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.

Figure 2 The PCM ramped up the TCC duty cycle to the maximum of 90 percent, trying to control TCC slip speed, when the TCC was enabled. A P1870 (transmission component slippage) was set because the PCM had no control over the TCC slip speed.

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).

Figure 3 You can see the definite TCC slip rpm and engine rpm drop when the torque converter clutch was applied. You can also see that the PCM is able to keep the slip speed near 0 rpm.

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.

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Travis Dewitz

Articles by Travis Dewitz

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Travis Dewitz