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Understanding TAP, Data Helps Diagnose Transmission Shift ProblemsPosted 8/13/2004By Jim Watson
In the same manner that the powertrain control module (PCM) adjusts fuel trim to achieve minimum emissions and maximum performance and fuel economy, the PCM adjusts transmission shift feel for minimum wear and maximum comfort. Transmissions capable of adaptive functions utilize a pressure control solenoid (PCS) to control the desired line pressure. The PCM controls line pressure by increasing or decreasing current flow to the PCS. Low current (0.1 amps) produces maximum line pressure; high current (1.1 amps) produces minimum line pressure. The pressure control current and duty cycle are both displayed on scan tools as PC solenoid amps and PC solenoid duty cycle percentage. Once an adaptable shift has been made, the PCM compares the actual shift time (in seconds) to the desired shift time. The shift time can be displayed through scan tool data. Shift errors can also be displayed through scan tool data as a range from negative 6.38 to positive 6.38. This data is the difference between the desired shift time and the actual shift time. If the actual shift time is too long (slipping engagement), the PCM will use the PCS to increase line pressure for the next upshift under the same conditions that were identified as the slipping engagement. The transmission adaptive pressure (TAP) is divided into cells numbered from four to 16, similar to the fuel trim cells used by the PCM to control for fuel delivery. Transmission adaptive pressure cells represent a torque range, with 4 being the lowest and 16 the highest. The TAP cell will be displayed in scan tool data as a negative or positive number. A positive number indicates the PCM is adding line pressure to decrease shift time, while a negative number indicates the PCM has seen too short a shift time and is decreasing line pressure to increase shift time. A code "P1811" will set if the PCM detects a long shift (shift time greater than 0.65 seconds twice during the same trip) that cannot be shortened by shift adapts during the same ignition cycle. The PCM will not illuminate the MIL light; it will command maximum line pressure and freeze transmission adapt functions. If this code is set, and you can't duplicate the problem, see if the customer has been overloading the vehicle or towing a trailer that could cause the code to set, but not show up during your diagnosis. Understanding how the TAP works, along with knowing what scan tool data is available, and what this data represents, makes diagnosing a transmission shift problem considerably easier.
When Is a Short Not a Short?By Dave Tidaback
A shop recently called about a problem with a late model Volkswagen, a 1999 Beetle with a 2-liter gas engine. The car had a running problem but not a serious one - rough idle and hesitation on initial acceleration. The technician had checked for fault codes and was surprised to find a total of seven - all indicating electrical components being shorted to ground. All four injectors, the evaporative purge solenoid, tank vent valve, and secondary air injection relay were indicated. When he tested these circuits, they all were operating normally and voltage and resistance tests showed no evidence of shorting to the ground. When I asked if he knew what had precipitated the original complaint, he said that the owner had left the car parked for a couple of weeks while he was out of town. The trouble began the next time he started it. I suggested that the tech get the owner's permission to keep the car overnight and test it the following morning, after it had been parked outside at a temperature in the 20s. The technician cleared the codes, put it outside, and then tried to start it the next morning. The engine cranked normally, but did not start immediately-then ran roughly when it did. Six of the original codes had reset. The technician tested battery voltage during cranking, and it dropped to 8.8 volts. A voltage test at the purge solenoid showed 8.2 volts. Though there was still no evidence of any shorting to ground, the battery failed a load test after being charged. So how, I wondered, does a control module such as the powertrain control module on this VW detect a short to ground? Since the module has no way to do a resistance test, it has to rely on seeing a great enough voltage drop on the circuit to interpret that as a short to ground. The critical voltage needed to keep most Bosch computers functioning is within a range of 9.0 to 9.5 volts. If the computer sees the voltage supply to a connected component drop lower than that, it automatically interprets it as a short to ground, and flags a code. After battery replacement and a basic setting of the car's electronic throttle module, the running problem and the codes were eliminated. In this case, a short was not actually a short at all. But remembering what a computer interprets as a short may help in diagnosing fault codes of this sort.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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