ECM TCC Methodologies Robert Rauscher 08/28/2001 Copyright 2001, Robert Rauscher, All Rights Reserved. This is not specific to any one ECM. The methods and madness applies to many of the GM ECMs. (Use at your own risk, no guarantee that any of this is correct). INTRO: The torque convertor clutch (TCC) is a mechanism that mechanically links the engine to the transmission. It is contained within the torque convertor. It is only used with automatic transmissions. The purpose of the TCC is to increase fuel mileage. A secondary benefit is that the torque convertor stall speed may be raised without excess slippage while cruising with the TCC locked. Control of the TCC is assigned to the ECM. There is also a brake pedal switch that will unlock the TCC whenever the brake is applied. The strategies used to unlock the TCC are as much a drivability issue as it is a reliability issue. The TCC clutch action needs to be smooth lest the consumer complain. It is also required to unlock the TCC for acceleration. For reliability the TCC can not remain locked under heavy load. The TCC is not designed to handle a high load. Also note that the GM 700R4 (4L60) transmission does not route fluid to the cooler unless the TCC is locked. GENERAL CONTROL METHODS: Every ECM handles control of the TCC differently. The parameters used differ according to vehicle type. An important aspect of TCC control is that the ECM does not check to see when the TCC should be locked. It does the opposite: it checks to see when the TCC is to be unlocked. The TCC logic goes through a series of checks to see if there is a reason to unlock the TCC. If there is no reason for it to be unlocked, it will then be locked. The only deviation to this logic is a mandatory lock above a particular vehicle road speed. Most ECMs also differentiate between the transmission being in a lower gear vs. high gear. There is a switch on the transmission that the ECM reads to know whether it is in high gear or not. Some ECMs/transmissions have switches that allow the ECM to know which mid-range gear it is in. Not just high or low gear. Items that can cause the TCC to unlock: a) Engine coolant temperature below a threshold. b) Road speed below a threshold. c) Throttle closing rate above a threshold. d) Throttle opening rate above a threshold. e) Throttle position below a threshold. f) Throttle position above a threshold. g) Transmission just shifted into high gear. h) Transmission just shifted from high gear to a lower gear. i) A relock delay is in effect. j) DFCO is in effect. k) Road speed above a threshold. l) Minimum run-time since startup. m) Engine RPM below a threshold. Items that can change unlock/lock thresholds: a) A/C clutch engaged b) Engine in overheat mode. c) Transmission gear in use. d) Vehicle road speed. DISCUSSION of ITEMS: Remember that not all of these strategies are used in every ECM. Different ECMs handle the TCC logic differently. However, any ECM with TCC control will use statagies similar to the ones discussed here. a) Whenever the engine coolant is below a set value the TCC will not lock. This is sometimes combined with a minimum run time. b) A minimum road speed is required before the TCC will lock. The actual speed threshold can vary depending upon which gear the transmission is in. To keep the TCC from chattering the required road speed will use two values. If the TCC is currently locked the unlock road speed threshold will be a value lower than if the TCC is not currently locked. This action is called hysteresis. These values are typically a couple of MPH apart. An example would be the road speed values of 38 mph & 36 mph. If the TCC is unlocked it will stay unlocked as long as the road speed is below 38 mph. Once the TCC locks the unlock road speed will be the 36 mph threshold. In this manner if the vehicle speed is held at 38 mph the TCC will not continuously lock/unlock causing chatter. If vehicle road speed falls below 36 mph the TCC will be unlocked. c) With the throttle closing rate above a threshold the TCC will unlock. A value might be a 32% change in TPS% over .1 second to force a TCC unlock. d) With the throttle opening rate above a threshold the TCC will unlock. This is typically done to enhance performance. A value might be a 12% change in TPS% over .1 second to force an unlock. e) While lifting the throttle the TCC will be unlocked once the throttle position reaches a threshold. The actual TPS% value can also be tied to a mph threshold. TPS% values might be 3.9% when vehicle speed is under 25 mph, and 8% when vehicle speed is greater than 25 mph. The TPS% threshold can also be tied to the gear in use. In other words whenever the TPS% is below the threshold the TCC will be unlocked. This is decel unlock. f) The TCC unlock parameter most commonly known is the TPS% required to unlock vs vehicle road speed. There may be one or more of these tables dependent upon the current gear in use. A table consists of two sets of values. One set is used when the TCC is unlocked. The other set is used when the TCC is locked. The table values are typically based on vehicle road speed. Many ECMs have an option to allow these tables to be vehicle engine speed based. The sets of values may have 12 entries ranging from 20 MPH to 60 MPH. This allows the amount of TPS% required to unlock to vary according to vehicle road speed. g) With the transmission shifting from high gear (overdrive) to a lower gear the TCC may be commanded to unlock. This is typically accompanied with a relock delay. h) With the transmission shifting into high gear (overdrive) the TCC may be commanded to unlock. This is typically accompanied with a relock delay. i) After the TCC unlocks it is common to have a delay before it is allowed to lock again. There may be different delays dependent upon the reason the TCC was unlocked. Relock delays may be as short as 0.2 seconds to several seconds (5 or more). j) With decel fuel cutoff in effect the TCC may or may not be unlocked. The ECM may use an option bit to define whether this occurs or not. k) When the vehicle road speed is above a threshold the TCC may be unlocked. This one is not too common. Typically it is the opposite. Once the vehicle road speed is above a set threshold the TCC is locked. No other ECM logic will unlock the TCC until the vehicle speed drops below this threshold. Sometimes this threshold has hysteresis. EXAMPLE: Ok, so how does this all fit together. Lets go through the logic on a single GM ECM, the 1228746. Used on TBI cars: F-body & Caprice. The parameters listed are directly from the 8746 ANLU bin. LD005: FCB $23 ; option byte: 0010 0011 ; b1: 1 == dfco tcc unlock This is an option byte. Bit 1 is used to designate whether the TCC is to be unlocked during a DFCO period. If set to a 1 (as above) a test will be made as to the state of DFCO. If the vehicle is in DFCO the TCC will be unlocked. If this bit is 0 then no test is made for a DFCO TCC unlock. (DFCO: decel fuel cut off). LD007: FCB $24 ; option byte: 0010 0100 ; b7: 1 = shift light (0 = tcc) ; b5: 1 = high gear sw polarity This option byte has two parameters that affect the TCC operation. The first, bit 7, needs to be a 0 when the vehicle has an automatic transmission. If set to a one the TCC logic is not run, shift light logic is run. (Having the shift light logic run the TCC is a bad thing. Something to be avoided). Bit 5 is used to define the state of the transmission high gear switch. This is so the ECM knows whether the high gear switch sends a high voltage (+12v) or a ground to the ECM when it is in high gear. If the high gear switch polarity is set to a 1, the ECM TCC logic expects the high gear switch to be at +12 volts while in high gear. LD4B6: FCB 102 ; 60c, min coolant for tcc lockup This is the minimum temperature that the TCC will lock at. At any engine coolant temperature below 60C the TCC will be unlocked. LD4B7: FCB 3 ; flag to use TPS% for decel TCC unlock This flag may be the value of 0, 1 or 3. It is not normally changed. The value of 3 tells the TCC logic to use the TPS% reading for the decel unlock limits. The others (0 & 1) signify to use MAP or VAC. LD4B8: FCB 40 ; mph thres for TPS% decel unlock limit, w/5 mph hyst This 40 MPH threshold is used to switch between two different TPS% thresholds for decel unlock. There is a +5 MPH hysteresis coded into the logic for this value. LD4B9: FCB 10 ; 3.9%, tps decel unlock limit when < 40 mph (LD4B8) LD4BA: FCB 21 ; 8.2%, tps decel unlock limit when > 40 mph (LD4B8) LD4BB: FCB 4 ; 1.6%, tps decel unlock hyst These are the TPS% limits that will cause the TCC to unlock during decel. This typically occurs while slowing down. As the throttle is lifted to slow the vehicle the TPS% will eventually drop below this threshold. Once the TPS% is below that threshold the TCC will be unlocked. Note that when the vehicle road speed is above 40 MPH the TCC will unlock with a higher throttle position. This reduces the stress on the TCC. The last of the three parameters is for hysteresis. Again this is to prevent the TCC from chattering if the throttle is at a threshold. LD4BC: FCB 0 ; N/U A parameter that is not used. Not unusual for an ECM to have locations that are not used. LD4BD: FCB 128 ; filter coef for MPH / 1, L0030 The VSS input is a pulse counting algorithm. Due to this the VSS input requires filtering to smooth the value. This is the lag filter coeficient for filtering the VSS input. LD4BE: FCB 1 ; 0.2 sec, downshift unlock to relock delay (tenths+1) LD4BF: FCB 6 ; 0.7 sec, re-lock delay (tenths + 1) These delays are used to prevent the TCC from going through quick lock/unlock/lock states. Again it is an abuse prevention technique. If the driver happens to hit the throttle, lift and quickly hit the throttle again, without these delays the TCC would have unlocked, locked, then unlocked again. They are also used to cushion the downshift. This makes it smoother for fewer customer complaints. The ECM logic is setup so that a downshift from overdrive to 3rd will cause the TCC to unlock. LD4C0: FCB 80 ; mph thres for mandatory lockup of tcc Once the vehicle road speed exceeds this value the TCC is locked. No amount of throttle will unlock it. This is often seen as a drop in RPM during WOT operation. ;----------------------------------------------- ; Low Gear tables ;----------------------------------------------- LD4C1: FCB 38 ; force unlock < mph thres (use if unlocked) LD4C2: FCB 255 ; force unlock >= mph thres (use if unlocked) LD4C3: FCB 36 ; force unlock < mph thres (use if locked) LD4C4: FCB 255 ; force unlock >= mph thres (use if locked) This table has two pairs of vehicle road speed parameters. The values at 255 basically disable the maximum vehicle road speed threshold for unlocking (can your car do 255 MPH?). The values of 38 and 36 MPH set the vehicle road speed that the TCC is forced to unlock. If the TCC is currently unlocked the vehicle road speed must exceed 38 MPH before it will lock. If the TCC is currently locked and the vehicle road speed falls below 36 MPH the TCC will unlock. Remember that other parameters may cause the TCC to unlock even when the vehicle road speed is above these values. ;----------------------------------------------- ; Low Gear tables ; ; TPS% required to/stay unlock vs. mph ; TPS% = VAL * .39 ;----------------------------------------------- ; mph: 20 24 28 32 36 40 44 48 52 56 60 ; min TPS% to unlock (currently locked): LD4C5: FCB 2 2 19 58 68 72 78 82 104 120 120 ; min TPS% to stay unlocked (currently unlocked): LD4D0: FCB 1 1 8 10 27 37 45 52 64 90 96 These two lines of values is the amount of throttle position required to unlock (or keep unlocked) the TCC. If the TCC is currently locked the upper row will be used. The value to be used is dependent upon the vehicle road speed. As a 2d lookup is used the speeds between the table values is interpolated. Once the TCC is unlocked the throttle position needs to fall below the value in the lower row for the TCC to relock. An example would be: with the TCC locked and the vehicle road speed at 40 MPH, the TPS% needs to exceed 28% (72 * .39) before the TCC will unlock. Once unlocked the TPS% needs to fall below 14% (37 * .39) for the TCC to relock. It will only relock after the 0.6 second relock delay has expired. It is these tables along with the functionally identical high gear tables following that have the most effect on drivability. ;----------------------------------------------- ; High Gear tables ;----------------------------------------------- LD4DB: FCB 32 ; force unlock < mph thres (use if unlocked) LD4DC: FCB 255 ; force unlock >= mph thres (use if unlocked) LD4DD: FCB 30 ; force unlock < mph thres (use if locked) LD4DE: FCB 255 ; force unlock >= mph thres (use if locked) ;----------------------------------------------- ; High Gear tables ; ; TPS% required to/stay unlock vs. mph ; TPS% = VAL * .39 ;----------------------------------------------- ; mph: 20 24 28 32 36 40 44 48 52 56 60 ; min TPS% to unlock (currently locked): LD4DF: FCB 2 2 10 56 74 88 97 112 120 120 120 ; min TPS% to stay unlocked (currently unlocked): LD4EA: FCB 1 1 8 22 41 51 69 84 90 90 96 --