Aisin Warner AW-4 TCU Datastream

Here is my reverse engineering work on the Aisin Warner AW-4 Transmission Control Unit found in 86-90 Renix Jeeps and 91-95.5 OBD1 Jeeps. This self contained unit is in charge of controlling the vehicles Automatic Transmission and has a data stream output that we can use for data and diagnostics.

TX output is found on Pin C4 of the TCU which connects to Pin D2-15 in the 15 pin Renix Diagnostic Adapter found in the engine bay. This may be some kind of inverted and protected open collector output circuit to vehicle ground, but the Receiver pulls the line high and the TCU will pull the line low to transmit. This gives us a signal that can be read with standard UART.

Data Protocol:

This TCU unit uses a Data Protocol which is as follows:
– Protocol: UART, Standard Logic
– Baud: 500
– Frame Length: 7 Bytes
– Frame Start: 70ms delay after last byte
– Special Case: data bytes are sent every 30ms

Data Framing

Byte Order	MT2500 Reading	Math	Dump
Byte 0	Module ID	DEC	1
Byte 1	TPS Steps	DEC	0
Byte 2	Output RPM	# * 34	0
Byte 3	Brake PRNDL Mode	Bit 0 Bit 4, 5 Bit 7	113
Byte 4	Solenoid 1 Solenoid 2 Solenoid 3 Current Gear	Bit 5 Bit 6 Bit 7 Bit 5, 6, 7	192
Byte 5	700 Solenoid 1 Fault (Stored) 700 Solenoid 2 Fault (Stored) 700 Solenoid 3 Fault (Stored) 700 Solenoid 1 Fault (Current) 700 Solenoid 2 Fault (Current) 700 Solenoid 3 Fault (Current)	Bit 1 Bit 2 Bit 3 Bit 5 Bit 6 Bit 7	0
Byte 6	705 Throttle Position Sensor Fault	Bit 0	0

Notes:
– Most Bit values use Inverted Logic (0 = True)
– Occasionally during bench tests, Byte 6 will have many bits active

Bit Logic Breakdowns

Byte 3 (Inputs)	FALSE	TRUE
Bit 0	C10 Brake Switch On	C10 Brake Switch Off
Bit 1	0	0
Bit 2	C5 Shift Point Logic Ground	C5 Shift Point Logic Open
Bit 3	0	0
Bit 4	C9 NSS 3rd Switch On	C9 NSS 3rd Switch Off
Bit 5	C8 NSS 1-2 Switch On	C8 NSS 1-2 Switch Off
Bit 6	C12 Mystery On	C12 Mystery Off
Bit 7	C11 Power Mode Switch On	C11 Power Mode Switch Off

NOTES:
Bench testing a 90 tcu shows bit 2 as true, but it is false on vehicle.
Bench testing a 92 tcu, bit 0 is false since C10 is held at 12v. Grounding reads true.
Bench testing a 92 tcu shows bit 7 as false

Byte 3 PRNDL	Bit 4 FALSE	Bit 4 TRUE
Bit 5 FALSE	???	1-2
Bit 5 TRUE	3	PRND

Byte 4 (Outputs)	FALSE	TRUE
Bit 0	0	0
Bit 1	0	0
Bit 2	0	0
Bit 3	0	0
Bit 4	0	0
Bit 5	Solenoid 1 On	Solenoid 1 Off
Bit 6	Solenoid 2 On	Solenoid 2 Off
Bit 7	Solenoid 3 On	Solenoid 3 Off

Byte 4 Current Gear	Bit 5 FALSE	Bit 5 TRUE
Bit 6 FALSE	2nd	3rd
Bit 6 TRUE	1st*	4th

NOTE:
– * 1st gear will read as “???” if Torque Converter Bit 7 is FALSE

Byte 5	FALSE	TRUE
Bit 0	0	0
Bit 1	–	700 S1 Fault Stored
Bit 2	–	700 S2 Fault Stored
Bit 3	–	700 S3 Fault Stored
Bit 4	0	0
Bit 5	–	700 S1 Fault Current
Bit 6	–	700 S2 Fault Current
Bit 7	–	700 S3 Fault Current

Byte 6	FALSE	TRUE
Bit 0	–	705 TPS Fault
Bit 1	–	705 TPS Fault?
Bit 2	–	705 TPS Fault?
Bit 3	–	705 TPS Fault?
Bit 4	–	705 TPS Fault?
Bit 5	–	705 TPS Fault?
Bit 6	–	705 TPS Fault?
Bit 7	–	705 TPS Fault?

NOTES:
Bench testing a 92 tcu, bit 0 starts as false if TPS ground is left disconnected. Connecting it will trigger bit 0 true. TPS reads 6 before, and 3 after.

Fun Facts:

– Although the RPM Byte has a full range, the TCU will not report values slower the 300 RPM.

– The Frame Timing comes out to a 1/4 second update time.

– Due to the oddly slow Baud Rate, Teensy Microcontrollers can’t read slow enough unless UART Serial 3 is used or the clock speed is reduced to 48MHz.

– There is a Jumper J2 that changes the module ID. On 2.5L, the Jumper is present and Module ID reads “2” instead of “1.”

– There is an unused Jumper next to J2 that changes the Module ID as well.
– Grounding unused changes the “2” to “1” on a 2.5L TCU.
– Grounding unused changes “1” to a “3” on 4.0L TCU. Grounding J2 makes it read “1” again.