Bendix 9 ABS Module Datastream

Here is my reverse engineering work on the Bendix 9 Adaptive Brake Module found in 89-90 Renix Jeeps. This self contained unit is in charge of operating the vehicles entire ABS system and has a data stream output that we can use for data and diagnostics.

TX output is found on Pin C4 of the ABM which connects to Pin D2-5 in the 15 pin Renix Diagnostic Adapter found in the engine bay. This is an open collector output circuit to vehicle ground, so the Receiver pulls the line high and the ABM will pull the line low to transmit. This gives us an inverted signal that will need to be flipped to be read with standard UART.

Data Protocol:

This ABM unit uses the standard Bendix Data Protocol which is as follows:
– Protocol: UART, Inverted Logic
– Baud: 62,500
– Frame Length: 16 Bytes
– Frame Start: value 0 is sent
– Frame End: value 255 is sent
– Special Case: data bytes must send duplicate 255 value to differentiate from Frame End. Duplicate must be discarded to maintain proper framing.

Data Framing

Byte Order
MT2500 ReadingMath
Start Byte0
Byte 0PROM 1HEX146
Byte 1PROM 2HEX20
Byte 2LF Wheel(MPH)#/3.10
Byte 3RF Wheel(MPH)#/3.10
Byte 4LR Wheel(MPH)#/3.10
Byte 5RR Wheel(MPH)#/3.10
Byte 6LF Sol
RF Sol
Rear Sol
Bit 0, 3, 6
Bit 1, 4
Bit 2, 5
Byte 7RF Sol
Rear Sol
Diff Press
Boost Press
Bit 0
Bit 1
Bit 3
Bit 4
Bit 5
Byte 8Low Accum
Modulator Rly
Sol Undervolt
P-Brk/Lo Flu
Bit 0
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Byte 9?17
Byte 10?0
Byte 11?0
Byte 12Error CodesBit 0 – 60
Byte 13Error CodesBit 0 – 60
End Byte255

– Bytes 9, 10, and 11 are not displayed on the MT2500. I affectionately refer to these as Mystery Bytes.
– Byte 9 displays a non-zero value while bench testing, indicating a possibly meaningful value.

Bit Logic Breakdowns

Bit 0LF OffLF Build
Bit 1RF OffRF Build
Bit 2Rear OffRear Build
Bit 3LF OffLF Decay
Bit 4RF OffRF Decay
Bit 5Rear OffRear Decay
Bit 6LF OffLF Isol
Bit 7

– Build and Decay can display simultaneously
– Isol will take priority over Build/Decay inputs

Bit 0RF OffRF Isol
Bit 1Rear OffRear Isol
Bit 2
Bit 3G-Sw ClosedG-Sw Open
Bit 4Diff P OpenDiff P Closed
Bit 5Boost P OpenBoost P Closed
Bit 6
Bit 7

– Isol will take priority over Build/Decay inputs

Bit 0Low Acc FalseLow Acc True
Bit 1
Bit 2Pump OffPump On
Bit 3Mod Rly FalseMod Rly True
Bit 4Sol UndV FalseSol UndV True
Bit 5P-Brk Flu FalseP-Brk Flu True
Bit 6Brake NoBrake Yes
Bit 7
Bit 0809 ECU-Test
Bit 1808 Modulator
Bit 2810 Solenoid Undervoltage
Bit 3813 Brake Switch
Bit 4814 Low-Fluid
Bit 5811 System Relay
Bit 6812 Motor Pump
Bit 7

– Bit 2 fault code was incorrectly labeled “801” on Cartridge MT25001099 Display

Bit 0815 RR Speed Sensor
Bit 1816 LR Speed Sensor
Bit 2817 RF Speed Sensor
Bit 3818 LF Speed Sensor
Bit 4805 Differential Pressure
Bit 5806 Boost Pressure
Bit 6807 Low Accumulator
Bit 7

Fun Facts:

– First off, I’d like to thank David for sending me this module all the way from the Czech Republic! It was due to his generosity that I was able to do this in the first place, so thanks!

– The Diagnostic Adapter Pinouts changed at the 88-89 crossover to make room for the new ABS Module features. Pin D2-5 was “Latch B+” but moved to Pin D2-12 to make room for the ABM TX line.

– These Bendix Units always seem to have a strange byte gap pattern to them. This has the most uniform pattern I’ve seen so far. Starting with frame end, Bytes will be sent in pairs of 2 for 5 sets, then send 4 single sets, then finish with 1 final pair of 2. Total Frame Length is 52.3ms with byte gaps of 1ms, 4ms, and 5ms.

– There is another ABM pin D6 which goes to 6 pin Diagnostic Connector D1-4 but is not needed for data. I noticed the MT2500 pulled the pin to 12v, where the data line was only pulled to 5v. I have no idea what this is for, but it’s there!

– There is another ABM pin C2 which goes to 15 pin Diagnostic Connector D2-14 labeled “BRAKE ALERT-A.L.M. C2”

– There are some missing fault codes according to a Bendix 9 Manual here:
800 – No voltage at ECM
801 – No serial data from ECM
802 – No parking brake signal
803 – Warning lights inoperative
804 – Yellow ABS warning light inoperative
819 – Open circuit at diagnostic connector

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