Select Monitor Adapter
Subaru Diagnostic Ports
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Engine Control Unit (ECU)
Reading Error Codes
Transmission Control Unit (TCU)
Reading Error Codes
Other Control Units
Air Conditioning Unit 4WS Control Unit Cruise Control Unit
Select Monitor Cartridge
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Custom TCU Development
4 Wheel Steer Control Unit
In 1992, Subaru produced a 4WS version of the SVX. Known as "Version L", they were only sold in Japan, and can be identifed by the applied model code "CXDA25E" on the VIN plate. I am lucky enough to have two of these rare and special vehicles in my collection. During high-speed cornering, or avoidance maneuvers, the version L feels much more stable and composed than a standard SVX.
There is a trick to getting the 4WS unit to communicate and it took me a long time to find it. As usual, you first send a reset command "12000000". The first 4WS address you ask for after the reset MUST be FFFF. Otherwise it just ignores you. So you send "92FFFF00" that makes the unit start talking and afterwards you can ask for other locations or the ROMID in the usual way.
The 4WS ROMID on both of my cars is 719210, but there doesn't seem to be anyway to download the ROM. I tried dumping the whole address space and it returns zero for every address except FF00 to FFFF. Looks like the unit only allows you to read the RAM portion of the address space via the SSM protocol. Here is a typical 4WS RAM dump, taken while the car was parked and the engine was off.
I found the 4WS parameters in the Select Monitor Rom. I've listed them below with my best guess as to what they might mean.
Subaru's parent company, Fuji Heavy Industries were granted a number of US patents for 4WS systems during the SVX development. The following documents appear to describe the SVX 4WS system. No other car was ever built with this system. Apart from these patents, there is very little information available in English.
5018594 "Rear-wheel steering system for four-wheel steering vehicle"
The 4WS control unit has a Mitsubishi MH73308CP8 microcontroller. As far as I can tell, it is a custom made and undocumented part. Currently, I cannot see any way of extracting or modifying the firmware in this unit.