94-95 ABS Actuation via ALDL?

[S.O.B.B.]

I am a licensed brake tech with abs experience but have been out of the biz more than 20 years.I've tried to stay familiar with systems and such. I am unfamiliar with the exact system u have but had this question.what exactly do u want to do and have done so far.do u just want to replace all the fluid as a maintenance job or was the system opened and got air in it.sometimes it's possible to mechanically trick the system.any difference in speed sensor values with brake pedal applied just enough for the brake switch it should trigger an event making the accumulater dump and charge just like an abs activation. I've never tried it with a vehicle raised and apply brake switch and spin just one wheel.the system sees zero speed and suddenly sees speed on one wheel while the brake switch says brakes are on it should trigger an event.I could be all wrong but hoped to help.

[NomakeWan]

Thanks for the response. The idea is to be able to perform the “auto bleed” function outlined in the 1995 Corvette FSM and 1995 TECH I manual, which energized individual solenoids to connect each ABS circuit to its associated corner in sequence so that the ABS system can be bled. This is because the Bosch 5 is a non-integrated system which is completely disconnected from the main brake hydraulics when inactive. As such, if air gets into the ABS circuit or one wants to flush old fluid out after 25 years, a certain procedure must be followed since there are no bleeder valves on the ABS unit itself.

In addition, there is a “relearn TPS zero position” command for the Bosch 2U and Bosch 5 ABS units which must be performed after replacing or altering the throttle body or TPS. These active commands are not provided in the GM serial datastream specifications, so I thought it would be a good idea to discover what the appropriate commands are and detail them for everyone’s benefit.

[NomakeWan]

Sorry for the delay in updating, but I had an opportunity to get my hands on both a Tech 2 and a protocol analyzer the other day and fumbled through using both to try to get some answers. The good news is I got a lot of good data out of it, but the bad news is that the EBCM is far dumber than I anticipated, and so I will need to go back and perform these tests again now that I know what I'm looking for. Not to mention I forgot to probe the command for relearning the TPS. D'oh!

First, the commands to initialize ABS communication are the same as the F-body--or rather, sending any command directly to the ABS once will initialize communication with it. Then any command sent after will actually be operated on by the EBCM. The Tech 2 just sends F9 57 01 00 repeatedly to make sure it's still communicating with the ABS. The rest of the Mode 01 commands are outlined in the DataStream definition files available on this very website, so I won't review them here. However, what I learned is that the EBCM is "dumb," or to be more accurate, it has no ability to run "programs" on request. It does not contain any code to perform an automatic bleed of the ABS. It does not have the ability to perform a self-test on demand; only when the vehicle speed exceeds 3.6 MPH and no other way. So, this being the case, how does the scan tool perform these tasks?

In the case of the automatic self-test, it actually sends a program to the EBCM, followed by a command to execute the program. Considering how small said program is, I imagine that all it's actually doing is creating a new interrupt vector that forces the EBCM to backtrack and perform its internal test on command. I'm not sure why they wouldn't have just programmed it to do that to begin with, but whatever. In any event, this is a Mode 06 command followed by a Mode 04. The command is as follows:

F9 79 06 04 00 00 12 1D 06 0E 00 EF 88 1C 71 BF 36 07 40 B0 14 7C B3 01 B1 20 14 EF 6F 1C B0 7C 14 F0 91 40 36 F0 87 ;Upload automatic test program
F9 56 04 AD ;Execute automatic test program

In the case of the automatic bleed, unfortunately the Tech 2 is in fact doing all the heavy lifting while the EBCM just responds to basic commands. The Tech 2 is sending Mode 04 commands to actuate specific solenoids and relays in a specific sequence at specific times in order to facilitate the bleeding of the system. This is disappointing as you can't just send a single command and be done, and also because my analysis of the communication is incomplete. My netbook couldn't keep up with the signal analyzer I was using, and had a nasty habit of dropping out when attempting to record for longer than a few minutes. The entire auto-bleed process can take several minutes per corner, so I don't have a complete dump of the process just yet. I will need to get access to both the Tech 2 and the analyzer (and a better computer) again so I can dump the commands and timestamps to know how to replicate this process on free software. In addition, I need to get the command to run the TPS relearn process.

I'll also try to get the commands for the '94 and earlier cars. While they don't have an auto-bleed, they do have an automatic self-test and a TPS relearn procedure, so it would be good to know those as well.

I'll update again once I have another opportunity, but it may not be for a few weeks.

EDIT: After further testing, I can't figure out what exactly the code above actually is. Nor does the code I received from my attempts to bleed the brakes make any sense. I have clear logs of three out of the four corners of the car, with the only missing log being the first corner (left rear). Yet attempting to send the commands to the EBCM results in nothing whatsoever happening. Sending the above Mode 06 command does receive the "acknowledgement mode 06" response, but after that the only thing that happens is that the ABS motor flicks on for a moment if you send any Mode 04 command (regardless of the content). It will still respond normally to a Mode 01 command, and of course will reset properly after being issued a Mode 00 command. This leads me to believe that my analysis missed out on some other piece of code that was transmitted earlier in the sequence, perhaps even once ABS was selected in the Tech 2.

As before, it just means I have more work to do. I'll update as soon as I have a chance to do more analysis with the tools.