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Thread: 94-95 ABS Actuation via ALDL?

  1. #1
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    94-95 ABS Actuation via ALDL?

    Hey everybody,

    Little side topic since itís not directly engine-related, but lately Iíve been reading spec sheets and manuals and noticed that one thing a Tech I can do that (as far as I know) no PC software can is talk to the ABS unit and get it to actuate. This is recommended as part of a full brake system bleed procedure. Itís just another message over ALDL so Iím not sure why this doesnít seem to be possible. All the research Iíve done shows that people either buy a Tech I, take it to a dealer with a Tech I, or do it sketchy by driving down gravel and forcing the ABS to come on.

    So, anyone know what ďmagicĒ the Tech I is doing on the ALDL for its ABS bleed procedure?

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    It is a matter of sending a command via ALDL. You can also log data. A definition needs to be add to eehack to enable logging.

    The commands might differ from abs to abs module. I have sniffed the commands for 94-95 f-body abs. Finding it is another matter.

    Tech2 and mdi also work for some of the 91-on vehicles. Somehow mdi have trouble connecting to some of the ALDL modules. So it is hit or miss there.

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    Hi
    Gary Dougs excellent 9495Scan software can communicate & control ABS on 94-95 F bodies as well as other engine controls (fans etc.)

    I wrote software to communicate with the ABS and SIR system about 15 years ago to access the stored fault codes & clear ABS History.
    It was fairly basic but worked.
    It used similar ALDL control messages to the PCM.

    Steveo's EEHack can send and receive messages to 94-95 LT1 PCM's (not sure what else it can communicate with ( possibly anything that communicates at the 8192 baud? rate)).
    If you know the message to send, you can decode the reply easy enough.
    The 160 & 8192 baud message streams are available on the internet.

    HTH
    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

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    Quote Originally Posted by kur4o View Post
    IThe commands might differ from abs to abs module. I have sniffed the commands for 94-95 f-body abs. Finding it is another matter.
    The message streams are floating around the 'Net.
    I have them saved away.

    I also acquired a GM factory 1993 F Body ABS suppliment which has lots of info.It also mentions the Tech 1 and 6 test modes.

    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

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    eehack is ready to request and log any device on the bus, that's why i made it support external definition files, but nobody has actually done any work to support the ABS controller or anything else yet, and i wont do it without a vehicle

    as far as actuation goes, if you have the commands, you can use the RAW COMMAND thing in eehack. it'll do the checksum and length bytes for you, you supply the rest. just make sure the thing is actually connected to the datastream when you use raw command as it sometimes has a total freakout if it isn't.....

    i did try scan9495 one time and never made it work.

    personally when i bleed the brakes on an ABS equipped car, i just pull the ABS fuses out and hammer the brakes a bunch of times. it's always worked for me to release whatever hydraulic device is trapping the air.

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    Thanks all. It occurs to me I forgot to mention in the first post that my cars are both Y-bodies. I've slapped a signature on now so this won't be a problem going forward. As such Scan9495 never worked properly, since it was designed to talk to an F-body, not a Y-body. EEhack of course works perfectly.

    Terminal_Crazy, since you seem to have the 'secret sauce' in your possession, and this being a hacking and reverse-engineering forum, would you mind just sharing said information? I don't mind doing manual commands or adding definitions, I had to do that back in my Subaru days when I was helping figure out communication with the Subaru SVX's various computers. But I'm relatively new to GM, so I wouldn't really know where to look or what magic search terms to use to get what I want. Any assistance you can provide would be appreciated.

    EDIT: If by "definitions" you mean the ones from this very website, I did already have those, but they only contain commands to read data from the ABS/ASR computer, not to force it to actually perform tasks. The relevant definitions to my vehicles are here:

    1992/1993/1994 Y-body Bosch ABS/ASR: http://gearhead-efi.com/gearhead-efi/def/aldl/A175.DS
    1995 Y-body Bosch ABS/ASR: http://gearhead-efi.com/gearhead-efi/def/aldl/A288.DS

    But yeah, if someone knows the proper commands to tell the ABS computer to actually do something like the Tech 1 does, I'm all ears.
    Last edited by NomakeWan; 01-30-2020 at 05:00 AM.
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

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    Quote Originally Posted by NomakeWan View Post
    Thanks all. It occurs to me I forgot to mention in the first post that my cars are both Y-bodies. I've slapped a signature on now so this won't be a problem going forward. As such Scan9495 never worked properly, since it was designed to talk to an F-body, not a Y-body. EEhack of course works perfectly.

    Terminal_Crazy, since you seem to have the 'secret sauce' in your possession, and this being a hacking and reverse-engineering forum, would you mind just sharing said information? I don't mind doing manual commands or adding definitions, I had to do that back in my Subaru days when I was helping figure out communication with the Subaru SVX's various computers. But I'm relatively new to GM, so I wouldn't really know where to look or what magic search terms to use to get what I want. Any assistance you can provide would be appreciated.

    EDIT: If by "definitions" you mean the ones from this very website, I did already have those, but they only contain commands to read data from the ABS/ASR computer, not to force it to actually perform tasks. The relevant definitions to my vehicles are here:

    1992/1993/1994 Y-body Bosch ABS/ASR: http://gearhead-efi.com/gearhead-efi/def/aldl/A175.DS
    1995 Y-body Bosch ABS/ASR: http://gearhead-efi.com/gearhead-efi/def/aldl/A288.DS

    But yeah, if someone knows the proper commands to tell the ABS computer to actually do something like the Tech 1 does, I'm all ears.
    Hi
    I guess you have all those data streams.
    That's what I had long before I read anything about anyone talking to the PCM. I think TunerCats was it at the time.
    I was only looking to access the ABS fault codes which are listed in the streams (F body anyway)
    Since I had Tunercats & could access the engine info I didn't see a need to access any more. I also didn't get to decode their raw output for other filtering software.
    It was Steveo's EEHack that rekindled my interest as I could access the log data.

    I'll mention Gary Doug again. His Scan 94-95 software accesses the 94-95 F Body pcm's.
    He can also actuate the ABS motors.
    I think he had a Tech 1 that he has reversed engineered (read the output code commands) to do this.

    He is on lots of Fbody sites, www.CamaroZ28.com www.Ls1Tech.com .

    Either contact him or see if anyone else has decoded a Tech 1.
    It was always my intention to get one but the genuine ones were dear.
    I think you can get a chinese copy for a couple of hundred $$.
    It would be good if we could get a full list of control commands, Anybody interested?

    Another thought:
    Have you tried shutting up the babble on the other units on the bus or as I've read here pulling fuses for other modules on the bus that aren't required.
    Can EEHack issue multiple commands?
    I recall having to send a couple of "quiet" commands before asking for data or I got corrupted replies.

    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

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    Thanks for the response! The issue with Scan9495 is that the bus addresses are actually different between the F-body and Y-body; so an F-body "shut up" command isn't the same as the Y-body "shut up" command and such. That being said it should be possible to fire up the program and just monitor the commands it's trying to send and see what those are doing, then try them on the Y-body with the appropriate address.

    Also, I had a look through my 94-95 service manuals and some documentation on the Bosch ABS/ASR system with the Tech 1, and it appears that the ability to automatically bleed the system was actually only available in the '95, when they switched to the Bosch 5 ABS/ASR unit. The Bosch 2U in the 94 doesn't have an Auto Bleed description in the Tech I, nor does the '94 FSM have a section about auto bleed (only a manual bleed for the pilot feed line). So it looks like it's just the '95 that would have this.
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

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    I have a '95 F body and haven't had any issues bleeding the brakes.

    I've used a vacuum unit that you fit onto each brake nipple & pull the fluid through.
    I generally do it the old fashioned way, pump up the brakes and hold, then release & shut the bleeder valve.
    There are also bleed nipples on the ABS unit at the front.

    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

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    The ‘95 F-body did not have the Bosch 5 ABS/ASR unit, and these being non-integral you can’t bleed them using normal methods because if the ABS is not operating, then it is not connected to the brake hydraulic system whatsoever. The only way to bleed the ABS is to actually have it actuate its solenoids to connect it to the brake lines, at which point conventional bleeding methods are used.
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

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    Isn`t there any manual way of opening the valves. On f-body there is procedures to start the engine for xx seconds twice and then shut off and start bleeding.

    On vettes you have to shut up the ccm module. It is the standard mode 08 command but the ID is F1 [ecm id is F4]. When the ccm gets quiet you can poll any module on the bus. One of things to do is to rip off mode4 commands for f-body ecbm. I doubt they will be the same for the ybody module, but you never know.

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    If there is a manual way, it is not detailed in any of the manuals I have access to. The unit is non-integral so it is totally separate hydraulically until activated for an ABS event. The only method detailed to make this connection for bleeding purposes is connecting a Tech 1 and following the Auto Bleed procedure. I was kind of hoping someone here just happened to know what commands it was sending already, but I’ll start by seeing what the F-body says and hope GM decided not to stray too far.
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

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    You can start experimenting. I bet the command will be mode 4 request. You have to find the ID of module. In the ds file it is defined as $F9.
    First send f1 xx 08 message to get the ccm quiet. You can dump the bus until a response is received.
    You will likely need to resend that every 2 seconds.

    That you can start sending
    F9 xx 04 [...] messages.

    First figure the length of [...] It can be 6,8,10 or 12 or ?? bytes. If it is shorter than needed it will be a problem. A longer message will be accepted without issues. The extra part will be ignored.
    Sending [....] filled with zeroes is also safe. That way you can check if the length is correct and the message is accepted. Than you can start sending random data like 01s, 02s, 04s, 08s, 10s, 20s, 40s, 80s and listen for unusual activity at the solenoids, lamps actuation and so on. Write A map with the discovered info for reference. Mode 4 is used as follow. 2 consecutive bytes. 1st byte sets bit $04 means enable control of XX solenoid, second byte bit $04 means switch on or off the solenoid.

    A sample message will be
    00 00 04 04 00 00 00 00 00 00
    or
    04 04 00 00 00 00 00 00 00 00
    And so on until you found some actuation.
    Be prepared to turn off ignition if something unusual occurs. It will clear the commands and reset the ebcm.

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    I got some free time and ripped some of the abs communication of 94 f-body. Here are the commands.
    Attached Files Attached Files

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    Thanks for that! Though the system is completely different (the Bosch 5 is a 4-channel unit which emulates a 3-channel in software) there is a chance that GM had enough influence on its design that the serial data commands may be very similar. I’ll have time to mess with it this Friday.
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

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