Different Diagnostic Modes?
1985, OBD1, Corvette..
I just discovered that my scanner lists (and can cause the car to enter) 3 different "Diagnostic Modes" They are: 1) "Field Service Mode" 2) "Backup Fuel Mode" 3) "ALCL Mode" As soon as one is selected, the tool (Mac Mentor) begins to stream data.. But I can do that anyway? Without selecting a special "Mode" What are these different modes and how are they useful? Anyone? |
I googled it and the only hit I got was from SnapOn, and specifically for a Chevy GEO?
*confused* |
Originally Posted by confab
(Post 1586455546)
1) "Field Service Mode"
Originally Posted by confab
(Post 1586455546)
2) "Backup Fuel Mode"
Originally Posted by confab
(Post 1586455546)
3) "ALCL Mode"
ALCL (Assembly Line Communications Link) was what GM called the ALDL (Assembly Line Diagnostic Link) back in the '80s. |
Originally Posted by Cliff Harris
(Post 1586458047)
This mode shorts the A & B terminals of the ALDL connector together. It sets the timing to a fixed advance and some other minor things.
Also known as "3.9K mode" because it puts a 3.9K resistor across the A & B terminals of the ALDL connector. This is what GM calls limp mode. It basically turns off the microcontroller in the ECM and the timing and injector pulse widths are controlled by the FMD (Fuel Modeling Device) IC using the resistor networks in the MEMCAL. The MEMCAL is MEM for program memory and CAL for the resistor network calibration. Also known as "10K mode" because it puts a 10K resistor across the A & B terminals of the ALDL connector. It sets the advance and the idle is set to 1000 RPM, plus some other miscellaneous stuff. ALCL (Assembly Line Communications Link) was what GM called the ALDL (Assembly Line Diagnostic Link) back in the '80s. |
There is also a "nothing" mode -- sorta "none of the above". That would be the normal mode. It is sometimes called ">20K mode" (where ">" means "greater than"), which can confuse people (I've seen it discussed on other forums). Basically that means "open" between ALDL terminals A & B.
The 10K mode has the least effect of the three modes originally listed. |
Originally Posted by Cliff Harris
(Post 1586516511)
There is also a "nothing" mode -- sorta "none of the above". That would be the normal mode. It is sometimes called ">20K mode" (where ">" means "greater than"), which can confuse people (I've seen it discussed on other forums). Basically that means "open" between ALDL terminals A & B.
The 10K mode has the least effect of the three modes originally listed. Also, is there an exhaustive list of everything that is different in the calibration in the various mode? Thanks, Jeff |
Examples of when you'd choose a particular diagnostic mode?
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There is a list of what the modes do in the FSM. Unfortunately it is very vague. Here's some info (all quoted from my '86 FSM, page 6E-7):
NORMAL OR OPEN MODE On engines that can be monitored in the open mode, it allows certain parameters to be obtained without changing the engine operating characteristics. The parameters capable of being read vary from engine family to engine family. Most "SCAN" tools are programmed so that the system will go directly into the special mode and the "open" mode must be selected when it is available. 10K MODE, SPECIAL MODE OR ALCL MODE In this mode, all information incorporated into a specific engine and ECM is obtainable. However, in this mode the system operating characteristics are modified as follows: - Closed loop timers are bypassed - EST (spark) is advanced - IAC will control engine idle to 1000 RPM +/- 50 RPM - On some engines, canister purge solenoid will be enabled - Park/Neutral restrict functions will be disabled FACTORY TEST MODE, BACK-UP OR 3.9K MODE When in this mode, the ECM is operating on the fuel back-up logic and calibrated by the Calpak (back-up chip). The Calpak Part of Mem/Cal is used to control the fuel delivery if the ECM fails. This mode verifies that the back-up feature is OK. The parameters that can be read on a "SCAN" tool in this mode are not of much use for service. --------------------- End quote --------------- As you can tell from the language, this section of the FSM is generic and applies to all GM cars. |
Originally Posted by jsiddall
(Post 1586517636)
Right, but I though "normal mode" didn't send ALDL data. I could be entirely off base on that though. That does raise a broader question: is there a difference in the ALDL data that is sent in the various modes?
The ECM has its own internal modes as well as the others listed. I have been meaning to create a spreadsheet which contrasts the differences in the modes. The basic mode spits out a bunch of data, but there are other modes that can output just the info you ask for. I don't know of any scan tool that implements those modes. There are 5 modes, numbered 0 through 4. I'll quote the 160 baud mode and mode 1 below because it is the one you normally see on a scan tool. Here's what comes out of my 1227165 ECM running BUA code (the hexadecimal number starting with the "$" sign is the address in RAM where the data comes from): Code:
******************************************************* |
Wait, I thought the 1227165 ECM ran at 8192 baud?
Looking through that list it seems much more extensive than what I remember seeing on the '85 only 1226870 datastream. But it has been so long I don't remember anymore. |
Originally Posted by jsiddall
(Post 1586569418)
Wait, I thought the 1227165 ECM ran at 8192 baud?
The scan software sends a magic series of bytes to switch it to 8192 mode. Those bytes are listed in the "Mode 1" list I posted earlier in post #9: Code:
* ALDL DEVICE MUST REQUEST MODE 1 BY TRANSMITTING THE FOLLOWING MESSAGE TO THE ECM: |
Got it. Any idea what happens on an 1226870 ECM? It doesn't have 8192 baud so presumably it also doesn't have a "mode 1".
Edit: Re-read your original post again. Does the 1226870 deliver the same info as the 1227165 in 160 baud mode (approx. 25 parameters)? |
Originally Posted by jsiddall
(Post 1586575783)
Does the 1226870 deliver the same info as the 1227165 in 160 baud mode (approx. 25 parameters)?
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No problem. Thanks for the detailed info you provided. I'll see if I can find out more about the 6870 and post my findings. It's currently snowing here so that may be a while yet!
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