I need to check my Base Timing - 1989 GTA TransAm - 350 5.7L TPI with 1227165 ECM.
Searching on ThirdGen and Google - people are disabling the Electronic Spark Timing (EST) module by disconnecting the Tan/Blk wire in the engine bay (after engine at operating temperature) and then using a timing light to set Base Timing (6 degrees BTDC).
In my 1989 Firebird Service Manual section 6E3-A-48 gives the EST circuit description and explanation for Code 42 - the Tan/Blk wire is annotated with "SET TIMING CONNECTOR".
But, in section 6D4-11 (Ignition System) - there is a procedure for Setting Ignition Timing - Step 1 says "With engine RUNNING at operating temperature ... ground the diagnostic terminal of the 12 terminal ALDL."
It then goes on in the next steps describing loosening the distributor, rotating it and using the timing light to set desired timing.
Step 4 says "... engine still running, unground the diagnostic terminal (if done before engine is shut off, no trouble codes will be stored)."
There is a diagram show the bridging of the A and B terminal.
Doing this places the ECM in "Field Service Mode". Field Service Mode is described in section 6E-6 Driveability and Emissions, but doesn't mention about the EST, only that the "Closed Loop" timer is bypassed and Open/Closed Loop is indicated by flashing the Service Engine Soon light.

Does anyone know if "Field Service Mode" disables the EST module - effectively doing the same thing as the physical Tan/Blk disconnection is doing?

Is this Timing procedure in section 6D4-11 correct?

Many thanks.

 

You can do it either way, doesn't really matter. 2 ways to do the same thing. Just unplug the ecm fuel pump fuse by battery when done setting the timing by unplugging the tan/blk stripe wire.
Tolerance stacking from everything will just get you a close setting anyway. With stock chips, i run 10-14 base timing. Depends a lot on the fuel type you use, chain stretch, balancer ring movement..

 

Thanks for the reply and your info.

My ALDL cable from OBD Diagnostics has 4 pins to jumper in different configurations - one for jumpering A and B.

Since I'm only checking the timing, I'd rather not induce a code 42 in the process and then have to remove the battery to clear the code. I'll retain the ECM's learnt tuning values as well.