JoBy

With the large HEI distributor you set base timing by disconnecting the EST wire. By doing that the ignition module uses the internal reference to fire the spark.

I just found out that the Camaro with small HEI and external coil uses another procedure to set base timing.

* start engine.
* Put a jumper between A and B on the ALDL connector.
* set base timing.
* Remove jumper.

As I understand the ECM is programmed to use a fixed timing when you short A and B on those cars.

What if you have a small HEI with an ECM made for the large HEI. How do you set base timing?

89RsPower!

sounds like someone wants you to fry your ecm.. thats a mixture of pulling codes and setting timing......

Red Devil

Sounds like monkey-spank!

Field Service Mode (heavily 'paraphrased' from some tin can head dude):

Insert a jumper wire in the ALDL connector between terminals 'A' and 'B', just like would do to scan for error codes AFTER the engine is running. As long as you don't crank the engine while the jumper is in place, you won't cause any damage to the ECM. Jumping the terminals while running force the ECM into Field Service Mode.

While in this mode, the SES lamp will flash regularly. If the engine is in open loop, the SES light will flash rapidly about 2 ½ times per second. If its in closed loop, it will flash about once a second.


Now, if I have this right...When in closed loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 base line, and more than once per second indicates the ECM is leaning the mixture below the 128 base line.

I have also seen in some books reference that you can set the base timing while in field service mode, however, I have never attempted to do so. For some reason I keep thinking TBI when it comes to mind, but again, I'm not sure.

JoBy

We disconnected the EST wire first to check base timing, but it would not start without it. And it seems it is not the only one.

https://www.thirdgen.org/techbb2/sho...ht=base+timing

I then got the information about using a jumper from a GM service book.

Red Devil

The timing may be too retarded then. You should still be able to start the car with the est disconnected IIRC this has come up before.

Try entering field servie mode and see what the timing comes up as.

JoBy

Ok, so the EST wire should still work like the large HEI with internal coil ?

Red Devil

Well, my TPI has a remote coil and I set it as discussed with the est diconnected.

JoBy

Problem solved.

The timing was way retarded and that is why it did not run with the cable disconnected.

Red Devil

Shaweet!

89RsPower!

I shouldn't respond to things I'm not positive about after a sleepless night.. shoulda read the first post a little better. I've always been told that you can damage an ecm by starting it with a jumper in and I've been told that by inserting it with it already running puts it into field service mode (which was clearly stated in plain english that this was his intention), just never heard about setting timing that way.. only way I've set timing on a 3rd gen is disconnecting the est and doin it the old fashioned way..

89RsPower!

Now for my question.. from what I'm reading in this post it seems to indicate that if your base timing is way off (so far off that the engine will not run with the etc disconnected) that putting the car into field service mode will somehow run a specific base timing allowing a car with inaccurate base timing to run when it otherwise wont? Hope that made sense..

Red Devil

Yea, see the problem is I've read it a few times years ago, never tried it.
Allegedly in field service mode it is supposed to run on base timing. I would find this hard to believe if you are stalling out when disconnecting the est wire.
I would logically think that if you force the car to run on base timing one way and stall it the same would happen the other way. Then again you are forcing it while running in field service mode as opposed to trying to start the car with the est by-passed.
The guys at diyprom or dfi/ecm may know, I just have never been motivated enough to figure it out as it has always turned out to be retarded timing as the culprit whit the est by-pass and no start issue related above.
Maybe someone smarter may know the answer to this one cuz I don't.