What Does TDC Offset Actually Do???

[CapnAmerica]

When you give an engineer a little information, often he wants more than is his share...

Can someone explain, in a technical manner, what the TDC offset actually does in truck operation?

We set actual timing to +3.5 deg relative to the commanded Desired timing of 0. When the truck resumes normal operation, the actual timing tracks the desired closely (at least on MY truck...). When I fiddle with the TDC offset, I am readjusting the setting which locked down(???) the Actual timing at +3.5!!! Even after I do that, the Actual timing doesn't seem affected.

(Of course in my case--see post below--I can't seem to get the TDC offset to settle down to -1.5, or -1.94, or even ANYTHING negative!!!)

Any help from you experienced and/or smart folks would be greatly appreciated: I'll dance at your wedding, adopt your teenagers (no, better not put them on the bus right yet), or something...

Charlie Jones
a.k.a. Capn America in TN

96 GMC Sub with "new" inj pump and less-than-new timing settings, Kennedy turbo boost controller (love it, since I plugged it in correctly!)

[gmctd]

Briefly - it is to compensate for total mechanical slop from timing chain\sprockets and indexing tolerances, as machined.

TDC OFFSET tells the PCM to make up for those inaccuracies.

So, if you tell it the timing is retarded, compared to original factory number, the PCM will electrically advance base timing to make up, giving better performance than factory dictates - warranty, environmental, etc.
Thus, the negative numbers: -1.5, -1.95

Changing injection pump usually requires stamped mark (static)alignment only.
Change timing sprockets\chain, camshaft, crank, harmonic balancer, timing cover, Crank Position Sender, etc, requires new TDC OFFSET numbers.

Almost fergot to tell you - so far, only genuine TECH-II or Snap-On 2500 have been able to successfully set up all timing numbers.

And, it would appear that OFFSET LEARN in the manual is only for new, or cleared by TECH-II, or reflashed PCM's. A PCM will not learn a new TDC OFFSET with a prior number in the register.

[ 01-07-2004, 07:59 PM: Message edited by: gmctd ]

[moondoggie]

Good Day!

gmctd: As always, thanks for the great technical input.

I have a further question: Once or twice I've read here of folks with OBD-I trucks like mine doing a TDC Offset Learn by turning the key to run (NOT crank) & holding their footfeed to the floor for ≥ 45 seconds. Is this so? If so, what does it set the TDC Offset Learn to?

TIA & Blessings!

Brian Johnson, #5044

[moondoggie]

Good Day!

Oops! Maybe this only works for OBD-II trucks. I accidentally bumped into one of the posts I asked about - it's in the topic Timing 6.5 TD (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005498), jjbouwens post.

The basic question remains: What do you know about this?

Blessings!

Brian Johnson, #5044

[GMCfourX4]

The procedure you're referring to is the one the Snap-On scanner instructs you to use to initiate TDC Learn. The TechII has 2-way communication with the computer, and can signal it to start the learn, but the MT-2500 does not, so you have to go through this procedure which tells the ECM to do the TDC Learn. Basically, you start with 3.5* of actualy timing, then do a TDC learn, then in VERY small increments, move the pump and re-try the learn until you get the figure you're looking for.
-Chris

[Uncle Wally]

In attempting to adjust the timing on my 94 with a tech1. I could not get the desired timing to read 0 degrees. Both desired and actual were both at 3.5 degrees.

How do you know for sure your at 3.5 if you can't get the desired to read 0?

Waldo

[CapnAmerica]

Many thanks for the info!

For TDC offset after pump change, the AllData procedure tells me to warm up the engine (>170 deg F), then...

1) ignition on/engine off--hold accelerator to the floor for 45 sec min.
2) ignition off--wait 30 sec min.
3) ignition on--verify TDC offset has gone to 0. (Engine still must be above 170 F)
4) start engine--computer "relearns" a proper TDC offset. In this step, I have the scanner on and watch it go wherever it's going--so far it's never gone to the right place, after making MANY careful adjustments.

If I can't get the TDC offset to set negative, could this tell me there is excessive slop in the chain, etc.? Everything in the procedure seems fine and healthy--operating as I would expect.

Thanks again.

Charlie Jones
a.k.a. Capn America in TN