What is this special timming proc on 98 an 99 PCMs

[AndyL]

While searching I have read there is a special timming procedure for 98 and 99 vehicles, what does it do and what is the exact procedure, most of the messages are hints only.

Thanks for the help

Andy

[Turbine Doc]

Andy,

Timing is best set with a tech 2, that said it can be done on a 98 & 99 for sure without T2, as I've done it myself several times using just a Snap-On MT2500, I've done it only on those 2 years which are theft deterrent equipped systems.

What I don't know is if this works for all OBD2 PCMs or just the ones with theft deterrent. It is for this reason I've not posted what worked for me in detail.

That said, for the general population, what follows is what is in the manual for my year model truck and is paraphrased for simplicity.

From what I read there are 2 different routines 1st is setting timing following pump replacement, this one requires a 2 directional scan tool, start engine bring to normal Op temp 170F or > coolant temp, use time set command to command timing desired to 0 deg, actual timing should read 3.5 deg on avg.

MARK IP BEFORE MAKING ANY ADJUSTMENT SO YOU CAN GO BACK TO WHERE YOU STARTED IF NEED BE. Also (1) mm rotation roughly a scribe line width = 2 deg. timing change, make small moves.

If above 3.5 avg., shut down engine, loosen mounting nuts and rotate pump toward pass side, if below 3.5 rotate toward drivers. These are hard to get at I moved the AC compressor off of it's mount hoses still connected to get better access, also I don't think you can access them without making a custom wrench or buying the Snap-On set which I did. IP turning wrench is also an asset. Continue this until 3.5 avg. is satisfied.

Best as I can figure this is the base timing, once out of time set commanded to 0; the computer commands where the timing needs to be. Can be greater than 3.5 avg seen in time set command mode.

The next procedure is the TDC offset which can be done independent of time set, and can be done with just a reader that displays TDC offset reading. Nothing in the manual directs that both be done at same time, but I did the time set before doing TDC offset just to see if I was in the correct ball park.

Same as above bring engine to coolant temp 170 or above, see that all codes are clear (clear codes before proceeding).

Shut off engine, then turn key to run not start and accelerator to floor minimum 45 sec,(this activates TDC learn), then key off 30 sec.

Restart truck if coolant is above 170 and rpm below 1500 PCM automatically relearns offset, may stumble on start 1st run until new offset is accepted.

If you want TDC to go more negative(-), shutdown truck as before and rotate pump toward driver side, if you want a more (+) offset go toward pass side, same (1) mm =2 deg be frugal with adjustment.

Bolt all down do key on key off routine pedal to floor as before, and restart to do learn, and repeat until at desired TDC offset.

As you can see, some trial and error this way so the T2 is the way to go; JK says it's a set and forget with that tool, but since I don't have access to one this is what worked for me, I've got more time than money.

You have to be on top of your game this way also, don't get too agressive and should have no problems,

All else fails, go back to original mark, and relearn that setting. If you want to talk more about itget with MP to set up off page contact. I'm not claiming this will work for all, but it did work for me.

[ 12-03-2003, 04:46 AM: Message edited by: tbogemirep ]

[AndyL]

That is good information, thanks for the detail.

[Turbine Doc]

I just found out this weekend the Snap On scanner does not display same value for TDC offset as the Tech 2 tools do, the T2 value for -1.76 to -1.94 offse on a MT2500 scanner is -5.4, and value -.25 to -.75 is shown as -4.8 on MT2500.

Compaired this to Billman's, Brian Alley's, & GMCTD's T2 tools. Matco, MAC and GM T2 all agree with each other, 2 MT2500s mine and 2 more agree with each other but do not match T2 so problem is MT2500 value.

I've reported the problem to Snap On still waiting on their reply why or just what are the reading for TDC offset, but -5.4 is number to use with the Snap On scanner, -4.8 will be the setting as specified in GM manual -.25 to -.75

[Govt issue]

That tdc relearn worked great on my 96. Went from .8 to 1.7 on my first try. My truck wouldn't powerbreak before now it will. Just remember to never let your foot off the floor. Had to try it 3 times. I was using a otc. Great info, The first time I had heard of the manual procdure. I could do the timing adj but couldn't figure out how to do the tdc learn. At that time I had borrowed a tech 2, must have had old software, because I did it on 2 other 95s.