TDC offset

[N9Phil]

After installing timing gears I did a TDC Learn and my TDC was -1.05. After moving the pump about 1/8 in, I did a TDC learn and it went to -2.02. I moved the pump back to where it was and it stayed at -2.02, of course this set an 88 code. Today I moved the pump back a 1/4 in to the left of the mark I started with and it went to -1.05 again. I tried bumping the pump back a little and I finally got to a spot that when I did a TDC learn it would go to -1.85 then the next time it went to -2.02 and then it went to -1.94. I am using a Tech 1A to set the timing. Am I correct in that when it stops at -1.94 and I shut off

[Billman]

I have heard the difference also. More Negative = More Noise.

-1.94 seems to be the what this board recommends.

I'm currently at -2.02.

[Kennedy]

You got it! For every different "lay" of teh pump, you will see a variety of offset numbers come up in increments of .07-.08. like 1.86, 1.94, 2.02, 2.11, and 2.20 all potentaially available from the same "lay". You will not see the numbers over 2.02 on the OBD 1 trucks.

[rameye]

I agree with Kennedy..

I have a tech1 and when you do the learn all the numbers start to flash. Stop the process at the number you want...thats it!

Seemed too simple to me also. Moving the pump will simply give you a different ramge of numbers.

I've also cheated and raised the rpm's to get the number to pop up that I want.

ram

[Bigg R]

When I raised RPM the offset went to a lower value, so I put mine in gear to drop RPM's and got my -1.94!!

[rameye]

Thats a good one!

I gotta try that trick next..

[N9Phil]

Thanks for all the info. I feel better that I finally got the TDC set correctly. smile.gif

N9Phil

[JohnC]

TDC offset is defined as the difference in degrees between the TDC indicator on the crankshaft and the "TDC" indicator within the pump. You don't "set" it with he tech II, you can only "learn" it. When you change the learned value by fooling wth the RPM you don't change the actual value, only the value "learned". This begs the question of what is the significance of "learning" a value other than the actual value? Seems to me there are 2 extremes: you can learn a more negative value while actually having a less negative value, or you can learn a less negative value while actually having a more negative value. What difference does it make? Your guess is as good as mine.

My guess is that the reason the value changes with RPM is due to the slop in the timing set. I'd be interested if anyone sees a bigger range of values with a chain as opposed to a gear set.

[ 08-01-2005, 10:09 AM: Message edited by: JohnC ]

[rjschoolcraft]

The service manuals give the TDC Learn procedure as a "backup" timing procedure. The learned value changes when the PCM expects the piston to be at TDC and, therefore, affects the injection timing. With higher negative offset numbers, you see correspondingly higher desired injection timing advance numbers. According to the book, with TDC Offset Learn set at 0 to -.5, the desired injection timing at idle should be 3.5 degrees BTDC. With my current TDC Offset Learn at -1.94, the desired injection timing at idle is 8.5 degrees BTDC.

I don't think the last statement about not "setting" a value is entirely true, at least in my understanding. I think you are "setting" an offset into the PCM. In other words, the PCM is "learning" the offset value that you want to set. The higher negative number means the PCM thinks the piston is at TDC before it actually is resulting in an earlier start to the injection event.

[JohnC]

Originally posted by ronniejoe:
..I don't think the last statement about not "setting" a value is entirely true, at least in my understanding. I think you are "setting" an offset into the PCM. In other words, the PCM is "learning" the offset value that you want to set... I agree. The question is what is the impact of "learning" a value other than the one it would "learn" on its own. if you advance the pump beyond the desired point then trick it into learning a value "only" -1.94, that's gotta be different than advancing the pump only to the point where it would "learn" say, -1.4 then tricking it into "learning" -1.94 Or, think abut this. I can make you "learn" that the earth is round. The impact, though, has a lot to do with whether or not it really is... ;)

[rjschoolcraft]

THE EARTH IS ROUND?!?!?!

:D

I understand better what you are saying now. However, I still think that learned offset value will cause the PCM to advance the timing.

If idle speed is not changed, moving the pump simply changes what offset values will appear and centers the stepper motor "window" so there is an adequate range of motion for the PCM to work with. Changing idle speed to achieve a desired number probably skews that window somewhat, biasing the range of adjustment (limiting it in one direction, but expanding it in the other) that the PCM can make. So, there probably is a bit of a detrimental impact, but it is probably not too significant.

[Bigg R]

During my timing "adjustments" I noticed that when the TDC offset was at -1.50 the desired timing was at 8.3, but when I got it to -1.94 the value changed to 8.5!

[rameye]

JohnC

I hear what you are saying however consider this..

When you run the Tech1 TDC learn it flashes a whole bunche of settings from -.5 to -2.02. If you're not quick on the draw sometimes your number may not show for a while. I vary the idle to get that number that once showed itself to re-appear.

I firmly beleive that by playing with this thing that my learned setting is actually changing the timing based on engine sound.

On another note, my desired injection timing is almost always is around 8.5 when I start. I reset it before my TDC learn procedure and it goes back to 3.5.

I'm new to this game, fortunately the service manual supports my tech1 and gets specific.

Another country heard from...your thoughts
take care

[Diesel Dan]

Originally posted by N9Phil:
I noticed that there is a much greater diesel rattle at -1.94 compared to the -1.05 setting. This makes me a little nervous. Should I be concerned or should I retard a little and if so, any suggestions as to how far?

N9Phil My truck is set at -0.7* and VERY loud cold. Not sure I want it any higher, maybe I'll get a buddy to bring home a techII someday and play with it. Currently you don't want to be standing outside of it when you start it up. My 12V Cummins is quieter.