I wanted to start a discussion about the max line constant in the PCM. I have noticed that a lot of people talk about increasing this over the factory value of 90 psi.

My understanding is that the 4L60E "line pressure" is generated by the front pump assembly and is controlled in part by the "force motor". The force motor is essentially a motor that controls a line pressure bleed off valve. The more current that is applied to it, the more line pressure is bled off. So, if no current is applied to it, "maximum line pressure" is acheived, and if the motor's maximum current is applied, "minimum line pressure" is acheived.

I will note that I am enclosing the line pressure terms in quotes because these are line pressure values from the PCM's perspective only - there are mechanical aspects to the line pressure control as well. Shift kits, special pumps, etc can all increase line pressure mechanically.

Now, there is a table called "Force Motor Current (ma.) Vs Press Vs Trans Temp". Across the top is the PSI value. In each column are numbers representing the amount of current to apply to the force motor to achieve that "line pressure" at the given transmission (oil?) temp. After the PCM has calculated the proper "line pressure" for the current driving condition, it uses this table to look up how much current to apply to the force motor. Note that this table ends at 96.

The max line pressure constant is supposed to be a "not too exceed" value. In other words, with the factory value of 90, the PCM will never command a "line pressure" greater then 90 psi, even if the line pressure tables command a higher line pressure. 90 is the top.

If this is raised to 120 psi, the PCM will use that value as the "not too exceed" value. But how is the PCM going to know what force motor current to use at a commanded "line pressure" of 120 psi? There's no 120 column in the table. It goes up to 96, that's it.

Furthermore, let's look at the values in the force motor table. In a tune I looked at today, the values for 90 and 96 columns are both the same. That means if a "line pressure" of 90 is commanded, or a "line pressure" of 96 is commanded, the same amount of current is applied to the force motor. That means the amount of mechanical line pressure bleed off commanded will be the same. That means even though the PCM is commanding 90 psi vs. 96 psi, the "line pressure" is the same.

Now, recall what was stated earlier about how the force motor works. The actual maximum "line pressure" is achieved when a 0 is in the table, when 0 current is applied to the force motor. In theory one could cause a higher commanded line pressure by zeroing out (or scaling down to 0 for the trans temp).

What would be the actual "line pressure" difference between the value 137, which is used when the transmission is hot, and 0? Well, if we look at other values and assume the change is linear, the most we could expect is an approximate additional 6 psi between the value 137 and 0.

I'm not even sure what the PCM will do if it tries to lookup a value of 120 psi since the table ends at 96 (but my guesses scare me - it might start referencing invalid data in the PCM's memory). But even if there was a 120 column, the lowest number it could contain would be 0, which, as stated above, might only add 6 psi of "line pressure".

So, I am wondering, does raising this constant really do anything other then allow the PCM to use the 96 psi column in the table, and if the force motor table values are the same between 90 and 96, does it have any real effect at all?

I don't pretend to know how all this works so I'm hoping somebody more knowledgeable can share.

 

Increasing line pressure is a big no no in my book unless it is necessitated for proper operation. Too much will kill the tranny in no time flat I have seen it time and again so save yourself and your wallet the trouble. There is also a debate as to what is too much etc. I wouldn't want to find out the hard way.

 

we use the 4l80Es in our hmmwvs (hum-v) and whenever it gets a code the tranny automatically goes to max line pressure and makes the tranny stay in 2nd gear. now if the tranny is drove on too long it will burn up the clutch packs and at that point we just replace the tranny. i dont know too much about the 60E but on the 80Es more line pressure usually results in internal failure. i wouldnt mess around with the line pressure too much. its just my own opinion i was sent to school for the 4L80E not the 4L60E

 

What if you change your gear ratio,would you change your line pressure?
If you change your shift points to match your gear change it will throw off your pressure tables too wouldn't it?

 

Yes, in theory it would, since the base line pressure tables are driven in part by vehicle speed. So I went and looked at two different BIN files, both from A4 F-body cars, but with a different rear axle ratio from the factory.

Interestingly enough the base pressure tables are identical between these two cars. However, the line pressure offset modifiers are different:

Car 1 (3.23 rear axle):

0 0 -10
0 -2 -2
-2 -10 -8
-4 -8 -8
-4 -6 -2
0 -7 -4
-1 -9 -3
-4 -14 -10
-1 -15 -11
-2 -16 -12
0 -15 -13
0 -12 -10
0 -15 -1
0 -11 6
0 -13 6
0 -11 6
0 -11 6

Car 2 (2.73 rear axle):

0 0 0
0 -5 -7
0 -8 -9
0 -9 -11
5 -7 -11
8 -4 -9
15 -2 -9
10 -7 -14
10 -7 -13
7 -8 -13
9 -7 -9
12 -4 -5
10 -2 -4
10 2 0
4 -1 -3
0 -3 -5
0 -2 -4

Generally speaking, the car with the 2.73 axle has more "line pressure". Now, I don't know what might have been different between the transmissions or anything else in these two vehicles, and I don't know if the BIN files are really stock (but I suspect they are).

A steeper (numerically higher) axle ratio is going amplify the shifts more, as it is, and there will actually be less stress on the transmission clutches because more of the gear reduction is happening in the rear axle, so it would make sense that you'd want more line pressure in the numerically lower rear axle to both make the shifts seem firm, and to reduce transmission clutch wear.

I'm only speculating what the factory did here, but there does appear to be a pattern.

 

i've been working on just that the past couple days. force motor gives harsh down shifts, not to mention what else is going on. if you change the constant force motor can be set to stupid. normal base shift pressure vs torque vs shift is getting me in pretty close.

 

got it dialed in pretty good -'02 camaro B4C 3:23 gears
here's some tunercat #s
http://img.photobucket.com/albums/v2...4203/4L60E.jpg
it's got some real good manners till about 1\3 throttle then it firms upreal nice.
gear ratio\eng torque deffinatly has a say in this torque steps are 22ft\lbs took three tries before it sunk in how much low end torque the LS-1 makes.
i started firming it up at 220ft\lbs.
just my 2cents i'm a rookie tuner