What is the PIS
The PIS, or point of initial slip is a important clutch parameter on any F1 or Cambiocorsa based transmission. It’s the parameter that tells the TCU (transmission control unit, or the computer) where the bite point of the clutch starts. Measure in millimeters, and along side the Self-Calibrated Clutch Closed Position, it give the information to the computer as to where it should begin the fethering of the clutch against the flywheel.
Having this parameter too large means that the clutch spends a lot more of its time slipping against the flywheel during engagements. Set too low, it means poor drive-ability (e.g. stalls) and worse yet may allow the clutch disk to slip against the flywheel when in first gear but not moving.
To understand better what setting the PIS is doing, it’s important to look at the basics of how the clutch in these cars works.
When the clutch is in closed position (e.g. you’re driving in gear), the actuator is releasing any pressure on the throw out bearing. This in turn allows the springs on the pressure plate to compress the clutch disk between the flywheel and it’s surface. When this happens, all the torque between the engine and the torque tube is transferred.
When the vehicle needs to switch gears, the computer signals the actuator to push the throw-out bearing inwards which relieves the pressure from the pressure plate. This causes the surfaces between the clutch, flywheel and pressure plate to separate. This allows them to turn independently and so now the engine is mechanically separated from the torque tube. Gear changes and/or sitting still at a stop light are now possible.
The parameter we are playing with controls how much the actuator, and in turn the, the PIS or how much the throwout bearing should move when it comes time to re-engage the clutch.
There’s actually a second parameter, called the Clutch Closed Position that needs to be carefully set when a new clutch is installed. This indicates the final distance between the centerline of the throw-out bearing and the flywheel face when the clutch is closed.
Here we can see the general idea of how the clutch position changes if we overlay the open and closed states on-top of one another. The clutch actuator is moving it between the two points using feedback from the clutch position sensor (hall effect type). The position is opposite of what’s maybe intuitive as the action between the throw-out bearing is such that as the throw-out bearing gets closer to the pressure plate, the pressure plate moves outwards.
If we focus in a bit now to what’s happening between the open and closed position, we can appreciate the point of the PIS. As the clutch moves from the open to close position, it moves fairly rapidly from the Open position to the PIS (point of initial slip). Once at the PIS, the computer feathers the clutches engagement slower until we are fully in the clutch closed position. This is naturally the same way a human driver would feather the clutch as they started from a standstill to prevent stalling, and prevent burning up the clutch.
Below is a EXAGGERATED view to give a better idea of what’s going on.
Note that the units of clutch position/time are not representing any true units of measure and are more for illustration.
At Time = 0, the clutch is fully disengaged and the start of the driver wanting the clutch to close (e.g. starting out from a stop). The clutch rapidly closes between that and Time = 30. Here, Time = 30 also represents the setting the TCU holds for the PIS, or the point that the clutch first makes contact with the flywheel. From this point on, the clutch is more slowly engaged until being fully closed. During this time, it’s let to slip to prevent the engine from stalling/etc.
As you can see, setting the PIS to too high of a value allows the clutch to slip for a much longer period of time. Likewise, if the PIS is set too low, there isn’t sufficient slip being allowed which is likely to cause the engine to stall (engagement is too abrupt).
PIS is set from the factory, as well as any time the clutch is changed out. Some earlier cars (such as some Ferrari 360’s) had a automatic PIS learn function and can’t be set manually. These screens only apply to those that can be set manually.
Although not related directly to the PIS, I thought it would be valuable to look at how this all plays into measuring clutch wear. The CCP can be viewed by looking at the clutch wear information on the vehicle (this is used in the calculation).
As you can see on this vehicle, the NEW clutches closed position measured 18.95mm. As the clutch wears, the distance between the flywheel face and the centerline of the throwout bearing when the clutch is engaged gets LARGER.
Again, in this example, that distance now measures 20.27mm on these vehicle. The TCU knows this as it takes a measurement of the clutches position whenever it’s fully closed.
This means, the clutch thickness has shrunk by 20.27mm – 18.95mm = 1.32mm. The usable throw-out movement on a 4200 is about 5.62mm (note different vehicles have different starting clutch thicknesses). Using the math of 1.32mm / 5.62mm * 100 = 23% clutch wear. Note that the Launch percentage is slightly out as it’s assuming a incorrect throw of 7mm. 5.62mm was found by back-calculating printouts from the factory SD2 tool. Note that other factors play into the condition of the clutch system too (e.g. if the flywheel is badly hot-spotted, you may start slipping well before 100% wear. You’re also likely never to actually get to 100%).
Setting The PIS
NOTE: Incorrectly setting the PIS can cause serious drive-ability or damage. Do this procedure only if you are comfortable with what you’re doing. Certain versions, including some Knock Offs don’t read or set these parameters correctly either which can unexpected results. Follow these directions at your own risk. If you’ve never done this have your dealer/mechanic perform it.
In my case, I’m using the Launch X431 Mini PROS to do this configuration. This is a small android-based tablet solution with wireless OBD dongle. They are available for ~$600 USD (with 2 year license) from various outlets like Amazon. There’s a lot of debate about the quality of this tool. It’s a low cost option, but of course comes with a lot of caveats and limitations. Using a SD2/3 is the right way, but the cost is prohibitive for a hobbyist.
Also of note is the very of Launch software you are running. In my case I was originally using v10.96. Earlier versions had issues handling decimal points. Later versions (v11.00 seems to be the most current).
First step is to get the clutch warmed up. This means going on a short drive and try and get a good number of shifts in. We want things warmed up as all the components will expand with heat and will be more likely to drag once warmed. A good 20-30 min drive around town is usually sufficient.
Next, connect the OBD dongle and fire up your X-431. We want to do Tradition Diagnose (sp?).
You want to select Maserati (or Ferrrari).
Then click SUBMIT to connect to the device. Before starting, you may want to note the version of the Maserati module and potentially update.
From here you want to go to the selection of available modules.
And select the automatic gearbox (TCU) option.
The next screen may show options for various versions of the TCU. There doesn’t seem to be much rhyme or reason to which works and which doesn’t. I suggest you just try them one by one until one successfully connects. In my case, my late model Gransport is a “First Version” according to the tool.
It will take a few seconds to connect. If it fails, like I said, try the other version.
As you can see, this is what it then detects as my modules information which matches with the data in the Maserati shop manual.
From here, we want to access the Special Functions.
We want to select the PIS menu next and acknowledge the warning screen. There’s a note about shutting off the power to the car but found this wasn’t actually necessary.
To read your PIS, then click Read Configuration. You want to ensure this is in a sensible range (say 4-5mm) to ensure the tool is communicating properly with your TCU. In my case, I’m currently at 4.37mm. This aligns with a Leo tool so I’m fairly confident it’s reading accurately.
Next, open the Writing Screen menu by pressing the Hamburger button and the Write Configuration option.
A dialog will pop up allowing you to enter a new value. Enter the value into the text box and proceed with submit.
At this point, the new PIS will be written to the TCU. Verify the write worked by clicking Read Configuration again.
Next is to verify your PIS setting. This is an iterative process where you want to slowly reduce the PIS until the clutch starts to drag, and then increase it slightly (0.1mm is what I’ve found works well).
To do this next step, with the car running, LEVEL GROUND, PARKING BRAKE OFF and the clutch still warm, put the car into N and then into 1 while holding onto the brake. Then go into the data read for the TCU and look at the Gearbox Primary Angle Speed.
Clicking OK here will show a live view of the data.
Now release the brake.
The trick here is that this value should remain at 0 rpm with not slight jittering around (this means that the clutch is starting to make contact with the flywheel and trying to turn the torque tube). Some people mention you can also tell that the PIS is too tight if the car starts to creep forward. This is a valid observation too, but feel that it means the clutch is dragging heavily and wouldn’t recommend letting it get to this point.
Once you’ve found your drag point, I recommend adding a bit of margin. e.g. if you found the drag to start at 4.4mm, increase the PIS to 4.5mm and leave it there.
Once you’ve dialed in your final setting, get the clutch nice and warm again and monitor whether there’s any remaining drag.
From the Workshop manual, there seems to be a-varying range of what the expect PIS should be depending on the version.