Bypass Valve Opening Late?

On the subject of the "Potentiometer values" table, the +/- percentages shown merely mean cumulative manufacturing tolerances of the resistors forming the TPS potentiometer. Every resistor has certain tolerance which is usually 1%, 5%, 10% or even 20%. In the case of the TPS, each given resistance measurement goes through 2 or 3 resistors resulting in the cumulative tolerances being rather high. I measured the following resistances on 3 new genuine Bosch 0 280 122 004 TPSs that I have (presently selling them on Ebay):

Pot 1: (1-2) 2.24 K , (2-3) 0.93-2.93 K , (3-1) 2.93-0.93 K
Pot 2: (1-2) 1.97 K , (2-3) 0.91-2.67 K , (3-1) 2.67-0.91 K
Pot 3: (1-2) 2.01 K , (2-3) 0.89-2.69 K , (3-1) 2.69-0.89 K

Obviously, the above values are well within the given tolerances.

However, as the ECUs do not work with the TPS resistances but with the voltage received from the Pin 3, the resistances and their tolerances are actually irrelevant where the TPS function is concerned. The voltage provided by the middle pin of a potentiometer (being a voltage divider) is the same for a given potentiometer rotational position, irrespective of whether it is, for example, a 2K, 5K or a 100K potentiometer. Therefore, the specified TPS resistance tolerances (20% and 18%) have no meaning where the voltage on the Pin 3 is concerned but are just given as part of the TPS manufacturing datasheet.

In view of the fact that the ECU reads the voltage coming from the Pin 3 of the TPS, it makes no sense to do any TPS adjustment based on resistance, especially since their resistance tolerances are rather large. Forcing a TPS to show any particular resistance at, for example, its idle position may actually cause the TPS Pin 3 voltage to be out of the specified range (400-600 mV in the case of the 348) and/or cause the max. difference between left and right TPSs to be exceeded (80 mV in the case of the 348).

 

My TPS is fine off the car, but reads just outside the resistance range when fitted. I just assumed that the ECU needed to see in range (current or resistance) at engine turn on and based its idle/mid/full calibration on that. On the 5.2, battery resets don't reset the typical learned values, so I assume this parameter adjusts every time I turn on the car.... or is like long term fuel trim and gets adjusted as time progresses.

So I'll probably need to read the current, rather than the resistance. I have no fault messages, so perhaps it's ok. If it is out of range, I'm just wondering what difference there is between idle and mid range during normal closed loop operation. FWIW, idle speeds are within tolerance.

 

Just measured the voltages on my 3 TPSs - 5.0V between Pins 1 & 2, measured on Pin 3:

Pot 1: (1-2) 2.24 K, Voltage on Pin 3: 0.26 to 4.80
Pot 2: (1-2) 1.97 K, Voltage on Pin 3: 0.26 to 4.80
Pot 3: (1-2) 2.01 K, Voltage on Pin 3: 0.26 to 4.80

A scan be seen, the voltages on the TPS Pin 3 (that the ECU reads) are identical on all 3 TPSs irrespective of them having different resistances (Pot 1 having about 12% higher resistance than the other two). Obviously, the TPSs are factory calibrated to voltage, not to resistance.

Of course, when fitted to the TB, the TPS will show higher voltage at the butterfly idle stop because the TB does not allow the TPS to be rotated all the way down to its internal stop as it would risk the TPS damage. In the case of the 348, the voltage on Pin 3 at the idle stop is 400-600 mV, as compared to 260 mV as lowest to which the TPS can go.

With regard to setting the TPS on the 355, I presume that there are instructions in the manual (I have not tried to find them) but I am sure it goes by voltage on the Pin 3, not by any TPS resistance.

 

These are the voltage readings I took on the TPSs of my 348 after setting the butterfly idle stops at exactly 0.05 mm and making some fine adjustments to the TPS positions:

Left:
1-2: 5V (supply from the ECU)
1-3: 0.48V

Right:
1-2: 5V (supply from the ECU)
2-3: 0.48V

I managed to achieve zero volt difference on the Pin 3 whereas up to 80 mV is acceptable.

 

That's the problem. There are no current/voltage values for the F355 2.7 or the 5.2 listed.

For the 2.7 it says:
Each time the engine is started, the control unit regulates itself on the potentiometer position as follows:
it reads the angle for idling position and acquires it, memorizing it with the name "OFF SET Angle", then takes the idling value acquired +72deg as the maximum value.
The engine must therefore be started without pressing the accelerator pedal.
For absolute angles exceeding 17deg, the control unit keeps the idling value (absolute) at 17deg. [????]
The throttle potentiometer is used by the ECU in two different ways:
1- Normal operation: the ECU recognises only 3 positions (Idling, intermediate and fully open). [Note that for the 5.2, it only recognizes two positions, idling and fully open]
2- Emergency operation: if the airflow meter fails, the ECU uses the signal from the potentiometer to decide the amount of air drawn in by the engine and therfore to calcuate the fuel injection time. This means that the engine can run even if he air flow meter breaks down....

.... Relative angle = absolute angle minus offset angle
Therefore, using relative angle, the ECU becomes insensitive to any initial differences in the positioning of the potentiometer, which are compensated by the off-set angle.

Elsewhere, there is a statement about when Open Loop operation activates....

...3) Accelerator pedal fully depressed (throttle open 72 deg).

Somewhat misleading.

Regarding...
Each time the engine is started, the control unit regulates itself on the potentiometer position as follows:
it reads the angle for idling position and acquires it, memorizing it with the name "OFF SET Angle", then takes the idling value acquired +72deg as the maximum value.

I assume this is the current/voltage value rather than the resistance value. Does it have a table for what the current/voltage value should be at 72 degrees? (Is the current/voltage value linearly related to angle, even if the idle angle/current may be different?)

 

I just realised that the 355 has only one TPS. This is probably why there are no instructions with regard to TPS setting which, I believe, is required on the 2.7 348 in order to synchronise the banks to within 80 mV difference at the idle position, not so much to set any exact voltage on each TPS. The given range of Pin 3 voltage at idle is rather wide anyway, 400-600 mV. The reason why the "base" voltage (voltage at the idle stop position) does not have to be any precise value is because the ECU will read the actual TPS voltage at idle and, whatever it is, adopt it as the "base" voltage then calculate from there at what voltage is the end of the idle zone and at what voltage is the 72 deg. butterfly position (start of the "fully open" zone). It seems that the ECU has certain permanent fixed voltage values stored/programmed which it needs to add to the read "base" voltage in order to obtain the voltage representing the end of the "idling" zone and another stored voltage which it needs to add to the "base" voltage in order to obtain the voltage representing the 72 deg. position of the butterfly.

I would not bother trying to fully understand the description in the manual as it appears to be somewhat deficient. It can possibly be understood better if one is looking at the display of the Ferrari's "Diagnosis System" while reading this section of the manual. Perhaps it also has some typing errors and errors in translation (as is also the case in the 348 manual).

So, for a single TPS arrangement, I would not worry about the voltages on the TPS, as long as it has 5.0V between Pins 1-2 (supplied by the ECU) and the Pin 3 voltage at idle stop is somewhere above the 260 mV minimum that the TPS can produce (say, at least 350 mV to avoid the internal TPS stop being stressed at the butterfly idle stop position). From there, the ECU will "know what to do".

 

I just realised the WSM is not specifically saying 72 deg throttle angle, but idle plus 72 degrees. That’s 82 degrees if the idle throttle position is 10 degrees “absolute”

Are they saying that if the engine is started with your foot past 17 degrees, that max throttle is 17 + 72? But how does it know the pedal is beyond 17deg? I thought it would establish 10 degrees by the throttle position at ignition on. If your foot is pushed to the floor at key on, isn’t that the new 10 degree reference point?

Never a truer statement spoken.

 

"it reads the angle for idling position and acquires it, memorizing it with the name "OFF SET Angle", then takes the idling value acquired +72deg as the maximum value" and
"For absolute angles exceeding 17 deg., the control unit keeps the idling value (absolute) at 17 deg."
are exactly the parts I could not understand so I tried to make some logical interpretation looking at the graph with the help of descriptions in the 348 manual.

Once the ECU has read the voltage on the TPS Pin 3 with the butterfly at idle stop, it will know the positions of the butterfly by reading the Pin 3 voltages as the butterfly moves over its range. This is why the manual warns that the engine "must be started without pressing the accelerator pedal" (I presume that this actually means "the pedal should not be pressed at switching the ignition key to on" as this is when the ECU will read the voltage on Pin 3 of the TPS) in order that the ECU can get the correct TPS voltage representing the butterfly positioned at the idle stop. What happens if you keep the pedal pressed, say, half way open, I don't know; perhaps you will confuse the ECU.

In conclusion, it seems that Ferrari do not want you to do adjustments on the TBs and TPSs yourself but want you to bring the car to an authorised service centre where they will hook it up to their "Diagnosis System" and do it for you for a "small fee".