F355 Aircon Control Panel

My dealer tells me my aircon control panel is faulty and that a replacement from Ferrari costs $3000. It has three controls (temperature, fan speed, air distribution) and two switches (aircon on/off, recirculation on/off). It also has backlighting for the controls.

Inside, there are three rotary switches which connect to resistor networks and they work by converting the switch position into a voltage which is read by the aircon ECU to determine the switch position. There's a 7 pin connector from this panel in the center console through to the aircon ECU.

Question is, does anyone have a schematic for this panel? It's proving tough to reverse engineer because the board is covered in protective paint.

 

Thanks, the dealer has swapped the panel for a known good one and the problems (no heat and incorrect air distribution) have gone away. This follows a repair to the ECU where two of the power op-amps were replaced and my follow up to sort out some poor soldering of them.

The fan speed control is working as is the aircon STOP button and recirculation button.

It seems there are three control lines, one for each of the three rotary controls and I expect there’s a voltage output depending on the position of the control. In the ECU, an analog to digital converter will measure the voltages and determine the control setting accordingly, for example 1.8-2.2v = Position 1, 2.4-2.8v = Position 2 and so on. Seems clear the resistors which are high precision and very specific values are used to form a voltage divider.

I’m wondering if the diodes are zener diodes to provide a relatively stable reference voltage or whether that is done in the ECU. I assume the PTV varistor is to protect the light bulbs from switch on surges and replacing them with LEDs sounds a good idea.

thanks everyone for your thoughts and information.

 

It turns out my car, which is 28 years old needs quite a lot of work to fix oil weeps, sagging engine mounts, a noisy water pump and, and, and, so it will be a few weeks before I can take a closer look.

There are 5 lines which go to the ECU, I assume one each for the three rotary controls and two for the push buttons and then 3 lines to ground, power and I think the instrument lighting feed.

In the pictures above, the circuitry seems the same for the left and right controls. They both have 6 positions and one is working, one not. I’ll be looking at the two to see what differences there are.

I’ll write more here when I’ve tested it. Funny thing is, the dealer quoted the replacement cost as if it was completely normal, $3000. I was an electronics engineer before I retired and the component cost here is, what, $10?

 

Yes, thanks for the resistance measurements, I will make the same when I get it back. Was your panel working when you made the meaurements? It's strange, the resistors are high precision, precise value which is they are being used to generate high precision voltages requires a highly accurate voltage reference and I dont know where that is. Not sure what the diodes are for, if they are zeners, driving then through a 33 ohm resistor (orange orange black) will drive them well into voltage saturation but at the expense of power dissipation and the burned out resistor.

Anyway, I'm on the case and will share whatever I find out.

 

An update. My car is now back from its rather expensive service - £12000, $15000 and I've had the chance to trace out the schematic of the HVAC Controller.

The circuit is divided into two - the two switches and the three rotary controls.

The two push buttons return a voltage of around 4v via pins 4 and 6 to the ECU when NOT pressed. When pressed, the voltage is zero and the switch is then used to illuminate the small bulbs in the switches or increase their brightness if the panel illumination is on. The ECU will use the presence or absence of the voltage on the two lines to switch the AirCon on and off and to open/close the recirculation flap. The PTC varistor is used solely to slow the switch on of the light bulbs to prolong their life. The diodes and 33 ohm (org org black) resistors is used to dim the small switch bulbs. If the resistor has burned out (as in the pictures here), the problem is the small bulb has shorted out.

The three rotary controls are entirely passive in operation. The rotary switches tap into a series of resistors so that pins 5, 7, 8 present a resitance to ground which varies according to the switch position. The resistance measurements (pin 7 and 8, to ground) helpfully provided by Qavion match mine exactly for the fan and direction controls. At the fully clockwise position, the resistance ground is zero and increases to 182k at the fully counter clockwise position. The figures for the two 6 position controls are the same.

The temperature control is entirely different in my unit compared to Qavion's. My resistance is 0 at max temperature and 282k at the minimum temperature. Qavion's measured values were 1.2 ohms (essentially 0) at maximum temperature and 3.071k at the minimum temperature, so a massive difference which I cannot currently explain.

Electrically, this unit is very simple and in no way justifies the replacement cost. I had to sacrifice the blue PTC varistor to be able to see the resistors installed underneath it. It was glued to the board with silicon. No problem, easy to find a replacement. If I replace the light bulbs with LEDs, I'll leave it out.

 

From what I’ve found out, the most important resistance values are those to ground, resistances to the positive power are in effect going through the ground and back up to the positive. Definitely the case that a poor ground connection between the control panel and the ECU will mess things up so that is something to check.

That said, the resistors in my panel match their colour codes but I need to do more work on which ones are in use at each position of the temperature control. There are ten in all, four on one side of the switch, three groups of two on the remaining three sides. I’ll post a closeup picture tomorrow.

 

Image Unavailable, Please Login

Here is a closeup of the temperature control switch and the 10 resistors which it uses to create a resistance which varies by setting of the control and which is interpreted by the ECU. In this way, a single line is used to communicate the setting of the temperature dial in the centre console to the aircon ECU in the front trunk…

Component/Marking/Nominal Value/Actual Value

R25 BBBB 1100 1097
R24 BYBB 1400 1398
R23 BPGB 1780 1780
R22 ROPB 2370 2372
R21 OORB 3320 3308
R20 YWWB 4990 5021
R19 GYGB 8450 8430
R14 BBWR 16900 16850
R11 YWWR 49900 49790
R8 BWBO 191000 192100

 

Image Unavailable, Please Login

Here is the schematic of the HVAC controls. For simplicity, I'm only showing one of the push buttons and its light, the two are identical. Same for the three rotary controls, there are 5 resistors for the Direction and Fan controls, 10 resistors for the Temperature control. When each of the control knobs is fully clockwise, the corresponding output pin is grounded directly, otherwise through one or more resistors. The 33 ohm resistor dims the illumination of the small bulbs in the switches. When the switch is pressed, the bulb is connected to a higher voltage and is therefore brighter. The purpose of the diode is to block this higher voltage from affecting the panel illumination elsewhere in the car.

 

Yes, at least it’s only two layer…

Qavion, any chance you could repeat your resistance measurements between pin 5 and ground as the temperature is changed? I’ve checked mine again and while my readings for the direction and fan match yours closely, the temp ones are very different.

 

No problem then!

As for 2k48, you’re right, this is standard shorthand notation for resistance and capacitance values, you just need to know that a resistance of 4M7 is a million times more than 4R7!!

I put my control panel back in the car and measured the voltage on pin 5 (temperature) which showed my panel is working perfectly which I will explain in another post tomorrow. Still no heat from my car so most likely the fault is in the aircon ECU. Just as well I didn’t waste $3000 on buying a replacement panel…

My car is now in winter storage as the bad weather bites here in the UK but I did remove the control panel and ECU to investigate over the winter.

 

Yes, the motorised valve was replaced by the dealer at a time when it was no longer available as an OEM part with a fix package using a functionally equivalent part. It certainly worked well for a while so it’s possible the replacement valve is not working or the ECU has developed a fault. The control panel looks fine so I have to start looking at the ECU which has 6 chips to drive motors. I found them online and bought 10 so I will be trying to test the ECU. Difficult to do when it’s in the car so will be trying to bench test.

This is pre-CAN bus so none of the luxury of plugging in a diagnostic box to exercise the different components…