New owner with ventilation problem

It is certainly safe to connect 12V power supply to pins 1 and 3 (+) but, before doing that, I would first test with an ohmmeter the function of the "STOP" button - connect the ohmmeter between the pin 1 (ground) and pin 4 (black) and see if the "STOP" button connects and disconnects the two pins. Then, connect the ohmmeter between pin 4 and pins 5-8 (one at a time) and see if you get varying resistances when you turn the knobs. If you don't get sensible results, try with the ohmmeter between pin 1 and pins 4-8. With this test, you should be able to find out which of the pins 4-8 is for the air direction control and what the other pins are for.

You can do another test with 12V supply connected - check the voltages on the pins 4-8, with the negative voltmeter probe on pin 1, by turning the knobs and find out the voltages on which pin vary with the rotation of which knob. If this does not give yo sensible results, move the negative voltmeter probe to pin 4 and measure the voltages on the pins 5-8. Do both with the "STOP" button on and off.

Let us know what you get.

 

Looking at the photos posted on the forum, the air direction control knob seems to select a resistor or combination of resistors (located below the knob).

It's very difficult to check resistances without pulling them off the board. The only way to check that you are actually checking a particular resistor is to check its colour code and see if that matches the ohmmeter reading. I guess it's a relatively good sign if none of them look burnt.

I tried to follow the tracks on the circuit board (from the photos posted on the forum), but the resolution of the photos wasn't great enough. Pin 6 seems to be connected to R2 on the board, but I can't figure out where it goes from there.

Pin 5 seems to go to pins on the air direction control knob. The poster of one photograph has labelled this as a common ground on the switch. I haven't been able to confirm this.

I'm wondering if the diodes are affecting your voltage results.

 

"26" on the "2000m" range of the voltmeter (I presume you have correctly used the DC Volts, not DC Amps on your multimeter) means 26 millivolts (0.026 Volts), practically nothing. I am surprised that you get no volt readings that vary when you turn the air control knob. I suggest you do the following further tests:

1. Connect the power and switch the panel "on" (STOP button up, as I understand); set the multimeter at 20 volt range and, with the negative multimeter probe on Pin 1, measure the voltages on the two diodes (D1 and D2) at the points as shown on the picture
Image Unavailable, Please Login

2. See whether there is any component under the large blue disc.

3. Repeat the earlier test measuring voltages on the pins 4-8 but this time turn the fan speed knob.

4. With no power connected, set the multimeter at the continuity check position (200 Ohms range or sound/beep position, if you have it) and go through all resistors at the left side of the picture (near the air control knob). Put one probe on one side of the first resistor and measure the continuity against all other resistor points, left and right including the vertical resistor to the right of the knob. Then put one probe on the other side of the first resistor and repeat the continuity checks. Go to the next resistor and repeat etc. When you move to the next resistor, you don't need to check the continuity back, just forwards. During the process, record between which resistor points you have direct links (zero resistance or you get a "beep") then try to show it on a sketch and post it.

5. Read the values of all resistors near the air control knob. If you are not familiar with the resistor colour coding, just right the colours of their stripes and post them.

Well, you now have something interesting to do over the weekend.

 

Great photo, thanks for the link. I tried something similar with the image reversed.

The forum reduces the size of larger photos.

Unfortunately, the left hand terminal of R2 still looks like it goes nowhere. And why is there a terminal under R2? Does the track going to R2 extend under the resistor?
Plug pin 3 (main power) is still untraceable because it sits under the plug. In theory, it should go to all knobs.

Here's a link to a colour decoder for four, five and six band resistors:

https://www.digikey.com.au/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band

Unfortunately, the lighting is still not good enough to pick the colours from the photographs.

e.g. R3 = orange, orange, brown(?), gold = 330 ohms with 5% tolerance

Miro... on boards like this, would the blue capacitor be a main power voltage stabiliser or something to help reduce sparks when switches are activated?

 

I don't believe the blue component is a cap. To quote. It's a "resettable fuse, a polymeric positive temperature coefficient (PPTC) device that is a passive electronic component used to protect against overcurrent faults in electronic circuits." At least that's what PTC stands for.

Or it might just be positive temp coeff thermistor.

 

The PC board is double sided so there are many links under the white face that are difficult to follow. If there is a small solder point on one side that seems not to be connected anywhere, it is usually a link to a line on the other side.

Are the resistor colours indicated on this extract correct? (first 4 colours shown, ignore the 5th) I have also attached the PDF of this pic where you can directly do the corrections and post back.
Image Unavailable, Please Login

 

Could you have a bad sensor and the air diffuser stays in the default defroster vent position.?
What about a bad ground? I've had this problem for years.

 

I'm not sure how you managed to isolate the resistors in the circuit from the other resistors (I guess it's just the way the circuit is wired).

Did you check the diodes D1 and D2 as per Miro's instructions (below)?

The only reliable way to check the resistance of a diode is to pull one of the diode ends out of the circuit. Having said that, did you swap your black/red test leads around anyway and compare the readings? Does your meter have a specific diode checking function?

Did you also try Miro's suggestion of disconnecting the control panel to see if the air direction motor ran?

Thanks for your efforts producing the word doc!

 

The reason why I asked to measure the voltages on the diodes D1 and D2 (at there ends where the black stripe is) is to determine whether they are Zener od ordinary diodes. If Zener (voltage stabiliser), the voltage readings should (most likely) be 5V. If no voltage or some higher voltage is present on the diodes, and if there is no voltage stabiliser under the large blue disc (like LM7805, TA78L005 etc.), then the control panel is probably getting stabilised 5V from the AC ECU and that could be the reason why there are no changing voltages on the (disconnected) control panel outputs when turning the knobs.

How basically the air direction control works is that the microchip runs as an Op-amp voltage comparator - it monitors the voltage on the middle pin of the flap motor's potentiometer ("pot") and the voltage coming from the control panel's air direction switch. The range of these voltages is usually 0-5 V. Example:

The air direction flap is positioned in the middle and the voltage on its motor's pot (middle pin) is, say 2.5 V. If the air direction switch is also set at middle flap position, the output voltage from the control panel for that flap position should also be 2.5 V and the flap will stay where it is (no power is applied to the flap motor by the microchip).

If then the switch is turned to a lower flap position, the control panel will change the output for the flap control to, say, 1.5V. When the microchip sees the difference in the voltages between the motor's pot (2.5V) and the incoming 1.5V from the control panel, the microchip will run the motor (and it's pot) in the appropriate direction until the voltage on the motor's pot reaches 1.5V (equal to what's coming from the control panel switch) and then the microchip will stop the motor.

Next change of the voltage from the control panel's switch (up or down) will result in the microchip running the motor again, in one or the other direction, in order to make the voltage on the motor's pot equal to what it receives from the control panel's switch. The motor's pot will have a specific voltage for each position of the air direction flap, in the span 0-5 V (or probably more like, say, 0.5 - 4.5 V). The resistors of the control panel's switch are accurately calibrated so that the switch produces the same voltages for its various positions as are required on the motor's pot for those flap positions. The panel's switch is actually just a potentiometer but, instead of continuously varying the voltage (like standard pots), it varies the voltage in fixed preset steps using various combinations of its resistors. As the accuracy of the voltages in the switch positions is important (to place the flap exactly where it should be), most of the resistors are of just 1% tolerance.

It appears that the test of the air direction switch output voltages has to be done with the control panel connected to the AC ECU so that the panel can get the stable 5V reference voltage which the switch then varies (in steps) and sends it back to the ECU to control the flap position.

Very good suggestion by "F355Bob". There could be something that causes the AC ECU to peg the air direction flap to the top (defrost) position. To test this, I would rotate the flap motor using a separate battery on the motor's (disconnected) wire plug and move the flap to its lowest position (feet). Then plug everything in, switch the ignition and the AC on and see whether the flap moves to the defrost position or stays where it is. If it moves, the problem is elsewhere, as suggested by Bob.

 

It just seems odd that there are so many different voltages going to the control panel.

I would have thought that manual selection of outlet position would override the sensors (?). In Auto, I believe at some ambient temperatures it will divert the air to the windscreen for a certain time period to prevent discomfort to the driver.

Unfortunately, my car is working perfectly at the moment (knock on wood) and I'm not tempted to pull it apart to compare resistances/voltages

 

The fairly standard voltage range for control of things by microchips is 0-5V. Also, the fan speed controller gets a varying 0-5V signal from the AC ECU to vary the fan speed from zero to full (the full speed comes on at about 4.7 V). So a 5V supply needs to be provided to all potentiometers (mechanical or digital) which is then varied by them 0-5V for control, via the microchips, of various functions (fan speed, heater valve opening-closing, air direction flap motor etc.). In the case of the air direction control, the 5V supply to the motor's potentiometer and to the selector switch on the control panel (the "step" potentiometer) most probably comes from the same source (a voltage step-down regulator) to eliminate any inaccuracy in the flap positioning due to any voltage fluctuation of the 5V source.

If the diodes on the control panel are not Zener and if there is no voltage regulator on it, then one of the 5 wires (pins 4-8) between the AC ECU and the control panel probably carries the 5V supply to the control panel. The other four wires, carry the control signals from the panel to the AC ECU (three rotary switches/potentiometers and one RECIRC switch). The STOP switch then probably does not send any signal to the AC ECU but just switches the incoming 5V to the control panel on and off.

 

Unfortunately, when diodes are in a circuit, you can never be sure you are checking the resistance of the diodes. If a diode's resistance is the same in both directions out of a circuit, you have a faulty diode.
If there were no visible faults with the resistors, I would have recommended changing the diodes, but Miro is usually correct with his diagnoses (and your problem will lie outside the control panel).

I assume Lorenzo was able to positively identify chips 4, 5 and 6 because there were circuit board tracks going from these chips directly to ECU pins (which are attached to specific external components)

If the "U" numbers are readily identifiable, then perhaps this would help the technician changing the chip, but I don't think he/she would have too many problems if he/she had access to Lorenzo's FChat photographs.

There are companies in the UK which specialise in the repair of the ECU. Unfortunately, we are still not 100% sure that the ECU is faulty and it might be difficult to find a volunteer to give you a loan ECU to see if it works on your car. Faulty external components are known to destroy ECUs.

When you did your motor checks, was it fitted to the tambour when you saw the smoke?
Did you check that the tambour rotated freely with the motor removed?

 

Thanks again for the great photographs. It's interesting to note the 8 dip switches (top left hand corner). This suggests the device may be suitable for other cars, but the large IC labelled "FE25" kind of dashes any hopes that you might find a cheaper version of this unit on, say, a VW.
"FErrari"?

Note that component checks on the ECU are not recommended (for beginners). The voltage produced by your multimeter may be enough to damage some of the more sensitive devices. Indeed, make sure that you don't handle the board or touch the plug pins too much. Static electricity may damage some components. You may even have received your new chip in a bag which reduces the possibility of static electricity damaging the component in transit. Speaking of which, it seems your repairman has made it rather difficult for the next person to replace that chip again (and the components around it).

 

This confirms that the control panel is getting the "control voltage" (5V) supply from the AC ECU and this is why you couldn't get voltage readings on any of the pins 4-8 which vary with different position of the air direction control knob when testing with the control panel disconnected. To do this test correctly, you would need to connect the control panel to its loom and then, with the ignition and the panel "on", measure the voltages between Pin 1 and each of the Pins 4-8 and find out the voltage on which pin changes with the rotation of the air direction knob.

But, before doing this, you should do a test to establish whether the AC ECU operates the air direction motor or not, as follows:

1. Rotate the air direction motor using a battery (on the bench, as you did before) and position its shaft to where it would be when the air direction flap is down (feet); this would be rotating the shaft that engages the flap clockwise when looking at it.

2. Connect the flap motor to its loom but don't plug it into the flap and place it where you can see it (it's shaft).

3. Plug the control panel in and position the air direction knob to "feet".

Then switch the ignition and the panel "on" and see if the flap motor will turn. It may turn anticlockwise and stop at a certain point (which could be at the "defrost" position) or it may not turn at all. If the motor does not turn, you can try switching the air direction knob to other positions and see what happens, Depending on what the motor does in this test, further investigation can be narrowed down.

 

Just a thought. On my 95 the air direction flap cycles through its movement range the first time the ignition switch is turned on when ever the battery is switched back on after being disconnected.

 

John , does the a/c have to be switched on for this cycling to happen?

 

I don't recall, but I have spider so pretty much the stop button is always depressed and fan speed set to zero with dash vent selected for direction control. I don't know if it's supposed to do that or not, but that's what if seems to do.

 

Looks like a Japanese owner has had problems with resistors on his HVAC ECU circuit board (chip replacement didn't work?):

https://ameblo.jp/j-wave2004/entry-12383909220.html

(Rough Google Translation... https://translate.google.com/translate?sl=ja&tl=en&u=https://ameblo.jp/j-wave2004/entry-12383909220.html )

 

For the electronic gurus... Some data on the Dual Power Operational Amplifier chips on the HVAC ECU circuit board:

http://www.htmldatasheet.com/siemens/tca2465.htm

I find it surprising that these surface mounted chips can handle 2 amps. I couldn't understand how these tiny chips could drive large devices such as recirc and air direction actuators.