355 HVAC ECU question

Michael, before I dropped any coin on a new one, I would call Ricambi, as there was mention that David had a way to fix the Motronic ECU's, maybe he will give a shot to this? Or, in the alternative, call around to one of those places that specialize in repairing various ECUs..I imagine there is simply a bad solder joint or bad component on the board.

 

Could we have clear pictures of both faces of the PCB please.
Anyway let the switches in the initial position. The failure is not there. This kind of switches may be used for a lot of functions: address configuration of this particular ECU for diagnosis purpose, configuration of the microcontroler depending on the country or whatever particularisation after manufacturing, fine tuning of the thesholds of the temperature regulation loop, ....
Only a complete diagram of the board or some reverse engineering could tell really what they are used for.

Have you also dismounted and checked the control board, in the car?

 

Can you say that again?? You guys are well over my head!!

 

With your "stepper" (BTW it is not a stepper motor but a servo-motor ie a motor connected to a potentiometer reading its position) installed can you perform some measurements from the ECU large connector:
resistance between pins 16 & 17 : you should see the motor winding resistance

Now, disconnecting the sevo-motor connector:
resistance between pin 7 of the ECU and pin 3 of the motor : check of potentiometer good connection
resistance between pin 12 of the ECU and pin 4 of the motor : check of potentiometer good connection
resistance between pin 2 of the motor and ground : check of potentiometer good connection

On the servo -motor:
resistance between pins 2 & 4 : potentiometer total resistance
resistance between pins 2 & 3 : potentiometer partial resistance (cursor)
resistance between pins 1 & 5 : motor winding


Also, on the PCB, try to follow, and inspect, the tracks from pins 16 and 17 (motor command). Try to identify on which component (IC or transistor) they are connected.

 

What about the other functions driven by this ECU? Is something working or not?
- Main fan commanded by the ECU through a power module
- Same for the circulation pump
- Air distribution servo-motor (top-bottom) is also directly driven by the ECU exactly in the same way than the hot/cold servo
- AC compressor clutch is also commanded by this ECU

If similar motors are not working it may be a common driver (IC) or power line which failed. Try another ECU, it may work or ... you may fry the new one !! I would rather perform an exhaustive check of the harness. There are not so many wires around. Just my opinion!

Oh btw, I don't think the failure rate of Bosch stuff is so bad and a lot of progress in car electronics are issued from Bosch and often manufactured under their license. No, I do not work for them but I do respect this company!

 

So were the DIPs set correctly in your junk part?

 

Michael, Sorry i didn't see this post earlier. I have a spare ECU from a '98 355. The switches are in the same position as yours. You are correct it is not a Bosch part mine has a Modulo sticker on the side and the board itself has Procond Elettronica printed on the left edge near the switch block ie; must be Italian.

 

Cool.

Now, about your dead unit. If you are convinced that it's trashed, a neat trick for debugging is to remove the circuit boards, place inside sealed zip lock baggies, and freeze them in your fridge.

Once frozen, immediately remove them, reassemble the boards, and install your box back into your car for a quick test.

If your A/C unit works while the boards are frozen, then you have a failing chip. You can track down the failing chip in a variety of ways (e.g. temperature readings with a laser thermometer, voltage readings on the board, etc.), and you can narrow down which board is failing by freezing only one board at a time (or half of the boards, etc.).

If the unit doesn't work when frozen, the problem is often a bad solder connection or non-chip electronic component failure such as a bad capacitor, resistor, et al.