Project: 2.5 20V GLT full OBD-II; Motronic 4.3 to 4.4 conversion; M4.4 ECU mods:

I converted my 850, which originally had a Motronic 4.3, to a Motronic 4.4, with full OBD-II.
My 850 is an automatic. So I had to find a 4.4 for an automatic, without an immobilizer and with as few additional options as possible. My original motronic is a motronic without anything, the only thing that my 850 has original is the EVAP valve, which every 850 has.

So what to do if a Motronic 4.4 is to be installed now, which even in a minimal version definitely has EVAR and PAIR and the tank pressure sensor as extras? You can still outwit any ECU with electronics! You can get the Motronic 4.4 installed without it giving out a single error code.

Since my 850 is an automatic and also has ECC, I have chosen the 91 35 726 / 0 261 203 077 as my new ECU. For a manuell car, the 91 46 560 / 0 261 203 076 should work. These two don't have an immobilizer.

There are 2 different ECC versions for the M4.3, one version does not have the pressure sensor 7/8, but only the sensor 8/3. My M4.3 no. 0261 204 224 originally has the sensor 7/8, then the exchange for the M4.4 is plug & play. If you have the M4.3 without this pressure sensor, you could possibly damage the a/c compressor, as it will not switch off if the pressure in the system is insufficient, if the system works at all without the sensor. It worked for me, but I don't know how the M4.4 behaves without the sensor.

So I bought my new ECU via Ebay from a junkyard in the US. Since I wanted to know what error codes the ECU gives out, I first installed the ECU as it is. After the first start I got the following error codes, read out with VOL-FCR:

EFI-445 Pulsed secondary air injection system pump signal (SAS pump & relay - 6/54 & 2/53)
EFI-447 Pulsed sec air injection system solenoid valve signal (SAS valve - 8/43)
EFI-616 EVAP canister shut off valve signal (EVAP Valve - 8/44)
EFI-621 Fuel tank pressure sensor signal (tank pressure sensor - 7/84)

First impression, not so funny, but it's what i have expected. Question is, how to solve it without having to get all the stuff out of a scrap car and install it? Now my excitement was aroused, the plan was not to retrofit a single screw. I then first listed all the errors, why does the error occur and what is the signal that the ECU would see as OK?

EFI-445 Pulsed secondary air injection system pump signal:
The ECU determines that there is no relay at the control output. In order to put the relay 2/53 under the ECU, solder a 80 ohm resistor between the connections A-27 - B-38. Now the ECU has received its relay, and to simulate the functioning of the virtual air pump on the virtual relay, you still need the SAS Delete mod, a diode 1N4001 between the connections A-32 - A-37, the cathode (-) on connection A-37.

EFI-447 Pulsed sec air injection system solenoid valve signal:
The ECU finds that the solenoid 8/43 is not there, shall get it :)
Solder a 15 ohm / 25 watt resistor between terminals A-37 - A-27.

EFI-616 EVAP canister shut off valve signal:
The ECU finds that the solenoid 8/44 isn't there, shall get it :)
Solder a 15 ohm / 25 watt resistor between terminals B-19 - A-27.

EFI-621 Fuel tank pressure sensor signal:
The ECU detects that the Fuel Tank Pressure Sensor is malfunctioning. Well we give her the expected signal, UB/2 from port B-15. For this we need 2x 5KOhm resistors, one resistor is soldered between the connections B-15 - B-31, the other between the connections B-28 - B-31.

Prepared in this way, the ECU runs without any problems, of course all other functions are fully there and error codes can be read with an ODB-II reader. So I converted my 850 to the newer Motronic 4.4, benefit from better engine management and got full OBD-II as a bonus.

With these mods, original systems that are in a suboptimal condition can also be brought back online. I'm also for environmental protection and sustainability, but some systems in cars are rather pointless....


set Error codes
set modified ECU
set ECC plug-n-play
set ECC non plug-n-play