Project: Improving fuel economy of my 850 GLT

Less consumption and more driving pleasure with the AW 50-42LE
The project consists of several sub-projects:

- Modifying the behavior of the torque converter
- Adjust intake camshaft to optimize consumption
- Conversion to newer Motronic 4.4, including ECU mods
- Engine coolant temperature sensor, ECT resistor mod

Modifying the torque converter lock-up behavior:

I have developed electronics that take control of the lock-up clutch in 4th gear without the ECU noticing that it has lost control of it.

Lock-up control is in 1st - 3rd gear by the AW control unit, in 4th gear the AW control unit just thinks it is in control. In the 1st - 3rd gear, the control commands of the AW control unit are passed through 1:1, in the 4th gear my electronics completely take over the control of the lock-up clutch (SL).

The lock-up clutch is activated from 65km/h and remains active as long as the 4th gear is active and the speed does not drop below 60km/h. If the brake is pressed and the speed is not less than 70km/h, the lock-up clutch also remains active.

If the control unit switches back to 3rd or 2nd gear at a speed higher than 65 km/h, my electronics immediately transfer control to the AW control unit.

The lock-up clutch is activated as original, my electronics sends a request for torque limitation to the motronic, when the motronic sends the confirmation for this, the lock-up clutch is activated as original with a pulsed signal.

Above 65km/h, the driving behavior corresponds to that of a manual gearbox in 4th/5th gear. The engine is firmly interlocked with the gearbox, you have an engine brake and a correspondingly lower engine speed. Since no more unnecessary energy is burned in the torque converter and the engine speed remains permanently lower, there is an advantage in consumption.

All control is active as long as the transmission is in ECO mode. If you switch to Sport mode on the selector switch, the original AW-ECU has full control, the additional electronics are completely deactivated.

If you want to rebuild the whole thing, the circuit diagram and the hex file for the Tiny13 are available here. The installation is relatively easy, 12 cables have to be soldered to the plug of the AW control unit, from which the cable to the solenoid (SL) in the gearbox has to be separated. It's definitely not an extremely difficult action.

One more note: I built the mod for my n/a GLT with 170hp, which has 220NM of torque. In principle, the circuit should also work in the turbo, but I do not want to guarantee that the closed converter bridge can cope with the torque of a turbo.

Adjust intake camshaft to optimize consumption:

If the loss of maybe 10% of the 170hp is not a problem then recalibration of the camshafts is possible. In order to get more torque in the lower speed range, the intake camshaft must be set to "advance".

When the intake camshaft is advanced, the valve opens earlier, but more importantly, it also closes earlier before BDC. This ensures better filling in the lower and middle speed range because less gas is pushed back into the intake at the end of the intake stroke, but peak performance also suffers because this earlier intake closing at higher speeds is counterproductive for the recharging, which engines do a real increase in performance at high speeds.

Procedure: Adjusting the intake camshaft using the slotted holes in the camshaft sprocket.

The standard is that the screws are located approximately in the middle of the slotted holes. So you have about 8-10 degrees of camshaft angle in both directions, which corresponds to about 16-20 degrees of crankshaft angle in both directions of adjustment.

In order to advance the intake camshaft, proceed as follows:
Make the crankshaft pulley accessible. Remove the camshaft sprocket cover. Slightly loosen the intake camshaft bolts, remove one bolt completely to better see where the camshaft is located. Place the tool on the crankshaft and rotate the engine counterclockwise slightly in reverse. The bolt hole moves to the right in the slot, which advances the camshaft. If the camshaft is readjusted, tighten the bolts to 20NM. The valves are free within the slotted holes.

I wrote it a bit simply because you should only change camshaft timing if you understand what you are doing.

Motronic 4.3 to newer Motronic 4.4 conversion, including ECU mods:

Very interesting topic, at least for me.

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....

ECT resistor mod:

We all know that an engine consumes the most fuel when it is cold. My idea was to influence the engine temperature reported to the ECU. For this I soldered a 10KOhm resistor in parallel to the ECT sensor in the ECU (4/46). These are connections A-18 - A-31 of the Motronic.

The resistor soldered in parallel has a logarithmic effect on the original resistance curve. This means that the deviation is greatest when cold and almost non-existent when warm from operation.

In a very cold state at approx. -20 degrees, the sensor only reports -5 degrees to the ECU, at real 25 degrees the sensor still reports 30 degrees, and at an operating temperature of 90 degrees 90 degrees are reported. As the temperature rises, the reported value becomes more and more like reality. This means that the engine runs slightly leaner than normal when it is cold.

If there are starting problems due to low temperature, the resistor can be interrupted during starting by suitable means, e.g. interrupting with a button, a relay controlled by terminal 50, or a microprocessor which receives the 42Hz signal from 4/46 B-27 evaluates.

You should then drive the cold engine as normal and bring it up to operating temperature as quickly as possible. Gentle driving, i.e. stroking the gas pedal, has a consumption-enhancing and wear-enhancing effect on a cold engine, since it takes longer to warm up.


Lockup-Software: AVR hex-file

Lock-Up schematic
set AW50-42 schematic
set Lock-Up PCB
set ECUs
set Error codes
set modified ECU
set ECC plug-n-play
set ECC non plug-n-play

ECT-Mod-Software: AVR hex-file

ECT-Mod schematic