Mercedes Diesel Engine Swap Guide – Engine Mounts, Gearbox Alignment and ECU Integration

Mercedes diesel engine swap into a non-original chassis is straightforward when you understand the correct process for engine mount fabrication, gearbox alignment and ECU integration. Today we cover all three using an OM605 into a Mercedes SLK as the working example.

The Basic Premise

Most OM60x engines end up in cars that never had that engine from the factory. Whether it is a Mercedes in a different model, a Land Rover, a Jeep or something more exotic the fundamental process is the same. The SLK example works well because it illustrates the complete process clearly.

An SLK with a 230 Kompressor petrol engine has a six speed manual gearbox. That gearbox bolts directly to the OM605. This means the prop shaft, rear differential and entire driveline remain unchanged. The engine fits, the gearbox fits, the driveline is correct. The starting point is better than most swaps.

Engine Mount Fabrication – The Correct Process

Engine mounts in this context means the fabricated arms that connect the engine block to the chassis. The rubber mounts themselves – the actual vibration isolating components – are separate.

The critical measurement is gearbox angle. Before removing the original engine:

1. Put the car on a two post lift in a position it will not move from until the new engine is installed
2. Measure the angle of the gearbox at the front – the tilt relative to horizontal
3. Write this number down

With the OM605 installed and bolted to the gearbox adjust the engine height until the gearbox angle matches the original measurement. Add five millimetres of extra height to compensate for engine mount compression under the weight of the engine in use. When loaded the mounts compress slightly bringing the engine down to the correct position.

Now fabricate the engine arms to hold the engine at this position. Tube construction is strongly recommended over sheet metal. Tube arms take up less space, route around obstacles more easily, look cleaner and are at least as strong as sheet metal construction of any practical thickness. Three disadvantages to sheet metal, no advantages over tube.

ECU Integration – The Trick Most People Miss

This is the piece of information that makes or breaks a conversion. With a mechanical injection pump diesel in a car that originally had an electronic petrol engine the instruments and electronics appear to be a major problem. They are not.

Use the original car ECU. Connect everything as standard. The ECU will receive its normal inputs – water temperature, RPM signal, vehicle speed and all other standard sensors. Everything works normally.

The ECU will throw fault codes for missing components – no ignition coils, no petrol injectors. Take the car to any competent tuner and have those specific DTCs deleted from the ECU. After deletion everything functions normally. RPM gauge works. Water temperature works. All warning lights clear.

The only limitation is ESP electronic stability control. The system cannot cut throttle on a mechanical diesel because there is no electronic throttle to cut. The ABS function of ESP continues to work correctly and that is sufficient for genuine safety benefit. If the car asks you to lift off throttle in a hazardous situation – lift off throttle. The mechanical diesel will respond. The system works adequately in practice.

RPM Signal for Additional Components

If the build requires an RPM signal for other components – a shift controller for a 722.6 automatic for example – add an additional sensor. The dual mass replacement flywheel used with short input shaft six speed transmissions already has the correct trigger wheel on the back face. Connect a sensor to this trigger and the RPM signal is available for any additional controller that requires it.

Summary

• Mercedes diesel engine swap process: measure gearbox angle, fabricate arms to match, add 5mm height for mount compression
• Tube construction for engine arms beats sheet metal in every measurable way
• Original car ECU integration works with mechanical diesel – connect all sensors normally
• Delete petrol-specific fault codes through a tuner and all instruments function correctly
• ESP throttle cut does not work with mechanical diesel but ABS continues to function
• Short input shaft gearboxes need dual mass replacement flywheel with correct trigger wheel already fitted
• Additional RPM signals for controllers can be taken from the flywheel trigger sensor

Stay tuned for more practical build knowledge. Bye!