[1] The light Mercedes-Benz trucks L 1100 and L 1500 as well as the bus O 1500[A 2] were also offered with the OM 138 as an alternative to the standard Otto engine.

[3] Daimler-Benz started mass production of the six-cylinder inline truck diesel engine OM 5 in 1928.

Caused by the lower diesel fuel price compared to petrol, the W 138 was favoured especially by taxi drivers.

Daimler-Benz decided to use the truck engine again to develop a fitting powerplant for a car.

[9] In 1934,[3] the cylinder count of the truck engine was reduced to four, and the bore and stroke dimensions were kept.

Problems such as heavy exhaust emissions and rough engine running were solved, and mass production could begin in 1935.

[1] The OM 138 is a naturally aspirated and water-cooled[10] inline-four-cylinder diesel engine with precombustion chamber injection, [11] wet sump lubrication[12] and OHV valvetrain.

[10] The lower and upper crankcase parts are connected with pin screws on the horizontal crankshaft centre.

The lower crankcase part is strengthened with ribs and made of a light metal alloy.

[12] The upper crankcase part is made of grey cast iron, it reaches from the crankshaft to the cylinder head.

The bearings of the connecting rods are made of a lead bronze alloy and are fixed with a pin.

On its precombustion chamber side, the cylinder head also has the pushrods necessary for the OHV valve train.

The water pump, which also holds the fan, is mounted on the cylinder head on the front of the engine.