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I found and old posting from a few years back called 'Is It True?' The onan L series engines were designed and built by onan for use on their DL series of gensets. They came in 3,4,and 6 cylinders with displacements of 1.7L,2.3L, and 3.4L respectively. Model designations were derived by using engine series (L) followed by number of cylinders, then displacement in Litres. Therefore, the 4 cyl was called and L423. These engines ran generator sets in the range of 15kW to 30kW. They used the Onan YD generator end. When these engines were developed, Onan was its own entity, unrelated to the Cummins Engine Company.
Upon the purchase of Onan by Cummins in 1986, Cummins used its own designation on these engines, similar to those already used on the B and C series Cummins engines already in production at that time. They called this new, smaller engine the A series. Designations were number of cylinders, followed by series. Therefore the engine with an onan degignation of L423, became the Cummins 4A. These engines were phased out by cummins quite early, and are now nearly extinct, and very hard and expensive to get parts for. I still occasionally see some of these here in Alaska, and worked on a few when I worked for the Alaska Cummins distributor as a field generator tech. I also have a 4A myself.

The Cummins B and C series were developed by a joint venture formed by Cummins Engine Co. and JI Case in the early 80s. Both companies were headed for bankruptcy, and joined forces to create the Consolidated Diesel Co, with engineering performed mostly by Cummins, and manufacture handled by JI Case. The resulting engines created the new midrange family of engines for Cummins. They were called the 4B,6B,and 6C, with displacements of 3.9L, 5.9L, and 8.3L respectively. These engines were used by Case wherever possible, and found their way into almost all of their products. Cummins used these engines for industrial, generator drive, marine, and automotive applications in order to maximize their market coverage with minimal investment in the product. The 6BT found its way into the Dodge D/W250, and 350 trucks for the 1989 model year. The first Dodge/Cummins used a rotary injection pump (Bosch VE), and was rated @ 160HP. In 1991, the engine was intercooled, using a Charge air cooler, still using the VE pump, and still rated @ 160HP. 1994 saw an inline injection pump added (Bosch p7100), and a rating of 180HP, using a wastegated turbo. 1998.5 MY introduced the ISB 24V electronic version rated @ 235HP.
So there is no relationship between the Onan L series (Cummins A), and the B and C series engines.
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