- Published by: Murray Yacht Sales Technical Division
- Expert Analysis: Engine & Generator Specialist
- Category: Marine Power Systems & Auxiliaries
Across the entire Cummins Onan marine lineup, the MDK series forms the backbone of their mid-range marine diesel generators, covering outputs from roughly 4 kW to 29 kW.
An Onan model number is structured as a puzzle, with each segment specifying the chassis, engine platform, electrical layout, and engineering generation. The systematic layout of the MDK family architecture breaks down as follows:
Example: 9.5 MDKBM - 1101 C
│ │││ │───│ │
│ │││ │ └── Spec Revision Letter
│ │││ └────── Voltage / Hz Winding Code
│ ││└───────────── Power Scaling / Block Size Variant
│ │└────────────── Platform Generation & Controls
│ └─────────────── Core Series (Marine Diesel Kubota)
└──────────────────── Kilowatt (kW) Rating
1. The Core Series: M – D – K
Every model in this family starts with these three letters, which anchor the primary physical characteristics of the machine:
- M = Marine: Indicates it is engineered from the factory for a boat engine room. It features a freshwater heat exchanger loop, a marine-grade raw-water pump, sacrificial zinc anodes, a water-cooled exhaust elbow, and corrosion-resistant paint.
- D = Diesel: Runs on standard diesel fuel, avoiding the spark-ignition hazards of gasoline engines in confined marine bilge spaces.
- K = Kubota Engine Block Base: Identifies that the base industrial engine block chosen for this family is manufactured by Kubota. Onan adds their proprietary marine components, electronic governors, and alternator backend to this block.
2. The Platform & Controls Generation (4th Character)
The fourth letter tells you which “era” or engineering platform the generator belongs to. This dictates what style of control board, safety switches, and sound shield footprint the machine uses:
| 4th Letter | Era & Platform Type | Notable Features |
| A (e.g., MDKAA, MDKAB) | Mechanical Era (Late 1990s–Mid 2000s) | Relied heavily on mechanical governors, analog senders, and older relay-based control frameworks. |
| B (e.g., MDKBJ, MDKBL, MDKBM) | Digital “Quiet Diesel” (QD) Era (Mid 2000s–Present) | Shifted to highly isolated sound enclosures, digital control panels (like the e-Series), and electronic isochronous governing. |
| D (e.g., MDKDK, MDKDP, MDKDR) | Tier 3 / Modern Network Era (Current Production) | Advanced electronic controls featuring integrated J1939 CAN bus data links for smart helm displays, compliant with stricter EPA Tier 3 marine emissions. |
3. Power Scaling & Block Size (5th Character)
The fifth letter determines the size of the internal engine block and physical footprint. As you go down the alphabet, the displacement, number of cylinders, and cooling capacity increase to support higher kilowatt demands:
- H, J, K, L (e.g., MDKBJ, MDKBL, MDKDK): Compact, small-frame units. Typically 2- or 3-cylinder Kubota blocks powering lower-range loads (4.0 kW to 11.5 kW range).
- M, N, P, R, S, T, U, V (e.g., MDKBM, MDKDP, MDKDR): Mid-to-large-frame units. These utilize heavy-duty 4-cylinder engine blocks to support larger electrical demands (13.5 kW up to 29 kW).
4. The Winding/Voltage Code (4-Digit Suffix)
Directly following the dash is a factory configuration code indicating how the generator’s stator is wound and pre-wired:
- 1100 series (e.g., -1100): Built for 50 Hz applications operating at a lower engine speed of 1500 RPM (standard for European and international vessels).
- 1101 series (e.g., -1101, -1102): Built for 60 Hz applications operating at 1800 RPM (standard for North American configurations).
- Digits further down the line denote specific factory options, such as single-phase vs. three-phase configurations or isolated ground setups.
5. The Spec Letter (The Final Suffix)
Represented by a lone trailing letter (such as A, B, C, D), the “Spec” letter tracks minor production modifications made mid-lifecycle. If Onan upgrades the starter motor supplier, adjusts a belt-guard bracket, or tweaks the heat exchanger core layout, the spec letter advances.
Crucial Maintenance Tip: Always have this letter handy when buying spare parts. A Spec A version of a specific model can use completely different fuel filters or raw-water impellers than a Spec D version of the same model.