Overview#
Kohler's commercial diesel alternator family covers the REOZ generator platform from 100 kW through 900 kW. These alternators are designed and manufactured by Kohler (now Rehlko) and are paired with John Deere, Volvo, and Mitsubishi engines across the REOZ commercial diesel lineup.
Unlike the KH-series alternators used in the larger KD industrial platform, the commercial diesel series uses alphanumeric frame codes (4R, 4S, 4UA, 4M, 5M, 7M) that indicate the frame size and winding configuration.
Frame family reference#
| Frame family | Model codes | Approximate kW range | Typical gensets |
|---|---|---|---|
| 4R | 4R13X | 100–150 kW | 100REOZJF, 125REOZJ4, 145REOZT4, 150REOZJF |
| 4S | 4S12X, 4S13X | 150–200 kW | 150REOZJ4, 175REOZT4, 180REOZJG |
| 4UA | 4UA9, 4UA10, 4UA13 | 200–350 kW | 230REOZJE, 250REOZJE, 275REOZJE, 300REOZJ |
| 4M | 4M4019, 4M4021 | 300–450 kW | 350REOZJ series, 400REOZJ series |
| 5M | 5M4024–5M4036BF | 450–900 kW | 500REOZJ series, 550REOZVB, 600REOZVB, 750REOZMD, 900REOZMD |
| 7M | 7M4046–7M4054 | 1000–2000 kW | 1250REOZMD, 1600REOZMD, 1750REOZMD, 2000REOZMD |
Design characteristics#
All frames in this family share common design principles:
- 4-pole, brushless construction — no brushes, slip rings, or commutators to maintain
- Class H insulation — rated for 180°C continuous operation
- Permanent-magnet pilot exciter — provides independent excitation for stable voltage regulation and short-circuit support
- Single-bearing design — direct-coupled to the engine flywheel via SAE flange and coupling disc
Common service issues#
In our field experience with REOZ-platform generators:
- Rotating rectifier failures — the most common alternator-specific failure mode. Presents as voltage instability or failure to build voltage at start. Diode test confirms; field-replaceable assembly.
- Bearing wear — typically only in continuous-prime applications exceeding 10,000 hours. Most commercial standby units never reach this threshold.
- Winding contamination — coastal installations and dusty environments require periodic megger testing and air inlet screen cleaning.
OnPoint service notes#
We service these alternators as part of standard REOZ generator maintenance contracts across the central California coast. Common service items include annual megger testing, voltage regulation verification during load bank testing, and rotating rectifier inspection. We stock the most common diode kits for the 4R, 4S, and 4M frame families.