Somewhere in a barn, a shop or a backyard shed, an olive-drab Onan generator is waiting for the power to go out. If it is a 10 kW military set with a four-cylinder Onan diesel, there is a good chance an American Bosch Model 50 pump sits at the heart of it.
The Model 50 is a compact American Bosch plunger pump built for small diesels like those Onan fours. Surplus generators often spend years in storage before they reach a new owner, and the pump feels every one of those years. We test it, rebuild it and calibrate it on the bench, so your set holds speed and carries load when you need it.
Also Known As
Collectors, preppers and surplus buyers search for this pump as:
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American Bosch Model 50, or AMBAC American Bosch Model 50
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American Bosch M50
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“Onan 10 kW injection pump” or “military generator injection pump”
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“MEP-003A injection pump”
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Onan service numbers such as 147-0308
Did You Know? The frequency meter on a generator is an engine-speed gauge in disguise. On a 60 Hz set, the hertz only stay put if the engine holds a steady speed, so the injection pump and governor are doing a precision job.
How the American Bosch Model 50 Works
Think of the Model 50 as the generator’s metronome. It has to deliver the same measured shot of fuel, at the same moment, thousands of times a minute, while the governor nudges it to hold speed as loads switch on and off.
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A cam inside the pump drives its plunger to build injection pressure.
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A metering control sets how much fuel each stroke delivers.
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The governor adjusts that metering to keep engine speed, and therefore frequency, steady.
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Fuel goes out to each of the four injectors in firing order.
The exact internal layout varies by build.
Shop Talk: “Hunting” is when engine speed wanders up and down on its own. On a generator, you see it as flickering lights and a frequency needle that will not sit still.
What It Fits
| Make and model | Engine | Years | Notes |
|---|---|---|---|
| 10 kW military generator sets, such as the MEP-003A | Onan four-cylinder diesel | Some Onan 10 kW sets use an American Bosch PSU instead | |
| Other Onan four-cylinder diesels | Onan four-cylinder diesel | Check the pump plate |
Part Numbers We See
Model 50 plates carry a code that starts with 50, such as 50-2/4A-80A-9540A. Onan added its own service numbers, such as 147-0308, and military sets may carry their own part numbers too.
Send a photo of the pump plate plus the generator’s data plate, and we will confirm what you have.
Owner’s Tip: Surplus sets often arrive with old fuel. Before you blame the pump, drain any water, replace the fuel filter and confirm the supply pump is delivering fuel.
Why Model 50 Pumps Fail and the Symptoms
Storage is the big killer. Fuel that sits for years turns to varnish and sticks the metering parts. Water in tanks and cans rusts and pits the plunger. Seals dry out and governor linkage wears or binds.
Common symptoms:
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Hard starting or no start
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Hunting, with flickering lights or a wandering frequency needle
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Frequency or voltage sagging when a load comes on
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Black or white smoke
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Fuel leaks around the pump
What We Do to Your Model 50
| Service option | Typical evaluation or service scope |
|---|---|
| Test and report | Bench test as received, written results and a service recommendation |
| Rebuild and calibrate | Teardown, cleaning, plunger and metering inspection, worn parts and seals replaced, bench calibration |
| Exchange (when available) | Ask whether a compatible exchange unit is available and how the core process works |
What Model-Specific Verification Can Document
Evidence for AMBAC American Bosch Model 50: When an approved procedure exists for this exact component, useful documentation can distinguish identification, visible condition, observed wear or contamination and supported functional test results. Record the assembly number and available engine data before deciding which checks will answer the reported complaint. Bench findings cannot replace the engine-side diagnosis. See how pump testing is evaluated.
Repair Options and Technical Considerations
Repair decisions depend on the exact pump, its condition, the components required and the service approved after evaluation. Ask about any available exchange options and review the service scope before authorization. Plunger condition and parts availability for your code drive the service scope.
How to Ship Your Model 50
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Photograph the pump plate and the generator data plate.
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Note the set model, serial number and engine model.
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Drain the fuel and cap every fitting.
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Bag the pump and pack it in at least 2 inches of padding.
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Complete the mail-in form, include a copy and ship to the address provided during intake with tracking.
Ship It Right: Send the pump, not the whole generator. A boxed pump ships for a fraction of shipping the complete generator.
Related Pumps and Parts
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AMBAC American Bosch PSB, the classic single-plunger pump on vintage tractors and the M35A2
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AMBAC American Bosch Model 100 for the Allis-Chalmers 8070
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All AMBAC American Bosch pumps
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Injector repair
FAQ
Is my Onan pump a Model 50 or a PSU?
Some Onan 10 kW sets use one and some the other. The plate code tells us, so send a photo before you ship.
My generator runs but will not hold 60 Hz. Is it the pump?
It can be. Sticky metering parts or governor wear cause hunting, but so can dirty fuel and air leaks. Check the fuel side first.
The set sat in storage for years. Is the pump worth rebuilding?
Usually. Varnish often cleans up. Rust pitting on the plunger is the bigger worry, and we will tell you which you have.
Should I send the injectors too?
If the engine smokes or knocks, yes. Stored injectors stick and dribble too. See injector repair.
A backup generator that will not start is just a very heavy paperweight. Send us the Model 50 and get your set ready for the next outage. Start a Mail-In Repair or request a repair evaluation.