Generator Head and Engine Overhaul in Rancho Santa Margarita
Written by RV Systems Lead, RV Systems and Electrical Lead, OCRV Center. Reviewed by Fleet Account Manager
In shortA set that runs with no power at the outlets is a head side fault, not an engine fault. Overhaul and repair work spans $800 to $8,500+ at $260 per hour.
What does generator head and engine overhaul involve?
Overhaul work splits a genset into two halves and repairs whichever failed. The head side covers the rotor, stator, brushes and voltage regulator. The engine side covers compression, valves and the governor. Rancho Santa Margarita owners bring sets to Yorba Linda when fault codes and parts swapping have stopped producing answers.
- Engine runs, outlets dead: the fault is on the generator head
- Output frequency is set by engine speed, so surging affects both halves
- Fault codes flash at the stop switch and narrow the search quickly
- A reman long block sometimes beats a piece by piece rebuild
- Price range
- $800 to $8,500+4 to 35 labor hours at $260 per hour
- Labor rate
- $260 per hourMechanical and electrical department
- Typical turnaround
- 4 to 12 shop daysLonger when a rotor, stator or long block is on backorder
- Written estimate
- $150Credited in full against an authorized repair
- Disposal
- $45 flatApplies when fluids or LP are handled during teardown
Last verified
A generator is two machines bolted together. One of them burns fuel and turns a shaft. The other one takes that rotation and makes electricity. When somebody says their generator is broken, the first useful thing anybody can do is work out which of the two halves is at fault, because the parts, the skills and the cost are entirely different.
That split governs everything on this page. An engine that will not start and a set that runs beautifully while producing nothing at the outlets are both described the same way over the phone. One is a fuel or compression problem. The other is a rotor, a stator, a brush set or a voltage regulator, and no amount of carburetor work will touch it.
Deciding whether the fault is engine side or head side
Everything begins with one observation: does the engine run. A set that will not start or will not stay running has a fuel, ignition, compression or protection circuit problem, and the generator head is irrelevant to it. A set that starts easily, holds speed and produces nothing at the terminals has a healthy engine and a fault in the electrical machine attached to it.
Owners rarely make this distinction because the whole assembly shares one name and one enclosure. It matters enormously for cost. Head side faults are frequently inexpensive components with high labor visibility, while engine side faults reach into compression, valves and occasionally a complete long block. Knowing which conversation you are in before parts are ordered saves a great deal of money.
| What you observe | Which half | Where we look first |
|---|---|---|
| Cranks, never fires | Engine | Fuel delivery, spark, compression |
| Runs, no voltage at outlets | Head | Brushes, slip rings, regulator |
| Runs, voltage far too high | Head | Voltage regulator |
| Speed rises and falls rhythmically | Engine | Governor linkage and throttle shaft |
| Fine at idle, collapses under load | Engine | Compression and leak down |
The engine runs and there is no power at the outlets
This is the most common complaint we receive on this page and the one most often misdiagnosed as a dead unit. The generator head builds voltage by spinning a magnetized rotor inside a stator winding, and that process depends on field excitation reaching the rotor. On brush type sets that excitation travels through carbon brushes riding on slip rings, and brushes are a consumable item.
There is also a quieter cause worth knowing about. A machine that has sat unused for a long period can lose the residual magnetism it needs to start building voltage in the first place. Everything in it is healthy, and it will produce nothing until the field is flashed. We check for that early because it costs minutes to establish and prevents an unnecessary teardown.
- Brushes worn past the wear line, a common consumable failure
- Slip rings glazed or grooved so contact is intermittent
- Voltage regulator failed, producing no excitation at all
- Residual magnetism lost after long storage, needing the field flashed
- Stator or rotor winding failure, the expensive end of the list
Reading fault codes instead of parts swapping
Onan control boards store fault information and flash it back through the stop and start switch as a counted blink pattern. Those codes are not precise enough to name a part, but they are more than precise enough to eliminate half the possibilities in a couple of minutes. A code reporting overcrank sends us to fuel and spark. A code reporting overspeed sends us to the governor.
The thing that destroys this advantage is resetting. Owners and shops clear codes reflexively before anyone has looked, and with them goes the record of what the set experienced before it stopped. We read and write down every stored code at intake, and on a set with an intermittent complaint that record is frequently the only evidence of what happens on the road.
Before you proceed
Do not clear stored generator faults before a technician has read them. That blink pattern is the cheapest diagnostic information on the whole machine, and once it is gone it cannot be recovered.
Governor hunting, frequency and the voltage regulator
Output frequency in these machines is a direct consequence of shaft speed, so anything that lets engine speed wander shows up immediately as frequency wandering at the panel. A governor with a worn linkage or a throttle shaft that sticks slightly will overshoot and undershoot in a slow rhythm, and appliances with switching power supplies do not enjoy that at all.
Voltage is regulated separately. The regulator adjusts field excitation to hold output within a window regardless of load, and when it fails the result is usually dramatic in one direction or the other. High output is the dangerous case, because it will stress and sometimes destroy converter and appliance boards long before anyone gets around to investigating why the lights looked bright.
- Set governed speed with a frequency meter, never by ear
- Inspect linkage and throttle shaft for wear and sticking
- Measure regulator output across no load and full load
- Log voltage and frequency together at several load points
The point where an overhaul stops making sense
A compression test and a leak down test between them decide most of this. Ring wear that is general across cylinders on a set with substantial hours points toward a remanufactured long block, because rebuilding one item on a tired engine simply relocates the next failure. A single burned valve on an otherwise healthy engine is a head job and worth doing.
The other input is availability. Rotors, stators and long blocks for older sets can sit on backorder long enough that the schedule, not the price, becomes the deciding factor. We check availability before recommending a route, and where an owner is choosing between rebuild and replacement we quote both so the comparison is theirs to make rather than ours to assert.
How an overhaul is scoped and billed
Work bills at $260 per hour from the mechanical and electrical department, and the four to thirty five hour range on this page reflects genuinely different jobs. Brushes and a regulator on a set behind an accessible door occupy the bottom. A stator replacement or a long block on a tray mounted diesel set, with the set out of the coach and back in, occupies the top.
Parts carry standard markup, tax applies to parts and materials, and fluid handling during teardown carries the $45 hazmat and disposal charge. A written repair estimate is $150 and credits against an authorized repair in full. Deposits apply above two thousand dollars at authorization, and above ten thousand a further deposit applies when parts land.
On cost
We quote diagnosis before overhaul on every set. Spending two hours establishing which half failed regularly turns an eight thousand dollar assumption into a repair costing a small fraction of that.
Downtime planning for rental and fleet units
For a personal coach a generator sitting for a fortnight is an inconvenience. For a rental operator it is a unit off the board through a booking window, and for a mobile clinic or a command vehicle it can mean the whole asset is out of service. Those customers need a date more than they need the lowest number, and the date depends on parts rather than on bay space.
We handle that by establishing availability during diagnosis rather than after authorization. If a stator is on allocation and a reman long block ships in three days, that difference belongs in the conversation before a decision is made. Fleet customers also get the loaded output log with the unit, which becomes the baseline their next service is measured against.
What is included
- Fault code retrieval from the control board
- Stored codes are flashed at the stop switch and recorded before anything is reset, since clearing them first destroys the most useful evidence available.
- Head side electrical testing
- Brush condition, slip ring surface, regulator output and winding resistance are each measured against specification rather than swapped on suspicion.
- Engine side mechanical testing
- Compression and leak down readings are taken on every cylinder, which separates ring wear from valve sealing before any head comes off.
- Governed speed and frequency verification
- Engine speed is set with a frequency meter across no load and full load, because output frequency is entirely determined by how fast the shaft turns.
- Loaded output testing after repair
- The rebuilt set runs against a real load while voltage and frequency are logged at several points, not just checked once at no load.
- Long block replacement where a rebuild is uneconomicalwhere applicable
- A remanufactured assembly is quoted alongside a piece by piece rebuild so the comparison is visible.
How the work runs
Capture codes and establish which half failed
1 to 2 hours
Stored faults are read at the stop switch and written down before any reset. Then the set is run if it will run, and output is measured at the terminals. Those two pieces of information split the job into head side or engine side within the first hour.
Test rather than swap
2 to 5 hours
Brushes, slip rings, regulator output and winding resistance all have specifications, and so do compression and leak down. Measuring against those numbers costs an hour and avoids the parts cannon approach that makes generator repair expensive without making it effective.
Pull the set where access demands it
2 to 8 hours
Sets on slide trays come out on the tray. Sets buried in a compartment come out entirely, because a head split or a head removal cannot be done through a service door. This step is quoted honestly since it is real hours on any coach.
Rebuild the failed half
3 to 20 hours
Head side work covers brushes, rings, regulator and windings. Engine side work covers valves, rings, bearings or a complete long block. The half that tested sound is left alone rather than being refreshed to pad the invoice.
Reinstall and prove it under load
2 to 6 hours
The set goes back in, governed speed is set with a meter, and output is logged across the load range including a transition where both air conditioners start. Nothing leaves on a no load run and a hopeful listen.
What this costs
$800 to $8,500+
4 to 35 labor hours at $260 per hour, mechanical and electrical.
- Hazmat and disposal $45 flat when chemicals, refrigerant or LP is involved.
- Parts at $100 or less carry 100 percent markup. Parts over $100 carry 35 percent. Special order parts require a full non refundable deposit at order.
- Sales tax of 7.75 percent applies to parts and materials. Labor is not taxed.
- A 50 percent deposit is taken at authorization on any job over $2,000, with an additional 25 percent when parts arrive on any job over $10,000. Final balance is due at pickup.
- A brush set and voltage regulator on an accessible set sits at the bottom of this range.
- A stator replacement or a reman long block on a tray mounted diesel set sits at the top.
- Rotor, stator and long block availability drives schedule more than shop capacity does.
How estimating is billed
- Written repair estimate: $150, credited in full against an authorized repair.
- In depth diagnostic, scan and programming: $285 per hour, one hour minimum, credited in full against an authorized repair.
- In shop pre purchase inspection: $400 to $1,200 depending on unit size and systems count.
- Insurance walk ins are accepted during posted hours. All inspection and estimating work is performed in shop at the Yorba Linda facility.
What you will notice first
- The engine runs smoothly and there is no voltage at any coach outlet
- Output measures far below normal at the panel with the set running steadily
- Voltage swings high enough to make lights flare and appliance boards reset
- Engine speed rises and falls continuously in a slow hunting rhythm
- The set cranks and cranks without firing, then flashes a code at the stop switch
- It starts and shuts down after a few seconds with a fault indication
- Blue smoke from the exhaust on start up that clears after a minute
- A heavy knock under load that quiets when the load is removed
How this fails, and what we do about it
| Failure mode | How we diagnose it | The fix |
|---|---|---|
| No output with the engine running normally | Voltage measured at the set output terminals is zero or near it while engine speed is correct and steady. From there the search runs through brushes, slip rings, the regulator and finally the rotor and stator windings, each of which has a distinct resistance signature. | Replace the specific failed component rather than the head as an assembly wherever parts remain available. Brushes and regulators are common and inexpensive. A stator is neither. |
| Output voltage far above or below nominal | Steady engine speed with output that sits well outside the acceptable window. Correct speed with wrong voltage points at regulation rather than at the engine, and a set producing high voltage should be shut down immediately before it damages coach electronics. | Replace the voltage regulator and confirm output across the full load range afterward. Any appliance boards damaged by the excursion are assessed separately and documented. |
| Governor hunting | A slow rhythmic rise and fall in engine speed, audible in the exhaust note and visible as frequency swinging at the output. The linkage is inspected for wear and the throttle shaft is checked for a sticking point that lets the governor overshoot. | Replace worn linkage components and springs, free or bush a worn throttle shaft, then set governed speed properly with a frequency meter rather than by ear. |
| Compression loss under load | The set idles acceptably and collapses when asked to work, with a compression test confirming a cylinder well below specification. A leak down test then separates rings from a valve, which decides whether the head comes off or the whole engine does. | A valve job where the bottom end is sound, or a reman long block where wear is general. We price both routes so an owner can see where the crossover sits for their set. |
Why it happened
- Voltage regulator failure, which sends output either far above or far below nominal while the engine behaves perfectly
- Worn brushes and glazed slip rings on brush type sets, which interrupt field excitation and produce intermittent or absent output
- Loss of residual magnetism in the rotor after long storage, which leaves a healthy machine unable to build up voltage without being flashed
- Governor linkage wear or a sticking throttle shaft, which allows engine speed to hunt and takes output frequency with it
- Compression loss from worn rings or a burned exhaust valve, usually the end result of lash that drifted and was never adjusted
- Bearing failure at the rotor end, which shows as a rumble that changes with load and eventually as a rub between rotor and stator
Parts and brands we work with
- Cummins Onan control boards, voltage regulators and stop switch fault code readouts
- Rotor, stator and brush assemblies for Onan Marquis and QG series heads
- Carburetors, fuel pumps and stepper motors for fuel injected QG sets
- Reman long blocks where a piece by piece engine rebuild stops being economical
- Governor linkage kits, throttle shafts and springs
- Onan low oil pressure and overspeed protection switches
What not to try yourself
- Continuing to run a set that is producing high voltage. Every minute at excessive output is another minute of stress on converter boards, appliance controls and anything else plugged into the coach.
- Clearing stored fault codes before anyone has read them. Those codes are the cheapest diagnostic information available and resetting them turns a short job into a long one.
- Ordering a replacement head because output is missing. Brushes and regulators account for a large share of no output complaints and cost a fraction of a head assembly.
- Adjusting governed speed by ear. Output frequency follows engine speed directly, and a set running fast or slow will misbehave appliances even when voltage looks correct.
Which vehicles we do this on
Vehicles we perform Repair and Overhaul on
Diesel PusherDiesel pusher repair means working on a rear engine coach built on a welded Freightliner XC or Spartan rail, with a full body painted fiberglass shell and large basement baysClass A GasClass A gas motorhome repair covers the molded front and rear caps, the vacuum bonded laminated sidewalls, the EPDM roof and the F53 chassis underneathSuper CA Super C is a coach built behind a Freightliner M2 106 or International MV conventional truck cabBus ConversionA bus conversion uses a Prevost or MCI intercity coach shell, which is a stainless steel monocoque where the skin and skeleton both carry loadMedical ClinicMobile medical clinic repair covers the exam room shell, the wheelchair lift opening in the floor, the welded clinical floor seams, the lead lined radiology wall and the roof condenser curbsCommand CenterMobile command center repair covers the telescoping mast base and its roof penetration, satellite and antenna curbs, the conference slide opening, equipment rack floor mounts and the diesel generator compartmentThe call I get most often is that the generator is dead and needs replacing. Then I get there, it fires up on the first try and runs like a sewing machine, and there is nothing at the outlets. That is not a dead generator. That is usually a brush set worth very little or a regulator worth a bit more. I have pulled sets apart that three other people had condemned and found glazed slip rings and a set of brushes worn past the wear line. Test the head before anybody talks about replacing anything.
Appendix: questions and answers
- My generator runs perfectly but there is no power. What is wrong?
- The engine half is healthy and the electrical half is not. The usual culprits are worn brushes, glazed slip rings or a failed voltage regulator, all of which are modest parts. Occasionally a machine that sat for years has simply lost residual magnetism and needs the field flashed.
- Is it safe to keep using a generator that is putting out high voltage?
- No. Shut it down. Excess output stresses converter boards, appliance controls and anything else connected, and the damage accumulates the whole time the set runs. High voltage with correct engine speed points at the regulator, which is a repair rather than a replacement of the set.
- Why does my generator speed keep rising and falling?
- The governor is overshooting, normally because the linkage has worn or the throttle shaft is sticking slightly. Since output frequency follows shaft speed directly, the swing shows up at the panel as well. It is corrected by replacing worn linkage and setting speed with a frequency meter.
- Should I rebuild my generator engine or fit a remanufactured long block?
- Compression and leak down testing decides it. General ring wear across cylinders on a high hour set favours a long block, because rebuilding one item on a tired engine just relocates the next failure. A single burned valve on a sound engine is worth repairing on its own.
- What can an Onan fault code actually tell you?
- It will not name a part, but it eliminates half the possibilities in minutes. A code reporting overcrank directs the search to fuel and spark, and one reporting overspeed directs it to the governor. That is why we read and record stored codes before anything gets reset.
- How long will my coach be without its generator during an overhaul?
- Four to twelve shop days for most jobs, driven by parts rather than bay space. Rotors, stators and long blocks for older sets can sit on backorder, so we establish availability during diagnosis and give a date before authorization rather than discovering the problem later.
Open a file on this repair
Tell us the vehicle, what happened and whether a claim is open. We will tell you what the scope looks like and what it takes to get you back on the road.
Or call (949) 799-3387