The story of troubleshooting a NP-55G Generac 4.3 Kw generator with multiple issue’s installed in a large 1719 Mercedes Benz 4x4 motorhome with office. How one failure leads to a another caused by a backlog of maintenance and design issue’s.
Generac NP-55 generator with electric start
The Generac NP-55G is a V-shaped 4-stroke twin combustion engine powering a 4.3 Kw belt driven alternator. Steady RPM is maintained using a simple mechanical governor throughout load changes. This 80 / 90's generator design is rather simple with a minimum of electronics which makes it ideal for regions where parts are hard to get.
The NP-55G is designed for installation in RV, mobile home or campervan. The twin engine is manufactured in Japan by Waukesha, a big engine industrial. The engine type plate reads model 9205.
The NP-55 generator has an electric start powered by a 44 Ah battery.
|Generac NP-52G RV Generator - installed in a Mercedes Benz 1719 4x4 truck
The generator is mounted on a slide-out tray so it can serviced more easily.
The Generac NP-55G generator in this truck is used to power a mobile office and has just 570 hours on the dial. Not much on-time since it has been installed in 1998. With 2 laptops, a fixed desktop computer, 1 printer, a RAID disk array and a 3G router for internet access. The Generac NP-55G generator powers a battery charger – the battery powers a sine-wave inverter, simulating a UPS. (Uninterruptible Power Supply)
Generac NP-55 start problem
Once a generator equipped with electric start fails to run for some time – the external battery will inevitably drain resulting in even more difficult cranking. This condition caused damage to the starter sprocket or ring gear. The NP-55 has a starter sprocket made from plastic and was damaged badly by many start attempts with a low battery.
|Generac NP-52G RV damaged crank sprocket from starter motor - partnumber 76254
To overcome a draining battery, it’s connected full-time to a CTEK multi-stage battery charger. After changing the starter sprocket and starting with a fully charged battery a new issue appeared.
|CTEK battery charger to charge the Generac NP-52G RV starter battery
After long cranking sessions the NP-55G started and ran irregular in idle. Connected to an electric heater the generator stalled each time unable to maintain a stable rpm. Closer inspection showed a damaged spark plug cable on the right cylinder.
|Generac NP-52G RV Generator sparkplug cable repair
When the generator was started in the dark – sparks leaked from the cable to the engine cover. The sparkplug cable was then fixed using several heat shrinks. On the next start the generator ran better but still irregular with sporadic stalling.
Generac carburetor issue’s
With irregular runs with sporadic stalling it was time to open the carburetor for any clues – it seemed fuel supply was hampered. After disassembly it was immediately clear this Japanese TK carburetor had a history of serious tampering. One support of the flotter had been broken.
|Generac NP-52G RV damaged TK carburetor - partnumber 73112B
Apparently the flotter still worked somehow because the carburetor didn’t drown in petrol. A new carburetor was order at Generator Parts in the UK, together with a new startermotor sprocket, starter solenoid and choke solenoid.
New TK carburetor for Generac NP-55
The difference was instant – the new TK carburetor resulted in a steady idle RPM. The carburetor has only one adjustment for stationary rpm. Unfortunately short spells of irregular idling surfaced again – now with occasional backfire in the new carburetor.
The carburetor gaskets were checked on air leaks since carburetor backfire can be a result of slow burning lean mixture - messing up combustion timing. The problem stayed though suspecting an ignition issue – based on the fact that the carburetor has no further adjustment options.
Generac NP-55 ignition system
The Waukesha twin engine ignition system is remarkably simple. Just two coils produce sparks caused by passing magnets – there's no ignition timing adjustments possible. The sparkplugs seemed ok, same as the sparkplug cables. Starting the engine with test sparkplugs hanging outside the cylinders showed a thick spark. Note that the twin engine can run on just 1 cylinder, with just a subtle difference in engine sound.
Generac NP 55 ignition control can shut down the engine through a temperature sensor, oil pressure sensor and the stop button. The control line is extremely simple, either sensor or stop button pulls the ISD (Ignition Shutdown Module) to ground / mass of the starter battery.
|Generac NP-52G ignition schematic
Measuring resistance of the ignition shutdown lead showed a peculiar value - about 2k – 3k ohms. Indicating leakage to ground / mass of the battery. In the end the problem was reduced to the stop button lead of the remote start panel. When the panel was disconnected the NP55 ran perfectly for hours.
Generac NP 55G wiring problem
After some digging faulty wiring at the remote start panel was the cause of the problems and most likely resulted in heavy tampering with the carburetor and sparkplug leads. On top of that the low charged battery caused a broken starter sprocket.
|Generac NP-52G remote start panel
The fault was the result of connecting a voltage measurement display to the start / stop button at the remote start panel. The display shows the voltage of the generator starter battery. In stead of connecting the mass / ground of the voltmeter to the center ground / mass lead on the start / stop switch it was connected to the ignition shutdown lead (lead nr 18 in the schematic above).
The internal resistance of the voltmeter caused sporadic ignition shutdown – causing sporadic backfire and stalling of the engine.
Choke solenoid wiring
While inspecting engine wiring another potential and dangerous issue surfaced in the wiring of the choke solenoid. The voltage supply wire was bloated over the full length coming directly from the starter solenoid. At some points blank wire was visible – a potential dangerous situation which could result in engine fire. The choke solenoid sits directly under the carburetor and runs parallel to the fuel line.
|Generac NP 52G rv generator choke solenoid bloated wire
Apparently the choke solenoid voltage supply line has drawn a lot of current in the past. Possibly cable damage and accidental connection to ground might have caused high currents – the line is not fused and connected directly to the battery. The choke solenoid itself doesn’t appear like a high current device and probably dies before the voltage supply wire bloats. The choke solenoid and wiring was replaced as a precaution.
|Generac NP 52G rv generator choke solonoid dangling connection terminal - partnumber 91306|
Closer inspection of the old choke solenoid showed dangling connection terminals from the coil. The metal frame of the solenoid already has a short-circuit mark (top left corner) most likely making the choke wire heat up and bloat it’s plastic shield. A dangerous condition – there’s no fuse to anticipate a short circuit.
Checking Generac NP-52G engine noise
Internal engine noises in the NP-52G cylinders are tracked and compared with a mechanic's stethoscope. No difference in valve adjustments were found, both cylinder heads sounded identical.
|Generac NP 52G rv generator - checking engine noise with a mechanics stethoscope.
Removing the spark plug caused a different noise - indicating the fixed sparkplug cable actually fires the sparkplug.
Generac engine temperature checks
After running the Generac NP-55G generator for a few hours several temperature checks were done. First oil temperature with a Non-Contact Infrared Thermometer with Laser Targeting. The oil filter housing reads 75 degrees Celsius which is 167 degrees Fahrenheit.
|Generac NP 52G rv generator oil temperature - 75 degrees Celsius or 167 degrees Fahrenheit
Exhaust temperature reading 165 degrees Celsius or 329 degrees Fahrenheit.
|Generac NP 52G rv generator exhaust temperature - 165 degrees Celsius or 329 degrees Fahrenheit.
Read about a Kipor Digital 2000Ti Generator review
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Parts were ordered at GenPlus systems LTD in the UK.
Parts arrived 1 week later - all original Generac parts.
Read about Generac generator problems