Controllers & Sensors

In short. A generator controller acts on signals, not on the engine itself. So every low oil pressure or high temperature alarm raises two questions: is the condition real, and is the measurement chain healthy? Sensors are identified by signal type, thread, range and the controller input they feed. Controllers are identified by exact model, hardware revision, firmware and configuration file. Replace either only after the circuit has been tested and the settings and event log have been saved.

Who this page is for: For maintenance engineers, facility managers and technicians diagnosing alarms or replacing generator controllers and engine sensors.

What the controller supervises

On a modern diesel generator the controller combines the AMF module, engine protection unit, metering panel and sequencer in one box. It:

• watches mains voltage and decides when the supply has failed;
• commands the start, checks oil pressure and speed during cranking and disconnects the starter;
• waits for warm-up, then closes the generator breaker or contactor;
• meters voltage, current, frequency, kW and kVAr;
• protects the engine and alternator and logs every event;
• reverses the whole sequence when the mains return.

The architecture is explained on generator controller architecture.

Engine sensors and what each one actually reports

Sensor	Usual signal	Confirm before ordering	How to prove the reading
Oil pressure switch	Contact, normally open or closed, at a fixed low pressure	Thread, switching pressure, contact sense the controller expects	Mechanical gauge on the gallery while the alarm is present
Oil pressure sender	Resistive, with a maker-specific ohms-to-bar curve	Curve selected in the controller, thread, single or two-wire	Resistance at a known pressure against the curve; gauge comparison
Coolant temperature sender or switch	Resistive thermistor or fixed-temperature contact	Curve, thread, location (head or outlet)	Contact or infrared thermometer at the housing
Magnetic pickup	AC voltage from flywheel teeth, rising with speed	Thread, length, air gap, tooth count set in the controller	AC volts while cranking; at rated speed, pickup frequency should equal tooth count × rpm ÷ 60
Current transformers	Secondary 5 A or 1 A, proportional to primary current	Ratio, burden, polarity, which phase each one measures	Clamp meter on the primary cable against the display
Voltage sensing	Direct 415 V or via a sensing transformer	Fuse rating, phase order, transformer ratio if fitted	Calibrated meter at the terminals

Worked example: two alarms, two different answers (illustrative)

Case 1: low oil pressure shutdown on a 125 kVA set. The site wanted the sensor replaced. A mechanical gauge on the gallery read about 1.0 bar at rated speed, against a normal of about 3.5 bar hot. The sensor was right. The oil was diluted with fuel from a leaking injector return, and the strainer was part-blocked. Running on with a “new sensor” would have destroyed the bearings.

Case 2: kW reading 25 percent low on a 500 kVA set. A clamp meter on the primary cables showed about 600 A per phase with the load steady.

Item	Value
CT fitted (from label)	1000/5 A
Ratio programmed in controller	750/5 A (left from an earlier CT)
Secondary current	600 ÷ 200 = 3.0 A
Controller reads	3.0 × 150 = 450 A
Error	450 ÷ 600 = 0.75, so every current function reads 25 percent low

The overcurrent protection was therefore set a third higher in real terms than intended (600 ÷ 450 ≈ 1.33). Correcting the programmed ratio fixed metering and protection together. Measure first, then decide; see controller diagnostics.

Never open-circuit a CT secondary while primary current is flowing; short it first. And never reclassify a nuisance shutdown as a warning to stop it happening. Find the cause.

Low oil pressure: condition or measurement?

This alarm is the one most often dismissed as “a faulty sensor”, and the one that can destroy an engine in minutes if the dismissal is wrong.

• Real causes: low oil level, a worn pump, a blocked pick-up strainer, excessive bearing clearance, fuel dilution, wrong viscosity for the ambient.
• Measurement causes: a failed switch, a corroded connector, a chafed wire shorting to the block, the wrong sender curve selected after a controller change.

Speed sensing and crank disconnect

The controller needs engine speed to disengage the starter, to detect overspeed and underspeed and, on many sets, to permit excitation. Speed may come from a magnetic pickup, the charging alternator W terminal, the generator output frequency or the engine ECU over CAN bus. Confirm which method the installed controller uses before choosing a replacement sensor.

Current and voltage sensing errors spread everywhere

Metering, overcurrent, reverse power, load sharing and load-dependent start and stop all come from the CTs and voltage inputs. A single CT fitted backwards makes kW read about one-third of the true value; with all CTs reversed kW reads negative and can trip reverse-power protection on a paralleled set. Correct the connection rather than changing settings to hide it.

Replacing a controller without changing the installation’s behaviour

1. Record maker, exact model, hardware revision and firmware from the rear label and information screen.
2. Download the event log and the configuration file, or photograph every parameter page: timers, limits, sender curves, CT ratio, tooth count, alarm classes, input and output mapping, communication addresses, load-sharing settings.
3. Check the replacement terminal by terminal against the panel drawing.
4. Load or rebuild the configuration, then function-test every start, stop, alarm and trip.

If the controller is dead and no backup exists, the settings must be rebuilt from the installation drawings and the engine and alternator data. That is engineering work, not a plug-in. For the panel the controller sits in, see AMF panels and ATS and switchgear parts.

Warning, electrical trip and shutdown are different responses

Class	What happens	Typical use
Warning	Alarm only; set keeps running	Conditions the operator can correct
Electrical trip	Breaker opens; engine cools down, then stops	Electrical faults with no engine danger
Shutdown	Engine stops immediately	Low oil pressure, high temperature, overspeed

Obsolete controllers and modern replacements

When a controller is no longer made, the choices are a direct successor with migrated settings, a different controller with re-engineered panel wiring, or a full panel retrofit. Judge each on the set’s age, the availability of other panel parts and whether remote monitoring is wanted. Networked controllers need controlled access and current firmware, and advertised monitoring features must be verified on the firmware actually installed. See SCADA, BMS and DCIM integration and AVR, PMG and exciter parts.

Controller and sensor record

Item	Value
Controller model, hardware revision, firmware
Configuration file saved (date, location)
CT ratio fitted and CT ratio programmed
Oil and temperature sender curves selected
Speed source and tooth count
Event log downloaded before work (yes/no)

Common mistakes

• Calling a low oil pressure shutdown a sensor fault without fitting a gauge; the engine runs again and seizes a bearing.
• Fitting a new controller at factory defaults; crank disconnect, sender curves and CT ratio are wrong.
• Changing a CT without changing the programmed ratio, or the reverse.
• Clearing alarms before saving the event log, losing the evidence of an intermittent fault.

What to send us

Send photographs of the controller front and rear labels, the information screen showing firmware, the panel wiring diagram, and the sensor with its connector and label. Tell us the alarm text or code, when it occurs, whether it repeats and what has already been replaced. For a replacement, send any saved configuration file and the engine and generator nameplates. WhatsApp 98852 01608 or send a spare parts enquiry.

Evidence note. Pressures, currents and ratios in the examples are illustrative; the engine and controller makers’ data govern.