The starting and charging chain
Starting a diesel engine is a chain of events, and the chain is only as strong as its weakest link:
- The key switch or controller sends a start command, provided all interlocks are satisfied.
- The starter solenoid engages the pinion with the flywheel ring gear and closes the heavy-current circuit.
- The battery delivers several hundred amperes through the main cables to the starter motor.
- The starter spins the crankshaft fast enough for compression to heat the air above the fuel’s ignition point.
- Fuel, free of air and water, is injected and the engine fires.
- The alternator, through its rectifier and regulator, recharges the battery and supplies the controller.
A strong starter cannot make up for a weak battery; a new battery cannot overcome a corroded earth strap; and a perfect start is wasted if the alternator is not charging. Test the chain in order rather than replacing the most expensive part first.
Symptom map: what the engine is telling you
| What happens when you turn the key | Most likely area | First checks |
|---|---|---|
| Nothing at all, no lights | Battery isolator, main fuse, flat battery, broken cable | Isolator switch, battery voltage, terminal clamps |
| Panel lights but no cranking | Interlock active, start relay, solenoid, control wiring | Emergency stop, fault display, relay click, solenoid feed |
| Single click, no rotation | Low battery under load, poor connections, solenoid contacts, seized starter | Voltage at starter while cranking, earth strap |
| Slow cranking | Weak battery, high-resistance cables, thick oil in cold weather, worn starter | Cranking voltage, voltage drop across cables |
| Cranks well but will not fire | Fuel air-lock, empty tank, blocked filter, fuel stop solenoid, cold-start aid | Fuel level, bleed screw, stop solenoid action, glow/heater circuit |
| Fires briefly then stops | Fuel starvation, stop solenoid dropping out, protection trip at start | Fuel lift pump, water separator, fault log |
| Starts but battery warning stays on | Alternator, belt, regulator, charge wire | Charging voltage at the battery with engine running |
Batteries: test under load, not at rest
A lead-acid starter battery’s job is to deliver heavy current for a few seconds while holding its voltage up. Cold cranking amps (CCA) describes that ability. A battery can show a healthy open-circuit voltage and still collapse the moment the starter draws current, so a voltmeter reading on a parked machine proves very little.
- Cranking voltage. Measure at the battery posts while cranking. A heavy fall in voltage means the battery cannot supply the current; a good battery voltage with poor cranking points to cables or the starter.
- Load or conductance test. A proper battery tester gives a clearer verdict than a voltmeter.
- Terminals and clamps. White or green deposits, loose clamps and cracked cable lugs add resistance. Clean, tighten and protect them.
- Site conditions. Vibration from towing and rough sites loosens hold-downs and damages plates. Heat in an Indian summer shortens battery life and increases water loss on non-sealed types.
- Standing machines. A compressor parked for weeks slowly drains its battery through the controller. Use the battery isolator, or charge the battery before redeploying.
On 24 V systems with two 12 V batteries in series, test each battery separately. One weak battery drags the pair down and also overcharges the good one.
Starter motors: speed is everything
A diesel engine depends on compression heat to ignite its fuel. If the starter turns the engine too slowly, the air leaks past rings and loses heat before the piston reaches the top, and the engine will not fire however good the fuel system is. Signs of a failing starter include:
- slow cranking with good voltage at the starter terminal;
- grinding or clashing as the pinion engages, which points to a worn pinion or damaged ring-gear teeth;
- the starter continuing to spin after the engine fires, which means the solenoid is sticking;
- heat and smoke after repeated attempts.
Limit cranking to short bursts with a pause between attempts to let the starter cool. Long continuous cranking burns out windings and drains the battery further.
Voltage drop: the fault most often missed
Corroded or undersized cables are a common cause of slow cranking on older portables. Measure the voltage drop while cranking across the positive cable (battery post to starter terminal) and across the earth path (starter body to battery negative). Any significant drop on either path means resistance is stealing current from the starter. The earth strap between engine and chassis is a frequent culprit, especially after repainting or a chassis repair.
Glow plugs, intake heaters and cold starting
Many compressor engines use glow plugs or an intake-air heater to help starting in cold conditions. Even in South India, a machine at a hill site, at dawn in winter, or with a weak battery can need the cold-start aid. Check that the heater circuit draws current when the start switch is in the preheat position, that the relay operates, and that the preheat time is respected before cranking. A failed glow plug circuit typically shows as long cranking, white smoke and rough running until the engine warms.
Fuel air-locks and fuel supply faults
A diesel engine cannot run on air. Air enters the fuel system when the tank runs dry, when filters are changed, or through a cracked fuel line or a loose fitting on the suction side. The engine then cranks normally but will not fire, or starts and stops.
- Check the fuel level with a dipstick, not only the gauge.
- Drain water from the water separator bowl.
- Prime with the hand pump and bleed at the filter head, then at the injection pump, in the sequence the engine maker specifies.
- Check the fuel stop solenoid is pulling in when the start switch is on.
- If air keeps returning, look for a leak on the suction side between tank and lift pump.
Poor-quality or contaminated fuel blocks filters quickly on remote sites. Keep fuel storage clean and change filters at the recommended interval.
Safety interlocks that prevent starting
Modern portables will not crank while a protection is active. Before you suspect the starter, check:
- emergency stop buttons, including any remote stop;
- active shutdown faults, such as high discharge temperature, low engine oil pressure or high coolant temperature, that must be cleared first;
- service valves or pressure in the separator tank where the controller requires the system to be vented before starting;
- low fuel or low coolant level switches;
- a faulty sensor or connector that reports a false fault.
Never bridge an interlock to get the machine running. Find out why it is active; the protections exist to save the engine and airend. For airend-side faults, see our portable compressor troubleshooting guide.
Alternators and charging problems
Once the engine runs, the alternator must deliver enough current to recharge the battery and run the controller. Check the charging voltage at the battery posts with the engine at speed; it should be clearly above the battery’s resting voltage and within the range the engine maker gives for a 12 V or 24 V system. Common faults:
- slipping or worn drive belt, often shared with the fan or water pump;
- failed regulator, giving undercharging or overcharging (boiling battery, swollen case);
- loose or corroded charge wire and earth;
- alternator bearing noise or diode failure from heat and vibration.
A machine that starts every morning after a night’s charging at the workshop, but fails on site, very often has a charging fault rather than a battery fault.
Preventive checks for site machines
- Clean and tighten battery terminals at every service; check hold-downs.
- Check charging voltage with the engine running at each service visit.
- Inspect cables where they pass the chassis for chafing.
- Drain the fuel water separator daily on remote sites.
- Use the battery isolator when the machine is parked for more than a few days.
On hire jobs we service machines every 300 running hours, and these electrical checks are part of that visit.
How Reddy Compressors helps
We diagnose and repair starting and charging faults on diesel portable compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and other makes, as an independent multi-brand specialist (not an authorised dealer) based at Tarnaka, Secunderabad. We have tie-ups with all major and reputed brands. Our service teams work across Telangana and Andhra Pradesh; batteries, starters, alternators, sensors and fuel-system spares can be supplied across India, identified from the engine and compressor nameplates before quoting. If the machine is needed urgently, a rental portable can cover the job. See the main diesel portable compressors guide for the full range.
What to send us
- Compressor make, model and serial, plus engine make and model
- Nameplate photos of compressor, engine, starter or alternator if relevant
- What happens when you turn the key, and any fault code shown
- Battery voltage, and cranking voltage if you can measure it
- Running hours, last service date and site location
Frequently asked questions
Why does my diesel compressor crank but not start?
If the engine cranks at normal speed but does not fire, the fault is usually fuel or cold starting. Check the fuel level, drain the water separator, bleed air from the filters and injection pump, and confirm the fuel stop solenoid pulls in. If the engine is cold, check the glow plug or intake heater circuit. Slow cranking points to the battery, cables or starter instead.
How do I know if the compressor battery is bad or the starter is bad?
Measure voltage at the battery while cranking, then at the starter terminal. If the battery voltage falls heavily, the battery is weak. If battery voltage holds but voltage at the starter is much lower, the cables or connections are at fault. If good voltage reaches the starter and it still turns slowly or not at all, the starter itself is the problem.
Why does my compressor battery go flat on site?
The most common reasons are an alternator not charging, a slipping belt, a loose charge wire, or the controller draining the battery while the machine stands unused. Measure charging voltage with the engine running. Use the battery isolator when the machine is parked. A battery that is repeatedly run flat also loses capacity permanently and may need replacement.
How do I bleed air from a diesel compressor fuel system?
Fill the tank, drain water from the separator, then prime with the hand pump and open the bleed screws at the filter head until fuel flows free of bubbles. Continue to the injection pump if the engine maker requires it. Follow the sequence in the engine manual. If air returns after bleeding, look for a leak on the suction side between tank and lift pump.
Why will my portable compressor not crank even though the battery is good?
The controller blocks cranking when a safety interlock is active. Check emergency stop buttons, active shutdown faults such as low engine oil pressure or high temperature, low fuel or coolant level switches, and any requirement to vent the separator tank before starting. A faulty sensor or connector can also report a false fault. Never bridge an interlock to force a start.
What charging voltage should a compressor alternator give?
With the engine at running speed, the voltage at the battery should be clearly above its resting value and within the range the engine maker specifies for the 12 V or 24 V system. Voltage that stays near resting value means the alternator is not charging; voltage well above the range, with a hot or boiling battery, points to a failed regulator.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.