A diesel engine does not start on fuel alone. Before combustion begins, the battery must deliver heavy current, the starter must turn the engine fast enough, and once it runs the alternator must take over and restore the battery. On a diesel-driven portable compressor this chain is the most common reason a machine will not start on a cold morning at a remote site.
The power chain
Each link depends on the others. A strong starter cannot make up for a weak battery. A good battery cannot overcome corroded cables. A successful start does not last if the alternator is not charging.
Battery
A lead-acid battery must supply several hundred amperes for a few seconds while holding voltage up. Cold Cranking Amps shows how well it can do this in cold conditions. Open-circuit voltage alone is not proof of health: a battery can read normally and still collapse under load. Judge it by voltage during cranking, or by a conductance or load test.
Starter motor
When the start command arrives, the solenoid pushes the pinion into the flywheel and closes the high-current circuit. The starter must give high torque at low speed, so cable size, terminal tightness, earth quality and battery condition matter as much as the starter itself. The overrunning clutch protects the starter once the engine fires.
Cranking to combustion
A diesel needs enough cranking speed to reach the compression temperature and fuel atomisation required to light. Low cranking speed can stop a healthy engine from starting even with fuel and compression present. So a crank-no-start fault should not be blamed on fuel first; check battery voltage under load, starter current, cable resistance and cranking speed. Where fitted, glow plugs or intake heaters must also work.
Alternator
The alternator is belt-driven. Its rotor field induces alternating current in the stator; the rectifier turns it into direct current and the regulator holds system voltage as speed and load change. It must run the electrical loads and put back what the start took from the battery. Charging voltage is above battery voltage by design; the exact figure depends on the system, so use the engine maker’s specification.
Symptoms and likely causes
| Symptom | Check first |
|---|---|
| One click, no crank | Weak battery, loose or corroded terminals, solenoid contacts, seized starter, poor earth |
| Rapid clicking | Voltage collapse: discharged battery, cable or terminal resistance, undersized battery |
| Slow cranking | Weak battery, undersized or corroded cables, worn starter, cold thick oil, mechanical drag |
| Starts, charge warning stays on | Slipping or broken belt, regulator, rectifier, wiring, alternator earth |
| Battery keeps going flat | Undercharging, drain with the machine off, failed cell, sulphation, loose belt, short running periods |
Test in this order
- Inspect the battery case, terminals, cables and earth points
- Measure open-circuit voltage
- Record voltage while cranking
- Measure starter current
- Do voltage-drop tests on the positive and the negative cable
- Check cranking speed
- Measure alternator voltage and current
- Test charging with electrical load on
- Check belt tension, alignment and pulley condition
- Confirm the battery accepts charge and holds it overnight
Swapping parts without testing causes repeat failures. A new battery can hide a weak alternator for a while, and a new starter can still crank slowly through a corroded cable.
Good practice for compressor fleets
- Keep terminals clean and tight and check earth straps at every service
- Record battery age and replace on condition, not on appearance
- Use a battery isolator or disconnect when a unit will stand idle for long periods
- Do not crank for long periods; let the starter cool between attempts
- Start-and-run idle machines regularly so the battery is recharged
See also diesel portable compressors, fuel systems, controls and diesel portable compressor troubleshooting. For battery, starter and alternator parts or a site visit, contact Reddy Compressors.