The ignition transformer takes 120 volts AC and transforms it into 10,000 volts AC to ignite the oil droplets.
There are a number of ways to test the output voltage of the transformer. The most accurate is to use a voltmeter capable of measuring 10,000 volts. With 120 volts input to the transformer, the output should be approximately 10,000 volts. If below 9,000 volts, the transformer is weak and should be replaced.
There are other types of testers that have a scale which indicates whether the transformer is “good” or “bad”.
Some use a screwdriver to draw an arc to test the transformer. This is not recommended due to risk of electrical shock. However, if not prolonged, this method should not cause damage.
A wide electrode gap beyond the 1/8″ – 5/32″ could be a problem. Any gap beyond the specified dimension is not recommended for two reasons. First, exceeding any specification for set-up could cause improper operation of the oil burner. Second, as the gap is widened, the high voltage stress on the secondary coil increases and could shorten the life of the transformer.
Excessive moisture can cause problems. Surface moisture on ceramic insulators can cause arcing between terminals or to ground, eventually damaging the transformer. Moisture can also get inside the transformer. This moisture causes internal arcing inside the transformer resulting in damage or early failure.
An occasional problem is tar leakage caused by excessive radiant heat from combustion, and long-running cycles in high ambient temperatures. With some internal failures, the transformer can also overheat and cause the tar to melt.
Transformers will not cause radio and TV interference if properly set up. They are designed with internal shielding to prevent this. But if improperly set up, premature arcing in the ignition system (see chart items 9 and 10), can cause TV interference or decrease the energy needed to ignite the oil. Discoloration of the secondary output terminals indicates a bad connection.
| Designation | Description | Service Hints |
| 1. 120 V ac input wires | Brings 120 volt to primary coil. | Wires must not be pinched against housing when transformer is closed. |
| 2. Primary coil | Current in this coil generates a magnetic field. | |
| 3. Iron core | Transfers magnetic field into secondary coil. | |
| 4. Secondary coil | Magnetic field from core induces voltage in this coil. | |
| 5. Insulating compound | Keeps moisture out, conducts heat. | Should not be leaking out. |
| 6. Metal cover | Protects internals of transformer. | Should not be punctured or severely dented. |
| 7. Mounting base plate | Mounts transformer to burner housing. | Must not be bent and cause air to leak from around transformer. |
| 8. Transformer output Ceramic insulator | Insulates high voltage from ground and opposing terminal. Holds ignition spring dimensions. | Must not be cracked. Must be totally clean. |
| 9. Ignition spring terminals | Terminates high voltage to electrode rods. | Must make good clean contact to electrode rods. |
| 10. Electrode rods | Transfers high voltage to electrode tips. | Must be clean. |
| 11. Electrode insulators | Mounts electrodes and insulates each electrode from ground. | Must be clean and not cracked. |
| 12. Arc gap and electrodes | Specified gap (1/8″ – 5/32″) allows arc to jump to other terminal and ignite oil droplets. | Too close causes delayed ignition. Too wide results in no ignition and possible damage to secondary. Clean and properly adjust. |