Onan generator emerald III genset. I push the start button and the generator comes on as if it has started, as soon as I let the button go, it shuts down.

Here are the most common reasons for your condition.

CHECK IF ENGINE KEEPS RUNNING The most common complaint is that the engine starts but will not keep running when the START switch is released. This happens because the control board will not allow the engine to continue running if the generator is not producing voltage or if the oil pressure signal is not present. See CONDITIONS REQUIRED TO KEEP RUNNING on the Troubleshooting Chart for your model. If the engine will not keep running, do the following:
A1. Generator AC Output Check the generator AC output voltage as soon as the engine starts. Models showing "B1-B2 Volts" or "L1 AC Volts" under CONDITIONS REQUIRED TO KEEP RUNNING need this AC voltage to keep running. Read between the points shown on the Troubleshooting Chart. If the AC voltage readings are good, proceed to "B" below.
A2. Field Flash If there is no AC, check for 10-12 VDC at the FIELD FLASH pin of the control board during cranking. If the field flash voltage is not present, the control board is defective. If the field flash voltage is present, the wiring or regulator may be defective. To eliminate the wiring, check continuity between the FIELD FLASH pin of the control board and pin 7 of the regulator. If the continuity is good, either the regulator or the field circuit (rotor and brushes) may be the problem.
A3 Field Circuit Check the field circuit by unplugging the regulator and measuring the resistance between pins 9 and 10 of the regulator’s mating plug. This reading should be 22 to 28 ohms. If this reading is too high, check the brushes and/or clean the slip rings (use the Slick Stick, or a similar tool). If this reading is too low, there may be a short in the rotor. Also, check the resistance from pins 9 and 10 to ground. This reading should be very high or infinity. If not, there is likely a ground in the rotor. If these readings are all good, the regulator is likely defective.
3
A4 External Excitation To confirm that the regulator is defective, turn off the generator’s AC circuit breaker(s), reconnect the regulator and apply 12 volts through a diode to pin 9 of the regulator plug (see illustration) while attempting to start the engine. CAUTION: DO NOT attempt this without the diode. If you do, there will be fireworks! Remove the 12 volts within 1-2 seconds after the engine starts. If the genset now continues to run and produces normal AC voltage, the generator is OK and the regulator is defective. If the genset does not continue to run but produces about 40% of normal AC voltage, again the generator is OK and the regulator is defective. If no AC voltage is produced, the generator may have a problem that is beyond the scope of this guide. Re-check all wiring for security and signs of damage. It may be necessary to take the unit to a qualified repair shop.
B. Check the oil pressure switch as soon as the engine starts. Models showing "LOP to GND" under CONDITIONS REQUIRED TO KEEP RUNNING need the oil pressure switch to be closed to ground when running. If the voltage on the LOL/LOP SW pin of the control does not go to near zero, the switch is not closing. These switches sometimes stick open if the generator has not been used or exercised often enough. Multiple start/stop cycles and/or tapping on the oil pressure switch will sometimes fix it. It may be necessary to temporarily jumper the switch to ground to keep the engine running. This can be done at the switch or at the OIL LOL/LOP SW pin on the control board. Models showing "LOP Open" under CONDITIONS REQUIRED TO KEEP RUNNING have electronic governors and require the low oil pressure switch to be closed at starting but open during running. When the switch opens, the governor module places a ground on the oil pressure input of the control board (P1-5). It is OK to temporarily ground P1-5 on this model to keep the engine running. NOTE: See chart for oil pressure and oil level switch locations.
C. If there is AC voltage present on B1-B2 or L1 AC, as applicable, and the correct oil pressure/level signal is present at the OIL LOL/LOP SW input of the control board, and the engine will not keep running, the control board is defective.

This should help you get it going.

Kelly

Check the run relay

Overcharging