Cummins/Onan — Testing Thermostat
Testing & diagnosis
From a Cummins/Onan service manual covering: 0.6MAJ, 1.0MAJ, 1.5MAJ, 1.5MAJ-224R, 1.5MAJ-232R, 2.5MAJ. Check it applies to your exact model before use.
Applies to: Cummins/Onan 2.5MAJ
Procedure
- Remove thermostat from cylinder head.
- Heat a pan of water to approximately 150°F.
- Suspend thermostat in hot water so sensitive portion does not touch pan.
- Bring water temperature to 155° to 165°F; thermostat should start to open within this range.
- When completely open, lift out into air; thermostat should close within a short time.
- If thermostat does not operate properly, replace with a new one.
- Install new gasket when replacing thermostat.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 155° to 165°F
Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.