Cummins/Onan — Testing Thermostats
Testing & diagnosis
From a Cummins/Onan service manual covering: 2.5A, 2.5L, 2.7 AJ, 4.0CCK, 6.0CCKB, 6.5 NH, JC - 3CR / 96 T. Check it applies to your exact model before use.
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Procedure
- Remove thermostat from cylinder head.
- Heat a pan of water to about 150 F (66 C) and check temperature with a thermometer.
- Suspend thermostat in water at 150 F; it should start to open.
- After opening completely (about 165 F [74 C]), remove and allow to cool; it should close within a short time.
- Replace if it sticks or does not operate properly.
- Always install a new gasket when replacing the thermostat.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Starts to open at 150 F (66 C)
- Opens completely about 165 F (74 C)
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.