Cummins/Onan — Replacing the Thermostats
Maintenance · Cooling system
From a Cummins/Onan owner's manual — check it applies to your exact model before use.
Applies to: Cummins/Onan MDDC, John Deere 4045TFM engine, John Deere 6068TFM engine
Before you start
- Hot coolant is under pressure and can spray.
- Let engine cool before loosening pressure cap; wear safety glasses.
Procedure
- Disconnect negative battery cable and let engine cool down.
- Remove rear door if enclosure fitted.
- Remove coolant pressure cap.
- Drain enough coolant so it does not spill when housing is loosened.
- Remove four thermostat housing bolts and pull off housing and gasket.
- Clean the gasket area.
- Replace the two thermostats (matched pair), ensuring correct location and orientation.
- Secure housing with a new gasket.
- Replenish lost coolant, secure pressure cap and door, reconnect battery cable.
Tools required
- Insulated wrench Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Thermostats are a matched pair and must be replaced as a pair
Full service-parts list: Cummins/Onan MDDC →, John Deere 4045TFM engine →
Source: Cummins/Onan owner's 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.