Generac — Test 30 - Test Coolant Temperature Switch

Testing & diagnosis · High coolant temperature switch (Wire 85)

From a Generac service manual — check it applies to your exact model before use.

Browse Generac generators & specs →

Procedure

1. Testing Installed: disconnect Wire 85 from the switch terminal.
2. Set a VOM to its Rx1 scale and zero the meter.
3. Connect one test lead to the switch terminal and the other to clean frame ground; meter should read infinity.
4. Crank and attempt to start the generator with no electrical loads connected.
5. If engine starts, hold Wire 85 terminal against clean frame ground; engine should shut down after a short delay.
6. Switch Removed: immerse switch in a 50-50 ethylene glycol/water mixture and set VOM to Rx1.
7. Connect test probes to switch terminal and switch housing; meter should read infinity.
8. Insert an actuating thermometer into the fluid mixture.
9. Heat the fluid; at about 245-266 F the contacts should close and the VOM should read continuity.
10. Allow fluid to cool; below about 245-266 F the contacts should open and VOM should read infinity.

Source: Generac 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.

More Generac repair guides →