Generac — CTTS 100-400 Amp: Switch Will Not Retransfer to Utility
Testing & diagnosis · Contactor Assembly (CTTS 100-400 Amp)
From a Generac service manual — check it applies to your exact model before use.
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Before you start
- Disable generator from starting; disconnect negative battery cable.
Procedure
- Remove cover. Refer to Figure 1 for PCB layout.
- For testing, follow PCB numbering in Figures 2 and 3.
- Measure resistance on Trip Coil between J2-1 and J2-2 on PCB2. Roughly 2.2 ohms. If not proper, consult factory for replacement.
- Test ATS limit switch. Remove single Phillips head screw and PCB 2 to access wiring harness. Unplug connector and measure resistance between Red and Blue wires on female side. If greater than 1.0 ohms
- If steps 3 and 4 good and switch not tripping to neutral, replace PCB 2.
- If switch tripping to neutral but will not close to Utility, and utility voltage at A1 and A2, measure resistance between PCB1 J2-3 and J2-5. Closing coil resistance should be measured (67 ohms).
- Test AX limit switch. Measure resistance between J1-1(A1) and PCB1 J3-4. Measure resistance between J1-2(B2) and PCB2 J3-2. Less than 1 ohm. If INFINITY, replace AX limit switch.
- If closing coil and AX limit switch test good, replace PCB 1.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 2.2 ohms
- 1.0 ohms
- 67 ohms
- less than 1 ohm
- 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.