The fault is limited to DO1: with no load connected and the output commanded off, the measured voltage from DO1 to COM is 110 V, while DO2 to the same COM terminal measures 230 V. These energized, open-circuit voltage readings do not by themselves prove that the DO1 relay has failed. Test the isolated contact resistance before replacing the module.
Recorded DO1 and DO2 Measurements
| Test point | Commanded state | Load | Measured voltage |
|---|---|---|---|
| DO1 to COM | Off | Disconnected | 110 V |
| DO2 to COM | Off | Disconnected | 230 V |
DO1 and DO2 use the same stated COM connection at contact 14. The evidence does not identify the internal contact arrangement, external wiring, meter characteristics, or expected off-state terminal voltage, so the voltage difference cannot uniquely distinguish a damaged relay from a wiring or measurement-path problem.
Test the Relay Contacts Without Power
- Remove power from the circuit and verify that voltage is absent before using resistance mode.
- Disconnect the external conductors from the relevant output terminals so the field circuit cannot create a parallel resistance path.
- Measure resistance from COM to DO1, then from COM to DO2, with both outputs commanded or mechanically positioned in equivalent states.
- Compare the readings in the inactive state and again after changing each output state, if the equipment can be operated safely for that test.
A contact that changes between a low-resistance path and an open circuit as commanded demonstrates switching. If DO2 changes state but DO1 remains open, remains conductive, or behaves inconsistently under the same isolated test conditions, the result supports a DO1 contact or relay fault. Record the actual resistance readings rather than relying only on the relay click.
Check the Switched Load Before Replacement
Identify what DO1 was switching and review its starting or inrush-current demand. The evidence raises inrush current as a possible relay-damage mechanism but provides no load rating, relay rating, or measured current, so it does not establish that overload caused this fault.
If the isolated DO1 contact test is correct, continue tracing the external wiring and load circuit. If the contact test is incorrect while DO2 passes under equivalent conditions, service or replace the affected output hardware and correct any load-current issue before returning it to operation.
FAQ
No. The 110 V reading was taken with no load connected, and the available evidence does not define the contact topology or measurement path. Confirm the fault by testing COM-to-DO1 resistance with power removed and the field wiring disconnected.
Why does inactive DO2 measure 230 V while inactive DO1 measures 110 V?
The measurements confirm different terminal conditions but do not identify their cause. Isolate both contacts and compare their resistance in equivalent output states to separate an internal contact fault from external wiring or measurement effects.
What should I check if the DO1 relay contact has failed?
Do not reconnect the load until any excessive switching-current condition has been corrected.
