Cummins/Onan — Testing SCRs
Testing & diagnosis · Silicon controlled rectifiers CR1 and CR2 on VR1
From a Cummins/Onan service manual covering: 300.0 DFT, 350.0 DFU, 350.0 WF, 400.0 DFV, 400.0 WK, 450.0 DFW, 500.0 DFY, 600.0 DWV. Check it applies to your exact model before use.
Applies to: Cummins/Onan YB series
Procedure
- Remove voltage regulator VR1 cover to gain access to SCRs.
- SCRs on VR1 are identified CR1 and CR2, positioned closest to terminal board.
- Remove gate and cathode wires from SCR.
- Set ohmmeter to R x 10K scale.
- Measure resistance across anode and cathode; should be 1 megohm or greater.
- Reverse leads; reading should again be 1 megohm or greater.
- Set ohmmeter to R x 1 scale.
- Short gate to anode; resistance should drop to 25-ohms or less.
- Remove short between gate and anode; resistance should remain unchanged.
- Replace SCR if results differ.
Tools required
- ohmmeter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 1 megohm or greater across anode-cathode
- 25-ohms or less when gate shorted to anode
Full service-parts list: Cummins/Onan YB series →
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.