Cummins/Onan — Testing Reference Transformer T21
Testing & diagnosis
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Procedure
- Leads are marked H1, H2, X1 and X2. H1-H2 are primary leads. X1-X2 are secondary leads.
- Measure resistance between H1-H2 and between X1-X2.
- Resistance between H1-H2 should be 133 to 139 ohms, and between X1-X2, 133 to 163 ohms at 77°F (25°C).
- Resistance between coils and from any terminal to transformer frame should be infinity.
Tools required
- ohmmeter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 133 to 139 ohms at 77°F (25°C)
- 133 to 163 ohms at 77°F (25°C)
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.