Cummins/Onan — Transistor and diode replacement
Repair & overhaul
From a Cummins/Onan owner's manual — check it applies to your exact model before use.
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Before you start
- Use only enough force to provide a snug tightness. DO NOT OVER TIGHTEN. Do not allow the transistor to turn while tightening. Turning damages the insulating washers.
Procedure
- Remove the defective transistor and thin (.001) mica insulating washers. When removing wires from transistors, be certain not to change their length.
- If available, lightly coat the copper base of the transistor and heat sink with Dow Corning Silicon grease #4 or Dow Corning Silicon Oil #200.
- See Fig. 14 for replacing transistors. Tighten nut to a maximum of 20 inch pounds. Do not over tighten. Do not allow the transistor to turn while tightening.
- Check to see transistor is not shorted to heat sink. Use highest scale on ohmmeter between 'C' and heat sink. A high reading is necessary to assure isolation.
- When soldering leads do not overheat. Use a very hot iron and do not hold it to the connection for more than 5 seconds. Allow to cool before soldering another connection.
- Make sure of proper polarity when replacing diodes. When soldering, use a long nose plier to absorb heat from the lead.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Mica insulating washer .001 inch thick
- Tighten nut to maximum 20 inch pounds
- Do not hold soldering iron more than 5 seconds
Source: Cummins/Onan owner's 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.