Why Duramax trucks need the capacity

Diesel trucks start with a heavier electrical burden than gas trucks. Glow plugs or grid heaters pull hard on a cold start, and most of these trucks run dual batteries from the factory to cope with it. Add a plow, auxiliary lighting, an inverter or an audio system and the factory alternator runs out of headroom quickly.

Mechman builds large case alternators for the Duramax from 320 amps up to 400 amps. The large case is a different physical unit from the standard truck alternator, so make sure you are ordering the Duramax-specific part rather than a gas truck alternator that looks similar.

Side output stud

Some configurations use a side output stud rather than a rear stud, which changes how the charge cable routes in the engine bay. Check your existing alternator before ordering, since the cable length you have may not reach the other position.

Choosing a battery

These trucks run dual batteries. If you are replacing them, replace both at once and match them — mixing an old battery with a new one drags the new one down to the condition of the old.

AGM handles the deep cycling of an auxiliary load far better than a standard flooded starting battery.

Don't skip the wiring

A dual battery setup means longer cable runs and more connections, each of which drops voltage. Upgrading the main runs matters more on a diesel than on a gas truck. See our Big 3 wiring upgrade guide and Big 3 kits.

Browse all Mechman high output alternators, batteries, or cable and wire.