2007.5-2010 GMC Duramax LMM Charging & Starting Parts

The LMM charging system in a Sierra HD

A Sierra 2500HD or 3500HD ordered with the Duramax came with the heavy-duty 730-CCA maintenance-free battery with rundown protection. The 600-CCA auxiliary battery listed on the gas option sheets was never available with the diesel; the diesel runs two batteries in parallel instead. One 125-amp alternator is standard across WT, SLE and SLT. If you wanted a second one from the factory you had to order Snow Plow Prep with the Duramax, and that combination was regular cab only, so crew and extended cab plow trucks got the single unit. The 160-amp alternator referenced for plow-prep trucks belongs to the 6.0L gas engine. From 2007 on, charging is regulated through the PCM with Regulated Voltage Control, and the glow plugs are GM's fast light-up design run by a dedicated control module.

Where these trucks give trouble

Three things account for most of the no-start and slow-crank calls on a Sierra HD of this age, and none of them is the starter. First, battery condition: cold mornings and weak batteries compound each other, so both batteries get load-tested before anyone touches a glow plug. Second, parasitic draw. Owners describe trucks that go dead after sitting, including one measurement of 2.6 amps with everything off; treat it as something to look for rather than a known defect, and remember that a single failing battery in a parallel pair will take the other with it. Third, the glow plug control module: when it faults the symptoms look like bad plugs, and early 2007 trucks had glow plug problems of their own. On the charging side, corroded terminals and a heat-cooked cable set produce voltage drop that usually gets blamed on the alternator.

Replace with OE-grade, or step up?

Ask what the truck does. A Sierra that commutes and pulls a trailer a few weekends a year is happy with OE-grade parts: the GM glow plug or its listed NGK equivalent, the right CCA batteries, a correct solenoid. A truck with a plow on the front, a service body, an inverter or auxiliary lighting is a different story, and that is where the heavy-duty starters and high-output alternators on this page belong. Two cautions. Output upgrades need the electrical system upgraded with them, since the 175-amp factory fuse should go to 225 amps and the charge and ground cables to 2-gauge or larger. And a swollen glow plug can need a reamer and removal tool rather than more torque; those tool kits exist for the LMM for a reason.

What we carry

The parts grouped here are Duramax-specific starting and charging hardware. Gear reduction starters come from Mean Green and XDP, and both brands also offer a high-output alternator for 2007.5-2010 trucks. XDP supplies the heavy-duty battery cable set and a black-phosphate starter bolt kit for the reinstall. The Merchant Automotive starter solenoid covers a long run of 6.6L trucks. The PPE top-post battery heat shield keeps radiant heat off the terminals in a crowded engine bay, and Lucas Oil Sure Start starting fluid rounds out the cold-start items. More for this engine family is on the Duramax page.

Fitment notes

Check the body before anything else. The LMM only came in the new GMT900 Sierra HD; the 2007 Classic Sierra HD is the previous GMT800 body with the LBZ, which is why these pages start at 2007.5. Model years matter at the other end too: for 2011 GM replaced the LMM with the LML and revised the chassis heavily, so do not assume a 2011 part number carries back. Within the LMM years, watch for dual-alternator Snow Plow Prep regular cabs, whose brackets and wiring differ from single-alternator trucks. Starters and solenoids here often span many Duramax years while alternators are year-specific. Verify with the fitment checker on the product page, or give us the VIN.

Frequently asked questions

Will a high-output alternator bolt to my Sierra with the factory bracket and belt?

Check the listing. LMM-specific high-output alternators are generally catalogued to use the stock mounting and belt routing, so confirm it on the product page or with the fitment checker before you order. The wiring is the part people skip. Plan on the larger fuse and heavier charge and ground cables at the same time, or the extra output never reaches the batteries.

My Sierra HD has Snow Plow Prep. Does that change which alternator I order?

It can. Snow Plow Prep ordered with the Duramax included dual 125-amp alternators and was offered on regular cabs only, and those trucks carry different brackets and wiring than a single-alternator truck. Open the hood and count alternators before you order, or send us the VIN.

How do I tell a bad glow plug from a bad control module?

Scan it first. Control module faults produce the same hard-start complaints as failed plugs, so codes and commanded glow time tell you which side you are on. The GM plug for these trucks is 12639701, with NGK DRD33AS listed as the aftermarket equivalent, and a seized plug may need a reamer and removal tool rather than more torque.

What battery should go in a 2007.5-2010 Sierra HD Duramax?

GM's heavy-duty 730-CCA maintenance-free battery with rundown protection is the diesel spec, and there are two of them wired in parallel. Match or exceed that CCA rating, and replace them as a pair so a new battery is not sharing duty with a tired one.

Not sure what fits? Call the shop at the number in the header, or use the fitment checker on any product page. We install everything we sell.