Filters
1 product
Not sure it fits your truck?
Our Wildwood, FL techs will confirm the right part for your build before you buy.
Call or Text (352) 722-5295The Sierra HD electrical layout, 2007.5-2010 LMM
GMC built the Duramax Sierra HD of this generation around a single 125-amp alternator. Order a regular cab with the Snow Plow Prep package and the diesel came with dual 125-amp alternators instead, and those trucks carry their own brackets and wiring. Every diesel Sierra got the heavy-duty 730-CCA battery spec, a pair of batteries in parallel, and GM's Regulated Voltage Control, which lets the engine computer manage charging. The 600-CCA auxiliary battery listed for gas Sierras was never offered with the Duramax.
Where the trouble starts
Hard starts in winter get worse when batteries are weak. If your Sierra cranks slowly and sits through long glow cycles, load-test both batteries before you spend money on glow plugs. A failing glow plug control module can also act like a set of bad plugs. Some owners chase overnight battery drain caused by parasitic draw, and because the batteries share the work in parallel, one bad battery can take the good one down with it.
Stock replacement vs. upgrade
For a stock Sierra, replacement means healthy batteries and sound cables. XDP's XD446 HD battery cable set is made specifically for the 2007.5-2010 6.6L Duramax. Upgrading is a wiring decision as much as an alternator decision. The factory wire is rated for roughly 160 amps behind the 175-amp fuse, so a high-output alternator calls for a 225-amp fuse plus 2-gauge or larger charge and ground cables. Plan the cables and the alternator as one job.
What we carry
Cables and hardware here come from XDP: the XD446 HD replacement battery cable set for 2007.5-2010 trucks, and the XD533 black-phosphate starter bolt kit, which is listed across 2001-2022 6.6L Duramax engines. For cold mornings there is Lucas Oil 11238 Sure Start starting fluid. Use it per the directions on the can, and don't let it cover up a battery or glow plug problem that needs fixing.
Fitment notes
This page covers GMC Sierra 2500HD and 3500HD pickups in WT, SLE and SLT trim on the GMT900 body. The Denali HD did not arrive until 2011, and the 2007 Sierra Classic is the older body with the LBZ engine. If your regular cab has the Snow Plow Prep dual alternators, its brackets and wiring differ from a single-alternator truck, so check before ordering cables. Savana vans and TopKick trucks with the LMM use a different chassis. Verify by VIN or run the fitment checker on the product page.
Frequently asked questions
Does my Sierra HD have one alternator or two?
One 125-amp alternator was standard. Dual 125-amp alternators came only on regular cab Duramax trucks ordered with the Snow Plow Prep package, and those trucks use different brackets and wiring.
Is a hard start my glow plugs or my batteries?
Test the batteries first. Weak batteries make winter starts worse, with slow cranking and long glow cycles. If both batteries load-test good, move on to the glow plugs and the glow plug control module, since module faults can look like failed plugs.
Will the XD446 cable set fit a dual-alternator Sierra?
It is listed for 2007.5-2010 6.6L Duramax trucks, but dual-alternator Snow Plow Prep regular cabs carry different wiring than single-alternator trucks. Call us with your VIN before ordering and we will help you confirm it.
How much charging can the factory wiring handle?
About 160 amps, behind a 175-amp alternator fuse. A high-output alternator goes past that, which is why it should go in with a 225-amp fuse and cables of 2-gauge or heavier.
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 at our shop in Wildwood, FL.