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Around 2005 or 2006, GM started using a 58-tooth reluctor wheel, where before they used a 24-tooth wheel in the LSx based engines (LS1, LS2, LS3, etc.). This wheel is used as a way for the computer to detect where the crankshaft is at in its rotational cycle. I have done a lot of research, but have found no credible source to tell me exactly how to tell. I believe there was some overlap in model years where GM used both wheels. The reason this is important is the PCM is specific with respect to the wheel.

How do you tell which engine has which wheel without tearing the engine apart?

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All engines shipped from GM since January 2006 carry the newer 58x reluctor wheel. If you have an early LS2 (PN 12499750) then it's considered an '05 engine and will carry the 24x reluctor wheel like the LS1 and LS6 variants. This is important to know when you start shopping around for a computer to run your engine. Any LS engine can be converted either way by swapping out the reluctor wheel, but it's a bit of a pain since it has to be pressed onto the crank. http://www.superchevy.com/technical/engines_drivetrain/completebuilds_testing/sucp_0906_chevy_chevelle_ls_engine_swap/viewall.html#ixzz2oflNXNOl

EDIT: It would seem there are a few more variables at play here, but there are two irrefutable things to check for. First, it appears that all 58x reluctor wheel engines are Drive By Wire (DBW) and all 24x are Drive By Cable (DBC). Secondly, all 24x wheels are detected by a black sensor, while 58x use a grey sensor. This sensor is located at the back of the block, near the oil pan, on the passenger side of the engine, just above the starter.

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All 06 engines are not 58x. All fully Gen IV engines are 58x, and all pre-Gen IV engines (whether they are fully Gen III, or partially Gen IV) are 24x, for the crank. All 24x crank sensor engines are 1x cam gear engines. All 58x engines are 4x cam gear. Many 24x engines are DBW, including the LS1, LS6, and LS4, as well as many of the Vortecs. Some Gen III engines were DBC. All Gen IV are DBW. –  dobey Aug 1 '14 at 1:13

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