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when using a gen 3 ECM on a gen 4 motor i under the gen 4 knock sensors aren't in the same location and i believe don't communicate with the gen 3 ECM.... so from what ive found it seems like ill need to use gen 3 knock sensors and either re tap the gen 4 sensor threads or drill and tap holes in the valley for them.. has anyone ever done either & or is there another solution ?
also for the cam sensor - is the location of the gen 4 sensor on the timing cover okay or would it need to be relocated as well ?
Did you change crank reluctors (toothed wheel near back of the crank)?
Gen III uses 24 tooth, Gen IV uses 58 tooth.
Gen III only works with 24 tooth,
Gen IV only works with 58 tooth
There's a crank signal conversion box that makes running a Gen IV 58x crank reluctor engine with a Gen III 24x ECM possible. I run one with my 58x crank 416 in 99 Trans Am with the Gen III 24x ECM. It's available on LPE's site.
Mount as far away from heat sources as possible. We used Gen IV knock sensors with harness extensions if I remember correctly on mine. **Be sure you have a 58x crank sensor** otherwise it won't work. We used Gen IV coil packs so a different coil pack harness was required. If using an LS3 style MAF you'll probably need an adapter harness for the Gen III harness. There were a few other details I've forgotten.
The set up has worked flawlessly for ~7.5 years on my 99 T/A.
However, it requires getting used too, when you turn the key to crank he car the system must sync. This can be instant or take ~ 2 seconds. Usually, I bump the key once briefly and then again and the car crank almost like a normal 24x or 58x set up.
If you drive the car a lot on long trips having a back up LPE box might be worthwhile because if the box fails...you're stuck.
If your gen 4 motor has a 24x crank still and you dont need a 58x to 24x converter box, you just need a harness extension for the cam sensor and knock sensors. Drill/tap the sides of the block to replace the gen 4 knock sensor with the gen 3 sensors. This is how my car is setup.
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Gen 3 ECM for Gen 4 motor 
