BCM involved in TC shifts?
 

Hi everyone, first post here as I just joined the forum. I have a 2003 4dr blazer that I have swapped a vw 2 liter 4cyl tdi into and have been driving for a few months now. The 03 was a 4.3 with an auto and I wanted a manual trans so I picked up a 05 2dr blazer with 4.3 and 5 spd trans for parts. The 05 had the 8.6 rear diff with 3.42's vs the 03's 7.6 with 3.73 gearing, so I swapped both diff's along with the 5spd, ecu, bcm, instrument cluster, and engine compartment wiring harness from the 05 into the 03 along with the tdi engine. I made my own adapters for the engine to trans and vw crankshaft to chevy flywheel, fabbed up some mounts and stuffed it in. I am running the tdi kind of standalone with it's own ecu, wiring harness, and necessary sensors. The vw electrics are powered up by a relay when I switch the blazer ignition switch to on, and the starter is of course wired to the blazer ign switch. I teed the oil pressure and coolant temp ports on the tdi and installed both the tdi and 4.3's oils pressure and coolant temp senders so each system is getting info from its original senders. Everything else on the blazer harness that was plugged into the 4.3 was just left unplugged and zip tied out of the way. Runs great and the speedo, fuel gauge, oil pressure, coolant temp, and voltmeter all work in the blazer instrument cluster. Have been driving it for about 3 months now and taken it for a couple of road trips, one about 3000 miles round trip, and the second of about 2000 miles. with no problems. Hand calculated mileage on the road trips was consistently in the 40 to 42 imperial mpg, think that equates to about 33 to 35 us mpg so happy with that. The 4.3 struggled to get 24-25 ish on the highway(imperial #s again). Now to my question. When I shift it from 2hi into 4hi or 4lo, or back from either of those to 2hi it usually takes quite a while to shift, sometimes flashing for a bit than going back to a steady light on whatever range I was trying to shift out of, but after a few trys it will shift and it will definitely go into the range I select, and 4wd engages and disengages but the light on the 4hi or 4lo buttons keeps flashing continuously even after the shift seems to have been made successfully. Before the swap it would shift quicker and the light would go steady after the shift on whichever button you pushed. Now going back into 2hi it may take a few pushes to make the shift, but once it does the lights goes steady, but into either 4wd position the light keeps flashing even after the shift is completed. I kept the original tc and tccm from the 03 in it, which was the 4 button tc with the auto position, well the 05 has the 3 button tc. So is it possible the 05 bcm (3 button tc) doesn't play well with the 4 button tccm and tc in the 03, or is the bcm even involved in the tc shifting process? Would it be worth trying the original 03 bcm in it or will that cause other issues with it being from an auto and now having the manual in the truck? Also the 03 has power seats, windows, and mirrors, well the 05 was all manual, although all that stuff seems to be working fine with the 05 bcm in the truck now.
Thanks for any insight anyone can share.

Welcome to the forum, sounds like you have a pretty awesome blazer. Be sure to do a build thread so everyone can see what is only one of a handful of diesel conversions. Onto your question, im by no means an expert on BCM's but i believe the autotrac feature of the 4 button t cases is at least partially controlled by the bcm, not really sure its your issue but probably wouldnt hurt to give it a shot swapping the BCM. read around on here as theres tons of info on diagnosing problems with the 4x4 as blazers are famous for this, and it could very well be a problem with the vaccum axle lock or the tc motor. And im sure someone else with greater knowledge will give there advice.

Thanks Alexei, I don't have the Blazer at home as no road access to where I live, but I will be boating up to where I keep it parked tomorrow for a town trip so will bring the other bcm up and give it a try. As far as a build thread goes I didn't take any pictures as I was building it so won't likely start a build thread, but would be happy to answer any questions anyone may have about the build. Put a lot of hours into it but it was well worth it to me for the fuel economy gains. Drives very well and even though it should be a little down on power on paper(the diesel as tuned should be about 175hp), I think it feels at least as powerful if not more so than the 4.3 did. This is probably due to the fact that the 4.3 made it's max power at a higher rpm where I hardly ever drive it, well the diesel makes it's power lower on the rpm scale making it more usable for my driving style.