03 LB7 to 05 LLY

[gr8wing]

Not sure where to post this so hopefully I'll get some help from someone out there. Everything I read about swapping an 05 LLY into my 03 GMC LB7 is that all I need to do is, swap over the engine with engine harness and swap the ECM. I have read the fuel guage value swap. I got the engine with harness, ordered a cold intake kit for the 05, got a passenger side intercooler pipe and got a LLY ecm that had the fuel guage values swapped and the egr delete.

Here is the first problem I ran into while pulling the LB7; when I pulled the ecm, it is not the same style as the LLY and does not have the same plugs. I was had the understanding that the 03 LB7 would use the same style ecm as the LLY. I do have a under hood harness for the 05, but I'm really not sure what my next move is. Anybody know about this?

 

[Mpsoldier]

Get a 05 engine harness, unplug the 03 from under the fuse block and plug in the 05 rest is plug and play and your correct on the fuel values

 

[gr8wing]

the engine harness is on the 05 engine. The ecm's are different with different plugs. Would you be talking about changing the under hood harness?

 

[rfletes79]

You’ll use the LLY harness that plugs into the LLY ecm. If you follow that harness from the ecm to the under hood fuse box you’ll see where the lb7 and LLY have the same connectors. Remove the lb7 harness and install the Lly one, plug everything in like you unplugged the lb7 one. Then plug in your tcm, engine and trans. You’ll need an Lly nsbu switch, the electrical component on the side of your trans. Remove the lb7 one and install the new LLY one, research on how to install this correctly. Then plug it in.

You also need the Lly ecm mount/bracket, as you stated above it’s different then the lb7 ecm.

Your ecm will need a vats relearn or the anti theft disabled before you try and start it.

 

[rfletes79]

If you’re missing the Lly harness that plugs into the Lly ecm then that is your problem. Start by finding one, you cannot move forward without it.

 

[gr8wing]

I have the under hood harness that plugs into the ecm. I appreciate the info. Didn't know about the backup switch, just ordered one. Also didn't know about the vats relearn. I have 2 05 ecm's. One should be stock and the other has had the egr delete done and the fuel sender tweek. Maybe that one has had the vat tweek also.

 

[THEFERMANATOR]

You don't have to disable VATS, you can do the VATS relearn when you get it all together.

 

[TheBac]

Moved to Repower/Transplant

 

[gr8wing]

So, I've completed my lb7 to lly swap, replaced the wiring harnes, the nsbu switch and ecm. The truck fires right up, but runs like crap. The idle fluctuates, the throttle does not respond, the oil pressure Guage reads 0 (has 80 psi) and the computer is throwing a code p0642. I have swapped the ecm to the one that came with the engine,
And it won't even fire with that one. The ecm I am running came from a guy here on the board. It has the egr delete and fuel gauge swap. Any ideas?

 

[Dkatubi06]

You’ll also need the harness that goes to the trans for the neutral saftey switch to even plug in. Lb7s had a two plug harness the lly is one connector. Could be some of your problem. Where are you located?