01 tahoe remote start wont work

I had a Viper 790 didn't like it cause it didn't work as well as I thought it should. I had the remote start hooked up on that and it worked fine. A friend of mine has the Viper 550 he said that it is the same except the brain and antenna. I told him that if he wanted it he could have it for free if he gave me his 550. all the plugs from the 790 are the same so we swapped brains and antennas. I also have that 555l bypass module. Now the alarm works fine but the car wont start. It starts with the key in the on poistion but not if the key isn't in the cylinder. The car also turns over but doesn't start. It acts like the tach wire isn't hooked up or something. This is obviously not the case cause it worked before so let me know what you guys think. I even got a new 555l thinking that it could relearn the code but that didn't work either.

I believe on those cars there may be an accessory wire that must be on before and during start in order for the car to read the key. Make sure that on the new brain, all necessary ignition and accessory wires are powering up.csorb