Anyone who knows what kind of signal the ECM receives from the BCM. As far as i can see there is two wires from the bcm to the ecm. I am twisting my head with this. Have replaced the whole harness with new ignition housing, lock and keys + installed a refurbished bcm programmed to VIN. Stil have the same problem, just starts and then cut. Security light and charge light alternate when ignition is on and security light permanent with ignition to off. Due to this i do not think the problem is with the ecm but please correct me if i am wrong.
a few questions:
A. did you end up with a passkey II (where there is a visible resistor on the key)? Did you try the passkey II relearn procedure? [1. Insert ignition key and turn to the “ON ” position. Do not attempt to start the engine. Leave the key in the “ON ” position for approximately 11 minutes. The security light will be steady on or flashing for the 11-minute period. Do not proceed to Step 2 until the security light turns off or stops flashing. 2. Turn the ignition switch to the “OFF ” position for 30 seconds. 3. Turn the ignition switch to the “ON ” position as in Step 1 for 11 minutes. 4. Turn the ignition switch to the “OFF ” position for 30 seconds. 5. Turn the ignition switch to the “ON ” position as in Step 1 for 11 minutes for a third time. 6. Turn the ignition switch to the “OFF ” position for 30 seconds for a third time. 7. Turn the ignition switch to the “ON ” position for 30 seconds. 8. Turn the ignition switch to the “OFF ” position. 9. Attempt to start the engine]
B. did you end up with a passkey III (where there would be a transponder inside the key plastic part)? Did you try the passkey III relearn procedure ? Other folks will need to comment here, but if the truck originally had the Passkey 3+ system, then I think the PCM needs to learn the new "fuel continue password" when the BCM is replaced by doing the relearn test. [When performing this relearn all previously learned keys will be erased. Additional keys may be relearned immediately after the first key has been learned by inserting the additional key and turning the ignition switch to on within 10 seconds of removing the previously learned key. 1. Insert a master key (black head) into the ignition switch. 2. Turn key to the “ON ” position without starting the engine. The security light should turn on and stay on. 3. Wait for 10 minutes or until the security light turns off. 4. Turn key to the “OFF ” position for 5 seconds. 5. Turn key to the “ON ” position without starting the engine. Security light should turn on and stay on. 6. Wait for 10 minutes or until the security light turns off. 7. Turn key to the “OFF ” position for 5 seconds. 8. Turn key to the “ON ” position without starting the engine The security light should turn on and stay on. 9. Wait for 10 minutes or until the security light turns off. 10. Turn key to the “OFF ” position. The key transponder information will be learned on the next start cycle. 11. Start the vehicle. If the vehicle starts and runs normally, the relearn is complete. 12. If additional keys need to be relearned, turn the key to the “OFF ” position. 13. Insert the next key to be learned. Turn the key to the “ON ” position within 10 seconds of removing the previously used key. 14. Wait for security light to turn off. It should happen fairly quickly. You may not notice the lamp as the transponder value will be learned immediately. 15. Repeat steps 12 through 14 for any additional keys. NOTE: If the theft deterrent relearn procedure does not work, please try the procedure again]
C. Have you replaced the ground and/or positive battery cables recently? Other GM cars and trucks sometimes have corrosion inside the battery cables (not visible from outside) that causes voltage issues that can be a fail to start and presumably a BCM not responding properly. Other folks would need to comment on this idea, but I'm thinking that's what's preventing my Tahoe from starting part of the time (I have two types of intermittent fail to start: sometimes no interior lights/no dash lights/etc OR sometimes have interior lights/dashlights but decides to not crank at all -- I'm suspicious of the battery cables on the no interior lights/no dash lights occurrences.)
To answer your question, I believe the BCM sends a serial data signal to the PCM ( the "fuel continue password" ) that is a coded signal -- supposedly the relearn procedure above would let the PCM learn to accept the "new" "fuel continue password".
From what I have seen the Tahoes sold in USA that are model year 2002 have an ignition switch that is integrated to a wiring harness and sells for about $65 USD. The Tahoes sold in the USA that are model year 2003 have an ignition switch that unplugs out of the wiring harness and replaces separately (with no harness available) sells for about $15 USD.
