I bought a 1999 Yukon 5.7, 4WD.

Supposedly, the previous owner swapped the engine and transmission from a '99 Silverado. It runs but seems to have lots of issues with the computers:

1. The transfer case controls are dead (no lights, but do get a click from the TCCM when switching between 2WD to 2WD.
2. Cruise control is inoperative.
3. It reports a Evap. purge vale circuit fault.

A bit of searching says the TCCM never gets a wakeup call from the ECU.

I'm thinking they swapped the ECM from the Silverado so the TCCM and cuise modules don't match.

THe evap code is a real problem as I can't have it pass emissions. I'm wondering if the ECM wiring is different on the ECU (Purge valve has been replaced.)

I would like opinions on a path to go:
1. Have dealer or shop flash the correct codes to all the modules.
2. Go to salvage yard and get matched ECM and TCCM.
3. What is the likelihood of the purge connections are different between ECM/*?

TIA.

Your trans and engine are controlled by the ECM, your transfer case is controlled by the BCM. The push button tells the BCM what mode to put the TC into. I also believe your cruise is also controlled by the BCM. Any EVAP codes are ECM relevant only.

This has a TCCM. Have already looked at it and checked replacements online.

The schematics show the TCCM and a Transmission Shift Module. Have not located the TSM, but have not looked as it drives fine.

On a 99 Yukon? If you can, post the schematics.

Here'* them on autozone'* site:
| Repair Guides | 1999 GMC TRUCK YUKON 4WD 5.7L SFI 8CYL | Drive Train (1999) | Transfer Case | AutoZone.com

If that does not work:

The schematics also show serial comms between the TAC module and PCM and instrument cluster. I suspect similar mismatched issues there.

Wow, guess your right. I'm more familiar with how the S10'* and Blazers are setup.

Just to close out this thread.

I purchased another main ECU. Huge difference.
Transfer case controls now work except LED'* were dead. Resolder board fixed that.
Drove over 200 miles on old computer and still 3 of the drive cycle readiness status lines showed "Incomplete" It then threw the purge vale wiring error again. I had never seen the catalyst test go ready.

After install, did the 30-minute "security procedure" to get it to run. Reset codes and drove it. Within 8 miles 7 of the 8 drive cycle status lines went ready. Next day started it and a minute later the last one, (O2 sensor heater test) went ready.

It is now throwing a "CKP System Variation Learn Procedure" code. The seller of the unit said that would probably happen and gave the steps to fix. I'll need to find someone with a scan tool advanced enough to send the learn command.

I bet when the previous owner swapped the engine and transmission from a Silverado, they also swapped the ECM. Probably from a 2WD so unit did not even try to talk to the TCCM and it expected different wiring connections and sensors.

Went to emissions test place and asked if it would pass with no emissions-related codes. Any error code will fail He also noticed the check engine light did not work (also a fail). I bet it is missing.

This has been frustrating. Spent 2 weeks on Craigs list and saw tons of scams and liars. Guess there are Liars, Damn Liars and Craigs List sellers Sill with all the probs needed to be fixed, paid less than blue book.

Well, all I can say is good job on what you accomplished....and thanks for posting your fixes....