Automatic 4wd Problem

[jon99z71]

I have a 1999 GMC Sierra 1500 Z71 with automatic 4wd. The "SERVICE 4WD" is desplayed in the information center. When I connected the code reader to the truck I got code "U1026 LOSS OF CLASS 2 COMM WITH ATC". The 4wd does not function at all and no lights are on in any of the push buttons on the selector switch. The only time I get any lights on in them is when I start the truck, they all light for a split second and then they stay off. I have replaced the selector switch with a new GM one before I put the code reader on the truck because I read several articles from guys having what sounded like the same problem and replacing the selector switch solved it. In my case it sounds like I have more than one problem, I can't access the 4wd module using the code reader because of the loss of communication between it and the PCM and the 4wd will not function at all.

Any advice would be greatly appreciated.

Thanks

[minibike132]

A switch will not cause a Loss of comm code, not that you wont find it bad once you repair the comm issue but first thing first, the modules have to talk. Give me a few min to look over schematics and I can help you out. First what is your experience level. If you need to take stuff apart can you, and do you know your way around a test lamp and voltmeter?

[minibike132]

In order for a module to communicate you need to verify all powers and grounds then finally the communication ckt which is one wire from 1 module to another. Quick checks for power are in the under hood fuse box check ATC fuse 20 amp fuse and in the left(driver)side of the dash there is another fuse panel check 4WD fuse 15 amp. Thats the easy quick stuff. If they are good post again and we'll go deeper. I'll be out for most of the day so I wont be able to repost till late tonight or tomorrow. Good luck Jonathan!

[jon99z71]

I'd say my skill level is high, I'll check those two fuses, but I'm pretty sure they have power and are good.

Any info you have would really help, I have alldata but it doesn't seem to get real involved with this type of 4wd issue.

Thanks

[jon99z71]

Both fuses have power and are good.

Do both the computers have to talk for the 4wd to work?

[jon99z71]

I found that the wire that goes between the two computers is a grey/Black wire. Is there a voltage that this wire carries?

I appreciate your help Mike.

[jon99z71]

The wire that goes between the PCM and the 4wd Control Module is good, ohm meter verified 0 ohms no brakes or cuts.

[jon99z71]

Hey Mike where'd you go?

[minibike132]

sorry been away all day five me a few to look at these diagrams...and field my daughters million untimely questions...might take an hour.

[gmtech4]

The wire that goes between the PCM and the 4wd Control Module is good, ohm meter verified 0 ohms no brakes or cuts.

 

 

I see mike and johnathan watching now,looks like the TCCM has possibly piled up

[jon99z71]

So far it looks like it, but Mike may know more

[minibike132]

Sorry for the wait Johnathan. The wire you tested is the 4WD LOW command to the Pcm not the data line. The data line we want is ATC module connector C1 (32 way orange connector) terminal E9 dark green/white wire. It leaves the atc module and goes to splice pack 205 That should be under the left kick panel to the left of the steering collum and looks like a grey connector with 6 wires going in and a white top(the white top is the splice if you take off the white part youll see what I mean) the atc wire is term c. Terminal B @sp205 dark green is the line to the PCM it goes through connector c100 terminal h (c100 is under the underhood electrical center if we need it) and goes to the pcm connector C1(blue connector) terminal 58. That is assuming you do not have a diesel or 4.3 motor, you don't do you. Before all that though if you have the atc module down double check powers and ground. Your powers are C1 term e10 brown wire, and C2 term D orange wire. Your ground is C1 term C black wire. Then check continuity on the previously described data line. I like to do a simple ohm check first cause that will point out a basic open. If thats ok we'll look at a few other things before The powered voltage drop because if done wrong you can fry all the modules, anyone who reads this might try one in the wrong situation and cost them selves a whole lot of money. Let me know what you find. I'm new to this site so I'm going to play around and figure how to upload a diagram that might help you but you may find all this info in alldata if you look in the right place. I'm sure somewhere there is a Data link schematic or 4 or 5 like we have here. And I am all but sure it is the atc module but your here to make sure you spend the right money to fix it right the first time, or we can just guess and throw parts at it but that gets expensive.

[minibike132]

Sorry I just re read that post it seems like alot to remember if you dont have a diagram. Basically check the 2 powers @ the atc module then check the ground at the module then do a continuity check from the module to the splicepack. Ask if you can't deciper my last post.

[jon99z71]

It took a couple of re-reads but I figured out what your talking about.

I checked the two power sources on the orange wire and the brown to the black ground wire, I have 12v on both.

 

I also removed the blue connector from the PCM and put the ohm meter between terminal 58 (drk green) and terminal E9 (drk green/white) at the TCCM, 0 ohms no problem I see with that wire.

[minibike132]

Did you find that splice pack I mentioned?