Hi gang!

Long time no post. My wife's 2000 Blazer has been running perfectly for the last couple years or so and I haven't had to post anything... until now. We were out to dinner tonight and got it the truck to drive home and as soon as she started it up, the Service 4WD light came on and the five lights on the switch lit up (2HI, 4HI, 4LO, Auto 4WD, and Neutral). She drove it home slowly because it felt like the transmission didn't want to shift into second gear and it was bogging down under moderate throttle in first gear. When I got it home I put my BlueDriver scanner on it and got the following codes:

P1875: 4WD Low switch circuit fault
U1026: Loss of ATC Class 2 Communication
C0223: RF Wheel speed signal erratic
C0221: RF Wheel speed sensor circuit open or shorted
B2647: Ambient light sensor circuit low

The last three codes don't have anything to do with the 4WD system. What should I check to troubleshoot this problem and how should I check it?

 

Now clear the codes and see which reappear.


This thread might be helpful if the U1026 comes back. Maybe a corroded TCCM connector or bad wiring preventing communication between the TCCM and the PCM.

https://blazerforum.com/forum/2nd-ge...tc-code-69458/

This thread might be helpful of the P1875 comes back
https://texags.com/forums/46/topics/2240969

 

Hello again,

I had some other vehicle work to do so I'm just now getting back to the Blazer. I cleared the codes, drove the truck, and the only one to come back was B2647, the code for the ambient light sensor circuit. The "Service 4WD" light is still on so I pulled the TCCM and there was some corrosion on the upper four-wire connector. I cleaned it off the terminal and drove the truck. The "Check Engine" light went out but there were no other changes. I didn't get any other codes and neither the U1026 nor the P1875 came back.

What next?

 

A little more info...

I got ahold of a Tech2 scan tool today and tried to scan for codes. It had no problems communicating with any other systems but couldn't communicate with the TCCM. Hopefully that info will help someone get me on the right track for troubleshooting.

 

I had same issue on my 99 blazer I just bought a few months ago. The service 4WD light was on when I started it and then off the next time. When the light was on the truck would not go into 4WD.
New 4X4 switch in the dash and it was all good. Might want to try that if you can get one cheap off eBay or a junk yard.

 

I think a new 4X4 button assembly (4 button) was something like $35 for mine at Advance.

 

I didn't think the dash switch was the problem but just to rule it out I grabbed another one and tested out the system with it. I got the same result. All the lights on the new switch were on, the transfer case wouldn't shift, and the Tech2 scan tool couldn't communicate with the TCCM. I won't be able to do any proper troubleshooting until I can figure out why the scan tool won't communicate with the TCCM. Whenever I try to access the TCCM with the scan tool I get an error message telling me that the tool can't communicate with the TCCM and I should check the ignition to make sure it's on (which it is). Anybody have any idea why the scan tool can't communicate with the TCCM?

 

OK, I'm making progress with the Blazer. I took the TCCM back out again and cleaned the pins and connectors again and it's working again and communicating with the scan tool. The "Service 4WD" light is out and it's OK in 2WD.

Now it's back to where it was before this problem started. It's OK in 2WD. If I press any of the buttons for the other modes the TCCM will click, the transfer case will shift, but the light for that mode will continue flashing. After that, all the lights on the switch come on briefly along with the "Service 4WD" light and then they go out and the light on the button I pressed flashes. That continues until I press 2WD again when it goes back to 2WD. There's only one code that comes up with the scan tool when that happens, U1000, serial data communication failure.

What's the next step to diagnose?

 

I know this is an old post but what was the fix. I'm having the same problem...