Hello,

My Serv 4wd light is on and I am trying to figure out how to diagnose it.

05 Chevy Colorado LS
3.5 l
Z71 Crew Cab
about 38,000 miles

I took it to an auto zone, but they were unable to read the codes for the 4wd. I disconnected the battery to try and reset the codes, but it came back on almost immediately. I can't switch to 4wd, in fact there isn't even an amber light on saying it is in 2wd. I am stuck feeling like the only possibility is to take it to a dealership. Any ideas on how I can pull the codes on the 4wd system? Thanks!

 

There will usually be a code e.g. DTC C0327 encoder motor circuit fault. But you need tech2 to pull the codes.

The transfer case shift control module has 4encoder circuits, and each circuit is supplied 12volts. These 4circuits are A, B, C, and P, and each circuit runs to a switch located inside the encoder/motor assembly. When a particular encoder circuit is active, the switch is closed, and the 12volts are grounded by the switch inside the encoder.
The transfer case shift control module supplies 12volts on all the circuits. The module can interpret the location of the transfer case shift position when the circuits are grounded.
This DTC detects a fault when it compares the shift sequence/strategy to a chart located in read only memory (ROM), within the electronic control unit (ECU), within the transfer case shift control module. This is accomplished by the module reading the different combinations of encoder signals being grounded while others remain high depending on what mode or range the transfer case is in.

The transfer case shift control module sets the DTC when the encoder signals do not match the chart contained in ROM. This DTC can be set while no shift is requested or while a shift is being performed.

 

other mom and pop shops should be able to read the codes for you. AutoZone will probably just have a cheapo code reader, where as another shop will have some money invested into a good scan tool. If you don't want to go to a dealership, i'd try a local shop

 

where is the transfer case module located