I have a 99 Blazer with Auto 4x4 (236 transfer case) and 116k miles. It is spitting out C0300 code and "Svc 4x4" light on dash intermittently appears. If I'm in 2Hi, I can engage 4Hi and 4x4 works. From 2Hi, I can also select Auto but when the rear slips, it doesn't engage 4x4. In either case, about 1-2 minutes after putting it in 4Hi or Auto (while driving), the "Svc 4x4" light comes on. Once the light comes on, I can't put it back into 2Hi unless I shut down the Blazer, turn back on and then select 2Hi...then it goes right back to 2Hi. 4Lo also works as long as the "Svc 4x4" light isn't on.

Here's what I've done so far:

1. Checked the actuator under the battery tray- no rips/tears/holes, it holds vaccum and the cable moves freely.

2. Replaced 4x4 switch on dash. all the connections looked good, no corrosion but the back lighting on the Auto button doesn't light up so I figured I would change out anyway after reading several posts about bad dash switches.

3. There are 2 rear speed sensors on the output shaft of the transfer case. I put a new one in the easiest spot first which is the passenger side. Still got the same code- C0300. I moved the old sensor to replace the 2nd sensor which is on the drivers side. Still got the same code- C0300. I then moved the previous drivers side sensor (original one) to the front output shaft sensor. These 3 sensors are all the same but the front one is for the front drive shaft. I figured if the rear sensor was bad and I put it in the front, the trouble code would move from the rear to the front. Unfortunately, this didn't happen and I still got the C0300 code.

This leads me to believe there are two remaining possibilities:

A) The wiring from the rear sensors to the TCCM is bad somewhere (I didn't have enough time to check this)

B) The TCCM is bad- is it possible that the TCCM would go bad and kick out a code for the rear output shaft sensors?

Any other thoughts or suggestions?

Thanks!

 

That code indicates that there is a problem with the circuit for the rear prop sensor. Only one of the two speed sensors on the tail housing of the transfer case is the rear prop sensor. The other is the VSS sensor. The rear prop sensor should have a light blue wire and a dark green wire.

The diagnostic sheet below describes some additional testing:
C0300 - Rear Propshaft Signal Circuit Malfunction

I would definitely recommend checking the wiring between the TCCM and the rear prop sensor.

 

Wiring is good...found corroded pins on the TCCM. I couldn't find an aftermarket replacement TCCM (GM part # 12577412) or a reman'd. GM wants $538 for a new one and a used one is $200- OUCH!

 

Pictures of the TCCM where moisture corroded the pins. In the middle picture, I think the two pins on the left (less corroded) are for the front propeller shaft speed sensor and the right two pins (most corroded- note the green) are for the rear propeller shaft speed sensor. I wonder if I can clean up the corrosion and voila...a working TCCM???

The left picture just shows the TCCM top & GM part #.

The right picture is just another view of the corrosion.

 

You should be able to clean them up with some electrical cleaner and a small wire brush or emery cloth. Just be sure to also clean the mating connector. Definitely try that first.

 

I just bought a 98 blazer which appears to be well taken car of. However, when I put it in 4x4, it goes in and then kicks out with a clunk. The 4x4 light is still on, but the front isn't pulling.

 

Use the search option...there are lots of threads about 4x4 issues. You will probably find something just like yours.

 

99blzr did you ever figure this out ? I have a 2000 doing the exact same thing.. Everything is showing that its in 4 wheel drive .. Encoder, actuator but front end will not engage . I pulled the codes and got the C0300 changed the sensors but it still will not engage. I checked the wiring, plugs .. and can't find anything wrong. My truck is doing exactly what you described here.