Hello,
I'm new to this group but have had many GM vehicles in the garage over the years.
I'm hoping someone could help with a VCM pin out and diagnostic chart for my 2001 blazer 4.3l, 4wd, auto trans, NP233 case.
Little background.
I bought the truck from my local firehouse over a year ago, original 68k, had a few minor issues to correct, dead short in radio draining battery, collapsed vacuum line with intermitten no 4wd, rotted brake line, but has been operating as it should since.
Recently out of nowhere, speedo stopped working (jumps a little randomly), ABS light on, Brake light on, and seems to be stuck in 2nd gear, code P0502, and no VSS/mph reading on scanner.
Past experience pointed me to VSS on driver side transfer case tailshaftshaft housing, purple/white- green/black wires.
Pulled the sensor, took a shot and installed aftermarket replacement. Same issues- no change.
So now I need to actually run thru the diagnostics and find the problem. My VCM/ECM is the " NEW" style with the two long connects held in with a small bolt in the center of each. I normally try to purchase the factory manuals but it seems this series Blazer are few and very expensive.
Any help would be greatly appreciated.
Thank you..

 

Here are your diagrams, I'll be able to get some diagnostic charts later today. The second one is the auto TC and the third one is the selectable TC control:




George

 

Thank you for the quick reply!

 

With P0502, the PCM does in fact command second gear only:






George

 

Thank you George. This information will be really helpful. I won't be able to follow up with you for a day or two due to work schedule.
One thing I did notice though is the VSS resistance spec in step 7 is no where close to the new one that i installed. As a matter of habit I regularly check replacement parts against what I remove. My original VSS was open( infinity) and the new replacement was 5ohms exactly. Mmmmm?

 

I don’t know the significance if any of the difference between the spec and your VSS resistance measurement.

Let us know if you need any help when you get back on this. Given the new VSS, the wiring is probably up next.


George

 

Well, I was able to run thru the diagnostics last night. The wiring from vss to pcm looked good with no opens, shorts to power or ground.
Resistance from pins 20-21 with vss connected was very low (5.5ohms).
A/c voltage across same pins while spinning driveshaft was .01-.003.
I removed the replacement vss and rechecked its resistance between the two terminals - 5ohms. I then checked continuity between each terminal and the metal threaded case- there was continuity.
So off to the local NAPA this morning and ordered a vss. Picked it up after lunch and installed it. Scanner showed mph reading which coincided with the speedometer and a road test found all is well in Blazer Land!
As a note the first vss was purchased on-line from a reputable auto parts supplier- I guess defective parts are more common now and as a lesson I will be more thorough with pre installation inspection and testing.
Thank you very much for all the help. The wiring and diagnostic charts really helped to pin point my problem.
Rich

 

Also- the NAPA vss measured 1275ohms with no continuity to metal case.....

 

Glad your fixed up and glad to help.

George