Hello All, I'm hoping you can help steer me in the right direction or confirm what I am thinking my issue is. I recently purchased a 2000 Blazer 4 Door, 4WD. It runs and drives great so far. The only issue I have is code P0753 "1-2 Shift Solenoid Electrical". No big deal, I dropped the pan, replaced the 1-2 shift solenoid, filled it back up, and drove it. The code came right back as soon as it shifted from 1 - 2. When I dropped the pan, I noticed the internal harness was original and the wires were pretty stiff, so I ordered a new internal harness, dropped the pan, and replaced it. Drove it and the code came right back again. I then checked continuity between Pin 6 on the red VCM connector and Pin A on the transmission pigtail and it seemed like I was losing connection. I ran a new wire thinking I found the issue, the continuity seemed good again. I cleared the code, took off, and it came right back again. At this point, I started researching a bit on the forums and found that there is a potential issue with the ignition switch not supplying enough voltage to circuit 1020 that powers all the solenoids in the transmission. I found where some had used a relay to supply a supplemental 12v from the battery, so I did that. Cleared the code and it came right back again. To summarize:

- New internal harness, New 1-2 shift solenoid, new wire between VCM and transmission pigtail connector (continuity checks good),

- I am reading 12v on Pin E at the pigtail connector for circuit 1020 (which I doubt was really bad anyway, considering I am not having any other problems with any other solenoids and all the solenoids share this 12v source)

I think I have it narrowed down that I have a VCM issue. I don't really know what else there is to this circuit. I have a good connection on the ground/signal wire, 12v power at the pigtail, and all new harnesses. Any help/opinions would be appreciated!

 

I'll take a stab at it, but don't hold my thought as gospel, check my work before you start poking the VCM. I'm just looking at a diagram real quick, I think you could check to see if the circuit is internally shorted in the VCM. This is a pull up, ground switched component where the computer is supplying the ground and it is internally switched (N/O switch). So my thought process is disconnect the VCM connector. Attach one lead from you DVOM lead (or computer safe test light) to B(+), key off, touch the pin on the VCM that feeds the ground to the solenoid with the other lead (or test light). My 98 manual shows it as circuit 1222. If you get any voltage, that would suggest to me that the VCM has an internal short since the switch is normally open. If you didn't get any voltage, then maybe replace the ignition switch like you saw suggested (though I haven't looked at this myself). Like I stated, I'm just taking a peek, so check your diagram to see if you agree. There's my shot.

Without a scan tool to command the solenoid "On", I'd have to dig to try and figure out how to test it doing some voltage drop tests.

 

Thank you! So I took your advice and did some testing. Here is what I found:

This section is all with the ignition OFF:
- I left the connector on the VCM and probed circuit 1222. I get no continuity to ground. I also tested for voltage and didn't get anything.

- I unhooked the connector from the VCM and probed the associated pin on the VCM and got the exact same result.

This section is with the ignition ON and the Blazer in Park:
- I left the connector on the VCM and probed circuit 1222. I had 12 volts on this circuit now, which appears to be correct, as I'm seeing that the 1-2 shift solenoid should be reading Hi when in park and if it is supplying the ground, I would see 12 volts on this circuit.

I obviously can't test it when I am driving, but according to information I can find, this code is set when the VCM is expecting Hi and receives 0 volts, or is expecting 0 volts and reads something above it. Considering the results of the test above (thanks for the guidance) and everything else I have replaced, I am thinking there is something inside the VCM that is "seeing" voltage on circuit 1222 when there isn't anything there or there is something shorted that is allowing a small amount of voltage into that circuit. The reason why I am thinking there isn't something shorted allowing full 12 volts on the circuit when it should be 0 is because the transmission shifts perfect. I have all gears and they feel to be shifting at the right points in the band. If I truly had 12 volts when I shouldn't, I would think I would lose 2 and 3 gear as that solenoid needs to be de-energized (or at least as close to 0 to allow the solenoid to release) to obtain those gears. This one is driving me mad trying to figure it out and I don't have access to a scan tool that I can command the solenoid on or off. Would you agree? If it continues to shift fine, I don't want to mess with the VCM, but I'd like to at least have it narrowed down if it does start to be a problem. Thanks again!