Stone Father

A few days an ago my truck threw this code P1860. I brought the truck to a mechanic because I figured it would be an easy fix, just replace the TCC Solenoid and done. Wrong.
$650 later, truck has all new tranny solenoids, and still no overdrive and a potentially ticking time bomb tranny.

The mechanic tested the wiring going from the VCM to the solenoid, said it was good. He said the signal from the VCM is not reaching the solenoid, and the truck needs a new VCM. He said he tried everything and ran every test he knew, then he called a transmission specialist and they agreed nothing more could be done and that the VCM needs changed. What’s your opinion on any of that? I may have left out some specific details, but ultimately my truck is in the same position as before, with the same code. Thank you!

Gumby22

It’s possible the VCM is malfunctioning but it needs to be verified.

I would be interested to know why all the solenoids were replaced if it only has a code for one.

The TCC solenoid receives battery voltage via the ignition switch when it’s in the on/run position. The VCM then provides a ground using pulse width modulation.

The wiring to the VCM can be verified by back probing pin #7 at VCM connector 2 with the key on, engine off and grounding it. If you hear the solenoid clicking/thudding, the wiring is good.

Stone Father

Whats frustrating is that I never thought I needed to know about maintenance on my own trucks because I thought mechanics would fix everything. I’m finding as I get older and “wiser”, that mechanics aren’t always most reliable. I don’t know why they replaced all 3. And I’m not sure why they wouldn’t have verified the VCM is faulty before telling me that was the problem. This mechanic is a very small town mechanic with lots of recommendations.
where can I find a diagram showing location of that pin? Please explain back probing, and how to ground. I’m not too familiar and maybe you’d consider walking me through that process. Thanks so much

Gumby22

As a professional mechanic, I can tell you that there’s a lot of factors when it comes to accurate diagnosis. Although a 1998 K1500 is outdated by today’s standards, it’s still a complex electrical system and depending on what’s actually happening, your mechanic may have been correct at the time the solenoid was replaced.

It’s important to remember that a control module can only perform its functions correctly as long as it has good electrical connections with the components it controls and a consistent supply of power and ground.

The VCM cannot provide a ground for the TCC solenoid if the primary ground circuits for the VCM are open or making intermittent contact.

Additionally, the VCM cannot control a given component if the connections to that component are open or intermittently making contact.

I still question replacing all the solenoids, but let’s focus on the one you’re getting a code for.

​​​​​I don’t have time at the moment but I’ll get a diagram of the VCM connector posted and provide more details on back probing. Should be able to do that later tonight.

Do you know how to use a volt meter and test light?

EDIT: Also, to ensure I’m looking at the correct info, what engine and trans are in your truck? If you’re not sure, look at the service parts label in the glove box and post the codes that start with “L” and “M”