Hey everyone. Like the title says I have the service 4wd light on my 2007.5 silverado. I cleaned the ground under the driver side door and that didn't work work sadly. I checked the ground continuity at the harness at the front differential actuator and that was good. I also checked voltage and was getting anywhere from 24-28 volts vehicle off in ACC. Tested battery at around 14v after it was running. The 24-28 seems a tad high. Im pretty sure it's not the **** but I ordered one anyway because mine is kinda "gunky" feeling.

I'm thinking TCCM problem? I haven't had a chance to read codes and have not had the TCCM flashed as some have suggested. If anyone has any insight or experienced something similar let me know please.

 

Your best chance to diagnose this is to have a shop do a scan of the TCCM for codes. Most off-the-shelf code readers only read from the PCM which may or may not have codes stored for the 4wd but an advanced/pro-level is capable of reading codes from all modules in the system.

There are multiple known issues with the 4wd systems on these trucks so without specific trouble codes to follow the only other choice is to check signals, power and grounds at the individual control components (TCCM, encoder motor and front actuator).

Are you certain the meter wasn’t reading 2.4-2.8 volts instead of 24-28?

 

So today I stand humbled. I tried to imagine what could cause a reading of 24-28v. I decided to go out today and check something. Recently I used my multimeter to ensure wires were disconnected in a home for a ceiling light I replaced. This, of course, is volts AC. I tested my truck in volts AC instead of DC. I don't know why or how it regesters any volts while in the wrong mode but it did. It's a cheap little orileys unit. I tested it properly on DC and got arouhd 12v which is consistent with proper voltage supply for the application.
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It appears the cleaning of the ground near the driver's door did the trick.

Simple stupid mistakes can be costly. Learn from my stupid error and step away and double check before loading up the parts cannon.

 

At least it was an easy fix.

The reason the meter registers voltage in AC mode is due to the internal circuitry that filters DC vs. AC voltage - what basically amounts to diodes and rectifiers because that’s how the meter is able to distinguish the difference, particularly if it doesn’t have separate probe connections.

It’s not that it’s a cheap meter so much as it’s not a “True RMS” meter which is what more expensive/professional level meters utilize.

This really only matters when in-depth diagnosis is being performed - particularly for signal analysis when troubleshooting automotive systems. For basic troubleshooting (checking power, ground, and reference voltages), cheap meters serve their purpose.

Additionally, AC voltage will register in the presence of EMF’s and any wire receiving battery voltage will generate a minor EMF. You can hold the leads near the engine while it’s running, without actually touching anything, and AC voltage will register on the meter.