Check Engine Light
 

New alternator, brand new yellow top battery, two different PCM's, three different ignition switches, same issue. Bouncing volt meter that sets a CEL, diag code says "low voltage condition detected".
Grounds look good, must be low voltage supply to the PCM but I can't find it!

Ideas?

I tried bypassing the PCM (with a brand new external regulator) and it did the same thing, although I did not try using a relay to bring power straight from the battery.
To me this suggests a power supply issue TO the PCM but I just can't find it.

Check this, best way is to back probe with a straight pin or something. See what the PCM is seeing with the truck running.

https://www.dieseltruckresource.com/...18&postcount=4

I remember seeing a similar problem from the ignition side, low volts causing code, related to failed wire group at fuselinks, we ran a relay circuit, like your suggesting, off the battery into the regulator "ignition sense" 12v wire and from seeing 12.5 volts, went to 14, and code problem went away,

Thanks for the reply. By "ignition sense" do you mean the hot wire from the ignition switch to the PCM?

When you bypass the internal regulator, it thinks it is defective and gives the low voltage condition code. I chased that rabbit around the tree a few times.
There are 3 power feeds into the PCM: Pin 3 is the direct battery feed, pin 8 is the ignition switch feed, and pin 57 is the auto shutdown relay feed. And then there is the internal regulator output. Any problem with any of these will CEL.

Thanks for the pin rundown, that will help.
I am not bypassing the regulator at this time. I only did it as a test to see if the voltage bounce would stop. It didn't.
So, since I used bus power I can assume there is a low voltage condition to either Pin 3 or Pin 8. Will check those with the Fluke.
Thanks.

yes, at alt, green is sbec pcm wire, controlling output, blue is from asd, ck volts on that one, add direct battery to blue to see what pcm does to volts at battery, ive seen both high and low alt output for same reason? I also know how to abandon the pcm control to external regulation, and satisfy pcm from setting code, after disconnecting it from service, vans and motorhomes used the same pcm control.

by the way, im remembering, a half of one volt changed up or down, from loss somewhere in the harness/connections, corrected the alternator output condition

I just put my Fluke on the battery and it is showing 16+ volts with the engine running.

whats the blue wire volts?

Will have to check that tomorrow. At an AutoDesk 3D seminar.