help, 89 7.3 E40D limp mode, OD light glows no tac

Not cool!!, my truck plugged in during work in single digits, truck started fine, warmed up for 15 minutes, unplugged block heater found no tac, od light dim glowing and trans seeming like it wanted to start out in OD. Transmission has been rebuilt 25,000 ago, by reputable local builder who has done 4 trans for me (other vehicles). The range sensor was updated to newest style, I have replaced VSS and ABS sensor on pumpkin and wiring in the last two years. Tac has never had issues, there is no corrosion or issues with the instrument panel. The truck has never had this issue, is well cared for. The charging system was updated from 1g to 3g two years ago and is charging correctly. I have read multiple issues on this site that are similar, but have yet to find solid answers by the comments. Batteries are up, new cables, positive and ground. Has someone traced out issues to a specific point besides the instrument panel?
Again, it happened suddenly with no indicators prior to last night with block heater plugged in, 9 degree weather. I have taken and given on this site and appreciate directional thoughts, thanks!!

Sorry I cannot be more specific, but it sounds like you have a voltage drop most likely on a ground connection. Cold weather, dampness, water, salt, etc. can cause a voltage drop on a connection. (Prevents the full 12 volts from passing through.). It will act up during the right conditions then work well enough that you do not see symptoms when temps change or the dampness goes away.

If related to your unplugging the block heater see what wiring your cord touches or hits. But having it plugged in or not (120v) should not have an effect on your trucks 12v electrical. Then inspect and clean any wiring connections and grounds your heater cord hits, touches, or can move.

Sounds like you have a E40D, it needs a Tach signal if I remember correctly from other posts. Some have shared water can go in the top of the Tach sensor or oil can come up through it from the crankcase causing it to not function properly. Inspect and perhaps if damp, use a heat gun or hair dryer to see if you can dry it out. Then check the electrical connector to it. And trace back from there. Check your grounds to the engine and frame to confirm dry and corrosion free.

My two cents for what’s it’s worth.

OD light glows there is no power to the TCM. Check the fuse and TCM relay.

For those of you not seeing it has already been resolved thanks for the replies it was a corroded relay to the TCM because it was no longer on the mount laying sideways water got in and rusted it again thank you for all your help