could low fluid cause limp mode

[Porno Joe]

Haven't been around much lately, but still have my 07 LBZ. have a built allison that has given me zero issues for years.

today i had to run to a couple meetings, leave in the truck all is well, when I come out of the first meeting driving to the second, i notice my check engine light is on. i was in the city truck was still running fine so i drove to my next appointment. come out everything is fine hop in truck drive home. cruising on the highway everything was fine for about 2 miles then all of the sudden bam truck drops out of 6th gear. limp the truck home and go to back up into my garage, as soon as I put it in reverse it just flashes. nothing. truck wont move.

Now here could be where I screwed up- truck stuck on the street, I grabbed my scanner- PO700- cleared the code and truck backed right up.

I did check the fluid when I backed in, and its not in the "hot" range, so it may need some.

I also now realized i needed my efi live to scan the TCM to see if there were any codes. i quickly tried using my efilive, but i didnt see any codes, so maybe I screwed up and cleared them?

just drove the truck yesterday, zero issues, wasnt beating on the truck or anything. so could the low fluid level have triggered the limp mode?

 

[Chevy1925]

what was the trans temp at when you checked it? was the fluid just below the hot mark?

i dont think the current level is your issue but the codes would have been very helpful.

 

[Porno Joe]

trans gauge on the dash was right in the middle. I did look at my scan gauge and I want to say it was like 175ish.

it was not right below the Hot area, it was definitely much lower.

I know I'm an idiot for clearing it now that I had time to think. in the moment all I was thinking about was getting my truck out of the street.

 

[Chevy1925]

so at that temp, you want the level just below the hot hash marks. you can try topping her off and seeing how she does from there. if it throws codes, throw them up and lets have a looksy

 

[Mike L.]

Low fluid level usualy acts up when cold on initial start up.

 

[Porno Joe]

Ok had time to look at it tonight.

started truck up went for a drive. No issues trans shifted perfectly normal. I got the trans temp up to 186. Checked fluid. Low. at that temp it was JUST above the cold hash mark area. I added fluid and it was dead even with the hot hash marks.
Is that a good level at 185? Should it be into the hot?

 

[Dozerboy]

I think your fluid level is OK but you could add a little bit. Most of the time is preferred to be on the low side of the hashmarks unless your trans is hotter.

 

[Porno Joe]

Ok it did it again and I managed to get a code

po979. Shift solenoid C if my googling is correct?

 

[Ron Nielson]

See attached

 

[Porno Joe]

Ok bumping up since I have more info now

truck has been sporadically throwing the code since I posted but yesterday it def seems to be getting worse.

last night didn’t even make it a mile before it went into limp mode
Checking codes now I have

The same 979 but now I also have 2670 showing up.
Searching the 2670 seems to be low voltage so maybe I have an issue with the internal harness?

 

[Mike L.]

You have a voltage loss or short. Chek voltage with KOEO. should have 12.5V to the trans. KOER should be 13.5V or better.

 

[Porno Joe]

you mean check it at PIN 71 at the TCM? Or can I use my scan tool(tech 2) to check trans voltage?

 

[Porno Joe]

Think I may have found my issue. Finally got a chance to look under the truck. Somehow 2 of my wiring harness clamps broke and the main harness was loose. It moved over and touched the drivers side exhaust manifold. I didn’t have a lot of time to look at it but I can see it was clearly touching the manifold it melted the plastic wire look. I assume the wires are likely burned and grounding out

 

[Mike L.]

Good man.

 

[JoshH]

I was just about to reply to suggest you take a good look at the harness. We had a similar issue with a truck that tended to act up and go into limp mode when it was hot. After a lot of digging and testing, we finally found a spot in the harness that had rubbed through and was grounding out. I'm not sure why it only acted up when it was up to temp, but you had to drive it a while before it started acting up.

 

[Porno Joe]

Just wrapping up that was def my issue. Caught it just in time as no wires were completely broken. Taped everything up and secured it away from the exhaust and no issues. Put roughly 50 miles on it