After arounfd 30 minute this came

Harness and solenoid is replaces

ready for driving home in limp mode

I post his issue in the Conversions & Swaps thread earlyer today, but i hope this post can be read wider in this thread for transmission
This is about a 1965 pontiac Bonnevile with Lq9 and 4l60e , https://ls1tech.com/forums/conversio...l#post20566996

I have a serious problem with my car, and it happens about
after 30-40 minute driving the car go in 3 gear mode, and the speedomter stop working, ( no vcc signal ? ) . The engine light come on, and the P0758 came up.... . After some hour the problem disappered, everything works fine, and i can drive 30-40 minute again.... . Solenoid and harness in tranny is new...

I place this post earlyer today

The PO758 is still following me. I thought I had checked out a grounding problem under the dash, but apparently it was a dead end. The short version is that after 30-40 minutes of driving, the engine light comes on, and the car locks in 3rd gear.( Failsafe mode) At the same time as the engine light is on, the speedometer stops working it tells me that the vcc signal is not reaching. And yes, the sensor was bought new a couple of years ago
Last drive I keep driving (to get it home) and after a while the car only works in 2 gears. Trust me, it's really fun sitting in this car, on a freeway, going 30 mph. . This 758 problem is driving me crazy and I can't figure out what the grounding problems are supposed to be. i feel like i'm fighting the phantom blot... . Could it be an ECU problem ? that it buzzes after this f...ng 30 - 40 minutes.. When i came home, i disconnect the battery, and after some hour ( next day ) the problem is disappered. Zero DTC ,and the car drive perfect....( first 30 min...) Something tells me, that this is not a gearbox problem., solenoid and cables are new... I am very happy for input here...

 

P0758 has been in the past been related to the PCM not supplying a ground path or having an erratic ground path. They found it with a scope whille watching the PCM gorund. I should give a square wave form as it changes states. The found a long slope to the wave form and determined that it was the PCM
Here is a P0753, P0758 diagnostic sheet that should help.

 

I am gonna go with PCM the VSS disappearing along with p758 at the same time and the cooling period. . A quick test to determine would be when the car exhibits the symptom spray the PCM with this real good , I the problem disappears in minutes instead of 30 to 40 minutes you have you culprit. I have used this and similar a number of times in my life when looking for temp related engine control and trans control issues and radio electronics too.

 

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thanks for your reply. When I read what you write, I think it's a good idea to try another ECU before I start troubleshooting as instructed in your attachment😌

 

thank you, I have not reflected on whether the element for the cooling fan(s) has a parallel here. but thanks for the tip and I will definitely include this in the elimination / get it checked (out)