78 th350 lockup or non lockup?

[wazzabie]

Would a 78 k5 th350 be a lockup or nonlockup tranny? How could I tell?

 

[tarussell]

GM K-series trucks didn’t get the lock-up TH350’s until ‘81 but considering the age of your ride it could have been replaced with a lock-up version at some point in the past 45 yrs.

If it is a lock-up TH350 there will be an electrical connection plug on the drivers side of the case near the shifter selector contraption.

Just for added info - non lock-up TH350’s are considered better or at least more durable. Differences in the case, pump, converter and related parts are different.

Hope this helps and answers your question.
Tom

 

[wazzabie]

GM K-series trucks didn’t get the lock-up TH350’s until ‘81 but considering the age of your ride it could have been replaced with a lock-up version at some point in the past 45 yrs.

If it is a lock-up TH350 there will be an electrical connection plug on the drivers side of the case near the shifter selector contraption.

Just for added info - non lock-up TH350’s are considered better or at least more durable. Differences in the case, pump, converter and related parts are different.

Hope this helps and answers your question.
Tom

The tranny is original to the 78 k5. The tranny has never been opened other then to change the filter. Also, how do you know when a rebuild of the tranny is needed. It runs without any issue currently. I would think the orings are no longer soft.

 

[tarussell]

There are several tell-tell signs that a trans is on its way out - but since yours is acting right judge it by the condition of the fluid and residual material in the pan and filter.

By all means your seals are most certainly aged to the point of needing replacement.
Also, things like bushings will most likely need replacement.
While getting it rebuilt consider upgrades like a better higher element count sprag and mild shift kit to firm up the shift engagements - these are cheap and well worth it.
Add a large ( high BTU rated ) trans cooler while you’re at it.

Better to plan for a maintenance rebuild rather then be stranded by a failure at the wrong time and location.