TH350 lock up inconsistencies
Thread Starter
Racer
Joined: Nov 2011
Posts: 319
Likes: 12
From: South Chase WI
TH350 lock up inconsistencies
My trans will go into lock up going up a hill or while lightly accelerating, but as soon as the transmission sees the load decrease the slightest bit it comes out of lock up.
Shifts fine, fluid is clean, no other abnormalities.
Google search has not given me any leads, anyone on here a transmission tech? Not a huge issue, but what was fine yesterday now is very inconsistent. Cable adjustment? Internal issue?
Thanks
Shifts fine, fluid is clean, no other abnormalities.
Google search has not given me any leads, anyone on here a transmission tech? Not a huge issue, but what was fine yesterday now is very inconsistent. Cable adjustment? Internal issue?
Thanks
Thread Starter
Racer
Joined: Nov 2011
Posts: 319
Likes: 12
From: South Chase WI
The one on the firewall? Happens at any and all speeds, I can make it go into lock up with slight throttle pressure, but as soon as I stop that increase in pressure / load it comes right back out.
Advanced
Joined: Dec 2012
Posts: 53
Likes: 1
From: North East Ohio
My 80 would shift in and out of the lockup constantly when driving 40 to 50 mph. I unplugged the green wire on the fire wall that went down to the th350c and the lockup would now never engage. That was fine for a year but now I use freeways more and wanted that extra help of the lockup to save fuel. I installed a switch to that green wire, mounted the switch under the dash by the headlight vacuum release button so I could engage the lockup on if I thought I would be driving at highway speeds. Ran the wires through the rubber bushing for the speedometer cable on the firewall so no drilling was needed.
Thread Starter
Racer
Joined: Nov 2011
Posts: 319
Likes: 12
From: South Chase WI
Thanks, I have a switch as well to do just that.
But as of late it will not stay engaged at all. Just locks up under slight load increase, once that load is removed it unlocks. I leave he switch off in town to stop the annoying lockup at low speeds, once on the highway I engage it for economy. Has been flawless up until last week when it would only lock up going uphill or with slight throttle increase. Then back off again.
But as of late it will not stay engaged at all. Just locks up under slight load increase, once that load is removed it unlocks. I leave he switch off in town to stop the annoying lockup at low speeds, once on the highway I engage it for economy. Has been flawless up until last week when it would only lock up going uphill or with slight throttle increase. Then back off again.
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
I would try a different vacuum source if it's plumbed into the carb, move it to the manifold. On my 82 it's controlled by the ECM but if you have a separate vac switch, you can replace it with a different rated switch. On Harley's we have what's called a VOES switch. Vac oper electric switch. That operates our vacuum advance on the electronic ignition. Jpcycles sells all different vac ratings for when you want it to active.
Hope this helps.
Hope this helps.
Advanced
Joined: Dec 2012
Posts: 53
Likes: 1
From: North East Ohio
Not sure if any of this will help but in my 1980 Chevrolet Corvette Shop Manual it says that if your th350c "locks up erratically" and "at different speeds" the malfunction could be a few of the following. A vacuum leak, defective vacuum switch, release oil exhaust orfice at the pump blocked, turbine shaft "o" ring damaged at solenoid, converter malfunction, solenoid bolt loose, governor pressure switch malfunction. I would try the simple stuff first but it may need an expert.
Thread Starter
Racer
Joined: Nov 2011
Posts: 319
Likes: 12
From: South Chase WI
Yup, seen all that in my shop manual as well. Lots of places to look, my hope was that I was not alone in this and others had experienced as well the same and I could narrow down the possibilities; certainly there should be a more common failure cause, and a least common failure cause, if I can start at the right end of the list it's always a bonus!!
Thanks for everyone's input.
Thanks for everyone's input.
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Advanced
Joined: Nov 2014
Posts: 63
Likes: 1
I have a 1981 th350 lock up trans , i have removed the onboard computer also replaced the intake and new msd dist and 6al , i want to make my trans work without using the computor my question is how ? just unplug the wires to trans and the vacume to it or what, or do i have to replace the transmission with a nonlock up th350 ,please help me