TH350 Intermittent Engagement Questions and Troubleshooting Steps

[GreyGhost85]

Good morning, First time poster and been lurking for about a month now. So long story short, I recently purchased a 71 c10 305 with a th350. The owner that I purchased from swore up and down that the trans was good and he seemed pretty legit. I had it trailered to the house so that I could begin working on it and getting it running. The problem is that the th350 is not always engaging properly or may take a couple of seconds to engage. It seems to get better the more I keep messing with it but then it just goes to no gears, no reverse, no drive,d1, or d2. The trans had been sitting for a while but connected to motor.

Here is my troubleshooting:

1. Changed trans fluid and filter. Saw a couple of leaks with old pan and Speedo cable so I changed pan, filter, orings for bullet, and cooler. No leaks.

2. Started her up again, poured 4 qts of fluid in and let her run for about 5 minutes and kept checking fluid to make sure it was within the hot crosshatches. And it was even after shifting thru all gears. Tried to drive and it moved forward only with a bit of revs and reverse as well. Drove around the block and no gears at stop sign. Pushed back to garage.

3. Drained pan and pulled filter and there was some sludge , I assume from sitting for a while, put new filter on and repeated the fill shifting through gears. This time I put the rear end on jack stands. I had strong reverse, strong drive, d1, and d2. Wheel spinning away. When appoint brake I put it in park and left it running, checked fluid and it was perfect. Hopped back in and put it in reverse and go nothing. No drive either???

4. I started engine with transmission cooler line off and trust me, it’s pumping, made a mess all over floor. Repeated the rear on stands process with fresh fluid. Popped it in reverse and bam, wheels turning hard through all drives as well. After going through gears once and back to park, I get no gears again while on jack stands.


Is this th350 toast??? Is a break in process for a th350 that has sat? There are 0 leaks, I’m just confused as to why it able to go in gear on second and then 0 gears the next.
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