Hey everybody, what's up? I have a 87 Monte LS with a mostly stock 5.7 that came out of a 91 chevy truck. I put a recently rebuilt th350 in it too. The tranny worked good for about a year and now I have a strange problem. Has anyone ever heard of a transmission not holding first gear? I unhooked the shifter linkage and put it in first gear on the side of the tranny. The action on the shifter shaft is firm and clicks just as it should 5 times. Take it down the driveway 1/4 throttle, it shifts to second pretty as you please just like I had the shifter in second gear. I hook the shifter linkage back up, put the car in drive, and drive the car normally. It shifts perfect in every gear, at every throttle position, with no slippage. I can hold it full throttle and it shifts at 4500 RPMs (stock truck cam) first to second and second to third with no slippage or anything else. It just won't stay in first. I thought it was kinda weird seeing that the tranny shifts perfect in drive. I was wondering if anyone else had seen this happen. BTW my kickdown cable is hooked up and adjusted properly and works just as it should, and also the vacuum line going to the modulator valve is brand new and looks fine I checked it out and made sure it was connected properly. Could it be the valve body? Band? Any help is greatly appreciated! Thanks in advance!

 

My first guess....
Faulty Modulator valve....
I would also consider changing the fluid and filter in the trans...
When you say it comes out of first.... does that mean it goes into neutral? Or shifts into 2nd?...
If it slips into neutral.. the first gear band is gone/worn out. If the kick down isn't set proper it will shift into 2nd quickly under normal driving...and it maybe worn down to the point that that adjustment is no longer operable. Under "Normal" driving conditions.
That could also be a sign..That the front pump is starting to fail also...
I'm guessing he did a soft rebuild... and didn't replace any of the "Hard" parts. How much did he charge you to "Rebuild" the trans? Was it by someone you know or did a shop do it?


Generally,
On 350 transmissions once 1st completely fails... reverse will also disappear. just a little FYI.

 

It shifts to second even if you have the shifter in first gear. I thought my linkage had started binding up so I unhooked the shifter linkage and put the transmission in first gear manually on the side of the tranny. Still shifts to second at the right time just like if I had the shifter in second. Put the shifter in second, same thing. Not holding first gear is the only thing this transmission does wrong. When you put the shifter in drive it works flawlessly and first gear doesn't slip at full throttle nor does it slip in between gears at full throttle. It's got me baffled. It's really not a big deal because the car drives perfect if you just put it in drive and go, but I can't do any good burnouts with it shifting to second unexpectedly lol. Cottman transmission did a hard rebuild even rebuilt the torque converter in 2009 but I never installed the transmission till 2012. If I can remember right, I think they charged me about a thousand bucks. It sat on a pallet in my garage wrapped in plastic until I put it in the car. Then the car mostly sat from 2012 till now while I fixed all the other stuff. I put like 25 miles on it driving it up and down our little lane to keep the battery from dying. However, I have took the car out this abandoned road close to our house and launched it a couple times and ran it pretty hard trying to see what it had. I'm stumped, but I can at least still drive it. Just no manual shifting, lol.

 

If you manually(by hand) shift it into 1st gear and it holds... Then its definitely a linkage issue...

I'm guessing its a cable shift linkage? if so...its either stretched out or adjusted improperly.... is it the proper linkage?

It sounds to me like a linkage issue...

since you said you can manually put it in 1st and it holds...