Automatic column shifter jumps down a gear on initial acceleration

[Accelo]

My truck is a 1970 Converted to a 4L60E behind a LS1. I used a aftermarket column shifter rod to clear the headers.
My automatic column shifter jumps down a gear on initial acceleration. Typically moves from OD to Drive. It does not jump out of gear; the shifter just moves down one gear. I have checked and I don't have a broken motor mount. I have also inspected all the bolts for torque. This includes the transmission mounting bracket.

I have always wondered why GM used the complicated linkage from the column to the transmission. (see attached picture of stock linkage) The single rod seems so elegant. However, I know GM didn't add parts just for fun. There has been a lot of discussion on how to make the automatic linkage work but very little on why it was built this way. Now, I am guessing, the stock linkage uses a bell crank from frame rail to trans for this exact reason.

As soon as I can get some help, I will start engine and hold my foot on the brake pedal. Then drop it in drive and goose the throttle while holding the brakes hard. While someone watches to see if the engine moves when you going from park to drive.

Maybe I will find my issue? I am going to look hard at reinstalling the factory style brackets. The factory stuff seems to get rid of the Micky Mouse feeling of a user modified ride.

For some unknown reason the transmission will not shift down correctly unless the selector is in OD. It works great in drive. Another reason I need to get this straightened out. I need to experiment with this to confirm. preliminary observations is the WOT shift points also change when the transmission is in any gear but OD? Weird.
Cheers

[Sheepdip]

Originally Posted by Accelo

My truck is a 1970 Converted to a 4L60E behind a LS1. I used a aftermarket column shifter rod to clear the headers.
My automatic column shifter jumps down a gear on initial acceleration. Typically moves from OD to Drive. It does not jump out of gear; the shifter just moves down one gear. I have checked and I don't have a broken motor mount. I have also inspected all the bolts for torque. This includes the transmission mounting bracket.

I have always wondered why GM used the complicated linkage from the column to the transmission. (see attached picture of stock linkage) The single rod seems so elegant. However, I know GM didn't add parts just for fun. There has been a lot of discussion on how to make the automatic linkage work but very little on why it was built this way. Now, I am guessing, the stock linkage uses a bell crank from frame rail to trans for this exact reason.

As soon as I can get some help, I will start engine and hold my foot on the brake pedal. Then drop it in drive and goose the throttle while holding the brakes hard. While someone watches to see if the engine moves when you going from park to drive.

Maybe I will find my issue? I am going to look hard at reinstalling the factory style brackets. The factory stuff seems to get rid of the Micky Mouse feeling of a user modified ride.

For some unknown reason the transmission will not shift down correctly unless the selector is in OD. It works great in drive. Another reason I need to get this straightened out. I need to experiment with this to confirm. preliminary observations is the WOT shift points also change when the transmission is in any gear but OD? Weird.
Cheers

I would say you are getting torque flex between your components. That complicated factory linkage was designed for misalignment and torque flex

[MikeB]

I wonder if the detents might be a problem. My 69 column has detents for a TH350, which are D, 2, and L. I don't how that would work for OD, D, L2, L1.

They are on the plate where the shift rod lever attaches to the column. Not a great photo, but you get the idea.

BTW, the frame to trans linkage keeps the trans in gear when the engine and trans move around. I snugged my linkage up a little by using a rod with heim joints on each end.

[Accelo]

Good point on the detents. I will check into it when I have time. May be a few days though.

[MikeB]

Originally Posted by Accelo

Good point on the detents. I will check into it when I have time.

I remember people saying all you need to do is change the gear indicator and you're good to go. But that's not true for my 69, because I have to pull back on the lever to go from D to 2, and then from 2 to L. I can't see that working with 4 forward gears, but maybe I'm missing something.

[Accelo]

Mine is out of adjustment I know that. However, the spacing and order are the same for a THM 400 & the THM 350's and the 700/4l60's. Only issue is the Low gear drops down further down on the 700/4l60/4L60E variants. There aren't and detents or adjustment for Low Gear.
I just need some help to adjust mine. That's first on my list. Second is to see if the motor rocks more than it should.
Cheers.

[MikeB]

So, once down into 2nd gear, no need to pull back again on the column shifter to drop down to 1st gear. Got it!

[Accelo]

It's still weird, to me, that I can put the vehicle in gear without stepping on the brake!

[68 P.O.S.]

I would investigate the motor mounts real well. Mine did the same thing when the mounts were failing....and when they completely failed it wasn't fun having the throttle stuck at WOT.

Sounds like you need to check your neutral safety switch as well.

[MikeB]

Originally Posted by Accelo

It's still weird, to me, that I can put the vehicle in gear without stepping on the brake!

You CAN put it into gear or can't? Has it always been that way?

I was reading that in a factory installation, there is a lockout that only releases when the brake light switch is closed.

[Accelo]

Veering off the subject but none of the early (1967-72) trucks had the brake pedal interlock.
A solenoid was required, by federal law, in the shift linkage beginning with September 1, 2010 production. Many manufacturers were installing them earlier. It requires brake application to shift out of park. It is a safety feature to assure small children can’t shift the car.

[tdangle]

Just recently I had an issue with the shift lever moving up when accelerating from a dead stop, but it went the opposite direction as your describing. Would almost go from OD (7004R) to neutral. The bolt had disappeared from the motor mount to the frame bracket. Don't know why it would be moving the other direction. When the engine torques, the drivers side moves up and would push the shift rod up moving the selector towards N. BTW, I also am thinking of reinstalling the factory linkage setup or going with the Lokar cable setup.

[MikeB]

Originally Posted by Accelo

Veering off the subject but none of the early (1967-72) trucks had the brake pedal interlock.

Yes, I know that. I was thinking that maybe whoever installed the 4L60E took advantage of the electrical interlock. But I guess you are the one who installed it.

[Accelo]

Issue solved:
I had checked the bolts and confirmed they were tight.
I inspected every thing again and noticed the washer was loose on the center motor mount bolt.
Turned out the center bolt for the motor mount was bottomed out.
I ended up replacing the bolt with a much shorter (7/16 x 3/4") Even then it took a thick washer to tighter to the mount.

I always appreciate the feedback. It got me to look at what I was absolutely sure was just fine!
It wasn't.
Cheers.