I found my 89 3 years ago w/64K miles on it. It had been babied and not driven regularly. (see more pics in the welcome section) We all know letting them sit will create unique problems of thier own.

The transmission will drop out of OD while cruising at speeds of 45-75. Any rise in elevation will make it drop down.

I'm considering a trying a shift kit and adjustable modulator before changin the torque converter. My 88 did not have this problem, but a friend had this trouble with his brand new truck back in 89.

Is this a common trait for the 89 model? Any other ideas on how to fix it? I'm driving it from Arkansas to Seattle in a couple weeks.

 

Any ideas?

 

Well I have heard (and this isnt so much of a fix as a way around the problem) if you wait until you are at 50 then shift it into od it will shift and stay this is a common problem on late 80s model vehicles the overdrive gear begins at around 45 to 50 so driving close to that speed confuses it and it wont stay in overdrive as far as fixin it Im not a tranny expert but maybe somebody on here would know more.

 

Are you sure that it is not just the torque converter unlocking? You can test this out by tapping the brake pedal as you drive down the road. The RPMs should jump up a bit, but the transmission will remain in OD (4th).

The early 700R4's would keep the TC locked up longer and GM made modifications to the programming to make things more "responsive". It all depends on your view of the new sensitivity whether they made things too sensitive...

Since you have an 89, the lockup should be controlled by the ECU.

 

I knew somebody would know more haha

 

Its not the converter locking. Not all of it anyway. I'll try your suggestion and see if it is more converter than I think it is. You know how it is, you get an idea in your mind and ignore all other posibilities...

Its been a while since I drove it, so I can't say for sure. I believe its dropping down into 3rd and out of OD. It happens anytime you would give the gas pedal a little more pressure to climb a small hill, etc. Not a big change in pressure, just a touch.

If you do not give it a bit more gas, the truck would slow and drop anyway. As I said, my 88 5.7FI didn't do this, neither did my 91.

My 88 got a shift kit and adj modulator when I bought it in 91. I did it for a more positive shift for towing.

 

If it is really dropping out of OD, then you have something amiss inside the transmission. Shifting is all manually controlled in the valve body. A weak or broken spring may cause quick drop out of OD.