91 350 SDL 4th to 3rd downshift
 

Any ideas what causes unwanted 4th to 3rd downshifts above 65 mph ? on a 91 350 SDL ? It starts doing it after driving about 6 miles

I'll guess a Bowden cable that is 'way out of adjustment. Does it upshift at higher rpm, like 3,500, with moderate acceleration?

That's my guess. $0.02.

Second guess would be a 'flaky' kickdown switch, activating on its own, or with the slightest touch. Test by disconnecting the switch and driving 65+.

Thanks for the info. Is the switch inline with the modulator line ?

The kick-down switch is an electrical switch located behind the gas pedal on the firewall. When you put your foot to the floor, it engages the switch which drops the tranny down one gear.

What is the function of the Bowden cable and what is the switch just below where it connects to the engine linkage ? I really appreciate everyones help.

I disconnected the kickdown switch, drained one qt of ATF out of the trans as it has been too full for about one year, went out on the highway again and it shifted great as I ran her at 70-100 mph for half an hour then I went to Walmart , left it running while I was in the store, took off in city traffic and it started the 4th to 3rd on its own again. I also noticed that I could accelerate to a 4th gear shift but when I let of the throttle and as soon as I applied throttle it would downshift. Then it started doing the exact same thing in 2nd and 3rd and it only does this after it gets good and hot. Any ideas ?

The Bowden cable lets the transmission know how far you have pressed the accelerator pedal. At lower speeds, pressing the pedal down, but not all the way, will not activate the kickdown switch, but will let the transmission know that you are accelerating. Depending on pedal position, gear and rpm, the transmission will downshift, or stay in the same gear.

If it is adjusted too 'sensitive' it will fool the transmission into 'thinking' that you have the pedal further down than it actually is, and may cause premature downshifts with less pedal travel.

You may want to adjust the cable housing in the opposite direction, so the cable does not move as far, if that makes sense. That should mean turning the adjustment of the cable housing so the housing gets 'shorter', or not as many threads are showing at the adjustment bracket. Be sure to loosen the locknut first, try 1/2 to 1 turn at a time and see what happens.

Of course, it could be something else internal to the transmission that is affected by heat...

Let us know.

Again thanks for the input. In an earlier post I had described how the heater/AC blower had stopped working at the same time that the Trans started acting up and after I adjusted the Bowden cable for one more turn of slop and I had checked all of the vacuum lines I decided to check the blower motor circuit to see if a fuse was blown. Well the external fuse that I had replaced a couple of months ago was still hot and so I was reading the fuse chart in the fuse box and noticed that the #5 fuse controls not only the climate control panel but also the "auto trans electrics" and it was blown. I installed another 8A white fuse and took her out for a more vigerous run and it shifted great ! The Bowden cable adjustment seemed to change the shift point to a lower RPM but I never looked at that in the past with much thought cause it always shifted to what I thought was OK. Now what I would like to know is what trans circuit does that fuse control ???? I still have the kickdown switch disconnected.