Downshifting transmission

[WStrong]

I have a '91. 5 4x4 with the 4-speed automatic with just over 100,000 miles on it. The problem is that it shifts from forth to third at anytime. Very frustrating when on the thruway. Most of the time it will shift back into forth and will stay there for awile. Have other people had this problem and what is the cure. Could you give me part numbers and where you got the parts. Thank you. Also has anyone changed torque converters and what brands did you use.

 

[Hostetter]

just read your post see that you are on this forum thought i would reply i do not have a auto mine is a 93 nv4500. that must be a major pain always down shifting and for no reason. sounds to be a problem maybe in valve body just a guess hopefully some will chime in good luck

hoss

 

[BushWakr - RIP]

That kinda sounds like the infamous TPS (throttle position sensor)

If it's outta wack the transmission will make a mess with the 3-4, 4-3 shift. You will need a voltage meter and a few minutes to check the thing.

The TPS is located on top of the inj. pump, just toward the front, ahead of the AFC cover.

It has a 3 wire connection attached to it and the center wire (orange) is + (positive).

You should stick a safety pin into the little socket that the wire goes into (+ side) and hook your positive meter wire here, and ground your meter to the frame or some such place.

Turn on the key (do not start truck) and read voltage. It should read between . 5 Volts and 1. 0 Volt DC. Also cycle the throttle slowly from idle to WOT. The voltage should go up in a nice smooth increase.

If it jumps around then the TPS is dirty inside or crapped out. I have heard of guys cleaning them by spraying contact cleaner from the underside, up into the housing but I've not tried it myself.

Underneath the TPS is a shaft which has two "flats" for a wrench. I can't remember what size it is, perhaps 1/2" or 7/16"

By adjusting it one way or the other you will affect your 3-4, 4-3 shift. Right now, mine is around . 7 Volts. If I go much further than that, it waits too long for the upshift to 4th and jumps back to 3 too soon.



If this is not it you may have a governor solenoid in the VB thats not happy or some other issue I'm not qualified to explain. I've had some experience with the TPS, so thats were I'm coming from on that one.



Good luck, let us know how you make out.



Bob

 

[cdrider]

The speed sensor on the transmission/transfercase will also cause this type of problem. If the TPS checks out OK, your problem could be there. I don't know if there is a way to test the sensor. I just replaced mine---I think it was about $45. 00 at the dealer. Before replacing the sensor, check to see if there is anything loose(the sensor screws on and there is an electrical connector that you could check).

 

[Troutdog]

speed sensor

I just replaced the speed sensor too. I could tell it was the problem by my speedo doing erratic readings. I would say your speedo is alright check that tps.

 

[hdm48]

bUSHWAKR is right there on this one. And the adjuster size is 10mm. Try backing it off to see if it will shift sooner and stay in gear. after 60mph it should not go back to third but on the way up it might stay until 65 or so. I like mine to shift at about 65 so it pulls harder on the back half. sorry, drag race talk.

If for some reason it still won't shift up or shifts down too soon check the line pressure along with the other stuff.

MAN, you want something that shifts wierd? Try driving my motorhome with the GM drivetrain. PAH!! All you gotta do is look at it and it shifts down and then right back up and then down and then right back up and then down and,, and ,,, and,,