1986 convertible not shifting into 4th gear
 

This is my first post in a very long time. My '86 has always shifted into 4th at about 45 mph with a light steady throttle like it's suppose to until recently. The car has 66k miles. The only transmission work that I know of has been changing the fluid and filter a few years ago. I've owned the car since Jan. of 2000. Currently, when I accelerate, I feel the change from 1st to 2nd, then 2nd to 3rd, but when I go through 40 to 50 mph, I don't feel or see the change in rpms like I used to when it should be shifting into 4th. I remember going about 50-55 mph at 1200 to 1300 rpms. Now, at 50 mph, I'm at 1600 to 1700 rpm. If I take my foot off the throttle at that speed, then the rpms drop, but go back up the second I apply throttle no matter how little. I'd appreciate any comments or easy things to check. Does anyone know of a reliable trans. shop in the upstate of South Carolina,( Spartanburg area)?

The trans will shift into OD long before 45 MPH. Mine will go into OD as low as 20 MPH.

I think what you were seeing is the torque converter locking. That is programmed to happen at 40 MPH. My car goes to TCC at 42 MPH, so the ECM isn't seeing the exact speed.

I suggest checking out the wiring on the transmission. One wire is the TCC signal from the ECM to the trans. The other is the OD signal from the trans to the ECM.

You can check out the TCC lockup by monitoring the voltage on pin F of the ALDL connector. The signal is "backwards" -- 12 volts when the TCC is OFF, zero volts when it is ON.

Sorry if Im intruding on your thread here, but while Cliff Harris is on this one, I would like to ask, do you think grounding the aldl pin with an scr would be a good way to activate the tcc manually if it has to be done that way? How much anode current would it take to pull it to ground?

The TCC is turned on by one of the quad driver outputs. The quad driver is only rated at 1 amp per output, so it wouldn't take much. Here's a link to the data sheet (the quad drivers in later ECMs are different, but I don't know what year they changed -- probably around 1990):

http://www.ti.com/lit/ds/symlink/tpic2404.pdf

These early quad drivers only have one \FAULT output, so there is no way to know which output has failed. They fault on over current. The outputs are open collectors, so there is no problem with putting another device in parallel with them (wire OR, as they say in the biz).

An SCR would not be a good solution because once it latches on there is no way to turn it off.

Wouldnt the brake pedal switch open the anode circuit?

Depends on how it's hooked up.

An SCR can only be turned off by reducing the anode current to zero. A lot of times that is done in an AC circuit by the polarity reversing.