Hyundai Repair: hyundai trans o/d, vehicle speed sensor, pulse generators

Question
I have a 1999 accent. dealer has replaced trans three times, trying to solve an upshift into overdrive problem. What controls the upshift into o/d. Is it the pulse generators or o/d switch or computer? I had a scan tool on it and it showed O/D SW off. The button switch on the shifter was in the on position and the econ button was applied. All fuses at fuse box are ok. Is there a fuseable link in the system?  The car has been in several frontend accidents, could a shorted wire be causing it? To me this seems to be an electrical issue that may also be senitive to ambient temp, hot weather no shift at first now all the time. Any suggestions would be greatly appreciated.  

Answer
Whether the transmission shifts into overdrive is controlled by the computer, but the computer considers data from the pulse generators, vehicle speed sensor, temperature sensor, range switch, and overdrive switch.

If your overdrive switch is on and the transmission computer shows the O/D switch off, then you've already found a problem.  The computer isn't seeing the input from the switch.  The O/D switch has a power wire (pin 1, green) entering, one signal wire to the TCM (pin 3, color code Be?), and a signal wire to the cluster (pin 2, white).  The colors represent the wires on the harness side of the connector.  I can't clearly make out one of the wire colors (color code appears to be "Be,"  but that's not one of the listed codes).  The switch works by simply switching power to the TCM (O/D enabled) and the cluster "O/D OFF" lamp (O/D disabled).  

Start by cycling the switch.  If the lamp in the cluster turns on/off appropriately, then you know the fuse and wiring from the fuse to the switch, and from the switch to the lamp are good.  If the lamp in the cluster is off, you'll need to suspect that the power may not be reaching the overdrive switch.

Pull the console out and check the three wires mentioned above.  You should have power in the green wire (pin 2) anytime the key is on.  If you don't, you'll need to go back and check the circuit from the fuse to the switch (presuming the fuse is good).  If you have good power, next toggle the switch and check for power alternating between pins 1 and 3 as you toggle the switch.  If you don't have that, the switch is bad.  If you do, you'll need to go to the TCM and check for receiving power there.  Chances are the switch is defective or the wires have been cut by the shifter.

If you'd like to see a schematic, you can register for a free account at www.hmaservice.com.  The site requires Internet Explorer, and you'll need Adobe SVG Viewer (free download) to see at least some of the schematics.

It's unlikely that the collision would be causing the problem.  If you've got power at the fuse for the overdrive switch, then the problem is definitely somewhere inside the car, since we already know the TCM isn't receiving the signal from the overdrive switch.