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Transmission downshift issue

Discussion in '80-Series Tech' started by Nemo, Feb 21, 2007.

  1. Nemo

    Nemo

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    Guys,
    I have read all of the forums concerning the TPS sensor, Throttle cable adjustments and I have turned my FSM into a big bunch of black pages, et cetera. All were very informative but has not relieved my issue.

    I have done the acceleration tests provided in the FSM, from you guys online and I don't have a problem with erratic shifting, or not holding gears when they are manually selected.

    How I do reproduce this issue is no matter what speed I am going the moment I touch (reiterate just touch - not enough to truely brake) the brake pedal the transmission will no longer "Kickdown" to a lower speed to accelerate. The only thing that seems to resolve my acceleration after that is if I come completly to a halt, and then hit the accelerator again or move the gear selector to 2nd and back to D.

    I have checked all fuses and fusible links in great length. To verify this craziness, I have also allowed the truck to come to a slower speed, ie 10mph from 30mph, 50mph to 20mph, 20mph to below 5mph and as long as I don't hit the brakes, I am fine with the kickdown.

    I just got through testing in 4 low, ie CDL engaged and ABS off and the problem doesn't happen? ABS speed sensor?

    Well I finally got out to troubleshoot the Stop Light Switch. Connects into the Brake pedal under the dash. Pulled the wiring harness to it and test drove. Problem goes away. I will be looking for a new switch and installing it to see if I have a wiring problem or if it is the switch itself. I guess I good do the ohm meter, but I am lazy.

    Thanks Bill. I have done the voltage check and the Stop Light Switch is def. sending the proper siganl to the ECM, so I am going from there right now. I do like the idea of clipping the wire. It could be helpful to fully isolate the issue. I think ultimately it will hurt because it registers to the ECM to cut the fuel supply / cancel the operation of the lock-up clutch so it may ultimately hurt in emergency braking where the engine is still torquing up??? Any idea on the lock-up clutch?

    Thanks for your input.
    __________________
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  2. Nemo

    Nemo

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    Just wanted to update everyone. My transmission problem is fixed as well as my TPS is solved. It was a bad wire from the ECU to the TPS. It was the IDL wire (Green / White) and must have been toasted somewhere between. The only other thing I have to work on is getting my Cruise Control ECU hooked back in which is a Splice from the Green / White to the IDL of the TPS. Not even sure how much I care being here in the beautiful rockies. CC doesn't do much anyways :)... Well I probably should see Christo and Team to get them to clean up my wiring. Have you seen one of them solder. Work of art. Mine... Blobs of metal, but functioning at least.