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Thread: Help 93 Silverado Won't Shift No Speedometer?

  1. #1
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    Help 93 Silverado Won't Shift No Speedometer?

    I pulled truck in the garage to change brake light switch. When I pulled it out the next day it wouldn't shift out of low gear. I noticed also that the speedometer wasn't working. I thought the trans had went out, went to my trans man an he laughed. He said if the speedometer doesn't work, it won't shift. I can't see that I've unplugged anything, does anyone have any idea what could be wrong? I changed the speed sensor in the trans today and it didn't fix anything. I have a 4L60E. I've chased wires, cleaned terminals, everything I can think of? It has to be something simple. You can pull the trans fuse out, and the truck will take off in high gear and run normal speed, it just won't take off. As soon as you put the fuse back it, it won't shift. I don't think the trans is out because it will pull as normal once you get it going with the fuse out. Please help, I'm pulling my hair out. Thanks Brandon.

  2. #2
    Senior Member brad4eva's Avatar
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    lol he is right if it wont shift , the throttle position sensor wont recognize no reading on speedo, for now you can drive it in 1st and 2nd, it isnt out I dotn think i just had a 4l60e rebuilt for 1 k maybe something you did when changing brake light switch ur grounding out something maybe?
    //B-rad

  3. #3
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    Welcome to the forum Brandon.
    The tranny won't shift out of first if the output speed is read as too low.
    That could have been caused by a bad VSS, but as you know
    this was sudden following the change of switch.
    The speed signal from the VSS goes to the VSS Buffer and ECM (PCM).
    The ECM is what tells the 4L60E tranny to shift.
    Since that's not happening and your speedo is dead, sounds like the buffer having trouble.
    This could simply be because the Brake/Stop fuse is out.
    Have you checked your lights ?

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    I had been having no brake lights. I went to replace the switch and they started working again. The fuses are all good. Where is this buffer located? Any other idea's? I've checked every plug near the brake switch and cleaned the contacts and all is well. I even replaced the VSS. PLease help. Local garage can't get it in for over a week. Is there anyway I can read the codes or anthing to find out what's wrong? Thanks Brandon.

  5. #5
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    Ran the codes with a paperclip, gave me a code 24 for Vehicle speed sensor? I have replaced the sensor, what can I check next?

  6. #6
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    budder

    if the code is stored in the computer, you can try disconnecting the battery a couple minutes and some times the computer will reset itself

  7. #7
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    The buffer should be located inside near the right hand end of I/P.
    Codes can be read by jumping "A" and "B" on the DLC/ALDL connector near brake release
    (see related threads for how) or having a local parts store read them for free in many cases.
    The buffer outputs two versions of the signal from VSS, modified for ECM and cruise, and unmodified for speedo.

    If you follow the wire from the VSS, I think you find two connectors on the way to buffer.
    One is near blower motor and the second is at buffer. Check them.

    Two other ideas: pull brake fuse (power for buffer) and see if any difference.
    With fuse back in, key off, pull large connector on antilock near master
    if you have antilock. See if any difference for shift/speedo.
    Antilock shares some wiring with buffer.

    Interesting that you got that code - it's for manual trans.

    Basic checks for buffer:
    Pull connector from buffer, then key on, check for voltage at plug pins c8 & c9.
    Those are Brn & Blk/Wht I think, don't confuse with Brn on c13 which goes to ECM.
    This makes sure buffer is getting power.

    Raise rear wheels with everything back together.
    Engine on, in gear, wheels turning slowly:

    Backprobe c7 and c12 on buffer (Grn/Blk & Ppl/Wht ?) for AC voltage
    using a scale that will read to 200v looking for volts to go up with speed.
    This is VSS signal reaching the buffer.

    Backprobe c11 on VSS (DkBlu?) for DC voltage...
    Should be basically varying up to 5vDC.
    If 0v, buffer may be toast.

    Do make sure you have done a good visual check for wires that may have been stripped/damaged
    by pulling or tugging and now are shorting to metal somewhere.
    This was a sudden problem following work, makes it likely.
    One little grounding short in several of these wires will take down the system.

  8. #8
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    Turned out to be the brake light fuse? I thought I'd checked everything, but evidentally I hadn't checked it. Thanks for everyone's help. So if your speedo quits working, check your brake fuse. Makes sense huh?

  9. #9
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    Glad you found the problem.
    Lots of times the fuses have several things covered,
    always good to double check fuses before buying parts
    or seeing the dealer. Simple stuff kills.
    Thanks for posting the update.

  10. #10
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    Similiar Problem: 95 Chev 2500 No shifting, No speed reading

    I had a similiar problem with the following: NO shifting , NO speed indication on speedometer, NO battery voltage on dash voltmeter. Tach OK, running OK (even after some bad gas -- 2nd time this month).

    Found that the Guages Fuse was blown. Brake fuses/relay were OK.

    Thanks.

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