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Thread: Speedometer fluctuating, bad vss?

  1. #1
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    Speedometer fluctuating, bad vss?

    My speedometer just started to act up on my 94 Chevy C1500.
    Manual trans, pcm:16197427
    Datalogging with Tunerpro shows the same fluctuating readings that the dash speedometer does.
    Started acting up in the 50 mph range, but now does not want to read anything until about 15mph, then fluctuates at times.
    Checked wiring, and grounds and have not found anything wrong, getting no error codes.
    Does this sound like a bad vss, is there a way to test a vss?
    Hate to just throw money at it and hope, so would like to know more.

    Thanks, any help and insight is appreciated.
    Been searching net, haven't found much yet, my Haynes manual says take to dealer to diagnose. That's not going to happen

  2. #2
    Super Moderator Six_Shooter's Avatar
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    Sounds like a VSS going bad to me. Thge only other thing it could be would be the DRAC/VSSB, but I don't think I've ever seen a bad DRAC, unless someone was messing with it.
    The man who says something is impossible, is usually interrupted by the man doing it.

  3. #3
    EFI GearHead ! EagleMark's Avatar
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    Loose wire issue?

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    Six_Shooter, I am not sure what DRAC/VSSB is, will have to do some research. No one has messed with it.
    EagleMark, if there is a loose wire, I have not found it. Tried to check everything.
    Is there anyway of checking the vss sensor, other than replace it on a hope?

  5. #5
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    I assume everything runs into the pcm, and signals go from there?
    Meaning, vss goes to pcm, then to speedometer, and cruise control?

  6. #6
    Super Moderator Six_Shooter's Avatar
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    No, VSS goes to the DRAC/VSSB then out from there to the PCM, cluster, cruise, etc.
    The man who says something is impossible, is usually interrupted by the man doing it.

  7. #7
    Fuel Injected! JeepsAndGuns's Avatar
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    Is it a magnetic pickup vss? I have seen buildup on those type cause flunky readings. Since they are magnetic, they attract metal filings that normally float around in the oil. After a long time they can build up into a sludge and block the readings from the tone wheel. Might try pulling it out and checking/cleaning it.
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    ABS wheel speed sensors have the same problem. i wouldn't be surprised by that being the case here too.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  9. #9
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    Quote Originally Posted by RobertISaar View Post
    ABS wheel speed sensors have the same problem. i wouldn't be surprised by that being the case here too.
    Not to argue, but the ABS shouldnt have any affect on the speedo in this situation. The abs gets its signal from the DRAC/VSSB.

    Does it bounce or move at al below 15mph? Might look at the aforementioned suggestion and check the speed sensor for excess debrisbuilt up but havent seen it cause such a problem in the 20 years Ive been fixing transmissions. Have seen instrument clusters fail at low speeds, high speeds, bounce all over the place for no apparent reason. The DRAC is either gonna work or it isnt so I would rule that out short of a wiring issue. If you have had the pcm out, added any stereo equipment, replaced the alternator lately, maybe you need an alternator(check for AC ripple). I have seen the connector at the sensor that the insulation has deteriorated and the wires "short " to each other.

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  10. #10
    Super Moderator Six_Shooter's Avatar
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    Robert was simply saying that a similar phenomenon happens with ABS sensors, not that it is related to this specific issue though.
    The man who says something is impossible, is usually interrupted by the man doing it.

  11. #11
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    yar. good example that comes to mind are the front wheel speed sensors on a W-body: there is a ~many tooth reluctor at the end of the axleshaft near the wheel, so small ferrous objects that pass near it stick to the sensor, eventually enough stuff can build up on it to where it no longer produces signals, or at least consistent signals and things go wonky.

    the rear speed sensors though..... they're basically sealed inside of the hub, so it doesn't happen from the same issue. if the hub started falling apart internally, that could do it.



    inside of a trans though? LOT of iron/steel parts that could either outright break or slowly wear down into a metallic powder and either end up in the filter, on the VSS or the pan magnet. maybe on the PCS/shift solenoids too? i imagine the rather fast moving ATF would flush them easily though.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  12. #12
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    Problem fixed, was the vss going bad.
    Thanks everyone for the information, tips and comments
    Greatly appreciated.

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