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Thread: '94 Vandura: where is the speed/ABS sensor located?

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  1. #1
    Join Date
    Oct 2005
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    Seattle, WA
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    12

    Default '94 Vandura: where is the speed/ABS sensor located?

    Hi, new member here. I've been messing around with diesels for many years now but so far have only dealt with small VWs and big Ford/Navistars. Needed a hauler so finally broke down and bought a 6.5L van since the GM motor's fuel economy is much better than the Ford's.

    My '94 Vandura runs and works great except for one issue. Increasingly often, something goes wrong with the electrical system involved with reading rear wheel speeds, I suspect. This causes a whole bunch of nasty problems in a lot of different places: speedometer, braking, and transmission. When the problem happens, the ABS light comes on and I notice the speedometer is staying on 0 even when moving. At the same time the truck bangs into 1st gear and refuses to shift out of it (makes sense, it thinks the car is going 0 mph...!). Last of all, it really messes with the braking system. It becomes impossible to modulate, the pedal is like an on-off switch. If you press it, the rear wheels will suddenly lock up and skid.

    Now, every once in a while, it will suddenly work fine: I'll have a working speedo, nice smooth brakes, and a properly shifting tranny. But then it will slip back into its bad mode again and I'll be stuck doing 20 mph everywhere I go and hoping I don't have to brake.

    I am suspecting an electrical connection issue. I got under the van today to try to find where the speed sensor(s) might be, but couldn't track it/them down. Didn't seem to be anything on the rear axle or hubs. Is it on the trans tailshaft? Or am I looking in the wrong place entirely? I am guessing either a frayed/broken wire, a bad plug connection, or some kind of grounding issue or maybe even just a bad sensor.

    Anyone have any experience with this kind of issue, or any guesses where that sensor might be?

    Thanks!
    '94 GMC Vandura 3500 6.5L, 189k
    '85 Volvo 740 TD wagon, D24T/M46, all original, 194k
    '81 VW Rabbit diesel, 1.6L, 52hp.... 244k
    '89 Volvo 740 wagon, 5.0L Mustang HO V8, 5-speed sleeper... gone but not forgotten

  2. #2
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,573

    Arrow

    Sounds like you have an issue with the VSSB (Vehicle Speed Sensor Buffer) or the VSS. The VSS is located on the rear housing of the tranny (the rear ABS isn't biased). I don't know where the VSSB is on a van, but likely in the dash somewhere. A poor ground (or any poor connection, for that matter) can cause what you are seeing.

    (advice)
    Tie a string to the ABS fuse and tape/tie it where you can reach it, so you can pull it in a hurry when your speedo goes to zero. Could be helpful if you have to brake unexpectedly. At least until you get the issue squared away.
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  3. #3
    Join Date
    Jan 2001
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    New Hampshire - Live Free or Die
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    6,057

    Default

    The trans problem is the give-a-way. (Confirm it is second gear, not first. If you can get over 30 mph it's second).

    Replace the ignition switch.
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC, John Deere X758
    New ride: MB GLS450 - most stately
    Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510

  4. #4
    Join Date
    Apr 2001
    Location
    Newberg Oregon
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    Default

    The switch could be an issue but I am more suspect of the output speed sensor.
    This is on the tailshaft of the tranny. Its held in with a bolt usually.

    The issue could be the sensor proper or the plug may have dirty connections.

    A scanner that can check stuff real time would be very helpful.

    The sensor is a "Hall effect" unit and simply generates current based on the speed that the reluctor (Cog wheel inside) passes the sensor.

    If the sensor has a short or an open the condition you describe will occur.

    Get the codes pulled and see what shows before spending much time beating your head on the wall.

    A real time scan that catches the beast in the act will make the fix easy.

    Buttttt this sure sounds like an output shaft sensor issue.

    Best

    Robyn
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2005 Suburban (Papa Smurf)
    THIS IS BOW TIE COUNTRY

  5. #5
    Join Date
    Jan 2001
    Location
    New Hampshire - Live Free or Die
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    6,057

    Default

    Put a volt meter on the "trans" fuse. If the power is steady from initial key "on" then the switch is likely NOT the problem. If it's flakey, it likely is the problem. Very common. This circuit is live any time the key is out of the "lock" position and controls the trans, speed sensor and speedometer.

    Also, if the gear indicator "light" flickers...
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC, John Deere X758
    New ride: MB GLS450 - most stately
    Gone but not forgotten: '87 F350 7.3, '93 C2500 6.5, '95 K2500 6.5, '06 K2500HD 6.6, '90 MB 350SDL, Kubota 7510

  6. #6
    Join Date
    Oct 2005
    Location
    Seattle, WA
    Posts
    12

    Default

    Wow what a great place. Thanks so much for all the replies.

    It is definitely first gear that it is dropping into -- I have pulled the gearshift down into "1" and verified that. It could probably get above 30 in 1st, I think, but not without scaring everyone within 50 feet from all the diesel roar...

    Sounds like I need to get out the DVOM and do some probing, which will probably happen this weekend once I get some of the snow that's falling today cleared away. As far as checking codes, is there a way to do it by reading the flashes of the check engine light, like on a Ford, or do I need to get a scan tool? (Sorry for the novice questions...)

    In searching the forums, I found some ABS-related threads. One of them had some advice that I thought might pertain:

    Pretty common issue and easy to fix. The speed sensor is held into the hub assy with a single bolt. What happens is rust forms between the hold down tab and the hub essentially "heaving" the sensor out of the hole just far enough that in low speed maneuvering it loses signal. Pull the sensors, sand clean, smear the surface with dielectric grease and reinstall.
    Any thoughts, is this pertinent or not? This was on a '95 Tahoe so different vehicle, maybe the mounting of the VSS is different too.

    I really appreciate all the help!
    '94 GMC Vandura 3500 6.5L, 189k
    '85 Volvo 740 TD wagon, D24T/M46, all original, 194k
    '81 VW Rabbit diesel, 1.6L, 52hp.... 244k
    '89 Volvo 740 wagon, 5.0L Mustang HO V8, 5-speed sleeper... gone but not forgotten

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