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

  1. #1
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    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
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    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
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    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
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    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
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    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
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    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

  7. #7
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    That ABS post applies to FRONT wheel sensors only. No relation to the rear, the speedo, or your problem.

    Yes on the code reading. With a '94, you can use the ALDL (Assembly Line Diagnostic Link) to read the codes. You can find the process HERE, along with a list of the codes you might see.

    Also, if you are in "limp mode", the tranny will stay in 2nd gear, if any forward gear is selected. It will not downshift to 1st, even if you select it. Unless you have 3.08 gears (doubtful) with tall tires, you can't get to 30 MPH in 1st, under power. This mode of limp mode will happen if the VSS signal is lost. The diagnostics for this circuit is not that difficult. Just ensure you have clean grounds and connections, then stay withing the circuit during your diagnostics. If you have no other symptoms, outside the intermittent loss of speedo and limp mode, the problem will be with the power or sensor return circuits (fuse, ign switch, connectors, sensor, VSSB). It could be a PCM issue, but only if all else has been eliminated. I suggest first cleaning connectors and grounds, and verify battery/alternator health. You can rule out speedo failure, as long as it isn't grounding the circuit, or feeding stray voltage back into the circuit (very rare).
    1985 Blazer 6.2
    2001 GMC 2500HD D/A
    dmaxmaverick@thedieselpage.com

  8. #8
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    When my transfer case was going south, it clogged the reluctor wheel with metal filings. Speedo problems and brake problems. You would be almost stopped then the brakes would go away. Lotsofun in stop and go traffic. Pull the sensor in the tail shaft to see if it is all fuzzy with metal filings. Just one more option that only costs time to check.

    And welcome to TDP!
    "The Constitution is not an instrument for the government to restrain the people, it is an instrument for the people to restrain the government."
    -Patrick Henry


    A5150nut
    2006 K3500 D/A
    94 6.5 4x4 5spd Sold

  9. #9
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    Quote Originally Posted by v8volvo View Post
    It is definitely first gear that it is dropping into -- I have pulled the gearshift down into "1" and verified that.

    I still bet it's 2nd. The trans is shifted electronically by solenoids. 2nd gear is all solenoids off, hence the default. Pulling the lever to 1 tells the PCM to switch to first, but if it's in limp mode already, nothing happens.

    Check the codes. If there are solenoid codes (81, 82 or 83) I'm still betting it's a power problem.
    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

  10. #10
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    Great, thanks for all the info. Sounds like I have a solid list of things to check. I don't think this will be too tough to work out. 20s today but they're predicting high of 51F on Sunday, so I'll dig it out of the garage and get busy with this stuff then. Will report back with what I find. Thanks again!
    '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

  11. #11
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    I've seen bad ignition switches that acted up more when it was cold...
    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

  12. #12
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    Default Finally back on it

    Winter held on a lot longer this year than I have ever seen before in New England. Temps are finally starting to climb out of the 30s so I'm going to get back on track diagnosing the van's trans/ABS/speedo problem. Sorry to bring this topic back from the dead...

    Let me preface this by saying that I have driven the van maybe 6 or 7 times since the last time I took on this issue, just to move it around and run a couple very short moving errands in the neighborhood (where I knew I wouldn't need more than ~20mph out of it). I'd say that it was almost exactly a 50% chance, over that period of time, of whether the speedo would work and the trans would shift. Some of the time it worked flawlessly, other times, it did not work at all. One time it start out fine, then after I got to where we were going and turned it off for 5 minutes to unload it, it stopped working right. It is hard for me to imagine what would be the determining factor in making it work or not work. There was no pattern related to temperature, humidity, time since last run, etc. Whether or not it worked was just completely unpredictable. I am about to go get it from where it is parked and bring it home to work on it, so we'll see what it does today.

    On the occasions when it does not work, no amount of fiddling with the ignition switch will make it quit acting up. Is there anything I can do to test the switch?

    I got under the truck back a few weeks ago to poke around and discovered there are many sensors and plugs towards the back of the trans. Which is the VSS one? What side of the trans is it on? Also, if this were a grounding issue in a location different than that sensor, where would the loose ground be? Just at the battery terminals, engine block etc? Sorry for my ignorance -- this is my first time dealing with an electronically-controlled automatic, and first GM vehicle.

    I guess my plan today will be to do the ALDL procedure and see what code it is that it's been throwing when that ABS light comes on, probe around for gummed-up connections at the sensor or loose/corroded grounds, and see where that takes me.

    Would appreciate any opinions or advice. Thanks again for all the help on this issue.
    '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

  13. #13
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    Also, does anyone have a link to how to check trouble codes via ALDL without having a scanner? The link that someone else posted earlier requires a registration to access the site.
    '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

  14. #14
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    Oct 2005
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    Never mind, found a good explanation of the DTC reading procedure. Will report back with findings.
    '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

  15. #15
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    Oct 2005
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    Seattle, WA
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    Default Progress

    I did a bunch of playing around with the 6.5 yesterday. Here is what I found.

    It was a little hard to figure out what was going on because the truck happened to be working perfectly yesterday. But I figured I'd do some experiments. Tried unplugging the rearmost sensor on the trans, which I figured would be the tailshaft speed sensor. I was right. When I unplugged the sensor, the truck exhibited exactly the symptoms it was doing before (no reading on the speedo, no shift out of first gear, uneven braking, and ABS light on). Plugged it back in and it worked properly again.

    It is definitely first gear, not second, that it is going into. I verified that by pulling the lever down in to "1" when the truck was working properly (ie speedo working, not experiencing its problem), and seeing how it drove, and compared that to what it did when that tailshaft sensor was unplugged and the symptoms were what they were.

    Intermittent problems are always tricky, and since it was working right yesterday I wasn't able to make much progress. The one thing I found that looked like it might be a clue was the ground strap going from the transmission (bellhousing bolt actually) to the floor of the van. The strap was very brittle and corroded. When I grabbed it, it almost fell apart in my hands. I tried detatching it and seeing if it caused the problem. It still worked OK, but yesterday was a fairly warm day. Do you think that in cold weather this could cause the problem?

    In terms of DTC's, the only one the truck threw was 63, which looks like something to do with accelerator position. Seems like not related to this issue.
    '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

  16. #16
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    Oct 2005
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    Seattle, WA
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    Sorry to bring this one back from the dead. I am about to drive the van cross-country and want to make sure I have this problem resolved beforehand. If it came back during the drive and made the trans downshift into first while doing 75 on the highway it would be bad.... I would be surprised if either the engine or the trans survived that.

    So, I cleaned the contacts in the VSS plug with QD. By disconnecting the VSS I can replicate the exact symptoms so I know the problem is somewhere in that circuit. It definitely is downshifting into first, not second. Since cleaning it, the problem has returned a couple of times. The metal ground strap going from the bellhousing to the floor of the chassis is very corroded. I ohm-checked it and it doesn't have much resistance... but that is without anything serious trying to flow through it. If that ground wire was bad, could it be causing the problem?

    Any other thoughts on this? Seems like def not ignition switch. Seems like VSS itself is OK because the problem is either there or not, not like a weird reading some of the time, just connected or not connected. I'm out of ideas.

    Thanks for 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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