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

[Robyn]

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

[JohnC]

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...

[v8volvo]

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!

[DmaxMaverick]

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).

[a5150nut]

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!

[JohnC]

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.

[v8volvo]

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!

[JohnC]

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

[v8volvo]

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.