TheDieselPage.com Forums  
2014 - TDP's 19th Year
What's New: | Feature Articles: | Product Reviews: | Member's Area: | Subscribe:
Duramax 6600 Diesel Page | Advertiser's Section | Classified Ads | Photo Album | Diesel Books, GM Licensed T-shirts


Go Back   TheDieselPage.com Forums > The Diesel Page Member Forums > 1982-2000 C/K Truck & SUV Chassis & Drivetrain
Register FAQ Members List Photo Album Mark Forums Read

Reply
 
Thread Tools Display Modes
  #1  
Old 02-02-2009, 10:37 PM
v8volvo v8volvo is offline
Enthusiast
 
Join Date: Oct 2005
Location: Seattle, WA
Posts: 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
Reply With Quote
  #2  
Old 02-02-2009, 11:03 PM
DmaxMaverick DmaxMaverick is offline
Administrator
 
Join Date: Apr 2001
Location: CA
Posts: 10,623
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
2014 Chevy Cruze Diesel
dmaxmaverick@thedieselpage.com
Reply With Quote
  #3  
Old 02-03-2009, 07:38 AM
JohnC's Avatar
JohnC JohnC is offline
Moderator - Enemy of the State
 
Join Date: Jan 2001
Location: New Hampshire - Live Free or Die
Posts: 4,636
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.
__________________
JC - Member #1346

"The natural progress of things is for liberty to yield, and government to gain ground." - Thomas Jefferson

2006 D/A extended cab LLY supercomputer
'68 BSA Thunderbolt
'73 Triumph T150V
'04 Triumph Thunderbird Sport
'07 Yukon rolling livingroom
'93 C2500HD 6.5TD-wrecked
'95 K2500 6.5TD, '99 K1500 5.3 SB, '87 F350 7.3 , '78 D150 318 (Sold)
Reply With Quote
  #4  
Old 02-03-2009, 08:30 AM
Robyn's Avatar
Robyn Robyn is offline
Missy Good Wench ( Moderator)
 
Join Date: Apr 2001
Location: Newberg Oregon
Posts: 8,059
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) 89 K5 Blazer Silverado
(1) 95 GMC Cowgirl Cadillac hauler Too much to list here but loaded to the moon with goodies
Reply With Quote
  #5  
Old 02-03-2009, 08:44 AM
JohnC's Avatar
JohnC JohnC is offline
Moderator - Enemy of the State
 
Join Date: Jan 2001
Location: New Hampshire - Live Free or Die
Posts: 4,636
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...
__________________
JC - Member #1346

"The natural progress of things is for liberty to yield, and government to gain ground." - Thomas Jefferson

2006 D/A extended cab LLY supercomputer
'68 BSA Thunderbolt
'73 Triumph T150V
'04 Triumph Thunderbird Sport
'07 Yukon rolling livingroom
'93 C2500HD 6.5TD-wrecked
'95 K2500 6.5TD, '99 K1500 5.3 SB, '87 F350 7.3 , '78 D150 318 (Sold)
Reply With Quote
  #6  
Old 02-03-2009, 01:41 PM
v8volvo v8volvo is offline
Enthusiast
 
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:

Quote:
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
Reply With Quote
  #7  
Old 02-03-2009, 02:03 PM
DmaxMaverick DmaxMaverick is offline
Administrator
 
Join Date: Apr 2001
Location: CA
Posts: 10,623
Arrow

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
2014 Chevy Cruze Diesel
dmaxmaverick@thedieselpage.com
Reply With Quote
  #8  
Old 02-03-2009, 07:58 PM
a5150nut's Avatar
a5150nut a5150nut is offline
Enthusiast
 
Join Date: Jan 2001
Location: Knoxville,Tennessee
Posts: 1,768
Default

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!
__________________
Growing old is mandatory; growing up is optional.



A5150nut
2006 K3500 D/A
94 6.5 4x4 5spd Sold
Reply With Quote
  #9  
Old 02-04-2009, 09:59 AM
JohnC's Avatar
JohnC JohnC is offline
Moderator - Enemy of the State
 
Join Date: Jan 2001
Location: New Hampshire - Live Free or Die
Posts: 4,636
Default

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.
__________________
JC - Member #1346

"The natural progress of things is for liberty to yield, and government to gain ground." - Thomas Jefferson

2006 D/A extended cab LLY supercomputer
'68 BSA Thunderbolt
'73 Triumph T150V
'04 Triumph Thunderbird Sport
'07 Yukon rolling livingroom
'93 C2500HD 6.5TD-wrecked
'95 K2500 6.5TD, '99 K1500 5.3 SB, '87 F350 7.3 , '78 D150 318 (Sold)
Reply With Quote
  #10  
Old 02-06-2009, 12:21 PM
v8volvo v8volvo is offline
Enthusiast
 
Join Date: Oct 2005
Location: Seattle, WA
Posts: 12
Default

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
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -7. The time now is 05:36 AM.


Powered by vBulletin® Version 3.8.2
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Copyright 1996-2014 by TheDieselPage.com - All Rights Reserved