Author Topic: vehicle speed sensor (VSS)  (Read 10550 times)

Offline Lt.Del

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vehicle speed sensor (VSS)
« on: September 18, 2007, 11:53:43 AM »
I am going to get back to the intermitant problem of my brakes on the 91 4x4 3/4 ton sub.  The brake light sometimes comes on, and sometimes blinks or just stays on.  It isn't the parking brake because when i depress that, the brake light gets brighter. The brakes sometimes pulsate when i come to a stop.  It is scary sometimes because when i must brake and she starts to pulsate, it takes longer to come to a stop.  It shouldn't pulsate.

I am looking at the VSS.  If this $119.00 part was bad, would the speedometer not work?  I don't know if the VSS is just for the RWAL system or also for the speedometer.  The cruise control and speedometer work fine. 

I am going to be looking at each part of the RWAL (rear wheel anti lock) system until i find the culprit part.  It is just that these parts are so expensive.

If it isn't the VSS, perhaps the ECU or combination valve. 

I need to fix this thing becuase when it does act up (which isn't all the time), i nearly hit cars in front of me when simply slowing down at a traffic light.  The dang pulsating starts and the stopping distance all of the sudden doubles. I hear that tell-tale sign also like a grinding noise when it does pulsate.

Does the VSS also control the speedo and cruise control?  If the answer is yes, it can't be the VSS since those other items work perfectly.

http://www.autozone.com/servlet/UiBroker?ForwardPage=az/cds/en_us/0900823d/80/08/6f/84/0900823d80086f84.jsp




 
« Last Edit: September 18, 2007, 11:56:53 AM by SgtDel »

Offline Captkaos

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Re: vehicle speed sensor (VSS)
« Reply #1 on: September 18, 2007, 01:52:42 PM »
The truck would throw codes if the VSS was bad the ECM uses that as an input.  The speedo and cruise wouldn't work either.

Offline VileZambonie

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Re: vehicle speed sensor (VSS)
« Reply #2 on: September 18, 2007, 03:18:06 PM »
Sounds like either a broken/cracked ABS tone ring or faulty ABS wheel speed input signal. In the meantime until you find the problem simply pop the ABS relay out.
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Offline Lt.Del

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Re: vehicle speed sensor (VSS)
« Reply #3 on: September 19, 2007, 11:58:50 AM »
So if the VSS was malfuntioning, the check engine light would come on?   That idiot light (service engine soon) does not come on after initial startup.

I am wondering if i have a mechanicl speedo (i guess i could get up under it and find out). 


Quote
ABS wheel speed input signal
Vile, is'nt that what the VSS is and does?  Or, do I have other sensors at the wheels?



Quote
ABS tone ring
Where might i find that?
 
I would just unhook the wires on the combination valve so it wont pulsate, but, the idiot light stays on then.

Offline Lt.Del

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Re: vehicle speed sensor (VSS)
« Reply #4 on: September 19, 2007, 07:22:50 PM »
Apparently the VSS also controls the cruise control.  So, the VSS must work fine--cruise and speedo work just fine.  I found this.....


"...On GM trucks, the speed sensor signal first passes through an intermediate module on its way to the antilock controller. This module is called the "Digital Ratio Adapter Controller" or "DRAC." The DRAC module translates the analog speed sensor signal into a digital signal that can be processed by the ABS control module. The DRAC module also divides the basic speed sensor signal into three separate frequencies of signals that are used by other vehicle systems:

A 4000 pulse per mile frequency that is used by the cruise control system and electronic instrument cluster on some vehicles.
A 2000 pulse per mile frequency that is used by the engine control module for various emission and torque converter lockup functions.
And a 128,000 pulse per mile frequency that's used by the ABS controller.
The DRAC module is calibrated to the final drive ratio and original equipment tire size of the vehicle. Replacing the original tires with ones of a different size or aspect ratio will change the speed sensor signal, which in turn can adversely affect the operation of the ABS system as well as torque converter lockup and the accuracy of the speedometer and odometer..."





"...In GM applications, the brake warning light will be brightest when grounded by the parking brake, a little dimmer when grounded by the hydraulic system and dimmest when grounded by the ABS controller.

To determine the cause of a red brake warning light in a GM application:

Set the parking brake. If the light gets brighter, the problem is not the parking brake.
Unplug the connector from the combination valve by the master cylinder. If the light remains on, the fault is in the ABS system and further diagnosis is required..."

I am thinking more of the ABS module.  I'll bleed the entire system, get rid of the moisture, reset the computer (unhook battery for 5 seconds) then see what i got.  Then look at getting a new module, we'll see.

http://www.aa1car.com/library/abs_kelseyhayes_rwal.htm








Offline Captkaos

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Re: vehicle speed sensor (VSS)
« Reply #5 on: September 19, 2007, 09:37:07 PM »
You must have missed the end of my post
Quote
The speedo and cruise wouldn't work either.

Offline Lt.Del

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Re: vehicle speed sensor (VSS)
« Reply #6 on: September 19, 2007, 11:20:55 PM »
At least I believe I can eliminate that...however, when the brake light blinks, it blinks 5 short times and then one long time.  According to the limited info i have read, that points to an inconsistent VSS signal.  But, again, cruise and speedo work.

I'll figure this thing out if it drives me to the grave (pardon the pun--brake problem, driving.... ;D). 

I think my next course of action is the ABS module.  $70....i guess it could be reading the VSS signal incorrectly from the DRAC because the module could be fried, therefore giving me the signal for a faulty VSS.  I don't believe the DRAC is bad because the cruise control and torque converter lock up get their signal from the DRAC as well.  The converter locks up just fine. 

 
« Last Edit: September 19, 2007, 11:32:08 PM by SgtDel »

Offline DnStClr

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Re: vehicle speed sensor (VSS)
« Reply #7 on: September 20, 2007, 12:16:45 PM »
Sgt. Del, Any chance that you have a bad brake rotor that's causing the pulsation?
 on your abs-I think each wheel has a speed sensor, but the controller is located at the rear right wheel. Doesnt it take special tools to work on the abs?
Don
87 Chevy Silverado

Offline VileZambonie

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Re: vehicle speed sensor (VSS)
« Reply #8 on: September 20, 2007, 02:08:19 PM »
Jump pins A and H in the DLC and see what you get for DTC's.

Did you check and adjust your rear brakes? Make sure the primary shoe is forward on both sides, all hardware looks ok
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Offline Lt.Del

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Re: vehicle speed sensor (VSS)
« Reply #9 on: September 20, 2007, 08:57:24 PM »
Thanks for the input guys.

Don, I don’t believe it is warped front rotors—I have had those before and replaced them last year.  This is definitely pedal pulsations along with a low tone electronic clicking (for lack of a better audible description).  I don’t think special tools are required.  The module has a few plastic clips on it—easy to remove.  I just have to be careful not to touch the terminals or let brake fluid get on them.  If the
DRAC is bad, I would have to look for that—somewhere in the dash near the glove box I think.  VSS seems easy to replace.  Combination valve should be easy to if I had to replace it---just would have to ensure everything is bled correctly. 

I yanked the wire from the combination valve and the same idiot light blinks---5 short, one long blink (6). 

If something was amiss in the rear shoes, I guess it could confuse the sensors/readings going into the DRAC—but, since the cruise control and torque converter lockup feature work fine, I am shying away from speed sensors. I’m going to remove the rear wheels and drums just to inspect them—ensure the adjustment is fine and bigger shoe towards front.   I believe there is only one speed sensor—I think the ’91 GM vehicles used the Kelsey-Hayes RWAL system.  That sensor would be in the transmission with the ring in the tailshaft.



Kelsey-Hayes RWAL….




I wish this thing didn’t have RWAL---this is aggravating.  I don’t have a code reader—perhaps that will be in the near future. In the meantime, I think I will replace the ABS control module. 

Vile, i see what you mean about jumping codes...

"ABS Fault Code Access
On GM vehicles, fault codes can be retrieved manually (flash codes) by jumping terminals "A" and "H" (or grounding terminal "H") on the diagnostic connector, or you can use a scan tool such as a Tech 1 or Tech 2. On Ford and Dodge trucks, fault codes can be accessed manually by grounding the diagnostic pigtail connector on the ABS control module.

The flash code sequence for Ford, GM, Dodge and import trucks with the Kelsey-Hayes rear-wheel ABS system are the same, but the flash codes are slightly different. If you're reading flash codes, start counting from the first long flash. Then add the short flashes that follow. The total of the long flash plus any short flashes that follow equals the number of the code.

Because the system can only do one code at a time, the first code must be read out, diagnosed and repaired before the system can generate any additional codes.

Beginning in August 1990, a software change was made in the ABS controller on all General Motors RWAL equipped vehicles. The change transforms codes 6, 9 and 10 into "soft" (non-latching) codes that only illuminate the warming light as long as the fault is present. If the fault is intermittent and no longer exists, the light will go out with the next ignition cycle. The conditions under which the codes are set remains the same, as do the basic diagnostic procedures.

A scan tool must be used on the newer GM applications to read any soft (non-latching) codes that might be present because grounding the "H" terminal on the ALDL connector erases soft codes.


RWAL Fault codes
Codes for Dodge RWAL, Ford RABS and GM RWAL applications are similar but have a few differences:

1. Should not occur (replace the controller if it does)
2. Open isolation valve circuit or defective ABS module
3.  Open dump valve circuit or defective ABS module
4.  Grounded or closed RABS or RWAL valve switch
5.  Excessive actuations of dump valve during ABS braking
6.  Erratic speed sensor signal
7.  Shorted isolation valve or defective ABS module
8.  Shorter dump valve or defective ABS module
9.  High resistance or open speed sensor circuit (resistance should be 1000 to 2500 ohms)
10.  Low speed sensor resistance (Dodge & Ford only)
11.  Open speed sensor circuit (GM only)
12.  Stop lamp switch circuit (Dodge & Ford)
13.  ABS module speed processor (Dodge & Ford only)
14.  ABS module program (Dodge & Ford only)
15.  ABS module RAM memory failure (Dodge & Ford)
To manually clear a stored fault code on GM and some Dodge trucks with RWAL, remove the ABS fuse or disconnect the battery for 5 seconds. Ford RABS codes will automatically clear when the ignition is turned to the off position"



I never tried this before, we'll see....


 
« Last Edit: September 20, 2007, 09:12:30 PM by SgtDel »

Offline VileZambonie

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Re: vehicle speed sensor (VSS)
« Reply #10 on: September 20, 2007, 09:02:59 PM »
Cavity A and H in the data link connector under the dash and then observe the brake light. Yes your Rwal only has the VSS input. The primary shoe goes forward and has less meat than the secondary shoe. If a brake is locking you will feel the abs stop active. Rwal ABS sucks and was a joke. I would fix your base brakes and eliminate it if I were you but if you want to fix it it is the easiest system out there to fix. But check your DTC's and check your brakes.
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