Speedo jumping and hard shifts + ABS light...bad VSS?

[tjg8675]

Hi all,
Having an issue with my 1995 bronco 5.8l...

My speedometer is jumping and showing incorrect speed while driving slow (0-25mph)
Above 25 it seems ok. Also ABS light comes on when speedo is jumping & also shifts are hard & not at normal shift points.
I'm thinking I have a bad VSS.
Hoping to get a confirmation or any pointers in the right direction.

Thx

 

[Tiha]

Pull the VSS and inspect it. If there is a bunch of metal stuck to it, then you need to pull the rear cover and inspect. If it is clean you can try just throwing a sensor in but I have had to rebuild a few bronco rear ends because of bad carrier bearings. Hopefully you catch it before the ring and pinion are bad.

 

[tjg8675]

Hi Tiha,
Thanks for your response, I kind of was thinking along the same line as you about some metal shavings on the vss, hopefully it's not though crossing my fingers...

 

[L\Bronco]

Hi Tiha,
Thanks for your response, I kind of was thinking along the same line as you about some metal shavings on the vss, hopefully it's not though crossing my fingers...

While you are down there checking the sensor, have a look at the harness in the left shock mount area. they used to rub through over there. An intermittent connection can give you a similar symptom.
Also, if there has been a repair in the past, check to be sure the harness from the vss is still twisted together. This is to cancel out EMI (electromagnetic interference) from the sensor signal. It needs to be at least 9 twists per foot or signal noise will occur. (Usually at low speeds)
I had one years ago that a guy spliced in a piece of extension cord for the repair. It
looked pretty, but at 15mph the vss jumped momentarily to 24,000 mph and caused a **** in the speedo and violent upshift.
Hope that helps
Cheers

 

[Motech]

Bad diodes in alternator can do this too. It creates a lot of RFI, and the excess AC voltage that goes everywhere can especially wreak havoc with analog sensors like your VSS and Wheel Speed Sensors (which your VSS does double-duty as)

If you strike out with above sage advice, try disconnecting your voltage regulator and take a quick run up the street and back. If all good, replace alternator.

 

[miesk5]

Yo tjg8675,
As Tiha advised;
"Removal

1. Pull the wiring harness connector from the vehicle speed sensor (VSS) (9E731).

Vss Location:

VSS is atop 8.8 Rear Differential

1. Remove the vehicle speed sensor hold-down bolt and remove the vehicle speed sensor from the axle housing.

Installation

1. Clean the axle mounting surface. Use care to prevent dirt from entering the axle housing.

1. Inspect and clean the magnetized vehicle speed sensor pole piece to make sure it is free from loose metal particles which could cause erratic system operation. Replace the vehicle speed sensor O-ring.

1. Lightly lubricate the vehicle speed sensor O-ring with motor oil, align the vehicle speed sensor bolt hole and install. Do not apply force to the plastic vehicle speed sensor connector. The vehicle speed sensor ****** should slide to the mounting surface. This will make sure the air gap setting is between 0.127-1.14mm (0.005-0.045 inches).

1. Install the vehicle speed sensor hold-down bolt and tighten to 34-40 Nm (25-30 lb-ft).

1. 
   
2. Push the connector on the vehicle speed sensor.
   
   

   Item	Part Number	Description
   1	390356	Vehicle Speed Sensor Hold-Down Bolt
   2	9E731	Vehicle Speed Sensor
   3	—	Connector Seal (Part of 2L373)
   4	NBR017	O-Ring
   A	—	Tighten to 34-40 Nm (25-30 Lb-Ft)

●
ALSO:
TSB 96-21-11 Programmable Speedometer/Odometer Module Pointer Waver
Publication Date: OCTOBER 7, 1996
LIGHT TRUCK: 1992-1996 AEROSTAR, BRONCO, ECONOLINE, F-150-350 SERIES
ISSUE: The speedometer needle may waver and/or a light surge may occur on some vehicles when speed control is used at highway speeds between 80-113 km/h (50-70 mph). This may be due to slight dents/chips in either the exciter ring or the Vehicle Speed Sensor (VSS) and air gaps between the VSS and the exciter ring.

ACTION: Replace the Programmable Speedometer/Odometer Module (PSOM) if required. New PSOM's come with increased immunity to system variability. Refer to the following Diagnostic Procedure for details.

DIAGNOSTIC PROCEDURE
BRONCO/F-SERIES - Perform normal PSOM diagnostics per Pinpoint Tests "H" and "J" of the 1996 F-Series/Bronco Body/Chassis Service Manual, Pages 13-01-30 through 13-01-32 and Pinpoint "B" of the 1996 Powertrain/Drivetrain Service Manual, Page 10-03-11.
ECONOLINE - Perform normal PSOM diagnostics per Pinpoint Tests "H" and "J" of the 1996 Econoline Body/Chassis Service Manual, Pages 13-01-29 through 13-01-31 and Pinpoint "B" of the 1996 Powertrain/Drivetrain Service Manual, Page 10-03-9.
AEROSTAR - Perform normal PSOM diagnostics per Pinpoint Test "K" of the 1994 Aerostar Body/Chassis Service Manual, Page 13-01B-31.

* Any slight dents, chips, etc., in either the exciter ring or VSS will create needle waver. Measure air gap between the VSS and exciter ring. It should be 0.38-0.51mm (0.015-0.020").
* Check exciter ring runout per the same ring gear backface runout procedure of the appropriate model 1996 Powertrain/Drivetrain Service Manual, Section 05-00.
* Make sure the exciter ring is mounted correctly to the ring gear. If runout is more than 0.1mm (0.004"), perform the differential runout check per the procedure in the appropriate model 1996 Powertrain/Drivetrain Service Manual, Section 05-02A or 05-02D, to find cause and repair as needed.
* If all items listed above check good, replace the Instrument Cluster Assembly. Obtain the correct service part number from the Parts Catalogue and then contact the Electronic Odometer Exchange Center at (800) 259-9700 for U.S. Dealers and (800) 663-9974 for Canadian Dealers.
NOTE: DEALERSHIP MUST TELL THE ODOMETER EXCHANGE CENTER THAT YOU NEED A PSOM3 LEVEL CLUSTER FOR A SPEEDOMETER NEEDLE WAVER CONCERN .

OTHER APPLICABLE ARTICLES: NONE
WARRANTY STATUS: Eligible Under The Provisions Of Bumper To Bumper Warranty Coverage

OPERATION DESCRIPTION TIME
962111A Replace Programmable Speedometer/Odometer Module (PSOM) - F-Series, Bronco 0.6 Hr.
962111B Replace Programmable Speedometer/Odometer Module (PSOM) - Econoline 0.7 Hr.
962111C Replace Programmable Speedometer/Odometer Module (PSOM) - Aerostar 0.8 Hr.
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