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Author Topic: VSS Code 29 Troubleshooting  (Read 7953 times)
El Hombre
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« on: March 19, 2011, 08:52:53 am »

VSS Code 29 Troubleshooting

Codes Definitions:

29 - Insufficient Input From Vehicle Speed Sensor -To 1992 (CM), Insufficient Input From Programmable Speedometer/Odometer Module -From 1993 (CM)

Vehicle Speed Sensor (VSS)

Background Information an Previous Misconceptions

Ever since the speed density systems came out, the VSS input signal was mistakenly given the Quote-Unquote "non-important/you can live w/out" tag. The only importance mentioned was always related to the use of the Cruise-Control system by mistake. The VSS signal was added to all Mass Air EEC-IV system EEC's and its input does have an effect for:

  • Engine idle control. The EEC uses the 6 MPH value threshold to activate idle dashpot strategy and prevent engine stalls coming to a stop and tip in when shifting.
  • VSS input is also an engine load determiner along with other inputs (TPS, MAF, etc).
  • Having a code 29 logged in the system can cause engine stalls in configurations with a manual transmission.
  • Its cruise control use has not changed.
  • Since VSS input is used to activate the dashpot function, it is normal to have a higher idle when transmission is shifted to neutral and the vehicle is above the 6 MPH threshold value. This is NOT an idle control problem (as it is erroneously tagged)... the system was designed to work as explained.

Tools Required: (in addition to the generic tools)

  • Digital VOM or DMM
  • Wiring diagrams for your year Mustang
  • Vacuum diagram for your year Mustang

Testing Procedure:

1. With ignition OFF, disconnect VSS connector. Measure the resistance between the sensor connector terminals. If the resistance is 190-250 ohms, the sensor is okay. Otherwise it needs to be replaced....if OK...

2. Check wire integrity by doing a continuity check for each wire from the VSS connector to their respective pin at the ECM (pins 3 & 6), resistance should be less than/equal to 5 ohms....not OK?...repair open wire condition....OK?....faulty ECM.

Hope this helps someone....



'86 Mustang GT, X-303 cam, Holley DP 700, RG Tran., 289 Heads   Ford Smilie 2

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