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Vehicle Speed Sensor Circuit Failure

Discussion in '1994-2004 V6 Mustang Tech' started by TNgineer, Jun 23, 2017.

  1. TNgineer

    TNgineer New Member

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    I have searched not only this forum but every forum, some Mustang, some Ford, some just mechanical about all cars. Google is getting tired of me I have searched so much so hopefully I can find some help in making sense of this. I have a 1998 3.8 Mustang with a rebuilt engine and rebuilt auto trans that I took on trade so I know almost nothing of its history. The speedo will slowly (like not at the same rate as the acceleration) until it hits about 30mph and then will kinda hang there a few seconds and go back down to 0. When you get to about 55-60mph the engine stumbles and sounds as if it is starving for fuel let off it runs fine until you get back up to speed. I pulled the codes and got the Vehicle Speed Sensor Circuit code which made perfect sense. I replaced both sensors on the trans(OSS and VSS) even though they both tested good and as I feared nothing changed. I noticed the front connector wires were loose and able to spin in the connector fitting so I replaced that and nothing changed. So I began to think maybe there is an intermittent short somewhere so it is some sort of limp-type reaction to save the engine so I completely unplugged the front connector (the one I replaced) and boom! No speedo, no cruise control, really high shifts (like around 4000rpm), but it runs and drives as it should with a bad sensor. I am thinking there is probably a semi-shorted wire or intermittent grounding issue so I plan to replace the grnd and pwr wires from the PCM to the sensor to eliminate whatever is going on in the wires. The problem is that there are several pink/orange wires at the PCM and I am not entirely sure which on to splice as my Mustang Manual only shows one in the wiring diagram. I cannot find a wiring routing diagram anywhere and obviously I don't intend to just cut and splice wires here and there with my fingers crossed. Does anyone have experience with this fix or even better a wiring routing diagram? I Need Help!
     
  2. TNgineer

    TNgineer New Member

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    Quick re-cap for clarity. 1998 Mustang 3.8 Auto trans. P0720 Code (Output Shaft Sensor Circuit). No speedo, no cruise control, very high shift points, and obviously terrible MPG. Replaced both sensors (OSS and VSS). Replaced OSS plug (or pigtail if you are oldschool). I plan to cut the positive and ground wire at the PCM and run new wires between it and the OSS, but I can not trace them due to being routed with many other wires wrapped in a bundle and the PCM has multiple wires of the same color. I know the Pink/Orange (powered) wire goes from the OSS to the PCM and from the PCM to the speedo/cruise control circuit, but I have counted three Pink/Orange wires at the PCM. My manual is almost worthless in terms of wiring diagrams so I need advice or even better would be diagram of the routing so I know what goes where.
     
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  3. 6 Shooter

    6 Shooter Well-Known Member

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    Send a private message with your email address and will send you wiring diagram with wire colors.
     
  4. TNgineer

    TNgineer New Member

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    Sending now. Thanks a ton.
     
  5. TNgineer

    TNgineer New Member

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    Update: I pulled the carpet and door sill on the passenger side to expose the loom and located the pink/orange stripe wire that comes from the OSS to the PCM and tested it with my multimeter and it shows 0.14 dc amps consistently when the car is on, but no power at the OSS plug itself which doesn't tell me much because as I understand it the sensor sends pulses to the PCM for the information. I checked the connections and re-connected the OSS plug. My speedometer goes up and seems to be pretty close to correctly reading speeds but drops back to 0mph when I let off the throttle as if it is rising and falling with the RPMS and as before the engine bogs and stumbles at exactly 55mph. Runs fine up to 55mph and if I let off to slow down it will run fine as soon as it is below 55mph. It is not shifting into OD, but the OD Off light doesn't come on or flash like it does when I unplug the plug from the OSS. If I unplug it the OD light flashes, no speedo, and it shifts very high and hard, but does shift through all four gears. I'm lost at this point and as it just started storming it looks like I'm done for the evening. My Haynes manual does not mention an OSS anywhere in the manual so it of no use. Basically there two sensors on our trans, one towards the front that parts stores call the output shaft sensor and the actual turbine/geared sensor towards the rear (that seems like it should be called the output shaft sensor) which is referred to as the vehicle speed sensor. Now as I am typing this I am wondering if it could be as simple as the guy who replaced the trans got the OSS and VSS plugs switched around?!? The one with the pink/orange wire is plugged into the OSS(front of trans) and the one with the green/yellow wire is plugged into the VSS(rear of trans). Any help is much appreciated.
     
  6. TNgineer

    TNgineer New Member

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    Ok Everybody feel free to have a good laugh at me. The plugs were swapped. I guess it just goes to show that we ought not overthink things. I gave the "mechanic" who installed the transmission the benefit of the doubt and told myself surely if he is competent enough to rebuild and install a transmission he knows what plug goes where and certainly would have thought to check that when he noticed his speedo, cruise control, and OD weren't working. All is well and the car and trans are fine. I guess as an engineer I tend to overthink things when I should probably sit on the porch with a beer and think simple.