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Old 09-16-2010, 12:16   #1 (permalink)
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AX4N Trans Swap -- VSS for OSS

2002 Sable, 137k

Transmission died on freeway. Otherwise, everything on car has worked fine. Found freshly rebuilt AX4N from 1996 for cheap and decided to do swap. Original equipment is 3.0 single cam (aka Vulcan) with AX4N (4F50N) trans.

Installed shifter position sensor and magnetic sensor near chain drive from 2002 trans into the 1996 box. As we know, older unit uses gear-driven VSS while newer unit uses OSS over reluctor wheel. With transmission installed, I decided to see what would happen if I spliced the VSS into the OSS circuit. Result was a car that showed zero at the speedometer and never got out of first gear. Is the circuitry incompatible enough that I fried a circuit in the PCM?

Next, I purchased a Speed-Cal from Dallas Mustang and installed it according to spec. This yielded a speedometer that would sporadically spike, but would not read consistently with vehicle movement. According to the spiking, the transmission would shift into higher gears. Pulling out the VSS while still connected and ignition on, I spun the gear by hand and noticed that the signal LED on the Speed-Cal pulsed appropriately, while the speedometer did the occasional spike. Was the box incompatible with the car? I reconnected the OSS to the wiring harness as it was originally, then holding it, moved it rapidly on and off of a large piece of steel to simulate reluctor movement. The speedo occasionally spiked. Did I fry the computer by trying to connect a VSS? I checked the voltage at the pins to the OSS and both were less than a volt, with the return side, apparently grounded enough to run the Speed-Cal circuitry. So far, so good?

Now here is the question: If the speedometer was working up to the time of transmission failure (a definite mechanical failure, as it would not go forward or reverse), is this a computer problem and should I replace the computer? Before that, what pins on the computer do the OSS wires connect to, so I can check continuity? What to do next?
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Old 09-26-2010, 07:50   #2 (permalink)
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Re: AX4N Trans Swap -- VSS for OSS

The PCM should be OK, it more than likely doesn't recognize what the trans is telling the PCM.

In this day and age it's always best to stay as close in year as possible when changing transmissions. I'm not sure about the pins but if you tinker around you should be able to get the speedo to work, it's just a matter of getting the PCM to recognize the input from the older transmission, in which case you might be euchred...
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