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4EAT speed sensor failure?


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#1 dashton

dashton

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Posted 03 November 2003 - 12:52 PM

The AWD on my auto trans 1990 Legacy has never worked since I purchased it two years ago. I’ve been though the whole system – removed the extension housing and checked the clutches, seals, etc. and still nothing. This summer I finally wired in a switch that cuts out the duty C solenoid to try and force it into four wheel drive. I finally had a chance to test it this weekend with the first snow and it works great! But, my AWD is still dead. Initially I thought it was because of a stuck or bad duty C solenoid, and that the switch just forced it loose, but I’m getting nothing from the “computer controlled system” even when it’s stuck in the snow – just one front wheel sits and spins. This leads me to believe that there’s something else wrong, because as soon as I hit the 4WD switch it digs itself out and goes like crazy. (very fun, by the way)

Now here’s a little blurb I found on the endwrench.com:

"4EAT Speed Sensors
If you are in the process of diagnosing delayed or erratic shifting on a 4EAT transmission, pay close attention to the comparison of readings between speed sensor 1 and speed sensor 2 as displayed on the Select Monitor. If these sensors are working properly, the two readings should be close, but are rarely absolutely identical. If, however, in the vehicle that you are driving, you notice that the speed sensor 1 signal and the speed sensor 2 signal are always precisely identical, there may be a malfunction in the signal or circuit from one of the sensors. When the transmission computer senses a failure in one of the speed sensor circuits, it may substitute the other sensor’s reading in the display of the Select Monitor. This is why the two readings would be identical.”

Could I have a bad speed sensor that is causing me to lose the AWD? It would seem to me that if the computer substitutes the signal if the sensor isn’t working then I would lose the AWD (since it’s supposed to engage when there’s a difference in the two signals). I’ve never had a trouble code for the AT (except when I put it in 4WD, because I cut out the solenoid) I would assume if the computer thinks the VSS is bad that it should set a code.

Has anyone else had a problem with a speed sensor? Does anyone know how to check it other than take it to the dealer and have it scanned? Any help would be greatly appreciated. Thanks. Dave

#2 Legacy777

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Posted 03 November 2003 - 09:29 PM

It's possible, I would think it would throw a code or something. You can check the TCU codes yourself. Instructions are on my site.

Without a select monitor it would be difficult to tell for sure.




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