2006 Mercury Milan Speedometer Sometimes goes to zero

Question

I have a 2006 Mercury Milan 4 cylinder, automatic transmission with about 142K miles. This is my daughter's car and she has seen this a few times. I have only seen it once after driving it 3-4 times. It seems to happen when it hasn't been driven for some time and when it's cold outside in Colorado.

The car will act normal, then jerk and the speedometer drops to 0. After some time it will jerk again and the speedometer will be correct. When I saw it happen the speedometer was zero after starting to drive it. It ran fine and it seemed to shift fine, even though this car has always had a rough shift. I drove it for about a mile and turned to a side street, put the car in reverse and backed up. Then got on the road again. Almost immediately after taking off the speedometer started registering normally and the car jerked. I noticed the wrench light on, but then it went off. I did a computer scan at the local auto parts store and the computer had nothing.

I'm guessing it's the output speed sensor (OSS).

1. Does that sound like the problem? Is it common for the OSS to work sometimes but not other times, especially not work when cold?
2. Does anyone know for sure where that sensor is located. Google has not shown me anything definitive. I think it's under the battery holder on top of the transmission but haven't take it apart yet to verify.

Thanks,
Mike

HillBilly · Answer

Yes, OSS is likely but I recall it having an input, output and speedometer sensor. First I'd do a trans filter and fluid change. Be warned tho this could accelerate trans failure...the jerk is being caused by fluid pressure changes due to tcm thinking it's at 0 speed and this could be causing damage. If your lucky a fluid change will clean up gunk/metal around the sensor reluctance ring... but the gunk metal is indicative of wear/damage that has already occurred.

First I'd get a scanner that you can read can bus and trans codes as likely there is a stored trans code the parts store scanner couldn't read (most only read P codes that AP stores use). A worthwhile investment imo.