Syptoms: Speedo reads 20mph in drive or reverse when stopped with foot on brake. Hard 1-2 shift. Took to a shop, they diagnosed as an output speed sensor, replaced.

4months later, same issue. I replaced the sensor myself this time. All was good for 1 mo.

Same symptoms again. I noticed the sensor was magnetic, and had collected quite a bit of metal dust on it (car has 90k). I thought this might be messing up the sensor readings. I extracted 5qt of fluid, replaced it, drove it around 50mi, extracted another 5qt of fluid, dropped pan, replaced filter, cleaned everything up really well, put it all back together.

Still doing the same thing. Was thinking I would clean the sensor one more time, but after that I am at a loss.

Couple other notes:

When I remove/replace the sensor, the car drives normal for the first 5-10 miles.

Wife said she saw the battery light flash twice, and one of the times was right when it started acting up again.

The car also has small and gross evap leak that have escalated to a GASCAP warning on the odo display. Gascap is new, I think something is cracked on the fuel pump in the tank, I guess it is common on these vehicles.

Codes are P0700 and P0720.

Any thoughts appreciated!

 

The P0700 code is a generic code indicating that something is stored in the computer relative to the transmission and you'll need to access the computer with a more sophisticated code reader to find out what else is there. A Chrysler dealer and a good trans shop would have the necessary equipment.

P0720 Output Speed Sensor Circuit Malfunction

There may be a problem in the electrical circuit associated with the output speed sensor.

Someone on the forum here might have some specific information but you'll need to at least tell what kind of car it is. There are no mind readers here.

 

Opps, my bad. It is a 2006 pacifica, 3.5 fwd, 90K on the odo.

 

I was tinkering with the Pac last night, and noticed something interesting. If you start the car with all accessories (headlights, stereo, AC) off, the 'vehicle speed' value indicated by my scanner would start @ around 25, and then quickly go to 0. This is accompanied by highish RPMs at startup (~1050rpm) that go down when the 'vehicle speed' value goes to zero.

It turns out that using some accessories - i.e. AC, headlights, or opening the door causing the interior lights to come on, would cause the 'vehicle speed' value to spike up again to 25 (sitting with the vehicle in park). When you turn off the offending accessory, the vehicle speed value returns to zero. Stuff like the stereo, driving lights, rolling up/down the windows wouldn't cause it on their own, but would cause it in a combination.

So, I will probably throw and aux ground cable on there tonight just to make sure it isn't a ground issue, and then start troubleshooting individual circuits.

The *REALLY* @%^&# frustrating thing is I took it into a shop for this problem, and they originally charged me like $120 for a 15$ sensor that I can now change myself in under 60 seconds. I then took it back 4 months later when it started having the same issue again, and they gave me the HARD SELL on ripping the transmission out to "take a look". I literally laughed at the guy when he started the conversation with "Well sir, the GOOD NEWS is you dont need a NEW transmission, but we really need to get in there and see what is going on. It will ONLY cost $600 for us to take a look".

Going up the corporate chain with that company today. They would have stuck me with a 2k$ rebuild, and I would be having the EXACT same issues now.

Sorry for the rant, but just wanted to post this up incase someone else has a P0720 that a new output speed sensor wont fix...

 

You may be on to something with that being a possible bad ground somewhere. Let us know what you find.
Unfortunately, most shops seem to want to do the most expensive fix first. The smart DIY guy looks to make the cheapest repair first and if that fails, start looking at the more expensive stuff.

 

So it turned out to be the alternator - which failed a test @ the local parts store. Turns out it was generating "excess diode ripple", and when they do that they basically put out EMI which can throw off sensors.

Grabbed a replacement from the junkyard for 40$, and the car is running fine again. Belt tensioner was a total PITA though, so now I am hoping the junkyard alternator has alot of life left, because I don't want to have to do that again any time soon!!