Replacing 3.5 liter engine speed sensors (Chrysler 300M and others)
After ten years or a hundred thousand miles, the 3.5 liter V6 engine should still be going strong, but the speed sensors tend to need replacement. Chris Carpenter replaced the output speed sensor on a 2000 Chrysler 300M; the process for replacing it on other cars with this engine (and Pacifica) are likely similar. Most of the time, owners discover that the speed sensor is bad because the transmission will only shift into first or second gear, and the “key dance” code reading method shows code P0700 (transmission error.)
You can tell which speed sensor fails by whether the speedometer is still working. If it works, the input speed sensor is bad. Take the car to a mechanic, pay him or her $100-$120 and maybe a little extra to replace the output speed sensor at the same time, and be happy if that was the problem (Can you do it? Probably. Is it worth the risk of damaging something else at the same time? Probably not.)
If the output speed sensor is bad, the speedometer will not work either. That’s a dead giveaway that the easy one needs to be replaced — the one most people can probably handle on their own. You will need a one-inch deep socket and (obviously) a driver. While others recommend an open wrench, we did not find that to be useful.
As a side note, given that our aftermarket switch failed within three weeks, it makes sense to get the dealer part — which is not much more expensive.
First, open the hood and find the fusebox on the driver’s side (it has a black cover and is clearly marked). Here it is, above, without the cover on. Yank the transmission computer fuse (not marked above, in case it moved around from year to year, or from car to car); the fuses are usually called out in a diagram on the underside of the cover.
Next, the car needs to be raised up somehow. On the 300M, we could not figure out where to put the hydraulic jack, and ended up using the scissor jack that comes with the car; we should have used jack stands as well. Taking of the wheels, as usual, involved trying a convenient “cross” wrench first, then standing on the jack handle supplied with the car until each nut slowly gave way. (One must be very careful to get the handle on “just right,” so it does not slip off, damaging the lug nuts along the way.) Then the wheel comes off, and we have some access. (The wheel removal trick was recommended by the 300M Club.)
If you can raise the car on a lift, or drive it onto ramps, you do not need to remove the wheel at all.
In any case, you should be wearing eye protection whenever you’re underneath, to protect you from getting transmission fluid or rust or whatever in your eyes.
Once you’re under the car — upside down — locate the two speed sensors. You will notice that the input speed sensor is cleverly placed right by brake or transmission lines, and now you know why we say you should just bring it to a mechanic. Some intrepid people can still get it out, with caution and perfectly sized tools. It probably is not worth the savings to remove it. (You might want to get both replaced at once, while it’s at the mechanic anyway.)
The output speed sensor looks exactly the same, but it is much more conveniently located, away from hard lines.
Carefully remove the electrical connector, and you should be able to use a normal 1 inch (yes, one inch) deep-dish socket to remove the output speed sensor. If the O-ring does not come out with it, make sure you take it out. A little transmission fluid will come out too, but not much.
Next, put in the new sensor; we were told, “make it finger tight, then turn it two complete turns.” However, you may want to check the service manual for the correct torque specifications — most likely in inch-pounds rather than foot-pounds. The sensor body is plastic, most likely to save weight and money, and to make it easier to remove when it goes bad.
At this point, reinstall the fuse you took out, and the car should be able to hit third and fourth gear again. The process should take around fifteen minutes; it took us around one hour. An experienced mechanic with a lift can probably do it in around five or ten minutes. The approximate mechanic price in 2012, when we did this, was $100.