05 Neon Tranny Problem P0700/P0720

[Chromehero]

This may get to be a long post, but if it saves someone money, it's worth it.
First, and most important, and big THANK YOU to all the contributors of this forum. The information is invaluable, and lets car drivers know they are not alone in their search for fixes to problems.

Long story short: CHECK THE WIRES!!! Apparently Dodge has used some cheap wiring in the Neon model, and the wires will break inside the insulation. Never hurts to check it, and very low cost to fix, so you have nothing to lose.

The long story:
2005 Dodge Neon SE stock. Been driving about 10k miles (got it used) and started noticing some missed gear changes and hard shifting. Soon after, the check engine light came on, and the dash reader threw a code of 0700. Learned that when the dash reader says that, it simply means 'There's something wrong in the transmission system', and you need to get an OBD2 reader to dig down into the transmission system codes to get a more precise code. Should have listened to my brother on this point. Get a reader. They're cheap, and you never know when you need that code to get the info you need. Got one on Ebay that connects to a computer by USB. $31 delivered 2nd day. It's worth it.

Before I knew about digging deeper, I assumed it was the computer. Checked with the dealer and they wanted $150 to diagnose the problem and $150 to re-flash the ECU/TCU. By the way, the 2005 model has the Engine Control Unit and the Transmission Control Unit (ECU/TCU) integrated into one module under the air cleaner and behind the radiator. Checked around and was able to get an updated ECU/TCU from http://dodgeneonpcm.com/ for around $200. Bought it, suffered with a very hard shifting Neon for a couple weeks. During that time, the car really wanted to go into Limp Mode, but if I could get the car above 30 MPH, I could shift into neutral (on a straight and level), shut off the car and restart, and the car would shift into the higher gear. Usually has to restart once per 10 mile trip to/from work, but at least my MPG was not suffering that much.Then the part arrived, and got it put in. But , no change. While installing the ECU/TCU, I checked on the input/output sensors and reseated the connectors....what could it hurt? Start it up and drive, and now the speedo is acting really weird. The tranny is shifting just as hard, and the speedometer is reading anywhere from 0 to 60, and I knew I was going no more than 30. The needle was just bouncing around the dial. Even worse now, I cannot restart on the run and get the car to shift into a higher gear. On restart, it would go into first. Very scary when I did it. Thankfully only had light traffic. Got the codes read for the transmission: 0720--Output Sensor Failure. It caught my attention.

So I start doing more research. One contributor in here (Eng3ff) noted to change the input and output sensors. Mind you the Chilton's manual I have calls them the Vehicle Speed Sensors (VSS), and *seems* to have their positions enchanged in the description and pictures.
By this point, the car is stuck in limp mode, and goes no higher than 2nd gear, but thankfully, O'reilly auto was not that far away. Got there, and had some difficulty explaining what I needed, since I was asking for Vehicle speed sensors, but what they had was Transmission Speed Sensors (Input and Output). But, for $20 each, I changed them both.

Start to drive from O'reilly, and everything is exactly the same. So no change with a new TCU, input and output sensors.

More research. Found a different forum (and I searched A LOT!!!) where one person noted to check each wire that goes to the input and output sensors. So, I dug back into the engine bay, put my original ECU/TCU back in and checked the wires. Found that the green wire going to the output sensor was broken inside the insulation. Was hard to find too. Used a multimeter and checked for continuity from the plug tips to where the two wires met the larger wiring harness. Kept moving the one lead up the wire until I got continuity--one half inch from the connector. It broke right next to the connector. Called O'reilly: they do not carry the 'pigtail' as they call it. It's a dealer only item. So, looking at the connector trying to figure out how to take it apart; it's molded plastic. In the end which plugs into the transmission, there is a small green clip which secures the connectors inside the plug. Using the lead from the multimeter, was able to pry this out (gently). Then put the multimeter lead into the round connector and gently pry up until I could get a grip on it and wiggle the connector out. After pulling the wire through the plug, I cut the wire, stripped the insulation back, opened the clip on the connector, and re-crimped the small clips around the bare wire. Put it all back together (remember to install that small green clip!!), re-taped the wires (had stripped off the factory electrical tape), re-installed the air cleaner box.
Car was as good as new. No engine code. Smooth shifting. Felt like I had beat my head against a wall for all the engine codes and problems all due to one small wire having broken inside it's insulation.

But in the end, this problem which cost me close to $300 could have been fixed with a multimeter and electrical tape. Hope to save the next guy some $$$.

[DeltaP]

Looks like you learned something here that most DIY people never get. Whenever a code is given its not always the part thats failed but the circuit. Good work!
I guess thats what keeps us in business.

[oavery]

Chrome hero, you are my hero. I had an issue with my daughters 05 neon. p0720, p1790 and p0731. I had replaced both speed sensors but the issue would not go away. Your post saved me a ton of time!! I found a broken wire right at the output speed sensor. same situation. the wire was broken inside, was not visible. fortunately I am a mechanic and have found many wires broken inside their casing. It was also right at the connector, and it was at 7 pm last night, so new end was not going to happen. I pulled connector apart and soldered wire. reassembled and problem solved. Thank you for taking the time and writing this long detailed post, I greatly appreciate it!!