by Sam Dillinger (comments: 0)
2001 Dodge Caravan 3.3 Liter Engine P0720
This Pcode is talking about the most dreaded system of car repair: the transmission. I have more questions about transmission related Pcodes and repairs than anything else. I even have professional shops contact me for diagnostic assistance with transmission Pcodes and other failures. The reason for this is because a competent technician is well versed in all systems of the engine, fuel system, A/C, brakes, etc. The transmission is almost a whole separate field. It’s completely different than any of the other vehicle systems because it combines hydraulics, mechanical components, as well as electrical components to operate collectively… Just to engage drive correctly! Ok that statement is slightly over dramatic. In reality the transmission uses all of those components to do pretty much everything it does across the range of it’s functions. Transmission diagnosis and repair requires a dedication to that specifically because the amount of information for that “single” component is staggering in scope. Realistically to become proficient at transmission diagnosis and repair, you have to dedicate yourself to that solely, and you have to spend a LOT of time learning. Enough about that, let’s take a look at what our customer is bringing to us today.
This Pcode is actually not too difficult to diagnose. It means “Output Speed Sensor Error”. At the surface of it, this Pcode names the output speed sensor itself as a culprit. In this case it’s the correct diagnosis that the speed sensor has failed, but why? How can you tell? First and foremost for this vehicle, and I mean this from the perspective of the driver, the output speed sensor is what tells the speedometer how fast you're actually driving. So when this sensor fails, the speedometer reads 0 no matter how fast you're driving. More often than not, that’s a dead giveaway that the sensor itself has failed. Like anything else though, there are several things that need to be checked before you actually spend any hard earned money to buy parts.
The way this system works is that there is also an input speed sensor, used in tandem by the TCM, Transmission Control Module, to determine electrically what gear the transmission is currently in. Both sensors read off of a tone wheel and relay that information to the TCM. NOTE: a tone wheel is a circular component that has notches cut into it at precise distances. The sensor sits in a fixed location and at a fixed distance from the tone wheel. The sensor is essentially a magnet that has voltage going through it. Without totally boring you on the minute details, the voltage reads one thing when at the top of a notch in the wheel, and it reads a different voltage at the bottom of the notch. The tone wheel spins at varying speeds in front of the sensor, and the sensor actually counts the teeth as they whiz by, no matter how fast they go. The TCM knows how many teeth are on the tone wheel, so it can calculate gear ratios and vehicle speed, among other things by rationalizing the speed of the tone wheels being reported by both sensors.
Now that we know a little bit about how this sensor works, we’ll look at the failure being reported. There’s an error with the output speed sensor. That can mean that the TCM is seeing the output tone wheel spin too fast, too slow, or not at all, when it compares to the input speed sensor. If the tone wheel is being reported as too fast or too slow, this is where gear ratio errors come from. It should be noted that when the output speed sensor fails, there will almost certainly be gear ratio errors reported. We’ll take a look at the other causes of gear ratio errors in a different article. If it’s being reported that the output isn’t spinning at all, that’s where this Pcode comes from in most cases. Another quick note about the output tone wheel: if it’s actually not spinning, the vehicle isn’t moving. Similarly, if there is speed detected on the output tone wheel, and the vehicle isn’t moving, there is an electrical problem in the circuit of the output speed sensor, not a mechanical failure. This can result in the speedometer reading an actual speed while you're sitting still.
So how do you accurately diagnose this failure? The same way you do with ALL transmission diagnosis- drive the vehicle and observe what it’s doing, provided you can do so safely. In this case, the speedometer wasn’t reading, and when checked with a scanner, the output speed sensor was reporting that the output tone wheel wasn’t moving even though the vehicle as being driven. At this point there is a decision to be made. You’ve either got to get some diagnostic equipment and load test the wires, or inspect the connector and wires as best you can, and then replace the sensor itself. My advice is to start with the sensor after checking the wires and the connector visually. Assuming you don’t find any corrosion or damage, the speed sensor is the most likely culprit. The only possible causes are the sensor, the TCM, and the wiring that goes between them. The sensor is by far the most common failure with the wiring being second, and the TCM being a distant third.
As always for my members, if you have any questions relating to this Pcode, if you want clarification on any of the things I’ve written here, for any other mechanical diagnosis or repair inquiries, or if you just want to say hi you know the spot www.mycheckenginelight.net
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Meet Sam Dillinger
My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.