by Sam Dillinger (comments: 0)
2007 Volkswagen Touareg 3.6l 09D Trans P0715 and P0720
Today we’re going to take a look at a transmission problem a customer contacted me about for a 2007 VW Touareg with Pcodes P0715 and P0720. These Pcodes mean: Input/Turbine Speed Sensor Circuit Malfunction for the P0715, and Output Speed Sensor Circuit Malfunction for the P0720. Most of my readers that are a little familiar with auto repair will be saying “wait a minute, those sensors don’t both go bad at the same time!” While in most cases that’s true, it may not be for this application. Let’s take a closer look.
Without boring everyone to death with the technical specifics, the input and output speed sensors work in tandem to tell the TCM, Transmission Control Module, what gear the transmission is in (if you want to get more in depth information on how these sensors work together, click here). They do this by monitoring separate tone wheels and counting the teeth of the wheels as they spin by. When this system is working correctly, this will tell the TCM the exact gear ratio the transmission is currently operating in.
When either of these sensors fail, the TCM can no longer correctly calculate what gear the transmission is in at any given point in the drive cycle. The TCM does not like this at all, and will throw the transmission into “limp” mode. This can be the fault of either of the sensors failing, the wiring from the sensors to the TCM, or the TCM itself. Assuming it’s an electrical failure. What if it’s not, and how do you determine that?
First things first, checking the electrical part of the system with a scanner is relatively easy. Monitor the input and output speed sensors and drive the car just a few feet. If they’re both reading RPM, then the sensors are reading, and more importantly reporting their readings to the TCM, correctly. By default if the TCM is reading the sensors, the wiring between the two is also functioning correctly. There are a lot of variables to this scenario, but for our topic today, this is as far as we need to look. When this vehicle was test driven with the scanner, both sensors were reporting correctly. After erasing both Pcodes and test driving, both came back and the transmission was in limp. Both sensors were reading at the time the Pcodes were set.
So what gives? What made both sensors report as having a problem? Unfortunately for this customer, the answer is that the sensors were reporting an incorrect gear ratio to the TCM. This means that one, or both, of the sensors was reporting that it’s particular tone wheel was moving at a speed that the TCM didn’t like. The only way for one of the mechanical components to move faster than it’s intended speed is if one, or more in some cases, of the clutch packs is slipping. In this case, that’s exactly what was happening. The transmission was slipping due to a failed seal that caused the clutch pack to lose hydraulic pressure. This transmission was overhauled, and the customer was back on the road in short order.
The important thing here is that the customer was told to replace the sensor set first, they came to me to get some further information before spending any money. This particular case can be tricky because the input and output speed sensors for this transmission are essentially the same part. What I mean by that is they’re two separate sensors that are wired together. When you get a replacement, it comes with both sensors, and the wiring between them. So if there had been some debris that cut one of the wires, this would’ve been the correct repair. One, or both, of the sensors would have been reading 0 RPM if this were the case. This was an expensive repair, but the customer was able to save some money by not needlessly replacing the sensors.
Even though this customer had an expensive repair at the end of the diagnostics, a few minutes of time with a scanner, and a couple of test drives, showed us that the sensors weren't the culprit in this case. So for my members, before you have any major repairs done, come see me at www.mycheckenginelight.net and let’s talk about it to be sure you’re headed in the right direction!
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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.