I took some of your ideas ( thanks!) and did a little more digging... on the fence currently about wether or not to sell a packaged unit that displays a few values such as oil/coolant temp and oil pressure (digital not analog). The electronics side is easy but I need to work on the display/wiring/mounting solutions that are OEM quality. At the very least I'll build something permanent for myself since my car was purchased mainly as a track car.

In the process I did find that my oil temp sensor resistance was out of spec vs. a new one. I wonder if others have the same problem and don't know about it, as we never see oil temp.

 

William - just an idea, but I heard there there some OBD2-Bluetooth dongle in the wild that can display those values and maybe even send them to a Car stereo as an app which run on Android ...or maybe I'm just dreaming

 

Hi William, I open sourced the info so anyone can build one if they like. Hope to see you can come up with a kit 'cause I'm sure a lot of folks will be interested.

Just wondering how did you know your sensor is out of spec? Did you take it out and try to calibrate it with a precision thermometer?

Since you seem to have the motivation and skills, I would write down here what I've found so you have a better starting point.

- You can only get the oil temp from the built-in sensor two ways:
1) By tapping the voltage of the sensor then make your own circuit like I did
2) By query the DME for the actual interpreted temperature

To achieve 2), you have to connect to the K-line and init the communication and query the DME like Durametric (or other Porsche scan tools) does. Basically reverse engineer Durametric. If it can be done without a computer, that would be the holy grail!

Don't waste your time on OBDII (bluetooh or not), the oil sensor PID is not exposed to OBDII.

 

True, I had durametric reading in mind, which obviously is proprietary

 

I've edited my posts. The oil temp sensor is actually on the same K-line as the OBDII K-line but the protocol to retrieve it is different than the regular OBII protocol.

 

What about pre-2002 cars like mine, no can-bus here IFIRC

 

I don't know who started mentioning CAN-BUS for oil temp sensor but it's wrong info. CAN-BUS uses a twist-pair physical wires and K-line uses a single wire. Totally different physical layer. Our car uses CAN Bus but it's for tiptronic and PSM, etc. modules.

If you look at the wiring diagram, it's very clear regardless of MkI or MkII. The OBDII port pin 7 (K-line #1) connects directly to the Alarm Unit, which then connects to the DME directly via the "W Lead". You don't talk to the DME directly (probably a security feature). The other K-line is on pin 3 of the diagnostic port, which connects to multiple modules such as ABS/tiptronic/airbag. That's the pin Durametric interfaces with to clear the ABS/airbag lights.

A generic OBDII scanner will only connect to pin 7 so that's one of the reasons why it will never be able to reset your airbag light.

 

Has anyone tracked the difference between the calculated and actual oil temp values? Is the calculated +/- a few degrees of the actual or is it a much larger delta than this?

 

The oil temp displayed by Durametric is not a calculated value on the 996. It's a true value measured by the oil temp sensor in contact with the engine oil. The sensor is combined with the oil level sensor and it's in the middle of the engine behind the a/c compressor.

The word "calculated" means it's derived. For example, in the Torque App, it displays the intake manifold vacuum, which is calculated because our engine doesn't have a MAP sensor.

 

So is the value displayed in Torque or Harry's Lap Timer through the Bluetooth ODB II dongle the actual oil temp?

 

Again, you can't get oil temp sensor reading from a generic OBDII bluetooth dongle. Torque does not display oil temp for our car. I don't know about Harry's Lap Timer but if it does display oil temp, it cant be read by the OBDII bluetooth dongle.

 

Ok, I hadn't verified they did with my car, only that they did general. Thx Ahsai.

 

Not sure how I missed your reply for so long...

I did perform a bit of testing with a precise thermometer. Unfortunately I also destroyed a couple oil temp/level sensors in the process. As fun as this was I ended up not using any of this data other than to find out that my original sensor was reading low by 30+ degrees (calculated temp... resistance was reading much too high at a given temp.). I actually did also find out that my lab skills aren't worthless.

Instead of the test data, my voltage to temp conversion now uses the Porsche factory equations/coefficients for the oil temp NTC thermistor (reverse engineered in a different way).

Like you did, I'm currently reading voltage that the DME sees from the oil temp circuit. I tried the same VDO gauges but decided that I didn't like them that much. I'm currently working on a mount that holds a digital display and gives oil and coolant temp.

I actually don't have a working oil temp sensor currently and will resume development when I get a functional one back in. BTW the DME uses coolant temp for oil temp when there is no oil temp sensor available... Jake Raby mentioned once that if you unplugged the oil level sensor the DME still returned a value for oil temp. This is the reason why. It makes sense because they should always be relatively close in value due to the oil/water exchanger.

 

Its supposed to run about the same, but it does;t on the track. Why? Because the factory great exchanger is thermally overloaded with high RPM. I see 60-80F differentials between coolant and oil temps, and ambient temps make a huge difference too. People that believe you have to keep the engine at redline to have fun are the other issue, as all that added RPM= oil temperature. This engine has 24 valve springs, engines post 2002 have dual springs on the intake that are of interference design. This alone adds huge oil temperature increases.

The factory heat exchanger is exactly that, it isn;t designed to keep oil cool, as much as it is to heat the oil up faster, due to emissions constraints. I throw it away whenever possible, and partition the coolant cooling from the oil cooling.

 

The stock sensor has a +/-5% accuracy per its spec. I also have the resistance to temp curve for the sensor published by Porsche. Once you can derive the internal resistance used inside the DME, you can get the voltage to temp curve (sounds like you did that already).

Sensor value substitution is pretty common (when it makes sense). e.g., when the MAF is unplugged, Durametric still shows MAF values because the DME uses other sensors and engine data to derive that.