dean1484
dean1484 UltraDork
6/2/12 6:21 p.m.

Figured I would ask this here as there are some really REALLY smart MS people on this board.

I was setting up my MS2V3 today. I was making the final calibrations to the Temp sensors (IAT - Intake Air Temp, and the CLT - Coolant Temp).

I am bench testing the MS through a engine harness that is plugged into a gutted DME (ecu) and then I have the MS wired into the pins in the gutted DME.

The IAT is a standard GM unit as I am going to get rid of the AFM and go with a MAP setup. The CLT is the stock OE Porsche unit.

In the settings to calibrate them you have a Bias setting (that I assume just scales things) and then three settings that are temp versus Ohms that creates the surveyor graph for the MS to pull values from.

Anyway when I was testing things I was getting all kinds of weirdness with the CLT sensor. I eventually determined that there was a bad connection in the harness at the sensor. I took the rubber boot of the plug and found a resistor in line. I soldered things up and things seemed to work ok EXCEPT I was getting temp readings that were about 10 degrees to low as compared to the IAT. So I got out my trusty multimeter and hooked up the temperature probe and set that between the two sensors on the bench and it was reading much closer to the IAT About 2 degrees less then the IAT. I then re wired the CLT bypassing the resistor that is in the harness and the temp for the CLT were spot on with the Multimeter.

SO this got me thinking.

I am betting that some one messed with the harness at some point and put the resistor in line to fake out the stock ECU to think it was colder and this resulted in more fuel being added. Being I got this harness from a fellow 944 racer that I use to compete against and is probably one of the smartest Porsche people I know. It would not surprise me that he did this as this was a spare harness from one of his race cars. Or is is stock (I doubt this) and for some reason Porsche put this in there. Porsche would not do this kind of thing they would have put the resistor in the DME. Also I don't see a resistor in the OE wiring diagrams for this circuit. I can verify this once the rain stops by looking at the wiring harness in my car.

Now the questions. Just for the fun of it I decided to play with the bias setting for the CLT in ms and I was able to drop them from 2490 to 1950. This correction caused the temps in the CLT to them match the other two temp readings I was getting. The interesting thing is I put my multimeter on the resistor to see what its Ohm rating was that was in the harness and got a reading of 540. Funny how that works. If you add 1950 and 540 you get 2490.

Thinking this was such a good idea I took a look at the IAT sensor settings and by raising the Bias setting from 2490 to 2560 it corrected the IAT the 1-2 degrees it needed to match the other two perfectly.

Ohya I forgot. The settings I got to start with came from two places. Since the IAT is a GM unit I used the settings that Turnerstudio has already in it for the GM sensor. The CLT sensor settings I got from DIY auto tune's web site. They have a P&P unit for the 944S. I checked the sensor applicability and it uses the same sensor as my car so I used there settings for the CLT that they recommend for there P&P unit.

Since I am assuming that the resistor I found in the CLT harness is not OE I am thinking that the drastic change I had to make to the CLT Bias setting will not be necessary. However the IAT sensor did need to be adjusted a little bit.

Does anyone know if the Bias adjustment in the MS is a linear sliding of the data curve up and down the scale? Or by doing this will it make the readings at the far ends of the spectrum way off? I will probably have to get a glass of ice watter and put the CLT and the temp probe from my Multimeter in it and see if they are the same. I could do the same with some boiling water as well. I was just hoping that some one would know the answer to this.

dean1484
dean1484 UltraDork
6/4/12 6:39 a.m.

No one? (Shameless bump)

MadScientistMatt
MadScientistMatt SuperDork
6/4/12 8:18 a.m.

The bias resistor is a physical component in the MS; are yours within spec? Using a false value would move the entire curve up or down, but not give as much adjustment as using the real bias resistor value and a three point measurement.

dean1484
dean1484 UltraDork
6/4/12 12:43 p.m.

Thanks Matt!!

Ya resistors are good. I figuared out that the resistor that is in line with the temp sensor is somthing that was added by the PO of the harnes so the settings I got from the DIY website for the temp sensor will be good with out me making any adjustment to the Bias. What about the GM IAT? That one only needed a small corection (about 70 on the Bias resistor setting) Should I just chalk this off to variances in the sensors since it was only about a 2 degree change. The following are what I am talking about.

MS settings

Bias Resistor setting to 2490

GM Intake Air Temperature

Temp F -----Resistance In Ohms

40 - - - - - - 100700

86 - - - - - - 2238

210 - - - - - - 177

Adjustment to bring it in line were:

Bias Resistor setting to 2560

GM Intake Air Temperature

Temp F -----Resistance In Ohms

40 - - - - - - 100700

86 - - - - - - 2238

210- - - - - - 177

MadScientistMatt
MadScientistMatt SuperDork
6/4/12 1:02 p.m.

It's probably just sensor to sensor variations. The official specs tend to have a fair amount of wiggle room.

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