tkacki

Hi,

In 86+ 944 / 924S there is a diagnose socket located near brake master cylinder. Some Porsche manuals claims that there is possible to connect LED to this socket. I have discovered that there are three pins used in this socket:

1. Ground (pin GND).
2. Throttle position switch input (pin VL) connected to the DME TPS input.
3. Pin T - connected to pin 12 of DME.

Analyzing internal Motronic DME schematic I have noticed that Pin 12 is DME processor output (port) buffered by the transistor based inverter and protected by resistor. Analyzing internal circuit it is clear that pin 12 on DME socket is digital signal output.

The LED should be connected like this:

+12V o------->|----------o PIN T on diagnose socket.

It looks like this is equivalent of CHECK ENGINE light.

Questions:

Does anybody know the tricky way to switch the DME to diagnose mode? In BMW (also Motronic) there is a need to push fully accelerator pedal 3 times after ignition is switched ON and LED starts to flash !

Does anybody know flash sequence codes meaning for 944/924 (there are fault tables in case of BMW or others) ?

Regards,

Tom

Ken D

Hi Tom, Danno has the fault codes and diagnostic process on his RacerX site. Here's a link:
Racer X

Choose the DME Fault Codes link about halfway down on the left under "944/951 Repair."

tkacki

Hi,

Thanks for the info. The report of my research below:

I made a testing tool (LED and connectors). The problem with my car is accidental revs oscillation. Car is 924S - non turbo. Tester is 100% properly connected.

When lambda is unplugged there is no error codes (LED is off). When I unplug the TPS switch it starts to blink but the blinking is nothing similar to 951 codes. There are 4 long blinks then a bit longer pause and again 4 long blinks (each equal) and again... Does anybody knows error codes for non turbo motronic DME ?

And here a mystery....
When lambda is pugged and engine is cold LED blinks in non-recognizable sequences. When engine is warm LED is ON. When I speed up revs it is still ON, when I close throttle it is OFF until idle is reached then it is ON again (related to CO level ?). When I unplug lambda LED is off immediately.



Need Your help with codes, testing scenarios, lambda behaviour !!!

led

Do you have a pic of the "testing tool"? It would be nice to make something up.

89magic98

Tom,

I think that the archives mentioned a procedure to bypass the idle stabilizer valve, so that you can prove that is or is not the cause of your wandering idle.

You should click on "search" then look for "wandering idle" as the search term.

The factory manual has information on Lambda sensor diagnosis, but not on the blink codes.

This page shows that your DME is a Motronic 3.1:

http://perso.wanadoo.fr/mtsa-atp/porsche.htm

This page shows that BMWs of approximately the same vintage as our Porsches used Motronic 3.1 systems:

http://www.bonnevillemotorwerks.com/dmeapps.html

And this one shows the blink codes for a BMW Motronic 3.1:

http://www.bonnevillemotorwerks.com/fault.html

I am still guessing here, but I would expect that the major motronic families of computers probably used the same diagnostic blink codes.

Kevin

89magic98

This thread provides some (conflicting?) information on whether any information is dumped to the "A" pin on the test port without modifying the DME code:

https://rennlist.com/forums/showthre...ight=dme+codes

Charlotte944

I may be way off here, but I seem to recall that blink style error codes did not show up until the '88 models.

I also believe that the "diagnostics port" on the late model cars has limited functionality, in that you can disable the Idle Stabilizer and get a visual indication of when the DME is in closed loop mixture control, but that's it.

Charlotte944

I found a Haynes Manual that has the blink codes and retrieval/reste procedures. If anyone wants a scanned copy via e-mail, let me know here:

Porsche944@carolina.rr.com

tkacki

Hi All,

Thank You for Your posts !

I want to present You the results of my research:

1. There is no blink codes / DME diagnostics in 924S/944 equipped with Motronic ML 3.1 DME, Bosch no 0 261 200 077. I am quiet sure that in this version of Motronic no diagnostic is present regardless of unit type (later models like 0 261 200 85). The diagnostics was introduced in 944S / 944 turbo. I was TOTALLY MISTAKEN considering blinks of the LED as fault codes !

2. The pin A in diagnostic socket is “Simplified CO setting” – description below.

3. The pin C in diagnostic socket is GROUND.

4. The pin B in diagnostic socket is DME’s throttle position input – grounding it results in ISV position to “middle” to allow idle adjustment.

5. Ping “+” and “LED” are not connected.

6. It is possible to completely adjust the engine (idle and CO) using this socket. The only tools are: LED lamp and RPM meter !

Diagnose tool – LED lamp. Picture below.
Simple LED diode. No resistor is necessary – it is added inside DME unit. I have examined the schematic. The anode terminal (crocodile clamp) should be attached to the positive lead of battery. The cathode terminal (screwdriver tip) should be placed in pin A of diagnose socket.

I attached the LED lamp and discovered that blinks have no sense (recognizable order) and are throttle position sensitive. When lambda socket is unplugged – LED is OFF permanently, otherwise it goes ON and OFF depending on throttle manipulation. I have checked my lambda according to the factory manual. Voltage on idle was 0.850 V. On DME side the input current was only 0.013 mA. When I manipulated the throttle voltage varied on Lambda side. I have prepared lambda voltage simulator: connected the 10 kOhm between battery terminals, set up the slider to the voltmeter and got 0.500 V on the slider (by moving the slider). Then, I connected the slider to DME (engine was running). The results:

1. When the voltage goes up form 0 V (LED is OFF) to 1 V (moving the slider), the LED lamp goes ON when passing 0.490 V.
2. When the voltage goes down form 1 V (LED is ON) to 0 V, the LED lamp goes OFF when passing 0.430 V.

Resume: The pin A is CO meter output. It switches ON/OFF around 0,450 V on lambda terminal which is Lambda ratio = 1 (see the mixture chart in factory manual). The difference of switch voltage (0,430 and 0,490) is just “insensitive zone” of the mixture regulator implemented in DME to prevent oscillating between richer / leaner modes.

So the procedure is simple: short the B and C terminals, connect LED lamp and RPM meter and set up the RPM using throttle adjusting screw and then CO adjusting the screw on the air flow meter. When adjusting CO set the screw nearest the position where the LED switches ON/OFF, but remember – RPM must be set exactly – correct in necessary as the settings must me done “together”.

Resolution for my unstable idle is just skip the lambda (disconnect) and see how car performs. If problem disappear i will replace the Lambda, if not – i will replace ISV.

Soon all the procedure will be available on my web page.

Regards,