Back me up, here.

[Bradical]

Alright, so my 924S developed this issue a few months ago. I was idling in line to start an AutoX and the engine died, immediately restarted. I ignored it.

Then a few weekends later, it died on the freeway. Again, started it up.

I swapped out the DME relay and also swapped DMEs. I keep spares of everything. It went away- and then, it stalled about 5 houses down the next weekend. Restarted immediately.

Well, I had a great autoX last weekend with absolutely no issues, but I was pretty exhausted when I got home so I pulled into the garage and shut the lights off. This weekend I decided to roll it out and wash off all the concrete dust I accumulated and she died AGAIN in the driveway, again- immediate restart.

I pulled back into the garage and DEAD again! This time, NO RESTART. I have to admit I was thrilled when this happened because I could finally troubleshoot it... AND, I was at home with all my tools, so thats a plus.

Anyway, I went through everything. I had no fuel, so I assumed that this was because of the DME sequence. I jumped the DME relay and the fuel pump ran and built 36psi. No tach bounce! I swapped DME relays, no luck. I swapped DME's... No luck. I don't have an oscilloscope because I'm not an amateur astrophysicist or anything, so all I could do is check resistance on the sensors and they both ohm'd at about 900 Ohms. Damn.

The DME wasn't telling the fuel pump to run or the injectors to fire. I tried 3 different DME and DME relay combos... It had to be the wiring or the sensors, but the connectors look totally fine. I got online and ordered 3 new sensors. (I love spares)

Went outside to pick my tools up and tried one last time and it STARTED. The PO put in FTE sensors during a clutch replacement. I am convinced it's the speed sensor- it would explain the random shutoffs if the sensor was acting quirky.

So, back me up. Do the experts agree? I have tach bounce now. It restarted 4-5 times now. Loss of that signal will kill the car dead via the DME. It's got to be the speed sensor.


I should probably say that working on this car is my favorite hobby- I am always itching to find problems and I have replaced nearly everything on it- the fuel system is entirely brand new from the pump & filter, to the lines, to the damper and FPR, the injectors. I am running an FTech9 solid-state DME relay. The engine bay is nearly immaculate. All of the vacuum lines have been replaced, the battery is brand new and the cables and connectors as well. The ignition switch, switch wiring and electrical has all been replaced. It's almost comical.

[Rasta Monsta]

Ignition switch?

[Pauld_94S2]

Try unplugging the speed and reference sensors, and then plugging them back together.
While they are apart, check the pins and sockets for corrosion or dirt.

Ohming out the sensors was the right place to start. The next time it fails try ohming them from the DME. If you have a bad connector or cable that will show the problem.

[v2rocket_aka944]

Watch this and test your sensor output...

https://www.youtube.com/watch?v=zeFOOcLSk60

Fuel pump doesn't run at key on, just when cranking over and engine running.
Unless your Ftech DME relay has a "prime" function?

[jhowell371]

Speed/reference sensor going bad/bad connections. 944online sells a new set of connections if yours are crumbling as they do sometimes.

[Bradical]

Originally Posted by Pauld_94S2

Try unplugging the speed and reference sensors, and then plugging them back together.
While they are apart, check the pins and sockets for corrosion or dirt.

Ohming out the sensors was the right place to start. The next time it fails try ohming them from the DME. If you have a bad connector or cable that will show the problem.

I did ohm them from both the sensor and the DME plug actually. It's most definitely the speed sensor failing me. The connectors look excellent. I'll probably order those replacement plugs and do them anyway.

[Bradical]

Originally Posted by v2rocket_aka944

Watch this and test your sensor output...

https://www.youtube.com/watch?v=zeFOOcLSk60

Fuel pump doesn't run at key on, just when cranking over and engine running.
Unless your Ftech DME relay has a "prime" function?

Right. I had no fuel pressure at the rail until I jumped the relay out. I had no tach bounce either- this is across 2 DME relays and 2 DMEs. This thread is serving to boost my confidence that the speed sensor should be replaced, which is exactly what I was hoping for. Thanks!

[Bradical]

Originally Posted by jhowell371

Speed/reference sensor going bad/bad connections. 944online sells a new set of connections if yours are crumbling as they do sometimes.

I agree. Thanks for reminding me where I saw those connectors, I could not remember where to find them.

[Rasta Monsta]

Second vote for $10 ignition switch.

[Bradical]

Originally Posted by Rasta Monsta

Second vote for $10 ignition switch.

Its brand new- I wrote that in the OP. Anyway, how does the ignition switch close the circuit for the starter motor but not for the DME? Because the engine was turning, but there was no fuel delivery. I would buy that if it was randomly dying and I didnt have the no-start problem finally. Unplug your speed sensor while your car is running- its a killswitch for sure

[944 Ecology]

Follow the factory service manual. Use an oscilloscope and measure the output at the DME. No other test is definitive.

[Bradical]

Originally Posted by 944 Ecology

Follow the factory service manual. Use an oscilloscope and measure the output at the DME. No other test is definitive.

For sure. Why does it seem like so many people HAVE an oscilloscope? haha.

[v2rocket_aka944]

Originally Posted by Bradical

Its brand new- I wrote that in the OP. Anyway, how does the ignition switch close the circuit for the starter motor but not for the DME? Because the engine was turning, but there was no fuel delivery. I would buy that if it was randomly dying and I didnt have the no-start problem finally. Unplug your speed sensor while your car is running- its a killswitch for sure

Ign switch sends power to starter and (via CE panel) to DME relay.

DME relay does a sort of loop-di-loop of power flow...power goes through one coil in the DME relay, grounds in DME then powers the DME, DME reads RPM>200 and trips the 2nd coil of the relay, which activates the fuel pump.

[944 Ecology]

Originally Posted by Bradical

For sure. Why does it seem like so many people HAVE an oscilloscope? haha.

Do you want to guess or do you want to fix the problem? If you want to guess, keep trying without the information you need.

Or, take your car to a real mechanic who has the necessary tools.

[Bradical]

Originally Posted by v2rocket_aka944

Ign switch sends power to starter and (via CE panel) to DME relay.

DME relay does a sort of loop-di-loop of power flow...power goes through one coil in the DME relay, grounds in DME then powers the DME, DME reads RPM>200 and trips the 2nd coil of the relay, which activates the fuel pump.

I have 12vdc on 86. Ignition switch is good I'd say.

[Bradical]

Originally Posted by 944 Ecology

Do you want to guess or do you want to fix the problem? If you want to guess, keep trying without the information you need.

Or, take your car to a real mechanic who has the necessary tools.

Oh man, okay. I'll drive my car to a Porsche specialist and have them check my speed sensor with an oscilloscope then. It's running now that I removed the speed sensor and cleaned it. I think because of the magnetic field over time small metal particulate can accumulate on the end of the sensor or something like that. I'll ask the mechanic to verify if that's correct.

[v2rocket_aka944]

Did you try the test in the video I posted? JUst takes a volt meter, not O-scope/

[Bradical]

Originally Posted by v2rocket_aka944

Did you try the test in the video I posted? JUst takes a volt meter, not O-scope/

I did check AC volts like that, this was after I removed/cleaned the sensor though. It checked OK. Reconnected the DME harness and it started.

[Bradical]

Do'h! If I wiggle this just right, DEAD! Its the speed sensor connector. Jeeze! Do I still need a osciloscope? Hehe.

[pfarah]

Fix that cable!