Speed versus reference sensors
 

I was wondering about the sensors. From what i have read it appears that these sensors are identical. Is this correct?
If I switch the connectors to the sensors, there should be no difference?????
When I switch the connections, I cannot ge the car to start at all. With them hooked up in the original configurtion, the car will start and run for approximately 10 to 15 seconds.
I put a new DME relay in, I have installed a new fuel pump and I still cannot get the car to run longer than 15 seconds before it shuts down.
Any suggestions??

I was able to remove both sensors and the looked good. What is it on the flywheel the these sense. As I was putting the sensors back into the car, my extension picked up a magnet that must have been sitting on top of the motor somewhere around the sensor bracket. This magnet is 1 9/16" long and 5/16" diameter.

The engine speed sensor picks up the teeth on the flywheel (as used by the starter). The speed sensor generates a pulse as each of the 130 teeth on the flywheel pass it. The reference sensor picks up an iron set screw in the flywheel. It generates a pulse once per revolution. Between the two signals, the DME can figure out exactly where in a revolution the engine is at any given time.

The magnet you picked up may be messing with the signals from the sensors, but it would have to be pretty close to the sensor's flat end (a few mm) unless it's very powerful.

The car will absolutely not run without the signal from the speed sensor. It can (but might not) run without the reference sensor signal, albeit very poorly since the ignition timing will be way off.

The sensors themselves are identical and interchangeable, you just need to make sure that the forward sensor is plugged into the reference sensor connector (BG) and the rearward sensor is plugged into the speed sensor connector (DG) If your harness connector on the back of the cam housing is unmolested, the reference sensor connector should be the one on top.

You can check the resistance of the sensors to tell you if there is an open or short in the circuit (Clark's has the procedure and the values your looking for), but the only way you can really tell if they are working is with an oscilloscope hooked up.

I swapped the sensors(reference to speed, not with new ones)and still the car will not run. It will start and run for a few second and then shut off. I replaced the DME relay and fuel pump. I have checked and have 12 volts to the coil and the fuel injectors. Not sure where to go from here. This car has been my daily driver for the last year. It ran great, with an occasional miss in the engine. Last week, It just died on me. It would start and run for about 15 seconds and then shut down.

Check the connection to the AFM. Without a signal from it the engine will run for ~10 seconds then stall. Does it die instantly if you give it some gas? If it does, then I'm 90% sure that your AFM or the connection to it is the culprit.

When you say check the connection to the AFM, not sure what you mean. Are you talking about the actual connector at the AFM

Here is a picture of the sensors thanks to Porsche; all right reserved.

http://forums.pelicanparts.com/uploa...1399938817.jpg

What is the flywheel made of? In my experience on working on cars for the last 35 years, most flywheels are made of iron. What is it about the set screw that make it different from the rest of the flywheel that would allow the reference sensor to pick up a signal from it? Since finding this magnet, the car will now start and immediately shot off. Since the original post, I have installed a used DME which i purchased online and installed a remanufactured
Air flow sensor. Neither of these has made any difference in the problem,

Power decreases with the distance squared. The stud is adjust to pass .08mm from the sensor - producing quite the signal compared to the flywheel which depending on after/market stock, can be quite the way away.

The set-screw trigger on the flywheel is higher than the flywheel. So when the flywheel is spinning past the sensor, it's about 5mm away. Then when the set-screw moves past the sensor, it's only 0.8-1.0mm away. This causes an electrical impulse to be generated on the sensor's coil.

Don't swap parts in troubleshooting. Get a scope and voltmeter and flow each step in the "DME/KLR Test Plan" and verify each component is working properly by measuring it's output voltages and waveforms. Don't skip steps, go in order and the problem will present itself along the way.

Never did figure out where that small magnet came from but, I got my 944 back on the road. After replacing the DME relay, DME, AFM, fuel pump and coil, my car would still start and run for about 15 seconds and then die.I discovered that if I sprayed starting fluid into my air intake that the car would run as longs as I kept spraying the fluid. I rented a fuel pressure gauge from Auto Zone to test fuel pressure. Had to do get inventive because the kit did not have an adapter that fit on the end of the fuel rail where I was checking the pressure. I used a piece of rubber tubing and hose clampsto hook up the gauge. Unfortunately as soon as I started the car the tubing ruptured (probably should have used fuel rated tubing). This led me to believe that indeed, I probably had to much pressure. I finally ran across a posting on Clark's Garage that stated I disconnected one of my injectors and sure enough, the car would stay running. I was able to pick up a new FPR from AutoZone that day and after some difficulty removing the old one from the fuel rail, I installed the new one. Car started and ran great.
Does anybody need a good used DME? After getting the car running, I plugged the old DME in and it still runs fine, so I don't need the one I bought.