StupEd05

Ok, so today the car didnt want to start, whats new right! I was at the bank the first time, I let the engine cool and wait a little and she starts up, so Im on my way. Does it again further in town, same thing wait and shes back goin. Starts a couple times just fine. Get it home to see whats goin on and she wont start. Jump the DME Relay no diff, its fine. Pull the Fuel rail no pulsing whatsoever of the injectors. Pull the cap and plenty of fuel making it to the rail. So I know the DME is NOT getting a start signal. Resistance check the Speed and Ref Sensors, both check. All injectors are getting the 12 VDC, just no pulse. Aside from the DME being bad, what else will cause a NO START signal to the DME? Ignition Switch? Engine turns over fine....



- Aaron McClintock

'83 944 NA

StupEd05

Scratch that! Speed Sensor didnt check the resistance check, I couldnt get anything on it, the ref. was showing 1050 ohms and the "spare" sensor I had read 1050 when placed against metal, when it isnt against metal it reads nothing. So I got it switched out and am now going to see if the injectors will fire... the speed and ref. sensors are interchangeable correct? i.e. same part number???

xsboost90

this could be a bad ref sensor plug or a ground problem in your injector harness, check your grounds!

StupEd05

how would I test to see if it is a ground problem in my injector harness?

StupEd05

injectors still not firing even after replacing speed sensor... shouldnt I also get no spark if the speed and/or ref sensors are bad???

StupEd05

ok yeah, so I have no spark either....

Danno

Hook up a scope to the sensors and see what their waveforms are. Compare to miminum voltage-heights specified in the manual. The sensors are the start of EVERYTHING in the Motronic system and should be checked first. Everything down the chain, the harness, the relays, the DME, the injectors, the fuel-pump, the spark-plugs, plug-wires, distributor, coil, etc. will all appear to not work if the sensors aren't putting out the correct signal.

StupEd05

I have no scope Guess its time to get one...

seb928s

Check if the wires are connected to the fuse/relay box. I had a problem like this they were connected most of the time but the connect tip broke off one day. It drove me nuts until we found the problem.

StupEd05

Do you know which wires at the fuse box?

Also, I just tried using my multi- meter to see if I am getting any voltage output from from the ref sensor at the DME plug and Im not so sure I plugged it in right. I believe its plugs 8 and 27 so I just put the pos lead in 27 and neg lead in 8 and cranked and I didnt get anything to show up... oh yeah and I had the meter set on 200 for AC V (didnt go to 20)...

H2

I'm experiencing a similar problem in my 968. I'm interested in how this works out since I may not be getting juice to my coil. I just did a tbelt/waterpump change and remember accidently stressing the wiring bundle that goes across the front of the engine by the water pump. I wonder if I might have broken a wire or something?

Harvey

StupEd05

GOT IT! I put the very brand new sensor up as the ref. and was just NOT getting the voltage out of it that I wanted, well it turns out that the sensors HAVE to be tightened WAY down! Do they ground through the bolt or something? Well anyways, I was getting the proper resistance readings at the DME plug so I knew the wiring had to be pretty good and I decided to go ahead and tighten the ref WAY tight and boom she bounced and fired! Thanks for all the late night help! I sure didnt get any sleep! So Im thinking the speed sensor was what went bad...

Thanks Again! Im goin for a drive!

hosrom_951

The torque spec on the sensors are 8 Nm (6 ft-lb), if you nbeed to go tighter, then i suspect that you need to adjust the clearance between the sensor and flywheel.......right guys?

StupEd05

6 ft-lb? Well thats tighter than I thought it was. I dont think I got them much tighter than that, what with the clearence issues and all I was barely able to turn them more than 1/8 a turn at a time. I would suspect that tightening them down past that prolly wouldnt help the clearance much at all, it would prolly crack the plastic ear before you got too far...

PCinDC

Hosrom is correct, if you had to overtighten the speed and ref sensors to get the car to work, the interval is likely incorrect. Check Clark's Garage for a procedure on checking and adjusting the spacing. This is of paramount importance, as these are magnetic sensors that if not spaced properly will not be able to produce the correct waveform.
With an oscilloscope on pins 8 and 27, the scope should show you a sine wave with peak to peak voltage of >2 volts. It should oscillate up and down once for each tooth on the flywheel. This is your speed sensor, it tells your DME how fast the motor is turning.
With the scope on pins 25 and 26, you should see a upwards spike of 2.5 volts every time the motor passes TDC. This is your reference signal and is what the DME needs to determine ignition and injection timing.
If the DME does not see values meeting the voltage thresholds from above, it will not start.
Also, another potential area of failure would be your ignition switch. If you are not getting +12 volts to the DME (IIRC, pin 24), your ignition switch is not working and will likely cause the same problems. One caveat to this is that without the ignition switch signal, the DME can still generate and engine start signal using the signals from the reference and speed sensors.

Ask me how I know this stuff. Still working on this myself, and I have a scope.