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Between changing out motors and reference/speed sensors for my 951 and NA a few times, I think to much of a big deal is made of the gap.
I usually leave the figure 8 washer out, tighten the the sensor to the bracket, gently push down the bracket until the sensor makes contact with flywheel, tighten bracket in place, remove sensor, add figure 8 washer, tighten sensor to bracket…done. Who knows, maybe I’ve gotten lucky but it’s worked every time.
Jumping the DME relay does not help, the only thing that changes is, that the car will inject fuel into the cylinder when cranking while the relays is jumped, but still no spark. (with the relays in place normally there was no fuel injected)
I did buy another reference sensor (just to be sure) and ordered the original washer from Porsche. I installed and regapped the sensor using tape that I layered until I reached exactly 0.8mm and glued to the sensor. I am pretty sure I am gapping in correctly, although it is super awkward to get im hand and wrench in place back there.
Still tach is not bouncing at all and the car is not starting.
I also measured everything again according to Clarks garages instructions. Power Supply to the DME is fine, power supply to the DME relay as well. Resistance values of the sensors are in spec, measured directly and at the DME plug. The only slight deviation I could find was measuring the voltage between Pin 8 and Pin 27, I did not see >1V when cranking, more like 0.5-0.6V.
I am leaning more and more towards the DME itself, simply because everything else seems fine and it's the one thing I cannot confirm. Sadly I don't know anyone having a spare one. I will try to call the shop I had been previously to see if they have one for testing. I also cannot find a single one for sale at the moment where I live...
I did find one more thing. The reference sensor plug on the DME side has two broken wires (the insulation). When I found this I was initially because I was sure that this must be the issue but sadly I don't think it is... I checked with the multimeter for any short in between the connections and there is none, also signal arrives at the DME fine. I put a small piece of rubber between the cables regardless to make sure there is no contact between the broken wires as a temporary fix. I did not make any difference sadly. Still something I need to fix, but as it seems not the root cause.
This is not great for troubleshooting, hopefully the shop you know can let you run a test with a known-good DME but if you really want to rule it out as a problem for the future, you might want to look into these: https://www.ftech9.com/new-products/...-944-sport-dme
There is also an option that includes OBDII, but I don't know much beyond that and neither of those things is applicable to my car.
the car will not start until you have a visible tach bounce. The DME is programmed not to ground the ignition until it has a speed signal value of over 225 rpm and a crank position value
Did you time the cranking speed yet ? if its not at least 225rpm you are simply wasting your time .
I would strongly recommend that you do not mess with the DME just yet unless you can see that it has got wet .........they rarely go wrong and they are extremely expensive to replace if you damage it.
you need to test and confirm both the speed reference sensor and crank position sensor first. Also test the engine temperature sensor, throttle position sensor, and ground points
the car will not start until you have a visible tach bounce. The DME is programmed not to ground the ignition until it has a speed signal value of over 225 rpm and a crank position value
Did you time the cranking speed yet ? if its not at least 225rpm you are simply wasting your time .
I would strongly recommend that you do not mess with the DME just yet unless you can see that it has got wet .........they rarely go wrong and they are extremely expensive to replace if you damage it.
you need to test and confirm both the speed reference sensor and crank position sensor first. Also test the engine temperature sensor, throttle position sensor, and ground points
How do you determine the 225 rpm? Oscilloscope? Engine temp sensor, TP sensor can also prevent starting like this completely? I will double check ground points.
Originally Posted by 944M3
My guess is bad DME. They are 30+ old electronics. I’ve had 3 fail.
To be fair, peanut makes a good point about getting wet. I know the first one had gotten wet for sure, the second one might have gotten wet, don’t know about the third.
To be fair, peanut makes a good point about getting wet. I know the first one had gotten wet for sure, the second one might have gotten wet, don’t know about the third.
My DME did not get wet in any way as far as I can tell.
I regapped my sensors very carefully once again and now I am seeing slightly over 1V between 8 and 27 when cranking. Still no tach bounce and no start. I rechecked and cleaned the grounds at the bell housing. Coolant temp and TP sensor are connected. I am running out of ideas...
Does anyone have experience with getting the DME tested? Is that reliable or is swapping for a known working the only option? I am struggling to find one...
Specialized ECU Repair does offer a testing service. I have not used their testing service, but I did purchase a rebuilt DME ECU from them for my '85 911 and it was perfect, and they were good to deal with. No affiliation.
You may be dealing with several problems here. First time around with no start, at least part of the problem is the injectors. There should be no pooling of gas on top of a valve. Did you fix that problem? Another cold no start could be the temp sensor - actually not the temp sensor but the wiring to it. The wires under the boot of mine were shorted out. Test output at DME, not sensor, to eliminate.
Second time around with no spark/tach bounce could be DME. The most common failure is the coil driver. The solder joints fail. This may start off as no start under certain conditions, usually hot, then progress to no start. Or could just go to no start. If you search you will find pictures of the three solder joints on the coil driver that fails.
Good luck, we have all been there!
Specialized ECU Repair does offer a testing service. I have not used their testing service, but I did purchase a rebuilt DME ECU from them for my '85 911 and it was perfect, and they were good to deal with. No affiliation.
You may be dealing with several problems here. First time around with no start, at least part of the problem is the injectors. There should be no pooling of gas on top of a valve. Did you fix that problem? Another cold no start could be the temp sensor - actually not the temp sensor but the wiring to it. The wires under the boot of mine were shorted out. Test output at DME, not sensor, to eliminate.
Second time around with no spark/tach bounce could be DME. The most common failure is the coil driver. The solder joints fail. This may start off as no start under certain conditions, usually hot, then progress to no start. Or could just go to no start. If you search you will find pictures of the three solder joints on the coil driver that fails.
Good luck, we have all been there!
While the engine was out, I send out my injectors to be cleaned and tested and got them back with positive result. I think part of the problem was (before the engine damage) that I kept cranking the car with the DME relay jumped. When it's jumped it is injecting fuel into the cylinder, I dont know how much is normal? When the relay is not jumped it's not injecting nor sparking. I will double check the temp sensor, and TP sensor in terms of values again tomorrow to make sure.
I think I found an DME to test, it has the number 951.618.121.08 while mine is 951.618.121.02 (Bosch No. are identical), that should be all right?
DME Temp sensor checked at the dme plug is giving a resistance value of around 2960 ohms (pin 13 to ground) which seems appropriate giving the temperature in the garage.
TPS signal measured at the dme plug Pin 2 to ground is giving around 0.20 ohms when closed at O.L as soon as the throttle is opened.
I bought a handheld oscilloscope of Amazon to check my ref and speed sensors again. I never used one before so it might also be user error, maybe some of you can confirm that. Does it matter which part of the probe goes to which pin?
between pin 25 and 26 I am not seeing anything one the oscilloscope at all. Resistance between them is 880 Ohm.
between pin 8-27 I am seeing a curve but peek voltage seems too low. (See picture)
if I did not mess up the oscilloscope reading, what could be the cause of this? Bad sensors? Tried 3 so far, all new, and also switched them around once.
I am pretty sure the gap is correct, I have done it 3 times now and I also don’t know how I would confirm other than doing it once again..
How do you determine the 225 rpm? Oscilloscope? Engine temp sensor, TP sensor can also prevent starting like this completely? I will double check ground points.
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I think you are overthinking it Tiger
Just crank the engine continuously for 10 seconds ....timing it with your watch or cellfone whilst counting the number of times that you hear the starter motor turn the engine over ,...whirr whirr whirr . Then multiply the number of revolutions in 10 seconds by x6 to find the rpm's per minute .It doesn't have to be a certain magical number its not a precision thing. ,
Cranking speed needs to be something in excess of 225 revolutions per minute for the DME to ground the second DME relay solenoid ,..which then supplies power to the Fuel pump, Ignition system, Sensors, Injectors etc NOTE ! When you do this test just pull the fuel pump fuse so that you do not flood the engine with fuel whilst cranking.
04-07-2024, 01:25 PM
