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Meh, maybe not on my theory. I was thinking the diagnostic sensor pins might be interfering with the signals in a way that killed every other pulse, but I'm not really seeing how that could happen. I'm wondering instead if the dme was losing track of the flywheel location and had to wait for the reference sensor to fire, resulting in one pulse per revolution.
For what it's worth, here are my old findings on the diagnostic pins in the flywheel from 2019:
While testing, etc., I kept noticing two recurring blips after the reference sensor pulse on the scope. When I'd spin the motor in the normal direction, the blips would come just after the reference pulse, but if I spun the flywheel backwards the same blips would appear just before the reference sensor pulse. They behave just like the primary pulse, just smaller. Among other things, that means the get bigger as the RPMS go up. I got up to over 2 volts peak to peak at about 800 rpm. I'm guessing at redline, they'd be a good multiple of that. After scratching my head a while, I finally realized the ref sensor was picking up the two pins on the flywheel that are there for the third diagnostic sensor (the one many people remove as useless). I can spin the flywheel by hand and watch the signal blip as those two pins pass below the sensor. The DME has no problem with them obviously, but I can't help but wonder if they are causing some of the trigger problems people experience with stand-alone systems? The modern VR sensor conditioning chips are "adaptive" and are pretty good at differentiating phantom pulses (especially the MAX9926 chip), but still. Those chips can detect pulse in the millivolt range, so seems possible they'd be fooled. I can't say for sure if it's causing anyone issues, but if I were experiencing triggering issues with the stock sensors on a stand-alone system, I'd sure remove those two pins and see it if helps....! They don't do anything, and removing them is free, so worth a try...
Tom wrote: Thu Nov 28, 2024 9:38 am
Great work finally getting to the cause of your bizarre issue. That one will go into my mental data base. I have to go back and look at my notes, but I might have a theory why sensors on the fringe of their range might fire every other time. In the meantime, here's an old video I did on the sensor signals that might be of interest.
That’s awesome! I remember watching this video a while back what a small world that it was you. It’s really bizarre. I wonder if it has something to do with magnetic fields being induced near that ground point since that ground is right behind the sensor bracket. But then again if it was random noise that couldn’t be it. My theory was the distance from the flywheel caused a very late short pulse to the dme, causing late ignition timing. If the timing is late the coil wouldn’t have a long enough “charge time” for the next pulse? I would love to keep trying to find what is exactly causing that just to know! If the dme is looking for a minimum voltage and the closer the sensor is to the flywheel the higher the “average” voltage is, the earlier the dme will get the pulse. If it’s further away you’d expect a lower “average” voltage and the pulse might only cross that “minimum” threshold much later in the cycle causing a delay between the dme reading and deciding to send a pulse to the coil. That’s my best assumption.
Tom wrote: Thu Nov 28, 2024 9:51 am
Meh, maybe not on my theory. I was thinking the diagnostic sensor pins might be interfering with the signals in a way that killed every other pulse, but I'm not really seeing how that could happen. I'm wondering instead if the dme was losing track of the flywheel location and had to wait for the reference sensor to fire, resulting in one pulse per revolution.
For what it's worth, here are my old findings on the diagnostic pins in the flywheel from 2019:
While testing, etc., I kept noticing two recurring blips after the reference sensor pulse on the scope. When I'd spin the motor in the normal direction, the blips would come just after the reference pulse, but if I spun the flywheel backwards the same blips would appear just before the reference sensor pulse. They behave just like the primary pulse, just smaller. Among other things, that means the get bigger as the RPMS go up. I got up to over 2 volts peak to peak at about 800 rpm. I'm guessing at redline, they'd be a good multiple of that. After scratching my head a while, I finally realized the ref sensor was picking up the two pins on the flywheel that are there for the third diagnostic sensor (the one many people remove as useless). I can spin the flywheel by hand and watch the signal blip as those two pins pass below the sensor. The DME has no problem with them obviously, but I can't help but wonder if they are causing some of the trigger problems people experience with stand-alone systems? The modern VR sensor conditioning chips are "adaptive" and are pretty good at differentiating phantom pulses (especially the MAX9926 chip), but still. Those chips can detect pulse in the millivolt range, so seems possible they'd be fooled. I can't say for sure if it's causing anyone issues, but if I were experiencing triggering issues with the stock sensors on a stand-alone system, I'd sure remove those two pins and see it if helps....! They don't do anything, and removing them is free, so worth a try...
test11.webp
test12.webp
test13.webp
That’s really interesting actually, i wish i pulled those out when i had her apart now haha. I could see that causing some problems as the sensor ages and induces a smaller voltage but i would think the dme can figure out the difference in voltage from each, might be worth in a case like that to throw a resistor in line to lower the voltage output if the diagnostic pins are creating a voltage high enough that the dme would be able to misinterpret as a ref pulse.
In the early cars, is the reference sensor pin at the sensor at cyl 1 TDC? I have a theory about the missing sparks, but I could not find this out by searching. I did find that the missing teeth of the single sensor cars is nowhere near TDC.
Hey all, now that the warmer weather is coming around in NY. I've had some time to get back to the project. From last time, i haven't been able to get her to intermittently run on 4 cylinders like before (not sure why the videos were taken down from Imgur).
Coming back to the previous posts from yalls help... My car being a model 83' actually does not have the diagnostic pins on the flywheel so i ruled that out causing TDC problems on the sensor. As well as... TDC is not where the flywheel pin lines up with the reference sensor. TDC actually occurs some time after the pin makes its way past the sensor. Couldnt give an exact degree measurement but to guess i believe its somewhere around 30 degrees apart.
Since then, I replaced the beaten sensor only to find out the new sensor i bought could not deliver a high enough voltage pulse as the reference and even the damaged speed sensor. I went back to using the damaged speed sensor since it still worked fine and even showed a good voltage on the O-scope. In terms of raw numbers. Im getting over 30v from baseline (amplitude) on the reference sensor and 2 volts amplitude on the speed sensor. It took me some time to get to such a high reference voltage. I was mainly concerned with the reference gap since i still cant get it to fire the second time in the cycle causing her to only run on cylinders 1 and 4. I have a short clip where i have probed both the ignition coil ground (DME pin 1) and the reference sensor output (DME pins 25 and 26) on another probe. Below you can see as the reference pin comes into pulse, the coil fires 10ms after and never fires until the reference pin comes back in. (The below video has a Timebase of 10ms and a voltage scale of 5Vac for the coil and 10Vac for the reference sensor. *O-Scope was calibrated prior to testing*).
Thus shows it's having trouble getting off that second firing in the revolution. So i have way above spec for the reference sensor but you could say the Speed sensor is low but definitely within spec with a peak to peak of 4v. One problem i noticed however, in the speed sensor, with the battery disconnected. I had about 0.75V (amplitude) of noise in the signal! I decided right here i just needed to re-run new sensor cables, new DME ground and re-pin it all on the DME connector. So thats what i did, new Sensor cables, new grounds straight to the battery negative and this is where i noticed a pretty crusty shielding wire for the speed sensor ground right at the DME connector. With the new wiring i was hopeful but, no difference still a noisy signal. I even thought this was due to that one being the sensor with the damaged casing so i unplugged the sensor and still have the same noise there! WTF. So i figured now i must be picking up some noise from the O2 sensor shielding given its spliced into the same DME connection as the Speed sensor shielding. I cut out the O2 shielding and... same thing, 0.75V Amplitude of noise. So weird especially since there is no sensor even connected to the wire. When compared to the Reference sensor signal, I get noise with just one end of my probe on DME pin 25, but the moment i add my probe's ground to DME pin 26, the noise flatlines to a perfect 0V. This never happens with the Speed sensor. I've tried both my DMEs and i get the same thing. Is Pin 26 not actually grounded? Is it normal that I'm not seeing any flat-lining on the speed sensor?. I wish i had a DME circuit diagram to figure this out but maybe someone here has more knowledge on the DME. To note why my Reference sensor voltage and Speed sensor voltage differs so much, My reference sensor pin is actually slightly damaged from either previous owner or when i rebuilt the engine. I essentially gapped my sensors to the flywheel pin and then cut up an old feeler gauge to act as shims for the speed sensor. I confirmed no contact using a endoscope in the bellhousing before cranking.
I'm excited to get back to this just sucks that ive been stuck as this dead end for so long now. My plan for tomorrow at least, is to add a smaller shim and try and get the Speed sensor voltage higher. I have some room to play with since i have a bunch of homemade shims from the feeler gauge. Otherwise im stuck here. I've ruled out my harness since ive replaced all ignition related wiring at this point but what are the chances of 2 bad DMEs too? Could use some well needed guidance here. Thankyou!
Given the damage to the reference sensor, I would consider getting another sensor so you have two, not mangled, brand new ones, and reset their height properly per the workshop manual. The signal strength is dependent on both the clearance and the engine speed.
There is a note in Volume 1 which states "It is important that the reference oscillation starts with a positive slope..." while in your oscilloscope video it starts with a negative. This is likely due to probing it opposite, and I can't tell from either the schematic or the manual which way it should be probed. It is of note though that they thought it important enough to mention. Hopefully you did not swap wires around while changing the harness.
For the noise issue, it clearly is not coming from the car since it persists with the battery disconnected. Looking at a similar vintage 911 DME schematic, Pin 26, along with Pin 25, go to the s100 chip which converts the sensor output to a digital level signal. If you are looking for noise from the speed sensor, you need to probe across Pins 8 and 27, which also go to the chip.
Just re-reading, I misconstrued the reference sensor pin damage for the reference sensor being damaged. Maybe you don't need another new sensor after all.
On the 911 DME schematic, there are resistors interconnecting the speed and reference sensors. It is possible the exceedingly high voltage from your reference sensor is messing up the S100 chip being able to read the speed sensor. Perhaps you should increase the gap for the reference sensor to reduce the voltage to a more normal level.
944er wrote: Wed Mar 05, 2025 11:30 am
Given the damage to the reference sensor, I would consider getting another sensor so you have two, not mangled, brand new ones, and reset their height properly per the workshop manual. The signal strength is dependent on both the clearance and the engine speed.
There is a note in Volume 1 which states "It is important that the reference oscillation starts with a positive slope..." while in your oscilloscope video it starts with a negative. This is likely due to probing it opposite, and I can't tell from either the schematic or the manual which way it should be probed. It is of note though that they thought it important enough to mention. Hopefully you did not swap wires around while changing the harness.
For the noise issue, it clearly is not coming from the car since it persists with the battery disconnected. Looking at a similar vintage 911 DME schematic, Pin 26, along with Pin 25, go to the s100 chip which converts the sensor output to a digital level signal. If you are looking for noise from the speed sensor, you need to probe across Pins 8 and 27, which also go to the chip.
Just FYI, the early DME's did not have the S100; they had a multi-component circuit to process the a/c pulses and turn them into square 5v pulses for the 8051 interrupt pins. Haven't had a chance to study the latest findings, but will.
The reference pin is offset so the DME can calculate ignition advance. No issue there. Something isn't right with the speed sensor -- the spec is an amplitude of 2.5v -- but it's all the same hardware so there's no reason it should be so much less than the ref sensor (unless not gapped right). What's the nature of the damage to the pin? The sensor is designed for a certain amount of ferrous metal spinning past it to generate the a/c wave. It also needs to protrude away from the rest of the flywheel adequately in order to work. I'm not following what you shimmed with the feeler gauge?
If you suspect the DME circuit (which seems unlikely since you tried two DME's) and/or if you want to see what the CPU sees, you can probe the interrupt pins on the 8051 (pins12 and 13) to see if the speed and ref pulses are being turned into square 5v digital pulses. My guess is your speed sensor signal is not generating a clean 5v digital pulse at the 8051, through no fault of the DME. I can't remember off hand which interrupt goes to which sensor, but can look it up if you want to try.
