Sounds good, what tool do you use to check for a pulse?

I haven't seen if you see any tach bounce in your thread. If the DME is operating and seeing the crank sensor signal you should see the tach needle bounce (or twitch).
If you are not seeing that, your problem is probably upstream of the ignition module, coil, distributor, etc.

In that case testing with a DME relay jumper is a good idea IMO despite whatever testing has been done with various DME relays.

Then the crank sensor can be tested for resistance and AC volts or with a 'scope. Testing at the DME end of the harness is a good start. Some multimeters even have a Hz function that will aid in testing a crank sensor.

You can even hook them up to an audio amplifier and get a tone, they are basically an electric guitar pickup. This won't tell you if you have a strong enough signal for the DME to trigger on. Kinda fun though.

when you get your new oscilloscope and start probing things, have a look at the manual and in particular at the input voltage limit. remember the coil can produce high voltage and that is likely to exceed your scope's input range.

also be aware funny things can happen with grounding, for example if you had a battery charger plugged in and set that down on the car, it may then connect your car to your house electrical ground. You can probably find some videos that explain the grounding potential and dangers associated with regards to test equipment better than I can explain here. grounds are often considered as dangers on a test bench and it is normally not conductive. in a nutshell weird and unexpected stuff can happen when you are sharing grounds through equipment.

the scope should be good for examining low voltage inputs from sensors, Just be a bit careful , I'm not sure what voltage you will see between the transistor on the output stage and the coil. you could check that with a meter first to make sure you are still within the scope's range, id keep it away from the high tension side of the ignition coil unless you have some protection mechanism involved as that may blow your scope..

sounds like you swapped the coil and other parts, and have eliminated some possibilities, partly through swapping in known functional parts and that's fine to do. now you can look for a pulse from the two sensors at the DME as Tyler suggested. Its a process of elimination and you are eliminating some possibilities.

you have no key switch on the outside of the car for the alarm but you found the module, I might have missed it but did you try the jumpers? I wondered if someone may have eliminated the key switch and did bodywork to cover where it was? could it have been moved? perhaps a PO put it somewhere out of sight?

as you dig around I;d keep an eye open for ground points, an example the fuel injector wiring is likely grounded to the block and the block likely has a ground strap. It wont hurt anything to remove the bolts or screws and make sure any grounds are clean or use a jumper cable to make sure parts are grounded, for example just take a jumper cable from battery ground post to engine block ground and if it should start you found an issue.

I had a similar issue in another car ( a volvo 240) and found my coil was fine but the little brass connector that is on the coil terminal that the spade teminals go onto had had a bad contact. the little bolt on the coil itself was rusty. I had been cleaning the wire spade connector and the brass clip thing but the problem was between the little stud on the coil ( which rusted) and the brass terminal with the "spade" it was tight but no contact there, hence no start. cleaning that threaded terminal on the coil fixed that.

that threw me for a while. I also had an issue just prior with a TDC sensor and the flex plate it reads both at once, swapping the sensor did nothing so I put the old one back ,, it took a long time to figure out I had both a bad sensor AND a damaged flex plate. It had gotten hit by the starter gear which had broken up and a chunk of the gear got carried up between the sensor and the flex plate, damaging both. (someone tried to move my car and gave it throttle with the key turned and that action blew up my starter pinion gear.)

in retrospect I had 3 issues all at once and was expecting 1 issue to be the cause. I had very limited access and ended up drilling a 1 inch hole in the bellhousing under the transmission to gain access from under the car, to bend the flex plate back into shape and that avoided the need to pull the engine or transmission...


if you do have a bad pulse,
you might check if the sensor has about the right clearance. I measured with a depth gauge down the hole and then measured the length it protrudes and subtracted to know the gap. dirt in the area might cause issues, you could look with a light down the hole where the sensor is. i dont know the spec but maybe 1.5 mm or so gap. if you suspect it could be bent you can check with a depth gauge at various positions of the engine. probably rare they get bent up. I also accidentally put a flex plate on wrong once and had to pull the tranny again to turn it to the right bolt hole. that car ran but would not rev up since the TDC was out by a mile.

I order an oscilloscope that should be here today and I am using that for the tests. I do have tach bounce. I also did test for resistance and voltage at the dme harness and the test showed the speed sensor was working, but I still would like to see the pulse from the speed sensor to the dme harness plug. I have a couple multimeters and neither of them show hz. I might have to look in to getting one that has that function.

the scope should be able to measure the voltage and frequency of the pulse and show a graphic display as you crank. once you get a trace that is proportional in size to the screen just look where your switches are and that gives you the number of volts per division You'll just adjust the trace so you can see the pulses and then you can read the timeline on a horizontal scale in a similar way. Voltage is vertical, time is across the screen. you'll just need one input probe unless you want to see the input of two sensors in unison. it will likely have a trigger function, try not to get confused into that stuff..

You also dont need a high frequency scope unless you plan to analyze things on a digital circuit board level, That's way beyond my knowledge.


- you have tach bounce so you have a sensor that is functioning,
Question - Does the tach bounce refer to the speed sensor or the TDC sensor?

If you have tach bounce you might try swapping the ignition module again. Some aftermarket ones don't seem to work. Maybe get a used OE one from a Volvo or something.
Maybe you can also look at the signal to the module first as well as power and ground.

Maybe also check for spark with a spark checker just in case your testing protocol is flawed. Plus without spark the plugs should be wet and smell like gas. If not consider trying fuel or starting fluid.

When things don't make sense it's usually apes and their assumptions. At least in my case.

Thanks for explaining how to use it. I have never used one before I’ll probably watch a youtube video to better my understanding. Tach bounce is the speed sensor I believe because it is the speed of the flywheel. The only the sensor on the S is a cam position sensor and if that’s broken the car will still run, but mines new. The Ignition control module I swapped mine out for was a used oem one, maybe I should try I new one but I’m going to make sure it’s getting power, ground, and also the pulse from the dme as you said. The plugs do smell like gas and are wet. I also have a fuel pressure gauge on the rail so I’m positive it’s getting fuel. I also tried starting the car with starting fluid just to see if anything would happen and no change. My scope came tonight so I’ll be doing the testing tomorrow and I will let you guys know the results I get.

Also is anybody aware of a way to test the ignition control module?

This looks on-point:
https://easyautodiagnostics.com/misc-index/no-start-volvo-case-study-3

it might not hurt to pull the plugs and crank it a bit. if it's flooded from trying it might confuse things.

I have 4 volvos just like the one described at the link above so I'm way more familiar with those than anything Porsche.. on one I had start issues so I bolted a spare ignition module so I can swap it if I need for testing. to be right it has some putty on the back for heat transfer and that should be renewed. maybe if it looses contact with the body it might overheat or burn up. I've never seen one fail on me, but they can. Ive seen the coil get really hot by just leaving the key on , never knew why exactly, but maybe a coil can get fried that way. Ive also never seen a coil fail and Ive been driving that kind of car and fixing my own for 30 years.. Verifying the pulse into the thing is probably a good next step.

I dont know if the Porsche one looks the same or may be the same part? I read that the idle air control valve is the same part. It's all Bosche , so similar in some ways...I have an 88 740 that uses a hall sensor in the distributor to get the timing pulse in 89 they went to the crankshaft position sensor. in that year they also introduced some means to read blink codes from the ECU, volvo 240's that year used the same engine and ECU basically. they used an ICU and an ECU rather than the DME . the ICU changed when the distributor hall effect sensor was changed to a TDC sensor. both use that ignition module. the reason its mounted where it is is to cool the thing so it must generate some heat and may be susceptible to burning up.

my 944 uses the two sensors, not a distributor hall sensor so Porsche seems to have used that sooner. i don't know if earlier ones used a distributor hall sensor but i know Volvo made that change in 88/ 89

a failed distributor hall sensor I have had happen in Volvo's a couple of times and it's a similar no spark situation. it needs the pulse otherwise it wont start. oil gets up inside the distributor and messes them up, or their wires.


if you pull your module off the fender you might want to look for some new heat transfer putty but you can swap the wires in as a test without removing yours too.

pulling the DME from a working car might tell you stuff but there is a danger , if there is a wiring issue that blew the other DME so there is some risk. sometimes not fixing a fault that blew a board can blow a new board and that's expensive. i once blew the ECU in my Volvo by accidentally connecting my battery charger backwards. i heard a pop, no more start, it killed the ECU. Stupid mistake. I don't doubt you could blow the DME like that.

if you do get inside to look at a ECU or DME you can observe any capacitors. look at the board near them for any discoloration, if you see that it can be a sign a capacitor leaked. ( 30 year old capacitors themselves fail and take out a lot of electronics so you can re-cap most circuit boards ) a leaky cap can leave a stain near it that is noticeable if you look for it carefully. they can be checked with a capacitor checker or there is a little electronic gizmo you can get to check them in circuit called an ESR meter. I sometimes just approach that in a simplistic way and "shotgun all the caps" and see if it helps things. replace with the same working voltage and microfarad ratings. I'll clip the cans off about 1/8" from the circuit board and reattach to what I can leave on the board of the leads rather than needing to work on the solder side of the board. some take a soldering iron and remelt every contact in the hopes of fixing a cold solder joint or bad contact.
roughly speaking, capacitors block DC but allow AC through, often they are described to be "leaky" in other words they can conduct some DC due to failure, that's different from leaking physically. they have a type of oil inside. electrolytic ones look like tiny tin cans, they fail , other caps are not electrolytic and look like little plastic blocks, they are less likely to fail. In old ( tube era) electronics the non electrolytic caps look like cardboard tubes and they do fail. I replace those because I have a hobby of restoring antique tube radios. the Non electrolytic caps got more reliable, but modern electrolytic caps do fail due to age.

good eyes help and I have trouble working on tiny stuff but otherwise all that can be done with no real serious electronic troubleshooting knowledge, just some careful soldering and hope! may prefer to try to find a replacement, its something you can try but I wouldn't go there at this point. i think I might attempt that with mine but if it ain't broke maybe don't fix that ;-) If you have time and determine the DME definitely does not work and then it's not a good one. then it's garbage.. at that point its stuff you might try before you spend hundreds on a new one or a rebuilt one.


it may be possible to change capacitors if you are so inclined. It would be a experiment to find out if it helped but sometimes it can fix things, the electrolytic capacitors are cheap to buy. don't turn them around they have a ground side ( look for a stripe) and polarity matters.

if you have a pulse at that ignition module then you know the problem is towards the engine , if no pulse than the issue is back towards the Porsche electronics so it's a good place to divide the issue into parts and break the problem down a bit. i think you are on the right track checking in that direction.

I had that module fail in my ford van also an 88 with bosche electronics. I parked it and it failed to restart after time. It caused a no start / no fire. and a mystery similar to that. a lot of ford owners pack a spare because they can and do fail suddenly.

I had a stuck injector in my volvo. it would empty the fuel rail into cylinder 3 after each key off and upon startup that cylinder was so full of fuel it would blow white smoke and stumble for a block until it started firing in that cylinder. .. I could see that wetting one plug, but it seems unlikely it would cause a no start / no fire situation because the other cylinders would fire up and it would be unlikely to see them all stick open. i think in that situation it will run completely crappy but the engine should still fire and it can run on 3

if you've been cranking it and it has been injecting fuel and not sparking that could cause all your plugs to be wet with gas so I'd remove them and give it some cranking to clear them out, blow them off dry because that may possibly cause it to not fire at least right at first. likely if you have no spark and you fix that, its going to fire right up and make you happy.

i once bought a VW and the previous owner couldn't start it. It turned out they had disassembled the distributor, put the gear that drives it on backwards and had then altered the timing by 180 degrees.
the car also had a power wire to the coil run under the car. (mickey mouse wiring) It would run but if I tried driving it the coil got grounded out by the wire rubbing on the axle shaft. the car would die as son as I moved it. Then as soon as I got out of the car to look at the engine it would start and sit there idling fine again ;-)

This is the signal I'm getting from the speed sensor

https://drive.google.com/file/d/1412ZiLybnrRF464CT5JgvzvMM7cq3ucS/view?fbclid=IwAR02F6Xoa752TJkjbKf43_cQllYmOx0gzDST 2vfd3Jbxo-YXpUvcaTV8Ziw[/IMG][/IMG]

It is supposed to look like this:

https://drive.google.com/file/d/1MReJQ8qxFxgeDPhbabO9ifRZo6Kxze4i/view?fbclid=IwAR2jko_uWoVVxwoLV0elRc3oWPF00LqdZhH5 Hfw3ziTmcbZRMzO7ggbb7W8

I am getting above 3 volts so that means the sensor is spaced accordingly, but it does not seem like I am picking up the reference mark signal. What do you guys think?

No access to the google drive links you provided, so we cant see the images. You might have better luck uploading to ImgBB and posting those links

I believe the speed sensor reads RPM and reference sensor sends TDC pulse. I would check both. Also sounds like you have already seen this page but for reference on spacing (maybe the speed is spaced correctly but reference isnt)
http://www.clarks-garage.com/shop-manual/ign-02.htm

Other possibilities are the harness and plug for one or both sensors could be degraded. Mine literally crumbled apart the one time I tried to unplug it.

So the green wire that connects from the ignition coil negative terminal to the ignition control module (the wire is labeled 1 on the wiring diagram) has no continuity. In the wiring diagram it shows that they connect. Im thinking that that wire might be corroded somewhere along the way.

with all of the changes and replacements you may have created a new problem you didn't have when you started.

I think you said you changed the sensor, are you sure you gapped it properly? I read a post once about gluing a piece of cardboard of the gap thickness to the sensor to gap it. once the engine starts it will wear the cardboard away. you can also use the dirt mark on the old sensor as a reference.

I can run a continuity test on the green wire tonight and see if my S2 also has no continuity to ICM, just to double check if thats normal or not.

Here is what its supposed to look like and then below it is what mine looks like.
https://ibb.co/16XVT8F

https://ibb.co/DVMv9C2

Im 99% I gapped it properly. The sensor I bought came with a tool that is .8 mm longer than the sensor. I installed the tool on the bracket pushed the bracket down as far as I could and locked it into place then I took out the tool and installed my sensor and tightened it down.

Thanks for checking that would be much appreciated.

I just tested and have continuity between pins 1-4 on the ICM plug and the green wire on the ignition coil (I didnt know which one is the green wire on the ICM so I tested all of the pins. All 4 registered varying levels of resistance). Pin 5 did not have continuity (resistance reading of 1), Pin 6 and 7 are blank.

I dont have a wiring diagram in front of me for the ICM. But I assume pins 1-4 correspond to each cylinder, and pin 5 must be an input signal from the dme?

For pin 1on the icm I have resistance and continuity to the green wire on the coil. For pin 4 I have no resistance and no continuity to the green wire on the coil.

pin 4 runs back to the ignition switch, maybe there should be 12V with the key on?

The icm diagram is posted above.

often with machines that use an encoder , they can count the pulses per revolution and verify that they see the correct number of pulses between each index pulse. the index pulse is once per revolution. If the machine doesn't count the right number of pulses it assumes something is wrong and shuts down , often producing an error. often referring to a driver or to the encoder. Its a similar system used in machienry to track various parts and positions of motors.


Its hard to know how the porsche "computer" "thinks" but I would not assume this index pulse or "TDC pulse" is not important, maybe if you unplug it from a working car , that could verify if it needs that pulse to start.

on my volvo I found that I had a situation wehre I could crank the engine and it would fire but not start. the issue seemed to be with a coil connection but it was definitely a repeatable issue and that proved to me there is something different between how it sparks upon crankig, as compared to running..

It led me down the path of thinking well the engine can fire up without seeming to need a complete revolution so maybe there is some "device" in there to allow the spark before it synchs up with the "index" or TDC pulse. in order to fire before seeing the index mark , , i think they can still fire , so there could be some difference between how spark is delivered when cranking as opposed to the way it is delivered when running and synchronized to TDC

I think it will need that index or TDC pulse in order for the electronics to define the position of the engine. unless the otheer sensor does have an index position it would look the same all around 360 degrees,, it only sees pulses.

the way Volvo does that, they leave a blank space in the holes the sensors reads, the blank space is recognized as the index. someone who has seen the flex plate , or the thing that the sensor reads, will know if it has a gap in the holes it reads or not. having never seen a Porsche one ,I can't answer that.



i remember long ago on an older car that they used something called a "ballast resistor" if that failed it would cause an engine to fire on cranking but not run. That is old technology from points style ignition systems.

when the car is cranking , a lot of the battery power is used by the starter motor which is putting a big load on the battery. Its starting in sort of a "brown out" as the starter may reduce the voltage available during cranking. there could be some way the manufacturer has gone about making sure there are still enough amps to fire the coil properly during cranking , perhaps some alternate circuit used when cranking but not during normal running.

maybe all that is more clear if phrased as a question. "can the car fire before the computer knows the TDC position of the engine? ( during its firs revolution) if it can, then how does the computer know the relative position of the engine to deliver that first 4 sparks?


I tink in very old cars, like model T's they had a vibrator, it would make and break the pulses to the coil at random, the only thing there was to deliver the spark to the correct plug was the position of the distributor rotor. I think contact breaker points came later.

It sounds like you guys may have it down to a bad wire, Lets hope that's it.