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furgo
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PostPosted: Mon Nov 20, 2017 8:41 pm    Post subject: TDC marker unit questions Reply with quote

Ever since I learnt about it, I've been wondering how to put the TDC marker unit to use without using expensive (and NLA) equipment. I'm particularly interested in how the functionality of the VAG 1367 meter to read RPM, the firing on cylinder #1 and the TDC marker signal could be replicated with modern electronics to read the advance angle without a timing light.

Figuring out how to do it is beyond the scope of this question, but while re-reading ratwell's article on the TDC marker unit, there was something that caught my eye: I would have thought that the sensor would return a pulse on each revolution when TDC is reached. However, the article seems to indicate that there are two points that would generate such a pulse: 12 BTDC and 18 ATDC.

Quote:
[...] dowels are located at 12 BTDC and 18 ATDC [attached to the back of the flywheel]


Why choose these two particular positions instead of one single dowel that would just generate one pulse at TDC? Have they got a particular meaning for timing measurement/setting purposes?
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PostPosted: Mon Nov 20, 2017 8:53 pm    Post subject: Re: TDC marker unit questions Reply with quote

The magnets are placed in the flywheel ahead of the actual point of reading, I guess to compensate for lag. There are usually 2 magnets, seemingly one for TDC and another for the specified timing setting at idle, they would trigger the sensor upon each revolution (reed switch or whatever is in electronic distributor pickups?), what the computer does with that data afterwards is above my pay grade.
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PostPosted: Mon Nov 20, 2017 9:25 pm    Post subject: Re: TDC marker unit questions Reply with quote

I am sure you could build some tool to make use of the timing sensor, but why? I put a light on my rigs about once a year to verify that the timing is still in spec or to retime the engine when the points get replaced. I see little reason to monitor the timing continually.

On a day to day basis when traveling I can also just turn the ignition on and rotate the engine by hand while watching the timing marks. When I hear the spark I will know whether or not the engine is still in time, which 99.9+% of the time it still is.
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PostPosted: Tue Nov 21, 2017 3:54 am    Post subject: Re: TDC marker unit questions Reply with quote

Thanks both for the replies.

busdaddy wrote:
The magnets are placed in the flywheel ahead of the actual point of reading, I guess to compensate for lag


I'm not sure I quite follow that. What makes you think they are ahead of the point of reading? (and what do you consider as the point of reading?)

busdaddy wrote:
There are usually 2 magnets, seemingly one for TDC and another for the specified timing setting at idle, they would trigger the sensor upon each revolution


I would expect one at TDC, yes. But if the second one were to be at the point equivalent to the specified timing setting at idle, that would mean that this second magnet/dowel would have to be in a different position depending on the engine type. That is, different configurations for different timing specs. I'm not sure that'd be the case.

Yet the ratwell article states that they are at 12 BTDC and 18 ATDC. The choice of these two particular points is what puzzles me. They are 30° apart, the middle point being at 3 ATDC, if that means anything.

Wildthings wrote:
I am sure you could build some tool to make use of the timing sensor, but why?


Let's just say I have an interest in electronics and as many others on this forum, an inquisitive mind Smile.

The post is not to question or discuss which timing method is best, I'm interested in understanding how the TDC marker could be put to use (and eventually use it).
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PostPosted: Tue Nov 21, 2017 5:16 am    Post subject: Re: TDC marker unit questions Reply with quote

This may be helpful

https://www.doityourself.com/stry/how-a-crankshaft-position-sensor-works
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PostPosted: Tue Nov 21, 2017 5:28 am    Post subject: Re: TDC marker unit questions Reply with quote

Also found people processing these signals with arduino

https://www.google.com/search?q=arduino+crankshaft+sensor
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PostPosted: Tue Nov 21, 2017 7:30 am    Post subject: Re: TDC marker unit questions Reply with quote

furgo wrote:
Thanks both for the replies.

busdaddy wrote:
The magnets are placed in the flywheel ahead of the actual point of reading, I guess to compensate for lag


I'm not sure I quite follow that. What makes you think they are ahead of the point of reading? (and what do you consider as the point of reading?)

busdaddy wrote:
There are usually 2 magnets, seemingly one for TDC and another for the specified timing setting at idle, they would trigger the sensor upon each revolution


I would expect one at TDC, yes. But if the second one were to be at the point equivalent to the specified timing setting at idle, that would mean that this second magnet/dowel would have to be in a different position depending on the engine type. That is, different configurations for different timing specs. I'm not sure that'd be the case.

Yet the ratwell article states that they are at 12 BTDC and 18 ATDC. The choice of these two particular points is what puzzles me. They are 30° apart, the middle point being at 3 ATDC, if that means anything.

Wildthings wrote:
I am sure you could build some tool to make use of the timing sensor, but why?


Let's just say I have an interest in electronics and as many others on this forum, an inquisitive mind Smile.

The post is not to question or discuss which timing method is best, I'm interested in understanding how the TDC marker could be put to use (and eventually use it).


You could remove the existing pins and install another one either on the same side of the flywheel or approximately 180° opposite and locate it exactly where you want it. I know that people have modified the pulleys on Type 1 engines so that they can run distributorless ignitions and on a Type 4 one could install pins in the fan and do the same, so lots of options. You could also just use the existing pin(s) and delay or advance the timing accordingly, this would be similar to what the Digifant ECU does.
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PostPosted: Tue Nov 21, 2017 8:30 am    Post subject: Re: TDC marker unit questions Reply with quote

Quote:
I've been wondering how to put the TDC marker unit to use without using expensive (and NLA) equipment

this subject gets discussed here about once to twice a year for the last 10 - 15 years. If you figure it out and build a circuit that works, please let us know. A search will reveal the other threads on it although it may take a couple tries to find them all.
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PostPosted: Tue Nov 21, 2017 8:40 am    Post subject: Re: TDC marker unit questions Reply with quote

furgo wrote:
I'm not sure I quite follow that. What makes you think they are ahead of the point of reading? (and what do you consider as the point of reading?)

As in neither magnet aligns with the sensor at TDC or the spec'd timing for that engine, they are both placed a few degrees ahead. One would assume the goal is to confirm a spark occurs at the same time the sensor is triggered when the timing is set correctly, wouldn't they? Magnet locations differ slightly on early flywheels and flex plates, I seem to recall later ones just had a single magnet (TDC?), maybe the machinery had advanced and it was now capable of calculating the difference between spark and sensor pulses?
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PostPosted: Tue Nov 21, 2017 12:02 pm    Post subject: Re: TDC marker unit questions Reply with quote

BayCreamPuff wrote:
This may be helpful

https://www.doityourself.com/stry/how-a-crankshaft-position-sensor-works


Thanks. I believe I know how the sensor works, but ultimately I'll have to measure the signal. It is probably two pulses per revolution at the specified points. I'm also guessing that it's a positive voltage peak as the dowel approaches the sensor, falling rapidly again with the same voltage peak but with negative polarity as the dowel moves away. Of the three pins, one is the shield, and one of the other two probably has to be grounded.

More modern setups seem to use the same principle, but with the inverse approach: the sensor is facing a toothed wheel, where one tooth is missing at TDC. As such, the sensor would output a continuous train of pulses, but just missing one on the missing TDC tooth.

BayCreamPuff wrote:
Also found people processing these signals with arduino

https://www.google.com/search?q=arduino+crankshaft+sensor


Indeed, that's the idea, but I didn't want to go deep into it on this thread, as the processing is a broader subject. I've got an Arduino, but mostly I do things with a Beaglebone Black, which is more comfortable to develop on for non-real-time-critical tasks.

Somehow related, and just for fun, here is an engine speed pulse simulator I put together recently:


Link


I initially built it to test a tacho that I had restored, but I've been thinking of using it for other purposes, such as feeding engine speed pulses to a spare ECU and monitor the resulting injector pulses' duty cycle. I can also reuse part of the code for a prototype to generate TDC pulses, a separate #1 cylinder pulse and then work out the timing advance/delay, before testing it on the bus.

Wildthings wrote:

You could remove the existing pins and install another one either on the same side of the flywheel or approximately 180° opposite and locate it exactly where you want it. I know that people have modified the pulleys on Type 1 engines so that they can run distributorless ignitions and on a Type 4 one could install pins in the fan and do the same, so lots of options. You could also just use the existing pin(s) and delay or advance the timing accordingly, this would be similar to what the Digifant ECU does.


Indeed, lots of options, thanks for the suggestions. I'd rather not modify the stock setup, though. If that means to simply accept that there is no TDC pulse, but one at 12 BTDC and one at 18 ATDC, that's fine. The TDC can then be determined relative to either of those.

SGKent wrote:
Quote:
I've been wondering how to put the TDC marker unit to use without using expensive (and NLA) equipment

this subject gets discussed here about once to twice a year for the last 10 - 15 years. If you figure it out and build a circuit that works, please let us know. A search will reveal the other threads on it although it may take a couple tries to find them all.


Definitely. For now, I just want to understand the system before setting any expectations Smile

busdaddy wrote:
Magnet locations differ slightly on early flywheels and flex plates, I seem to recall later ones just had a single magnet (TDC?), maybe the machinery had advanced and it was now capable of calculating the difference between spark and sensor pulses?


Ah, that's interesting. Would someone have a picture of a Type 4 flywheel showing the dowels/magnets? Or is there a "Type 4 flywheel ID" thread somewhere? (I could not find it after a search)
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PostPosted: Tue Nov 21, 2017 2:53 pm    Post subject: Re: TDC marker unit questions Reply with quote

lots of flywheel photos.

Go to the gallery or classifieds
Choose the gallery or section (bay for example)
Type "Flywheel"
Ads or photos will come up. Click one and you will see something like this. Is that what you were trying to find, and does this help? :

Image may have been reduced in size. Click image to view fullscreen.

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PostPosted: Tue Nov 21, 2017 4:21 pm    Post subject: Re: TDC marker unit questions Reply with quote

SGKent wrote:


Image may have been reduced in size. Click image to view fullscreen.


The picture did help, thanks! I've also found this looking at my books:

Image may have been reduced in size. Click image to view fullscreen.


You can then see the timing studs screwed on the left flywheel on the picture, at 5 and 6 o'clock. The flywheel on the right seems to have the holes, but no timing studs are screwed into them.

From that text, I understand that the timing studs came as a pair (as opposed to one single TDC marker), from model year '74.

That still does not answer the question of why these two particular points were chosen, but I have learnt an awful lot so far Smile

Thanks everyone.
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PostPosted: Tue Nov 21, 2017 4:29 pm    Post subject: Re: TDC marker unit questions Reply with quote

we've learned from your research too. Maybe someone (Telford?) can design a circuit that is triggered by that system and give a digital readout of the timing for those who want something like that. Me, I've got too many gauges already. Smile
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PostPosted: Tue Nov 21, 2017 11:47 pm    Post subject: Re: TDC marker unit questions Reply with quote

I love it when I see an oscilloscope to do technical analysis.
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PostPosted: Wed Nov 22, 2017 12:09 pm    Post subject: Re: TDC marker unit questions Reply with quote

Before starting to look at the electronics, I've drawn this diagram of how I believe the relevant signals fit together.

Image may have been reduced in size. Click image to view fullscreen.


The explanation of each trace is on the diagram itself. The only thing to mention is that the signal amplitudes are not to scale.

If I've overseen anything, feedback is always welcome.

I'm looking at options to convert the TDC sensor output to digital. The other input that will be needed is the spark #1 signal (secondary, HV side). The points signal at terminal #1 can be optionally used to work out the RPM more accurately, but it's not absolutely necessary.

With these signals pulse-shaped and attenuatted to a digital voltage, it shouldn't be too hard to work out the timing advance with a microcontroller. The signal conditioning is the tricky part.

Again, this is simply an exercise to see if the the TDC marker can be put to use, for the fun of it and the learning experience. It's not an attempt to change the timing setting practices (although if it's successful, it might provide an alternative way).

Btw, if someone has got a spare sensor they'd be willing to part with, I'd gladly buy it to do some testing (I couldn't find any on the classifieds).
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Last edited by furgo on Wed Nov 22, 2017 2:09 pm; edited 2 times in total
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PostPosted: Wed Nov 22, 2017 12:18 pm    Post subject: Re: TDC marker unit questions Reply with quote

I bet a sensor would be easy to come by. What you are wanting to do sounds like a standard type of problem. I've seen similar situations in other places.
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PostPosted: Wed Nov 22, 2017 1:02 pm    Post subject: Re: TDC marker unit questions Reply with quote

That's going to be a challenge, the sensor is pressed in and is destroyed in the removal process. If you could find one in a trash case and cut the case around it you could salvage the sensor.
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PostPosted: Wed Nov 22, 2017 1:21 pm    Post subject: Re: TDC marker unit questions Reply with quote

Not sure what your intentions are, but with the stock L-jet FI system the injectors batch fire twice per complete engine heat cycle, they are not timed as you show. I assume timing them and firing them individually would be better but that it didn't offer enough advantage when used on a mass produced vehicle.

It would certainly be nice to have a soft overspeed cutoff verse cutting fuel to multiple cylinders at the same time.
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PostPosted: Wed Nov 22, 2017 1:56 pm    Post subject: Re: TDC marker unit questions Reply with quote

busdaddy wrote:
That's going to be a challenge, the sensor is pressed in and is destroyed in the removal process. If you could find one in a trash case and cut the case around it you could salvage the sensor.


Argh, bummer, I wasn't aware of that. That said, it seems from this thread that one or two people have had success in punching the sensor out. Not clear if they destroyed it, though.

Even then, it could only be done after having dropped the engine.

Wildthings wrote:
Not sure what your intentions are, but with the stock L-jet FI system the injectors batch fire twice per complete engine heat cycle, they are not timed as you show.


Thanks for the feedback. I can see where the confusion came from and I've corrected the diagram accordingly.

The diagram will be somehow familiar to those who have read the L-Jetronic documentation, as it's based on the original Bosch one. The original text at the top read: "Ignition sequence of the cylinders and opening times of the injection valves", which I initially left unchanged.

However, I'm not trying to do or represent anything related to the injectors. The gray bars depict the time during which the intake valves are open, and the text now reflects that. Let me know if that's clearer now.
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PostPosted: Wed Nov 22, 2017 3:55 pm    Post subject: Re: TDC marker unit questions Reply with quote

As far as I know...the two pins in the flywheel are NOT magnetic. Of course I havd not checked any that I have to see of they are magnetic....but neither do they need to be.

This is a dirt simple magnetic induction switch....identical in function to the magnet sheel and pickup module in a Pertronix ignition.

The magnet units usually have a narrow range of distance they operate across. The pins are simply raised targets because the back of the flywheel is too far away by design.

I have catalogs laying around somewhere with thousands of different magnetic induction sensors....also called proximity sensors, prox sensors etc. They have many, many configurations....some set up to read magnets passing by...others where the sensor itself iw the magnetic waiting for a piece of ferrous metal to pass by and trigger it from open to close closed to open. Some are even signal generators that take the on/of signal then generate a seperate pulse.

These are used on millions of pieces of equipment in every industry...some you pass by every day...like escalators and automatic doors and elevatorzs.

Now how its circuit is set up.....all depends on what data they want. The two pins do not have to be at TDC to detect TDC. You simply program in the offset. If TDC is at 0° and due to space constraints you have to put the sensor at 18°.....then 0° = +18 and everything moves in sequence.
Speed/rpm is measured due the known distance between each pin and the time hack between impulses can be divided out in the circuitry.

If the pins are nkt magnetic.....you just need a simple magnetic induction switch. You need to decide if its nkfmally closed or open and what its operating range is.

As for the circuitry itself? No idea.....but I know you can buy things like thia pre-made. They are usually material or speed counters witj programmable options. Ray
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