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Discussion Starter #21
Not a wind up at all, it's just getting to the crank sensor could be easier hence my question about is there another way of doing it, not being particularly clued up on ECU stuff in modern cars and how each manufacterer decides to control them
 

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I think you are misinterpreting the readout. It is not showing advance from TDC, it is showing difference from the base advance. That 2 after is not 2 after TDC, it is 2 after the nominal firing point, which will be some degree of advance. Put another way the indicated 0 is already somewhere between 15 and 25 BTDC and your readout is showing difference from that reference. I doubt it would run at all if it were firing 2 after TDC at idle. WRT knock sensor - you don't expect knock sensing to be performed all the time do you? The only variable that the knock sensor is compensating is the quality of the fuel - this is most easily detected at idle and would be damaging if your engine was knocking at speed under load. The knock sensor sets advance parameter to get best out of fuelat idle and with that base figure the map can take care of the rest of the rpm and load range. Fuel does not change its characteristic except when you put more in so no need to continuously sense knock.

Do you honestly believe that you know the correct ignition timing better than the engine designers?

Tell you what - go right ahead and slot the sensors and report back here the results.
Thread a couple of years old but i came across it whilst searching for ignition timing and what controls it on my 54 picanto.
If you are still around could you tell my how you know what you have stated above? I too got weird ignition timing data from obd, once saw -11 deg which would be 11 degrees after TDC and it jumps all over the place. As -11 is such a ridiculous figure i wondered if it was just an adjustment like you have said - even so the range is too high but to check it i put the strobe on it and it confirmed the crazy readings obd was saying the ignition was bouncing around and sometimes even firing after TDC.
I don't know if its connected my issue is power drop off and even misfire around 2000 - 2800 revs,tick over reasonable and above 3000 rpm fine. It is hard to ignore erratic ignition timing when you have a problem but cannot find a reason for the ignition timing being like this and up to now this op is the first other person i have found that has read it and getting the same silly readings begs the question "is this normal".
Any body else read their circa 54 picanto ignition timings?
 

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The following is not based upon any particular insider knowledge of automotive electronics but draws upon understanding of digital electronics and computing techniques used in real time control systems of many different types.

It is important to recognise that the ECU uses values of various physical parameters to monitor and control the engine. These physical measurements are represented internally as a binary number. All computers have a limited number of bits which are used to store and manipulate these numbers, known as the "word size". Word sizes vary but are most commonly multiples of 8 bits. So a computer with an 8 bit word (other values are possible, typically 16 or 32 or more) can represent values from 00000000 to 11111111. There are means to extend this range (by using two or more words to represent one value) but they involve complications which greatly increase the time it takes to manipulate such values and in real time computing these things are avoided if at all possible since the time to process them is critical (it's no good working out the correct time to fire the ignition if the computation took so long that the time is now in the past!) . So with an 8 bit word the smallest number is 00000000 or "0" in counting numbers and the largest number is 11111111 or "255" in counting numbers - that is to say there are only 256 possible different values. A common computational trick to allow representation of negative numbers would result in a range of between - 127 through 0 to + 127 (the perspicacious among you would notice that this range has only 255 different values - this is because the trick results in unique representations of both +0 and - 0).

Thinking about engine temperature for e.g we can expect a working range of (just picking reasonable values here for illustration) -40 deg C to +120 deg C. It is pointless to waste the numbers between -127 and -41 and the numbers between +121 and +127 since we would never expect to get these values nor would we want our engine management system to operate in these areas. We can get more precision by arbitrarily deciding that the internal representation of -40 is 00000000 and +120 as 11111111, so we are now representing temperature in steps of approximately 0.63 deg C for better accuracy.

The full possible range of ign timing is from -180 degrees to +180 degrees, but as in the temperature example, we really don't need many of these values as we will never encounter them in the real world so a similar trick is used to give better accuracy within the expected range of values by picking an arbitrary reference point and expression all timings of interest in relation to that arbitrary reference.

To further complicate matters there is no reason why manufacturers of ECUs would necessarily pick the same arbitrary reference so unless the diagnostic tool is specific to the type, it's interpretation of the ignition timing point is not necessarily in degrees BTDC.

To even further complicate matters there is no reason why the stored values have to be a linear representation - it is also entirely normal to use non-linear scales so that particular precision is achieved in a particularly sensitive part of the range by having less precision in less sensitive areas of the range (the sizes of the steps are not the same over all the range)

Back to your problem:

Firing after TDC is unlikely to allow the engine to run so this cannot be the correct interpretation of the OBD data.

Using a strobe to measure actual timing is also a bit tricky since you probably don't have a good TDC reference point but in qualitative terms the strobe and the OBD do agree that the timing is bouncing around.

Is the idle stable or is it lumpy or does the engine "hunt". Does the OBD/strobe show reasonably consistent timing at idle?
 

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Thanks for reply, the obd11 data is not about what the ecu is processing internally but what byte of data it sends out down the k/l lines to the recipient. The ecu is supposed to be obd11 compliant ( if the ecu programmer hasn't got that right then i would be worried about the complicated stuff) so it should be sending a byte of data that is then divided by 2 and subtract 64 giving a range of -64 to 63.5 degrees BTDC. so accordingly TDC should be 10000000 or 128 in dec, now i don't know if 10000010 is 1 deg btdc or 1 degree atdc but it doesn't matter for this exercise it still should be outputting accurately the actual timing advance that the ecu is using.

I put a dti down the plughole the marks i used were quite accurate certainly less than 1 degree out.
The idle varies by only about 10 rpm normally occasionally a bit more but then no more than 20 rpm as read from obd, it sounds smooth enough certainly wouldn't describe as hunting.
 

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OK, so I am tempted to conclude from the absence of idling issues that the engine is idling correctly which implies that the readings you are getting are normal (but puzzling). The absence of stored codes indocates that the ECU thinks nothing is wrong. Beginning to wish that I had an OBD reader so I could look at my Sporty and see what is happening.

Have you looked at the timing when running off load at the problematic speeds? Does the issue only manifest under load or can you reproduce it off load?

Can you investigate the crankshaft sensor? I am theorising that there might be a small fault in there, insufficient to be detected as a malfunction but just giving some slichtly wobbly indications. Grasping at straws really.
 

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Fascinating discussion. A long way from my old way of timing . Advance it till it knocks and then retard it a bit, done at side of road of course. But I read the codes on my Piccy 1 but unless there is a fault to fix I would never try to improve the engine. I go on mpg. If this wavers dramatically there is a problem. Keep good records from new ( 10 years ) and it is very consistent. I doubt any dealer would understand the OP's question- or want to investigate it eve for a large cost.
 
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