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05/22/2006

  OBD2 - Refresh Rate

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OBD2 - Refresh Rate

 

OBD - Refresh Rate

Train Window Syndrome

 

Recently, an owner contacted me with a scan. His OBD system had showed a TPS (Throttle Position Sensor) error while he was actually scanning it. He ran through the data again and again, and could not find anything wrong with the TPS data, even though the DTC was time-stamped during the scan. Mystified, he contacted me and asked why his system had failed.

Of course, the OBD had not failed and there was nothing wrong. The DTC had been stored correctly, it was just that the refresh rate of the Vehicle Explorer had been too slow to capture the event. Picture yourself as the passenger in a High Speed Train: no matter how vigilant you are, you are only going to see a tiny fraction of the landscape detail as it flies past your window.

Vehicle Explorer is your window to the OBD events as they flash past: 0.1 Hz (ten times per second) is actually a very respectable refresh rate - some other OBD tools I have tried are much slower. But even this refresh rate means that 1 source can be sampled only slightly under 10 times per second, 2 sources five times, and so on. If you sample meaningful data, like RPM, MPH, TPS, HO2S11, HO2S12, HO2S21 and HO2S22 - and the fuel trims LTFT1 and LTFT2, each of these items will be sampled and recorded only perhaps once per second.

 

Refresh rate from a VE scan. The rate is shown on the bottom right of the screen, Sample Hz 0.1 Note that the data reflects this - the scan effect is confusing but the data recording is 0.1 seconds apart.

Good enough for idle, though, isn't it? Well, not really. When you consider that the EECV is capable of at least 1.5 million instructions a second (and some sources put it at 2.5 million) then you realise how much data is being missed. Even at idle of 800rpm,, the six cylinder engine experiences 40 cylinder events (power strokes) per second, (and 26 for the DOHC engine) so it is no wonder that the OBD can detect and record a DTC for a single sensor in between a scan. A second is a very long time in OBD !

Some sensors are much more likely to be captured misbehaving. An example is the HO2S (lamda) sensor, where a failure to 'switch' is quite likely to show up on a scan, because the time span for this will be several seconds.

 

Slow-responding HO2S sensors are easily captured -  the bottom trace is the Bank 2 sensor registering a DTC for slow-response (for over 2 seconds) and high voltage. The red trace above is a reasonable healthy sensor.

So when scanning do not expect to see the actual event recorded in the DATA. If you do it is a bonus and this should not to be expected. Instead research the DTC you find, then select the particular sensor on its own. Watch its behaviour to see if you can confirm the error.

 

EricR

 
 

 

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