OBD-II Permanently Connected

blake7984 · 09-15-2003, 01:55 PM

I am not sure if this has been addressed in the past or not, I did not see anything directly discussing this from searches I did. How safe is it to leave an OBD-II reader permanently connected and constantly reading/polling data from the ECU? I have heard that initializing the OBD-II connection puts the engine/ECU in a somewhat "special" mode which makes me wonder if leaving it consantly running is not good for the car. Let me know if any of you have any information on this.

jasonsp6 · 09-17-2003, 06:41 PM

I am also interested in this... it's my understanding the ECU must be taxed somehow by the OBD-II link constantly polling it for data...

Is there anyone out there that can provide any info on this?

cproaudio · 09-17-2003, 07:33 PM

Mines been connected since Nov 2002. havent notice any changes in performance, gas mileage. Check engine never came on due to OB-II plug. It only collects data when you're running the software.

blake7984 · 09-17-2003, 07:59 PM

cproaudio, how much have you had the software running and collecting data? also, what kind of car and year do you have? most practical applications of OBD-II in a car computer setup would involve polling pretty often. and just for confirmation, simply having the obd-ii reader connected to the car has *no* effect on the ECU until it is initialized and polling, correct? Thanks for the help,

-Blake

cproaudio · 09-18-2003, 04:45 AM

I have a 97 Toyota Tacoma. For my setup, the obd-ii has been connected since Nov of 2002. I use the obd-ii software for digital dash at least once a week. Everytime I used it is for at least an hour. Most of the time I use it on long trips 100+ miles per trip. So far I have not experience any effects on the ECU. It's been almost a year now. It only initialize the scanner when the software is running.

brrman · 09-18-2003, 10:17 AM

I have driven my Audi running OBDII diagnostic software for extended periods of time with no side effects.

wi77iam · 09-18-2003, 06:47 PM

Quote: Originally Posted by jasonsp6

I am also interested in this... it's my understanding the ECU must be taxed somehow by the OBD-II link constantly polling it for data...

Is there anyone out there that can provide any info on this?

A data logger or computer using an OBDII connector is not a polling device, i.e. it does not suck data out of your ECU. Data is just available at the connector for reading at the rate that the ECU outputs.

There should be no problem having a permanent connection.

gork · 09-18-2003, 07:20 PM

It's worth noting that some ODB II hardware are actually read-only .. ie they dont even have the ability to write to the port. From the standpoint of an auto technician, it would not make much sense for the car to change the way it performs when the connector is plugged in since it would hinder the ability of the tech to diagnose a problem.

Notwithstanding, many cars do have special modes that hamper performance. I Know my car has an "Emissions Dyno Mode" where it will not rev above 6K RPM, and that mode is probably accessible via the diagnostic connector, but I imagine that you have to send some special commands down the wire to get a car to switch to some other diagnostic mode.

blake7984 · 09-19-2003, 12:51 PM

data from the OBD-II connector does have to be "requested" from the ECU (i know this as a programmer....a few bits are sent, and then a few response bits are returned)....but overall it does sound like from these replies that it should be fine to leave it permanantly connected and running. i agree that it would not make sense to have the engine go in to a different mode when you are trying to diagnose information about what it is doing. anyways, thanks for the help, i believe my question has been answered now.

-blake

crash893 · 09-21-2003, 07:48 PM

if it was readonly how would they use it to clear check engine lights and such?

Magnezium · 10-06-2003, 01:29 AM

OK. This shoud clear EVERYTHING up. Computers have a VGA out. This is our output from the system. We all know that whether the monitor is plugged in or not, it doesn't affect the performance of the system. This is because the VGA adapter sends the information whether it's read or not by the monitor. It's the same in the case of an OBD II, the output is 2 pins (Far right pins). These are the equals to the VGA out on a computer. The software you get to read from these pins does not "poll" the ECM or ECU. The software only displays what is already being sent. OBD II is NOT necessarily read only. OBD I (One) was Read Only. It was an EPROM that was changable. OBDII (Two) is a perminant IC. Therefore it is programmable. Making it a non-read-only system. However, you wont damage it because the software that you use to read the info from the OBDII interface doesn't send any information. When you clear the "check engine" light, you're actually telling the computer to clear current data (we call this the "shut up" command") and the light turns off. But it doesn't stay off. If you still have a problem, the second it occurs, BANG! "Check Engine" is back to haunt you.

If you're just trying to get speed and rpms (simple data) from the OBDII, it's best that you don't use that kind of software. Instead, just use VB or C++ to read the 2 pins I talked about earlier. That way, you don't have to spend $200+ on software and an adapter. There's PLENTY of info out on the internet about this. Give it a search.

BigBoogieman · 12-13-2003, 05:37 AM

This is incorrect, OBDII logging software absolutely does poll the port. The way it works is that you send a specific request to the OBDII port, and it returns a single piece of information. For example, there's a command to request the current RPM's. The port DOES NOT constantly output data, only as it's requested. This is why OBDII loggers only allow you to log a certain number of parameters at a time, and why the sample rate is lower the more parameters you log. If your logger is logging RPM's, speed and ignition advance, for example, it constantly requests those values. The conversation would look like this:
Logger: Give me current RPM's
ECU: 6000
Logger: Give me current speed
ECU: 50KPH
Logger: Give me current ignition advance
ECU: 2 degrees
Then this entire conversation would repeat. The more parameters you want to log, the less often the logger will be able to obtain their values.

To answer the original question, I believe the reason why the OBDII port operates at a rate of 10.4Kbps is specifically because at that slow transfer rate, the ECU won't have any trouble keeping up with it. The protocol was designed for diagnostics, not really for long-term data logging, so this speed is reasonable. But there should be no problem using the port for data logging all the time because it requires very little processing from the car to answer these requests. Your logger is severely limited on how fast it can make requests, because both it and the ECU spend most of their time waiting around for the bytes to be transferred.

Quote: Originally Posted by Magnezium

OK. This shoud clear EVERYTHING up. Computers have a VGA out. This is our output from the system. We all know that whether the monitor is plugged in or not, it doesn't affect the performance of the system. This is because the VGA adapter sends the information whether it's read or not by the monitor. It's the same in the case of an OBD II, the output is 2 pins (Far right pins). These are the equals to the VGA out on a computer. The software you get to read from these pins does not "poll" the ECM or ECU. The software only displays what is already being sent. OBD II is NOT necessarily read only. OBD I (One) was Read Only. It was an EPROM that was changable. OBDII (Two) is a perminant IC. Therefore it is programmable. Making it a non-read-only system. However, you wont damage it because the software that you use to read the info from the OBDII interface doesn't send any information. When you clear the "check engine" light, you're actually telling the computer to clear current data (we call this the "shut up" command") and the light turns off. But it doesn't stay off. If you still have a problem, the second it occurs, BANG! "Check Engine" is back to haunt you.

If you're just trying to get speed and rpms (simple data) from the OBDII, it's best that you don't use that kind of software. Instead, just use VB or C++ to read the 2 pins I talked about earlier. That way, you don't have to spend $200+ on software and an adapter. There's PLENTY of info out on the internet about this. Give it a search.