My air pump acts up in winter and now the bearing is going (my car sounds like it has a supercharger some mornings). So now I have two options: the easy way is to replace it, not cheap, and I haven't been able to find anything used that would fit.

Option two is to 'bypass' it, which would mean deceiving the OBDII system into thinking that the pump was working. So my question is, what input tells OBDII that the pump is working? There must be an air flow switch or a pressure switch to indicate that the pump ran as requested. If I bypass that switch with a relay wired into the air pump power I should be good to go, right?

Folks,

I do not think you can "fool" the computer, with respect to the SAS system. Or, at least, it would be dam-near impossible.

From my understanding of the OBD2 system (and I am by no means an expert), the system is continually monitoring for certain kinds of failures, and occassionally monitoring for other kinds of failures.

The continual monitoring involves sensors which are used real-time. In other words, the system is always checking the validity of the readings it is receiving from the water temperature sensor, or the O2 sensors, or the cam position sensor, etc., etc. It needs that information to perform its most basic functions and if, all of a sudden, it stops getting a signal from one of these critical components, "it throws a code." I believe that the presence of the air pump is one of these continually monitored values and you probably can fool the system into thinking that the missing pump does exist.

But there is another side of the OBD2 system which performs occassional tests on some of the more complex systems, and the SAS system is one such system. These tests are run at random times and only when certain conditions are met. These are the tests that we've all read about that require that your 850 be driven for several weeks after you've had OBD2-related repairs and before you take your car in for testing. Unless you have encountered all of the circumstances that are required to run these tests, some of the tests will not be performed and you may fail emissions testing.

The SAS test, like some of the other tests, only initiates when the car is started in a certain temperature range. I forgot what that temperature range is, but it is possible to go a whole winter and never trigger this test because you may only start your car when it is below 30 degrees, and the test may require the car to be started at some higher temperature.

Anyway, in this occassional SAS test, the computer tells the pump to start and the vacuum solenoid to operate and the computer "looks" at the O2 sensor to see if it is responding as would be expected if the whole SAS system was operating properly. If the O2 sensor does not seem to report any change, even though the SAS test was initiated, then the computer concludes that the SAS system has malfunctioned and it throws a code relating to your SAS system.

So, you may be able to fool the computer as to the presence of the air pump, but I find it highly unlikely that you are going to fool the computer with respect to the occassional SAS test.

Not to go into too much detail, but I believe the OBDII system continuously checks sensors based on conditions and output status. Faults are reset if they have not re-occurred in x key cycles, or km, or hours of operation. This is also manufacturer specific. Some cars never reset themselves.

For example, an egr solenoid would have a flow switch that verified exhaust flow (or pressure) whenever the solenoid is actuated. This is typically monitored 'continuously', i.e. the computer checks it once every scan. If the solenoid is supposed to be open and the flow is not present in x seconds (xx scans) then the CPU throws the EGR code.

And while I thought the air pump had a sensor, I think Klaus is correct, the OBDII sytem just checks to make sure the downstream oxygen sensor does not read 'rich' after a cold start. If that sensor were to read rich it would indicate that the catalytic converter is still cold, meaning the air pump failed to heat it and reduce emissions.

So, the device Klaus suggested mimics the voltage that a happy downstream O2 sensor would produce. Whenever the OBDII system chooses to look at the voltage it sees a figure in range and it will never throw a code.

It's a very cheap fix, I think I will try it.

So, you are, in essence, going to disable your O2 sensor so that the computer doesn't "see" the issue with the SAS system?

And the O2 sensor is used solely for the purpose of testing the SAS system???

Good luck!!!

The downstream O2 sensor monitors the catalytic converter efficiency.

What's the harm in ignoring it?

Yes, you can. Read :

http://www.brickboard.com/AWD/volvo/1043078/V70/secondary_air_pump_can_bypassed_fool_computer.html

That should get you started.

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)

Thanks Klaus.

I found this generator link http://www.dsmtalk.com/forums/showthread.php?t=162906 and I'm guessing I only need to fool the OBD system when the air pump should be on.

I've read a few reports of $7 devices. I'll look for one.