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Technical_   Engine_  OBD II 101

OBD II 101
Chris Bishop

The following information is intended for E30 owners who plan to swap their motors for one from a late model BMW, specifically any BMW built after 1996. Since there are a lot of people wanting to use the M52/S52 engines from the E36 323i328i/M3 or E46 325i/328i/330i for the swap, I found Chris Bishop's article on OBD II equipped cars to be very useful for those of you interested. -FK

I've noticed quite a few questions about OBD II the last couple of weeks, so I figured I'd do a quick writeup. Here's a quick rundown on OBD II:

OBD (On Board DIagnostics) is an emissions related standards program for all U.S. produced vehicles which was implemented by the California Air Resources Board (CARB) to tightly control emissions from passenger vehicles and light trucks. The second generation of OBD (OBD II) became federally mandated in 1996.

The core purpose of OBD II is to prevent the vehicle from producing excessive emissions (1.5 times the legal limit for a given vehicle). The OBD II-programmed PCM controls all aspects of engine management and extensively monitors all electrical engine components, exhaust emissions, as well as evaporative emissions from the fuel tank. The PCM has an extensive memory and is fully adaptive, so it is quite intelligent. When the PCM does not recognize a component operating properly (or a proper operating condition) it can compensate for the loss of input to prevent the vehicle from producing excessive emissions. Depending on the component failure, the PCM may switch to "limp home" mode, allowing the engine to operate enough to drive the vehicle to a repair facility while still preventing the engine from producing excessive emissions. Believe it or not, on most OBD II vehicles, the engine can operate (although not very well) with virtually every electronic sensor removed from the engine (except the crank sensor) and still manage to keep the emissions output below 1.5 times the legal limit

Besides the basic engine management system components (crank and cam sensors, throttle position sensor, etc) there are many things the PCM must "see" in order to operate normally, such as evaporative emission purge/vent valve signals, fuel level signal, engine accessory load signals, brake and clutch position signals, etc. ALL of these things can affect emissions output, so it is very hard to "fool" an OBD II system. For instance, you cannot simply put a resistor on the ECT sensor to make the car run richer. It will simply compensate for the faulty signal and possibly set a component failure code, or even a "condition" code. The PCM not only monitors the resistance and duty cycles of the various engine management components, it "watches" the activity of them. If a steady 0.5 volt signal is fed to the PCM in place of one of the oxygen sensors, it will set a code indicating that insufficient activity has been detected at that particular sensor. The PCM also compares signals with various sensors and makes sure that they "connect" with each other. For instance, if the mass airflow sensor is indicating a small amount of incoming air and the signals from the TPS, MAP, and O2 sensors do not match accordingly, a code will be set, indicating an air leak.

OBD II cars aren't bad by any means. They are somewhat complex, but OBD II scan tools are very friendly and can be helpful to a skilled mechanic when diagnosing a problem. But OBD II becomes a major problem when one attempts to modify the motor. Since the PCM "knows" the engine, if it is modified then it may think something is wrong with the engine and set a code. In some cases, the PCM will adapt to minor modifications (intakes, exhausts, etc). But a modification like changing the fuel injectors may set a code since the PCM compares the duty cycles of the injectors with the input signals from the O2 sensors. If the fuel trim values become too large, since the injectors are flowing more fuel for a given duty cycle, a code will be set. The PCM may even set a code if the resistance of the fuel injectors does not match the originals.

Now, when it comes to E30 PCMs, Motronic 1.1/1.3 is extremely dumb compared to OBD II, (kind of like comparing an Apple IIc to a G4).

With all of that said, I hope I've shed some light on why it's so difficult to transplant an OBD II engine management system into an E30. OBD II doesn't "suck", it's just not very possible to work with when performing an engine swap. So, if you're hunting for an E36 engine for a swap, steer clear of OBD II.

Chris
cnbishop@rcn.com

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