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The Check Engine Light
There are some things in life we'd rather not think about. Things like paying taxes, a smog inspection, or that accursed check engine light. One flash from that light can completely ruin a weekend. Picture a beautiful Saturday morning. The road is clear, the car is running flawlessly, and you're enjoying life. Then it happens. You have to do a double take. You glance at the gauges, and they're normal. The car seems to be running OK. How can that light be on? Next come the mind games. There are a million questions that you don't want to face, but you have no choice. What could the problem be? Will the car get me back home? Is it OK to keep driving? Should I just stop it here and get it towed? Can my mechanic start working on this "first thing" Monday morning? Will the parts be in stock? How much is it going to cost? Some people tap the warning light with their finger. Over the years I've read a lot of repair manuals. I don't remember a single manual that recommended tapping the warning light. You might as well settle in until Monday when the shop opens.
I'm old enough to remember when OBD was first announced. The uninformed predicted doom and despair. After a little reading, I had a slightly different outlook. OBD stands for On Board Diagnostics. OBD is typically what causes the check engine light to illuminate. Having OBD meant modern cars actually kept track of what was happening under the hood. For the first time in my life, the check engine light would actually mean something.
My prior experience with cars and check engine lights typically fell into two opposing groups. In some cars, the "Check Engine" light meant absolutely nothing. For those cars, the light was only there in case you somehow missed the fact that the entire car was in flames. I have a friend whose 1970's era car had severe rod knock and no oil pressure. The engine sounded like a jackhammer, shook like a junkie going cold turkey, and had the power of a pogo stick, but the light would not illuminate until the engine stopped running. On the other end of the spectrum were BMWs. In the mid-80's BMWs were famous for "the light". Anything could trigger it. If an oil change was due, the brake pads were getting thin, or the moon was full, that light would come on. On a BMW, once that light was on, the "fun" began. Many independent repair shops could fix the problem, but they couldn't reset the light. That usually took the dealer's special tool. The tool that BMW dealers only brought out after performing the officially authorized four hundred dollar repair. I had friends who simply removed the check engine light's bulb. That was the original "Don't ask, don't tell".
Most cars weren't as bad as BMW's, but as fuel systems became more complex, they became tougher to diagnose. The basic idea behind OBD was to have the ECU (the car's engine computer) do all the work. The ECU would monitor various components, and if anything was amiss, the "Check Engine" light would illuminate, and the ECU would store a code. This was much better than the old system. With the old system, the car would start missing and someone with a four million dollar piece of diagnostic equipment told you that a $25 sensor was bad. Unfortunately, running diagnostics on the car cost ten times as much as the broken part. With OBD, those of us "hands on" types could run diagnostics without having to own mega-dollar test equipment.
As you've no doubt guessed, I like OBD-I. The problem with OBD-I, was the federal government came up with OBD-II. With the feds, if a little is good, then a zillion times that amount should be wonderful. That's the reason the feds aren't in charge of making sandwiches. Can you imagine the amount of mayonnaise on a federal sandwich? After one sandwich, you'd be sick of mayo. And that's exactly what happened when OBD-II first rolled out. With OBD-II, just about everything is checked. Maybe I'm overstating things a bit. Everything isn't checked. Just the "electronic powertrain components, systems, and subsystems that affect emissions during normal driving conditions." I think that eliminates the cigarette lighter and the glove box light. Everything else is fair game. Lets take a look at a couple of the major systems that OBD-II monitors.
The quality of the catalytic converter is monitored. The quality of the converter is determined by using two O2 sensors. The data from the first O2 sensor (before the catalyst) is compared with data from the second O2 sensor (after the catalyst). Between the two sets of numbers, the ECU can determine how good a job the catalytic converter is doing. If the numbers from the two O2 sensors are out of range, the check engine light can illuminate. The downside is this; the values supplied by the O2 sensors are based on the amount of oxygen the catalyst material can store. The less oxygen stored, the less effective the catalyst material of the OEM cat. That's good. We may want to know when the catalytic converter is worn out. Unfortunately, there are catalyst materials that may be more efficient than the OEM materials. Some of those new materials don't store oxygen. That means they're incompatible with OBD-II. They'll work fine with OBD-I systems, but cars with OBD-II will have to do without.
Another system that's monitored is gasoline vapor recovery. You know those signs on the gas station pumps that say you shouldn't breathe for the entire time it takes to fill your car? That's due to gasoline vapors. The EPA says gasoline vapors are a form of pollution, so those vapors have to be accounted for. The way most vapor recovery systems work is this – gasoline vapors are captured in a storage container. When the car is running in closed loop mode (at cruise and being monitored by the O2 sensor), those captured fumes are inhaled through the intake system. That's called a purge cycle. In addition to keeping track of purge cycles, OBD-II must also test the vacuum system used to capture the fumes. To perform the test, most systems create a vacuum in the system and then check to insure the vacuum doesn't leak down too quickly. If it leaks too quickly, OBD-II registers that as a problem. One common way leaks occur, is when the gas cap isn't tight. Unfortunately, there are other ways to artificially fail the test. If the test is running and the car passes over a bumpy section of road, the gasoline is agitated. That increases the pressure in the tank. The increased pressure means the vacuum is decreased. OBD-II views that as a failure. Fortunately, many systems require a number of failures in a row before illuminating the check engine light.
OBD-II also constantly monitors engine smoothness, or more accurately, the lack of engine smoothness. As expected, engine misfire can cause the check engine light to come on. We want that. If the engine misfires, something is wrong and the light should come on. Crankshaft speed is a good way of checking for engine misfire. That's actually what many owners use to determine a miss. They either watch the tach jump around, or they feel the car shake. That shaking is caused by abrupt variations in crank speed. The ECU has to watch the crank for the ignition anyway, so why not use crank speed to monitor misfires? This sounds good on paper, but there are some other things that have to be taken into consideration. If the AC cycles on while at idle, the ECU has to compensate by increasing the idle speed, or the engine may stall. If the sudden drop in crank speed that occurs before the idle speed is adjusted is viewed as a miss, the light may illuminate. The same thing applies when turning the steering wheel at rest. One way to confuse OBD-II is to stall a car with a manual transmission. A stall is often viewed as one big miss. ECU manufacturers have to take all this under consideration when designing OBD-II code. One way of lessening these problems is to use a heavier flywheel. The increased flywheel mass doesn't allow the crank to react to increased load as quickly, and that keeps the engine running smooth. The down side to using a heavy flywheel is just as it makes the engine is slower to decelerate, it makes it slower to accelerate. Some of us performance-minded types like our acceleration to occur as rapidly as possible.
The good news is this; some failures are viewed as non-reoccurring. They may cause the light to come on for a short while, and then it will turn itself off. In an OBD-II system, some of the failure codes are written to what's called short-term memory. That means if the problem doesn't reoccur within a certain time frame, the code is erased from memory. So if your light comes on for a short while, and then turns off, and you wait too long to take it in for service, the dealer may test the car and have it come back clean. It's not that the dealer missed it. There was nothing to see.
A great piece of news about OBD is that it allows those of us who like to tinker with our cars to be able to "dump the codes". Basically that means we can run some diagnostics. By running diagnostics ourselves, we can quickly see the results of the ongoing ECU tests. In many cases, this narrows down the list of possible problem areas.
With OBD-I, retrieving the codes can be simple. In some cases, the owner has to build a code checker. This can entail buying a pair of LEDs and some spade connectors from an electronic supplier like Radio Shack. It costs less than $10 and takes about 15 minutes to assemble. After assembling the checker, four wires are plugged in, the ignition is turned to the on position (without starting the car), and the lamps will begin to blink. The sequence of the lights determines the code values. Some OBD-I cars are even easier. I've seen cases where a single wire is grounded or a sequence of radio codes are entered, the ignition is turned to the on position, and the check engine light will flash the error codes. Once again, the sequence of the lights determines the code values. OBD-II is quite a bit different.
On OBD-II cars, a scan tool is required to look at codes. Not a set of wiring instructions that were cobbled together and hosted at "Joe The Hacker dot COM's" web site, but a real scan tool. In addition, every $50 generic OBD-II reader won't work. Mercedes-Benz owners have to use a scanner that utilizes the CAN (Control Area Network) protocol. The CAN protocol is currently being used in a number of vehicles and is scheduled to be mandated for all vehicles sold in the US by 2008. Earlier I wrote about using a single ECU to keep track of all the various components. CAN technology is a shift from using one computer to control and keep track of everything, to a distributed system of computers where each computer has it's own area of responsibility. Scanner tools that utilize the CAN protocol are readily available and not very difficult to use. You follow the instructions that came with the tool, plug it in, and if any failure codes are present, they will be displayed. However, if any of this makes you feel uneasy, then stop right now. You don't want to plug just any piece of electronic E-Bay wizardry into the DLC-2 port on a very expensive car. If you melt down an ECU, and have your car towed into the local dealer for "warrantee repairs" you may be in for a shock. A big shock. You may find out the warrantee expired the second you plugged that cable in. Read this carefully: There is a HUGE DOWNSIDE if anything goes wrong.
That having been said, there are some outstanding tuning aids available. Aside from the assorted standard scan tools that are available, there are things available to the OBD-II community that OBD-I owners can only dream about. Using a MS Windows based PC and less than $400 in software and cables, there are products available that will allow you to read and clear codes, run diagnostics, and turn off the Check Engine light. You expected that. What you may not have expected is the ability to see HP and torque measurements, 0-60 times, ¼ mile times and speeds, MPG figures, view multiple sensors simultaneously, record sensor data, freeze frame data, full gauge information, O2 diagnostics, and have the ability to upload all this data to a PC. This info is in real time, the plots are in real time, and it's acquired while the vehicle is running. This allows an owner to buy a tuner part, install it, and then determine exactly what the results are.
One last thing to keep in mind regarding "do it yourself" repairs: There is a fair amount of expertise required to repair a modern car. That expertise goes well beyond running OBD tests and throwing parts at a vehicle. If a part breaks, especially an electrical part, there is a reason why. In a best-case scenario, the part could simply be past is useful lifetime. However, it could be something else. Bad wiring, bad grounds, or other bad electrical parts that reside either upstream or downstream can all play a big part in the failure. It may prove prudent to determine why the part broke. If you don't know why, and you feel like you're in over your head, stop. There's no crime in taking a car to a shop for repairs. When choosing a shop, you may want to ask around. Any hack can hang an "auto mechanic" sign in front of their shop. To get the job done properly oftentimes requires specialized diagnostic equipment and a Mercedes-Benz background.