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Volkswagen

VOLKSWAGEN OBD II DIAGNOSTICS

edited from an article by Larry Bailly, ImportCar, October 2001

Volkswagen was one of the first carmakers to install an on-board diagnostic (OBD) system starting in June 1971. As of that date, all VW cars had a datalink socket installed in the engine compartment that was designed to connect to the dealer’s diagnostic computer, similar to a modern scanner but only in a much larger box. It really just reduced the number of separate connections needed to make the same basic tests. Of course, compared to the latest technology, this system monitored very few vehicle systems. The system was used to reduce the inspection time needed to find basic faults in the charging, starting, ignition and electrical systems, and provide the customers with a printed report on the condition of at least the basic operating systems of their car.

In the late ‘70s and early ‘80s, the system got more sophisticated, with the ability to make system checks and get basic faults indicated by a flashing light, the original MIL or Check Engine Light. These early OBD systems gradually added more monitored components and, as computer memory grew, more information was analyzed and more memory was retained. Data retrieval became possible if you had the scanner and adapters needed to download and analyze it to make repairs.

Now we have OBD II. In essence, a systems analyst is sitting inside a box, designed to constantly monitor and control vehicle operation, alerting the driver when something is becoming a problem, and even making adaptive corrections when possible to keep the vehicle in compliance with ever-tightening emissions requirements. In this article, we’ll take a look at the major components of the VW version of OBD II, and some common problems encountered in the shop and where to look to find the solutions.

GETTING CONNECTED
As required by law, VW’s OBD II system is standardized as far as the location of the datalink connector and the basic parameters of operation and monitoring. On the most common VW models (Jetta, Golf and Cabrio), the DLC is behind a panel next to the ashtray. Just remove the ashtray and then slide the little panel to the left. On other models, the DLC will be under the driver’s side of the dash, or there will be a sticker indicating the location of the connector.

Connection to the DLC can be made through any generic scanner with OBD II capabilities, however, depending on the software available you may be able to access only basic OBD II information. Additional functions and information can be accessed with either the factory V.A.G. 1551/1552 or an aftermarket scan tool. With the use of a special adapter, these scanners can also access fault codes on earlier OBD I cars. Other functions can be performed through the DLC as well, including resetting maintenance lights, adjusting basic settings, checking codes for automatic transmission and air bags, and getting actual data while driving. Blink codes are not available on OBD II-equipped VWs.

Once you retrieve fault codes from the scanner, you will need a list of DTCs and information to trace faults and resolve them. VW has its own fault numbers that are converted to OBD II "P" codes, as well as assigned numbers for individual components in the system, to help locate the part that has failed. VW recommends that any time you have driveability problems, the first thing you should do is check for fault codes. I agree, in that it will save a vast amount of diagnostic time for you and your customer.

Using the V.A.G. or aftermarket scan tool, you can also access block values (VW calls them channels) for various operating parameters, check actual values on individual components, and test individual components depending on year and model and supported components. The best source I have found for DTC identification is the factory Bentley manuals. The latest edition (for 1999 and 2000 Jetta, Golf models) lists more than 700 DTCs! I recommend that you also check for any applicable TSBs for additional diagnostic information and factory remedies.

The cost of the scan tool is related to how much it can do. For basic diagnosis, a generic scan tool might be okay, but we find we do a lot of sublet resetting of MIL lights for other shops that can’t do it with their generic versions. But, a more advanced scan tool, on the other hand, can be used to obtain a more in-depth diagnosis and for resetting faults.

After repairs are made to correct the faults, you’ll need to reset the faults and then take the car on an OBD II "drive cycle" to make sure that all readiness codes are set and then recheck for faults to make sure they are gone. We have learned this the hard way when customers come back in just a short period of time with the light back on and the same code set.

VW also recommends performing the OBD "basic setting" after a battery disconnect or repairs. Resetting the basics "zeros out" the adaptation process to prevent driveability problems. However, when you do this, let the customer know what you’ve done. In that way, if there is a noticeable difference, you can let him know that the adaptive capabilities of the system will adjust for best performance and economy after a short period of time.

DIAGNOSTIC TROUBLE CODES
Again, I recommend that if you get faults on any late-model OBD II car, before you do any more than basic diagnosis, check for applicable Technical Service Bulletins. Having access to available TSBs, which you can review on-line or print out, is a very important part of your diagnostic efforts. The car manufacturers see far more common problems than do individual dealers or shops, and can make suggestions for the most common faults. However, I have seen where even the factory engineers overlook really basic problems and come up with rather complicated repairs that can sometimes be corrected with a simpler "fix."

Camshaft Position Sensor (P0341/16725): This is probably the most common fault code we see on a regular basis. It signifies - "Engine quit while driving, no start." On 4-cylinder vehicles, the Camshaft Position Sensor (CMP), VW component G40, is part of the ignition distributor. We usually get this code at around 50,000 miles or when the car is towed to the shop. Pull the upper timing cover and you’ll find either a shredded belt or one that’s so loose that the timing has gotten too erratic for the computer, causing a misfire. Replacing the timing belt will usually correct the fault but, as noted above, make sure you do the "drive cycle" after erasing the code and then recheck to make sure the code is gone, or the car will come back with the light re-illuminated. Another point here, the distributor on OBD II engines is "locked" in place, so getting the timing dead-on after changing the belt is extra important. On difficult cars, the locking device can be removed to make timing adjustments possible. The 6-cylinder cars don’t have a timing belt, so the sensor may be bad. But first check the engine grounds and harnesses for abrasion problems.

Random or Individual Cylinder Misfire Detected (P0300, 0301, 0302, 0303, 0304): The most common reason for these codes is that a car is in serious need of a tune, or they result from doing a compression test during a tune-up. We’ve stopped doing routine compression tests on OBD II cars for this precise reason. Besides, if there were a compression problem, there would be other codes present as well. Another common cause is spark plug wire resistance or insulation breaking down. Even though the wires may look good, after 50,000 miles or so they may be "bleeding" to the engine block or other wires. We have also seen these codes when "other than specified" spark plugs are used. Obviously, carbon tracked distributor caps and burned-through rotors will also cause these codes, so remember to first check the basics. These codes usually reset themselves after repairs are made, but performing the "basic settings" will also help the car run better.

Throttle Actuator (B1) Malfunction (P1580), Mass Airflow Circuit (P0103): These codes may be set due to the driver attempting to start the car with a low or dead battery. Either replace or charge the battery, erase the codes, perform the "basic settings" procedure and road test the vehicle to make sure the codes are gone.

Numerous Adaptation and Fuel Trim Codes: There are too many of these codes to list, and some of them come up quite often. Usually, a look at the overall condition of the vehicle is an indicator of the main problem, which is a lack of maintenance. The adaptation process takes some time to get to a point where it will set a code and turn on the light; not usually a quick failure. The first things to look at if there are no codes indicating a component failure are: Crankcase oil dilution, oil collected on MAF sensor wire, leaks in breather system, and plugged air or fuel filters. Sell the obvious repairs such as an oil change and tune and filter replacements. Also, let the customer know that there may be a hidden problem, but that normal maintenance may stop the light from coming on again and even make the car run better, too! This is a place where a smoke generator will come in handy to find leaks in the breather system, and a fuel injection system service may make a big difference.

Evap System Malfunction (P0440/16824): This one happens often, so don’t overlook the obvious, especially when the cooler weather approaches. If the customer didn’t tighten the gas cap, left the car running while filling the gas tank, or left the cap at the station, the evap system will not work and the code will be set. Obviously, if the basics are okay, then look for vacuum lines that are off, then test the evap components per the manual.

FINISHING UP
OBD II undoubtedly makes diagnosing these increasingly technical systems easier. The new VWs are great cars and their OBD II systems are state-of-the-art. Those statements do not tell the whole story, however. These cars will rack up a lot of miles, require maintenance, service and repair over their lifetime, and certain parts will eventually fail. OBD II diagnosis will make the service industry upgrade to be able to access the information and make repairs.

If you are going to enter into VW OBD II territory, make sure that you do your homework, get the right tools and obtain the necessary information to make your repairs last. I caution you to research any equipment before you buy it, to make sure that it can be upgraded and have the ability to access more than just generic codes.

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