Pulling Fault Codes

When the magnificent W140 was introduced, the control systems in the vehicle were state-of-the-art. Even now, years later the W140 is still a sophisticated automobile. The car is filled with computers, modules, controllers, sensors, switches, etc. that all communicate on a Computer Area Network (CAN).

If you're serious about understanding problems that will come up with your car, and to be able to perform higher level repairs on your W140, I strongly recommend you equip yourself to be able to read and clear fault codes. I recommend this even if you do not intend to do the repair work yourself. It is always to your benefit to understand what may be wrong with your car when repairs are needed. A well-educated consumer is less likely to find themselves in unfortunate situations when getting their vehicle repaired.

The 1995 MB has what is called the OBD system. OBD stands for On Board Diagnostics. 1996 saw the introduction of OBD II, which is the second generation of OBD. Once OBD II was introduced, the original OBD also became known as OBD I. OBD and OBD I are the same systems.

When I first bought my S600, I had three separate occurrences of dash warning lights come on. Each time my dealer wanted $110.00 to hook the car up to read the codes. I wasn't really upset at the fee, after all, they are providing a service. However, I realized that a few more trips like this to the dealer, and I could probably buy my own scanner.

As luck would have it, I found Jim F's website K6JRF Auto Page. He describes how to build a home-made "scanner". It's really a simple device that uses an LED to blink fault codes. I made one of these following his directions (click here, and go to the bottom of the page). It works well, but only for the analog systems in the car. The 1995 W140 has a mix of analog and digital systems. For the digital systems, like the SRS system, it will not work. A scanner will be needed for these systems. You will also need to buy a copy of the fault codes in order to interpret the blinks. Jim has a nice bound manual for sale. Click here, and go 1/3 down the page.

Scanners are available in different capability levels. The basic scanner is able to read stored codes and clear them. The next level of scanner is capable of reading live data. In other words, when it is hooked up to the car, you are able to see data streams from the various systems while the car is running. This is very helpful for diagnosing faulty systems. The next level of scanner allows you to turn on various components, such as the fuel pump or air pump. The final and most expensive of scanners can program new modules, such as the air bag modules. Modules like these must be programmed to your specific car model. These scanners can also pull software from existing modules and upload to new modules. As capability increases, so does price.

The good news is that there are many inexpensive scanners for OBD II systems. So for those of you with '96 or later cars, you're in luck. The bad news is that this isn't so for OBD, '95 or earlier. There are several VERY expensive OBD scanners available, but I could not justify spending several thousand dollars.

After much research, I found a scanner for approximately $575.00 made by a company called Trisco. It's called the Palm Scanner, and it's easy to use. Think of justifying the cost this way...5 trips to the dealer to read codes, and it's been paid for. Rather than duplicate a lot of information here on the Palm Scanner, I would like to point you to Jim F's excellent website. He wrote a detailed review of the Trisco Palm Scanner. Click here to read. Keep in mind that the Palm Scanner is the most basic type of scanner you can buy. It is capable of reading stored codes and clearing them.

I found that the combination of using the home-made blinker, and the Palm Scanner covered almost everything in the car. There is at least one system that neither of these devices will read properly, and that is the infamous ADS (Adaptive Damping System). I am not sure why this is, but false codes are retrieved when trying to pull codes from the ADS module.

I would like to share some info on how to use the home-made blinker. First, always make sure you see 12 volts across pin sockets 1 and 3 on the 38 pin diagnostic connector. Some systems in the car are extremely sensitive to voltage. You could get faulty readings if the car's voltage is too low.

1. Hook up the blinker to the connector. Socket 1 in the connector is Ground, and Socket 3 is +12 volts. The third wire of the blinker is connected to the socket of the system under test.
2. Have paper and pencil ready.
3. Turn the ignition to ON. This is the last detent position BEFORE the key will start the car. All instrument cluster lights should be on. Leave the ignition ON.
4. Press the blinker push button for 2 - 4 seconds.
5. Count the number of times the LED blinks. If it blinks only once, this means there are no faults. That's a good thing.
6. If the LED blinks more than once, there is at least one fault. Count the number of times the LED blinks, and write that number down.
7. Press the blinker push button again for 2 - 4 seconds.
8. Again, count the number of times the LED blinks. If it repeats the previous number of blinks, that is the only fault in the system. If it blinks a different number of blinks, this is another fault. Count the blinks and record the number of blinks.
9. Press the blinker push button again for 2 - 4 seconds.
10. Again, count the number of times the LED blinks. If it repeats the first number of blinks, it is now repeating the same faults. If it blinks a different number of blinks, this is a third fault. Count the blinks and record the number of blinks.
11. Continue this cycle until you start repeating the same fault codes.
12. Refer to your manual to interpret what the different blinks mean.

Here is an example of what you might find scanning for fault codes in the EA/CC/ISC module (socket 7);

• 1st cycle... 3 blinks (Right EA/CC/ISC actuator (left cylinder bank) (M16/1))
• 2nd cycle...5 blinks (Stop lamp switch (S9/1))
• 3rd cycle...13 blinks (Left EA/CC/ISC actuator (right cylinder bank) or actual value potentiometer (M16/4r1 or M16/4r2) or actuator motor (M16/4m1) or magnetic clutch (M16/4k1))

To clear the codes, you must follow these steps;

1. Press the blinker push button for 2 - 4 seconds. The fault code will reappear.
2. Wait 3 seconds after the last blink, then press the blinker push button for 6 - 8 seconds.
3. Repeat for each fault code. Each code must be erased individually.
4. You MUST begin to clear the fault code within 20 seconds of of reading the fault. After 20 seconds, the fault will not clear unless you begin the process over again.

I spoke with a very knowledgeable Tech named Dominic at Technic Tool about scanning. He told me that with the Palm Scanner, one is able to pull codes from the car while it is running. I have not tried this. Perhaps someone can let us know if this works?

If you suspect that you're not seeing a fault code for a particular system, force an artificial code to see if one appears. For example, let's say you're trying to read codes for the MAF sensor. The scanner isn't picking anything up. Maybe there isn't anything wrong, maybe there is. How can we tell? Create your own fault. Turn off the ignition, and unplug the MAF. Rescan that system. You should definitely see a fault now. If you don't, there is a problem with the way the scanner is hooked up, or a problem with the scanner itself.

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