Taking Your Scan Tool To The Next Level

by Guy Garrett

For years, we have used our trusty scan tool to pull codes, check prom updates and troubleshoot. But are we using it to its full potential? Do we know every button and basic operational feature? Do we really understand On-Board Diagnostics (OBD) I systems? If you answered yes to the previous questions, you are ready to take your scan tool to the next level!

Let’s review some basic operational issues of your current scan tool:

  1. Will your current scan tool work on an OBD II system? OBD II systems, which first appeared in certain 1994 models, are now standard in all 1996 automobiles. If your scan tool doesn’t accept OBD II, can it be updated?

  2. Do you know how to operate your scan tool’s snap shot or movie function? This function is a must to duplicate an intermittent problem on a vehicle! We will discuss the OBD II freeze frame function later. This scan tool feature will help supplement this limited OBD II feature.

  3. Are you aware of all of your scan tool’s functional tests? Functional testing is a very important feature of your scan tool. Being able to turn on actuators to do functionality testing is critical. A properly run wiggle test on a Ford can help find a needle in an electrical haystack. A voltage drop test during an output state test can find the phantom bad ground. And an ATM test can help you find a defective shift solenoid in an A604 Chrysler electric shift transmission. You need to know your functional tests!

  4. Last, but not least, have you read the information that came with your scan tool? This can be one of your best technical information resources. You will usually find all of the needed operational information, as well as sensor and actuator data parameters.
Looking Inside OBD II Systems
Now that we have OBD I behind us, let’s focus on OBD II. These systems first started showing up in a limited number of 1994 vehicles. The new diagnostic link connector (DLC) first appeared under the dash of 1994 Cougars and Thunderbirds.

But there is much more to this system than a new diagnostic connector. OBD II uses a new type of diagnostic software that looks at system efficiency, and system failures. OBD II does this with a series of enhanced system monitors and faster processors. OBD II standardizes many of the diagnostic procedures, as well as common terms and acronyms. OBD II will even store a freeze frame of serial data in its own memory if an emissions-related fault code is set! But for all of its new high-tech advances, you will still need a high-quality DVOM and lab scope to properly diagnose this advanced system. The real value of OBD II to the automotive repair technician is the diagnostic monitor software.

The major change in OBD II equipped vehicles for 1995 and 1996 will be in the vehicle’s diagnostic software. The vehicles that are equipped with the "Leak Down" type of EVAP diagnostic monitors will have the most noticeable amount of new hardware. This is because of their new leak detection pump (LDP).

One other major change is the portion of the PCM’s software used to control the advanced system monitors, freeze frame data, MIL illumination and diagnostic trouble code (DTC) storage. Chrysler calls this their Task Manager Software, General Motors calls it their Diagnostic Management Software and Ford calls it their Diagnostic Executive Software. They basically do the same thing.

Trips And Drive Cycles
Technicians have had trouble understanding the difference between an OBD II trip and the drive cycle. Here are the primary differences between them:

  1. An OBD II trip is when the vehicle is started and brought up to 160 degrees Fahrenheit and at least 40 degrees Fahrenheit above it’s original starting temperature. It must then meet specific enable criteria for a diagnostic monitor to start, run and show a pass or fail.

  2. A drive cycle is the same as above, but, all of the diagnostic monitors must complete and show a pass or fail result. The OBD II drive cycle is the complete test running without interruption.

The value to the technician in all of this is that we can use these diagnostic monitors to confirm the effectiveness of our repairs. This is the first time that we have had this kind of diagnostic power in our hands. This should prove to be very helpful once technicians understand the concept of how the OBD II diagnostic monitors work.

OBD II Freeze Frame Data
Another advantage of OBD II is the freeze frame data function of the scan tool. Whenever an emissions related DTC is set, the MIL will illuminate and a freeze frame will be set in the PCM’s memory. The freeze frame should contain, but isn’t limited to, the following data:

The generic OBD II scan tool will be able to access this information any time a DTC is recorded. But will this information really be that helpful? I’m not so sure. If you have freeze frame data, you must already have a DTC to work with. So if it’s a hard fault like a Type A misfire, you probably won’t need this extra data to diagnose this vehicle. But if this is an intermittent problem, the freeze frame data could help narrow down the problem. But would you want to put all of your faith in one frame of serial data? I would use the freeze frame data to set up a road test with the operation conditions. This is what you would do with an OBD I vehicle and the customer’s detailed complaint. Then you can use OBD II’s next feature to do a proper diagnostic.

OBD II Data (No Codes)
In this generic scan tool feature, you can’t get DTCs. But you can get a serial data stream that will update a minimum of two times a second. More importantly, you will have access to at least 16 data parameters and a 66-frame movie of ECU/PCM data. The most important function of any scan tool is to diagnose intermittent problems. Listed below are my reasons why the movie/snap-shot is so valuable!

  1. On slower baud rate systems, an out-of-range sensor may take several frames of data before a DTC is set. But it would still cause a noticeable driveability problem on the vehicle. And it still might not set a DTC. Duplicating the customer’s complaint with a road test and a movie/ snap-shot may be the only way to properly diagnose the vehicle.

  2. On newer systems with faster baud rate systems, the problem can be worse! On these faster systems, the scan tool data updates so fast that the scan tool display can’t keep up. So if you are looking for that TPS to drop out for a frame or two, you may never see it happen. That’s because the scan tool is probably displaying only one out of every three or five frames of data. The TPS could drop out and you would never see it! Your scan tool is getting all of the data; its display just can’t keep up. So take a movie/snap-shot. Then you can play it back at a rate that your scan tool’s display and brain can handle.
Short- And Long-term Fuel Trim
To explain short- and long-term fuel trim parameters, re-visit block learn integrator values. Basically they are the same thing. Block learn and integrator both have a range of 0 - 256 with 128 the mid-point. These are fuel trim commands from the PCM. If the value is less than 128, the PCM command is lean. If the value is more than 128, the command is rich. If the value is 128, there is no change in the fuel trim. The integrator is the short-term or fast fuel adjustment in relationship to the O2 sensor reading. The block lean is the long-term or learning/ adaptive adjustment to O2 sensor readings.

OBD II uses short and long term fuel trim values. The only difference is that 0 percent replaces 128 as the mid-point for fuel trim adjustments. Any value over 0 percent the PCM is responding to a lean O2 sensor reading by adding fuel. In general, short- and long-term fuel trim commands should stay around +10 percent/-10 percent if the PCM is controlling fuel properly. Your scan tool troubleshooting or diagnostic software will help give you a better understanding of these values.

What other skills will you need?

As mentioned earlier, you need to understand electrical waveforms (lab scope) and have access to electronic repair information. This could be a CD-ROM based repair system or going online with your shop PC. Good training is out there if you look for it. Don’t let yourself or your shop get lost in the past!

Guy Garrett is an industry instructor based in Springboro, Ohio. He has more than 18 years experience in the automotive industry.

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AutoInc. Magazine ®, Vol. XLIV No. 11, November 1996