Scan Tools and You

Scan Tools and You

Bob Freudenberger

Talk about something you can’t live without! To paraphrase the anti-gun-control bumper sticker, “You can have my scan tool when you pry it out of my cold, dead fingers.”

In terms of speed, ease of use, and all-around helpfulness, no other piece of driveability diagnostic equipment can compete with a good scan tool. That’s why the typical technician has one pretty much permanently attached to the palm of his hand — it’s commonly the first troubleshooting aid chosen. It lets you inside that otherwise-inscrutable electronic brain, tells you what those critical sensors are reporting, and lets you capture a crucial moment, among numerous other things.


When scan tools first appeared in 1982, we often heard the pinion that they would just be used as trouble code readers. That was dead wrong, and downright disrespectful of techs. Since the beginning of the electronic engine management era, you’ve been using them to assess the data stream where possible (although you couldn’t do that on Fords and most imports for quite a few years), then engaging your considerable intellectual powers to interpret that information and, more often than not, arrive at accurate diagnoses. In fact, no-code problems have been a hot topic all along — we have a feature on that subject planned for the January issue of MS.

Now, of course, we’ve got OBD II. We’re not getting into the discussion about whether this is a step backward or forward, but it’s here to stay and it has certain advantages. You can now buy a pro-grade generic OBD II scan tool that’ll work on any late model for less than $600 (DIY models are even cheaper). This is what the EPA had in mind.

There’s no arguing with the statement that also having the capability to access the proprietary info of each make will increase your chances of nailing down the problem, and that’s what enhanced OBD II scan tools are all about. They include the software necessary to read hundreds of brand-specific parameters and “personality keys” or “smart inserts” that adapt the J1962 DLC connector as needed to accomplish this.

Data galore

Most of the user’s manuals for the popular units we’ve perused don’t actually list all the things you can read with them. We had to do some hunting to come up with the following, which applies to a pre-OBD II GM car read through a typical aftermarket unit, and is nothing if not impressive:

-Engine rpm

-Desired idle rpm

-Coolant temperature

-Intake air temperature

-MAP signal in kPa and volts

-Barometric pressure signal in kPa and volts

-Throttle position volts

-Throttle angle as a percentage

-Oxygen sensor signal in millivolts

-Rich/lean flag

-Oxygen sensor cross-counts

-Injector pulse width in milliseconds

-Fuel integrator counts (now known as Short-term Fuel Trim, or SFT)

-Block learn counts (what’s now called Long-term Fuel Trim, or LET)

-Block learn cell number

-Block learn enable (yes/no)

-Air/fuel ratio

-Spark advance in degrees

-Knock retard in degrees

-Knock signal (yes/no)

-Open/closed loop indication

-Converter high-temperature condition (yes/no)

-Air control solenoid (port/atmosphere)

-EGR desired position as a percentage

-EGR actual position as a percentage

-EGR pintle position in volts

-EGR duty cycle percentage

-EGR auto-zero (inactive/active)

-Idle air control counts

-Park/neutral position

-PRNDL switch position

-Commanded gear

-Brake switch on/off


-TCC/shift light on/off

-4th gear switch on/off

-A/C request (yes/no)

-A/C clutch on/off

-Batt/ign voltage

-Fuel pump volts

-Int. tune valve on/off

-Purge duty cycle percentage


-Time from start

-Diagnostic Trouble Codes/DTCs

That’s not all of it, either. One major scan tool maker faxed us a 22-page list of capabilities!

As you can see, there are four different types of readings here. One, input directly from sensor circuits coming into the PCM; two, output commands from the PCM to actuators; three, calculated readings, which include such things as throttle opening in percent (the TPS obviously doesn’t send a percentage signal); and four, DTCs.

You can beat the vast majority of driveability, performance, and emissions problems with that much data. Only the most mysterious and obscure cases will manage to escape solving if you use all that information wisely.


How can data stream information help you actually fix cars? In many, many ways. We’ll use one of our favorites as a prominent example: fuel trim. You probably first heard of adaptive strategies when you ran into the terms “block learn” and “integrator” in GM C3 literature years ago. They’re now officially known as “Short-term Fuel Trim” and “Long-term Fuel Trim” to comply with SAE standardized terms document J1930. These two temporary (compared to ROM) memory arrangements make minor closed-loop adjustments in fuel delivery on the basis of the history of oxygen sensor activity.

A typical normal numerical reading for this as displayed on a scan tool will be about 128. If it’s higher, the PCM is adding extra fuel to the base calculation because the system is running lean. If the number is lower than 128, the module is subtracting fuel to make up for a rich condition. Integrator is a short-term correction (“SFT”), while block learn is long-term (“LFT”) and will only make a change if the integrator has sensed and corrected the same condition for a calibrated period of time.

The 128 value doesn’t necessarily represent stoichiometric, but rather the middle of the range of control. The system will always be trying to trim itself back to this, and is more and more unhappy the further it gets from this number. Although theoretically you could see 0 or 255, actual range will usually be between 85 and 170.

To explain further, SFT (or integrator) sees oxygen sensor voltage low more than half the time, it starts marching upward — 129, 130, 131, 132. LFT (or block learn) watches, then backs up what SFT is doing by going up one. If SFT keeps going — 133, 134, 135, etc. — LFT will increase another step. Then, if SET is satisfied that it’s seeing O2 signals higher than 450 mV over 50% of the time, it turns around and marches down — 135, 134, and so on. At a certain point, LFT gets the idea and backs down, too. Remember, both are always trying to stay at 128.

As we hope you already understand, GM isn’t the only car maker with such strategies by any means — everybody’s got ’em (although not everybody displays them).

So, how can you capitalize on adaptive info to improve your rate of troubleshooting success? First of all, the only practical way to read these values is with — you guessed it — a scan tool.

To continue with our GM example, it’s become common to take readings at both idle and 3,000 rpm. Use LFT (block learn) as a baseline, although SFT (integrator) can be useful, too. Interpret the following relative to the 128 mid-point mentioned above:

-High at idle, normal at 3,000 suggests a vacuum leak

-Normal at idle, high at 3,000 indicates a fuel flow problem, possibly a weak pump.

-High at both speeds points to dirty injectors or low fuel pressure.

-Low at idle or 3,000 is evidence of a rich condition, perhaps due to gasoline in the crankcase, a leaky fuel pressure regulator diaphragm, or high fuel pressure.

SFT can help you locate a vacuum leak. Pinch off hoses, block seams, etc. while watching the number. A sudden drop means you’ve found the unintentional intake path.

Engage contents of head bone

As we’ve said in other articles, you can have the best scan tool in the world in front of you and it simply won’t help you fix cars unless you understand what the data means in terms of how the engine runs.

Unfortunately, even long and hard-won experience won’t teach you very much if you don’t have a matrix of understanding to put it in. We hope the following examples will contribute to. the establishment of that framework.

-Suppose your shop is at sea level, hut your scan tool shows a BARO reading of 78 kPa with the engine off. No fault codes are present, but you know that at your altitude barometric pressure should be close to 100 kPa (one bar, or 14.7 psi). It shouldn’t take much thought to realize that this faulty signal could cause lean running because the PCM will be supplying the proper amount of fuel for high elevations where there is less oxygen in each cubic unit of air, so the mixture will be lean at, sea level.

-Similarly, if you see a high resistance/low temperature reading from the intake/manifold air temperature sensor, yet the day is warm, you’ll know there’s trouble in the IAT/MAT sensor or its circuit because the information it’s providing simply doesn’t agree with current conditions even though the OBD system has not recognized that fact. This common-sense approach could also apply to a low temperature reading from the coolant temperature sensor when you can feel that the engine is hot.

-Signals from mechanically-operated sensors can be checked against reality, too. Say you’re driving at cruising speed, yet the VSS registers 0 mph. Something’s obviously amiss. Or, maybe you see the at-idle voltage specification for TPS output even though you’ve put the pedal to the metal.

-Besides comparing data stream information to actual conditions, you can compare it to what other sensors are telling you. For example, suppose when you open the throttle half way your scan tool tells you that the throttle angle is 50%, yet when you switch to the throttle position sensor voltage you get a reading of 1.2 volts. Since the reading should be around five volts at WOT (Wide Open Throttle), half throttle should he between two and three volts. By making an intelligent comparison, you’ve probably located the cause of a hesitation problem.

-Another way to make good use of the data stream is to cause artificial conditions and see how the computerized engine management system responds, which is called, logically enough, “response resting.” For instance, if you introduce a large vacuum leak (say, by removing the brake booster vacuum hose), you should see a lean flag, then an increase in fuel injector pulse width milliseconds. If you cause a rich condition (you can pull the vacuum line to the fuel pressure regulator; with a sensor simulator, you can inject a signal of about .9 volt into the oxygen sensor wire), you should see a rich flag, then a decrease in pulse width.

With an ordinary NTC CTS, if you disconnect the sensor’s lead you should expect to see a very low temperature reading in the data stream. Grounding the lead should give you a high temperature indication. If you get these results, it is probable that the CTS circuit and the PCM are both okay. The trouble, then, is probably in the sensor itself, or deposits that keep it from warming up as it should.

In much of what we’ve said so far, knowing what’s normal is supremely important. It makes sense to experiment with cars that run well and to jot down the scan tool readings they produce.


If you’re not recently from a dealership, the concept of fixing cars with software may seem odd. After all, aren’t we in the business of snooping out bad parts and replacing them? Sure we are, but this is a logical addition to that, and it involves scan tools.

Reprogramming can address an almost unimaginable array of driveability and emissions issues, both subtle and not-so-subtle. One that gave us a laugh was a police car that was delivered with the regular civilian program and was being outrun by everything from Yugos to diesels because the speed-limiting action kicked in so early. Reflashing let it get up to its true 130+ mph capability.

Although nothing’s sacred in this wild-west area of service, these are the definitions of reflashing procedures that are emerging:

-Remote: You attach a suitable scan tool to the DCL to retrieve the current calibration, then take it to a PC, which looks up the calibration and, if available, uploads a new one from CD to the scan tool. Go back to the car and let it download the fresh program from the tool’s memory.

-Pass-through: Drive the vehicle up to your in-shop computer work station, attach cables from the DCL through a scan tool or dedicated reflashing device and on to the PC, which will identify the calibration, offer any new ones that are available, then upload your choice through the scan tool or reflasher (either of which acts as a conduit) and into the BEPROM.

-Off-board: Remove the PCM from the car, take it to your computer-equipped work bench, attach power and ground, connect to your PC through the reflashing unit and requisite cables, and fire up the software.

The main companies that are making reflashing available to the aftermarket are, in alphabetical order, EASE, Hickok, OTC, and Vetronix. Each has its own approach, and the details would fill up a whole article, which is exactly what we’re planning to do.

Wireless wonder

At the AAPEX show in November, we were introduced to a whole new approach to integrated diagnostics and shop management: the Delphi/ATRI Atrisys. Using EASE PC-based scan tool and wireless technology, an E-notepad such as the Hitachi 600, and a server hooked up to the Internet, this gives you much more than just outstanding access to the data stream, including tag files, tech info from the likes of Alldata or Mitchell, a diagnostic assistant application with a table of “known goods,” etc. Sounds like a future MS feature to us.

COPYRIGHT 2000 Adams Business Media

COPYRIGHT 2001 Gale Group