DOING IT ALL WITH GENERIC DATASTREAM
By Bell, Sam | |
Proquest LLC |
A resourceful diagnostician knows complicated and expensive equipment isn't always needed. Tools with a narrower focus, combined with an enlightened approach, allow him to get the job done.
The Greek root gen underlies many words in common parlance-generic is one of them. Your medical insurance provider and your pharmacist both know that when it comes to prescriptions, generic equivalents can save us all big money, with identical results. In the last few years, generic has, for complex reasons, become a pejorative term, often used to convey the idea of something of lesser quality than a socalled name-brand alternative.
Yet, for diagnostic purposes, the generic datastream sometimes offers a better window into powertrain management operating conditions than even the name-brand "enhanced" or "manufacturer-specific" interface can. In fact, even though I own several much more powerful and expensive scan tools, I routinely use the generic interface residing on the cheapest of the bunch as my go-to choice for initial code retrieval and data analysis.
This particular machine, an aging AutoXray EZ6000, offers no bidirectional controls above code clearing, but has the signal virtues of speed and a very high overall connectivity rate. It also quickly compiles a printable report which includes current operating PIDs, DTCs (including pending codes) and freeze frame data, all of which are obviously useful. Individual monitor completion status requires a separate query, as do both Mode
While substituted values are prohibited in the generic datastream, calculated values are not. Thus, for example, an ECT PID of - 40°F reflects the calculated temperature of an open ECT sensor circuit. In such cases,
As a technical consultant to our state
An additional advantage of using the generic datastream becomes apparent when you're working on a vehicle for which your scan tool doesn't provide an enhanced interface. Don't laugh; I've had students call me up to ask what to do because they didn't have a scan tool that offered, say, a
Let's take a look at what the generic interface typically offers these days (see the screen captures on this page). The J1979 SAE standard specifically defines 128 generic data PIDs, but not all manufacturers use or support all of them. Some, such as Mode
Most of the PIDs in the list are probably familiar to you, but a few may have you scratching your head. As you see, starting with a model year 2005 phasein, several new parameters have been added to the original generic data fist. These include both commanded and actual fuel-rail pressure, EGR command and EGR error calculation, commanded purge percentage, commanded equivalence ratio and a host of others, including many diesel-specific PIDs.
In-use counters may also indicate how many times each of the various onboard monitors has run to completion since the codes were last cleared. The list on page 24 includes most of the generic PIDs currently in widespread use. However, since not all manufacturers support all PIDs, and since their choices may vary by model, engine and/or equipment, the fist given here represents only a portion of the PIDs potentially supported. Additionally, manufacturers are free to establish and define supplemental modes and PIDs which may or may not be accessible via a generic interface. All ECUs with authority or control over emissions-related issues, however, must be accessible via the generic interface.
From our generic PIDs list, I want to focus on commanded equivalence ratios first. In essence, this is the PCM's way of reporting how rich or lean a mixture it's commanding. The PID is presented in a lambda format, with 1.0 indicating a stoichiometric (ideal) air/fuel ratio. Larger numbers indicate more air-a command to run at a leaner air/fuel ratio-while numbers less than 1.0 indicate a correspondingly richer mixture. If you have a gas analyzer capable of displaying lambda, it should coincide extremely well with the Commanded Equivalence PID.
As with all fuel trim-related issues, it's best to check this PID at idle, at about 1200 rpm and at about 2500 rpm. If your actual tailpipe measurements don't coincide with the PID, be sure to check for any exhaust leaks first. If there are none, you'll have to check for factors that could account for the discrepancy, such as fuel pressure faults, vacuum leaks or a biased oxygen or air/fuel sensor. If you observe a close correlation with lambda, you'll be able to use this PID with confidence in lieu of actual lambda readings while conduofing additional tests.
In general, you should expect this PID to read very close to 1.00 at idle in closed-loop operation with conventional oxygen sensors in the upstream positions. (Wide-range air/fuel [WRAF] ratio sensors may target alternate values under various driving conditions, typically targeting a leaner mix under lightthrottle cruise, for example. Additionally, vehicles using gasoline direct injection [GDI] may deviate from stoichiometry even at idle or under light-throttle cruise conditions.) Keep in mind that the name says a lot: This PID reports the command, not necessarily the effect of the command.
Once in a blue moon you may find that commanded equivalence ratio seems to travel exactly opposite from lambda, so that a Com Eq Rat of .95 corresponds to an actual lambda value of 1.05, for instance. After the one instance in which I've encountered this, I eventually learned to think of the PID value as a deviation from 1.00, then move exactly that far in the opposite direction. (An unfortunate computer crash led to the loss of my notes from that vehicle, and I can no longer remember even which foreign nameplate make it was, much less the year, model and engine. What I do remember is that it sure threw me for a loop! I also remember rechecking this at the time with another scan tool with the same result, so I suspect that it was simply the result of a mistranslation somewhere along the way, and not a tool glitch per se.)
One more note on the commanded equivalence ratios PID: You'll find it in use for diesels as well. Stoichiometric conditions for gasoline engines result in an air/foel ratio of approximately 14.7:1. The advent of oxygenated fuels has accustomed us to seeing lambda values showing slightly lean, up to as high as 1.04 in some cases, with no apparent fault. Since fuel blends vary both regionally and seasonally, normal values for your area may differ. With diesels, the ratio is closer to 14.5:1, with propane running best at 15.7:1 and natural gas working out to about 17.2:1. If you're using your gas analyzer on a vehicle burning one of these fuels, you'll have to reset your lambda calculations accordingly.
Most gas analyzers with a computer plotting interface readily accommodate multiple fuel types, usually from the setup menu. In the case of flex-fuel cars, check the ETOH PCT PID to help your analyzer figure out the correct stoichiometric ratio. Once you've made the proper selection, you can work from lambda without bothering to know or remember the exact stoichiometric ratio involved.
I was certainly glad to see the appearance of purge data in the generic list, as knowing the commanded purge status can assist in diagnosing several types of driveability faults above and beyond evap leaks and malfunctions. Remember, however, that this PID reflects only the current commanded state, not necessarily what's actually happening.
The PIDs for EGR Command and EGR Error are likewise helpful. Depending on the interface you use, however, EGR Error may be reported "backwards," with 100% indicating that command and position are in complete agreement and 0% indicating that one shows wide-open while the other shows shut. (I've seen this on numerous Hondas, where a 99.5% "error" actually meant that the valve was closed as commanded.) As usual, a few minutes checking known-good vehicles can help avoid many wasted hours hunting problems that aren't really there.
Other new PIDs inform us of the mileage since the last time the codes were cleared as well as the distance driven since the MIL first illuminated for any current codes. Both of these pieces of information can be useful, especially if yours is not the first shop to look at a particular problem. In the case of intermittent faults, they can also help give you a better idea of just how frequently the issue does arise.
Beyond PIDS
Potentially both more helpful and more problematic are the new Permanent DTCs found in mode $0A. These cannot be cleared directly via a scan tool, but will be self-erased once the corresponding monitors have successfully run to completion. Attempts to circumvent plug & play emissions tests by simply clearing codes without fixing the underlying causes led to the development of these Permanent DTCs. While there are times when I would rather just "kill the MIL," the PDTCs make me take the extra time to more fully educate my customers and to verify the efficacy of my repairs, often by resorting to Mode
The key thing to remember when working with Mode
Probably 90% of the MIL-on complaints we see in my shop are resolved using "just" a generic scanner, coupled, of course, with a few decades of experience! Nevertheless, since a generic scan interface can take you only so far, there are certainly other times when we break out one of our more sophisticated scan tools with bidirectional functionality, access to additional PIDs, guided diagnostics, etc.
Especially in an older vehicle, the generic communications data rate (baud speed) may also seem slow by todays standards. After an initial scan, this limitation can often be overcome by selecting a relatively small number of PIDs relevant to the problem at hand. All vehicles since 2008 support CAN communications even in the generic interface. The effective data transfer rates here are plenty quick enough for almost any practical purpose.
Since OBD II generic standards do not apply beyond P-codes (and some Ucodes), any full-service shop needs one or more scanners to deal with B-, Cand most U-code issues. Remember, though, that many OEMs illuminate TRAC, VSC and/or ABS lights in response to any P-code. This is nearly universally true in the case of drive-by-wire (electronic throttle body) applications, but may be found in many other instances as well. In all such cases, you must resolve the P-code issue first, before worrying about any of these sideeffects codes. If you have an appropriate interface, once you've killed the MIL, clear those extra codes as well, so the next tech doesn't find them still in memory if and when a legitimate Bor C-code ever does set.
The bottom line is that there are several potentially important advantages to using a generic scan interface for initial code retrieval and data analysis, so don't be afraid to get your feet wet! Since the generic datastream focuses on the most important inputs and commands, where the bulk of problems occur, and since all PI D values reflect their associated sensor states without substitution, you're less likely to be capsized by a flood of irrelevant data.
As always, checking known-good vehicles will help keep you on an even keel and familiarize you with what "good" looks like. While you may occasionally wind up switching over to an enhanced interface, you'll likely find that routinely starting in generic using a fest and inexpensive basic scanner results in much greater efficiency. Whether your shop is large or small, this practice also lets you avoid excessive wear and tear on the more expensive and advanced scanners and keeps them free for those longer-term diagnostic challenges where their enhanced features are actually needed.
Generic PIDs
The five starred (?) critical PIDs in the list below are the most influential inputs. Virtually all the others function merely to fine-tune (trim) the basic spark and fuel (base map) commands mapped out in response to these PIDs. The cause of any fuel trim corrections (STFT, LTFT) beyond the range of approximately ±5% must be investigated.
Standards Compliance - such as OBD II (Federal), OBD II (CARB), EOBD (
MIL - malfunction indicator lamp status (off/on)
MON_STAT - monitor completion status since codes cleared
DTC_CNT - number of confirmed emissions-related DTCs available for display
?RPM - revolutions per minute: also, engine cranlahaft (or eccentric shaft) speed, sourced from the CKP
?IATintake air temperature
?ECT - engine coolant temperature
?MAP and/or *MAF - manifold absolute pressure or mass airflow, respectively
?TPS or *TP - throttle position sensor, usually given as calculated percentage; see absolute TPS below
CALC_LOAD - calculated, based on current airflow, as percentage of peak airflow at sea level at current rpm, with correction for current BARO
LOOP - status: closed, closed with fault, open due to insufficient temperature, open due to high load or decel fuel cut, open due to system fault
STFT_x (per bank) - shortterm fuel trim; the percentage of fuel added to or subtracted from the base fuel schedule (for speed, load, temperature, etc.) in order to achieve stoichiometry as determined by the relevant air/fuel or 02 sensor
LTFT_x (per bank) - long-term fuel trim
VSS - vehicle speed sensor
H02SBxSy - heated oxygen sensor, Bank x, Sensor y, such as B1S2 for a bank 1 downstream sensor
IGN_ADV - ignition timing, measured in crankshaft degrees
SAS or SEC_AJR - commanded secondary air status off/on; may include information such as atmosphere, upstream or downstream of converter, commanded on for diagnostic purposes
RUNJÏME - seconds since last engine start; some manufacturers stop the count at 255 seconds
DISTANCE TRAVELED WITH MIL ON-in miles or km
FRP - fuel rail pressure relative to intake manifold pressure
FRP_G - fuel rail pressure, gauge reading
02Sx_WR_lambda(x) - wide range air/fuel sensor, bank x, equivalence ratio (0-1.999) or voltage (0-7.999)
EGR - commanded EGR percentage
EGR_ERR - deviation of sensed or calculated position from commanded position, percent
PURGE - commanded percentage
FUELJ.VL - fuel level input percentage; can provide especially invaluable information in freeze frame diagnostics of misfire codes set under "ran-out-ofgas" conditions; unfortunately, not universally implemented
WARMUPS - number of warmups since codes cleared; a warm-up is an ECT increase of at least 40°F in which the ECT reaches at least 160°F
DIST SINCE CLR - distance since codes cleared
EVAP_PRESS - evaporative system pressure
BARO - absolute atmospheric pressure (varies with altitude and weather)
02Sx_WR_lambda(x) - equivalence ratio or current - wide range air/fuel sensor, position x, equivalence ratio (0-1.999) or current (-128mA to +127.99mA)
CATTEMP BxSy - catalyst temperature by bank and position (may be wildly unreliable)
MON_STAT - monitor status, current trip
CONT_MOD_V - control module voltage; usually measured on the B+ input for the KeepAlive-Memory (KAM) but may be measured on a switched ignition input line
ABS_LOAD - absolute load, percentage, 0-25,700%
RELJPS - relative throttle position percentage
AMB_AT or AMB.TEMP - ambient air temperature; where used, usually measured in front of the radiator, while IAT or MAT (manifold air temperature) are usually collected in the intake ductwork, or inside the throttle body or intake manifold, respectively
ABS_TPx - absolute throttle position, percentage, sensor B orC
APP_x - accelerator pedal position sensors D-F
TP_CMD - commanded throttle actuator percentage
MILJÏM - time run with MIL on, minutes
FUELTYPfuel type
ETOH_PCT or ETH.PCT - ethanol fuel %
ABS_EVAP - absolute evap system vapor pressure, 0327.675kPa
EVAP_P or EVAP_PRESS - evap system vapor pressure (gauge), from-32,767 to+32,768Pa
STFTH02BxS2 - short-term secondary (postcatalyst) oxygen sensor trim by bank
LTFTH02BxS2 - long-term secondary oxygen sensor trim by bank
HY_BATT_PCT - hybrid battery pack remaining life, percentage
E_OIL_T or ENG_OIL_TEMP - engine oil temperature
INJ_TIM - fuel injection timing, in crankshaft degrees from -210° BTDC to +302° ATDC
FUEL_RATengine fuel rate in volume per unit time-e.g., liters per hour, gallons per minute, etc.
TRQ_DEM - driver's demand engine, percent torque
TRQ_PCT - actual engine, percent torque
REF_TRQ - engine reference torque in Nm (0 to 65,535)
TRQ_A-E - engine percent torque data at A=idle; B, C, D, E = defined points
AFC - commanded diesel intake airflow control and relative intake airflow position
EGR_TEMP - exhaust gas recirculation temperature
COMP_IN_PRESS - turbocharger compressor inlet pressure
BOOST - boost pressure control
VGT -variable-geometry turbo control
WAST_GAT - wastegate control
EXH_PRESS - exhaust pressure
TURB_RPM - turbocharger rpm
TURB_TEMP - turbocharger temperature
CACT - charge air cooler temperature
EGTx - exhaust gas temperature, by bank
DPF - diesel particulate filter
DPF_T - diesel particulate filter temperature
NOX - NOx sensor
MAN_TEMP - manifold surface temperature
NOX_RGNT - NOx reagent system
PMS - particulate matter sensor
Glossary
OBD - on-board diagnostics.
OBD II - second-generation OBD, as specified by SAE J1979.
EOBD - Euro-specification OBD; slightly different from SAE-spec.
JOBD - Japanese-specification OBD; slightly different from SAE-spec.
DTC - diagnostic trouble code; Pcodes refer to powertrain management faults; U-codes flag communication network errors; B-codes relate to faults in body system management; C-codes are chassis system based.
PDTC - Permanent DTC; one that cannot be cleared directly via scan tool command; such codes will selfclear after the affected monitors have successfully run to completion with no further faults. PDTCs are written into a section of nonvolatile memory, so they persist even if the battery is disconnected and all capacitors are discharged.
PID - parameter identification; a value found in current or freeze frame data; may indicate a sensor reading, calculated value or command status. In a nongeneric (enhanced) interface, may indicate a substituted value.
$- or -$ - prefix or suffix indicating that an alphanumeric string is hexadecimal (presented in base 16.) The J1979 specifications which establish the OBD II protocol are written using hexadecimal notation throughout.
Freeze frame - a set of PID values indicating then-current data written into the PCM's memory when a DTC sets, similar to an aircraft flight recorder. Note: Freeze frame data is erased when codes are cleared; be sure to read and record before clearing DTCs.
CAN - controller area network; also, communication via the same.
Monitor - one or more self-tests executed by the OBD system to determine whether a specific subsystem is functioning within normal limits. Monitor status changes to incomplete or "not done" when DTCs are cleared, and returns to complete or "done" once all relevant self-tests have been run. A monitor status showing completion is not a guarantee of a successful repair unless there are no codes and no pending codes, and unless the vehicle has been operated under conditions similar to those under which a previous fault had occurred (see freeze frame).
This article can be found online at www.motormagazine.com.
Copyright: | (c) 2014 Hearst Business Publishing |
Wordcount: | 3780 |
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