HESITATION/LACK OF POWER DIAGNOSTICS STRATEGY
THEORY OF OPERATION
Diagnostic strategies are employed to speed up and
accurately find specific electronic faults that affect all automotive
systems today. The following strategy is meant to be used by any technician
using a multi-channel oscilloscope/ diagnostics equipment. In today’s
automotive diagnostics field, the proliferation of electronics has led to
the need for using multi-channel scopes and test equipment in an increasing
way. The use of such equipment speeds up the diagnostic process and allows
sure diagnostics that will not result in the replacement of unnecessary
parts, therefore, reducing repair costs. The following are strategies used
to detect and confirm electronic faults in today’s vehicles. Provided that
the engine has no mechanical problems and all cylinders are working without
any misfires, a no-power or hesitation complaint will usually fall into 5
major categories of problem areas. These are the categories in order of
delivery. ( pressure and volume ) - Possible symptoms are lean/rich misfire
or flooded engine.
• Spark reserve
or enough magnetic buildup (saturation) in the coil - Possible symptoms are
misfire, hesitation & lack of power.
backpressure.( clogged cat. ) - Possible symptoms are bogging down & lack of
timing. - Possible symptoms are: Pinging and no power.
timing.( jumped timing belt ) - Possible symptoms are: Backfire. This
systematic fault detection order will lead you in the right direction.
GUIDELINES FOR USING A MULTI-CHANNEL/DDC SCOPE and
REPARING ENGINE PERFORMANCE FAULTS IN GENERAL
THE FOLLOWING ARE STEP-BY-STEP DIRECTIONS ON USING A
MULTI-CHANNEL OSCILLOSCOPE TO PERFORM ELECTRONIC DIAGNOSTICS ON MODERN
AUTOMOBILE SYSTEMS. THEY ARE MEANT TO BE FOLLOWED IN A SEQUENTIAL,
STEP-BY-STEP MANNER. BEFORE PORFORMING ANY TESTS, A STARTER CURRENT
COMPRESSION TEST SHOULD BE PERFORMED FIRST TO VERIFY THE MECHANICAL
INTEGRITY OF THE ENGINE.
PRE-CONDITIONING STEPS and PRE-DIAGNOSING WITH THE SCANNER
First a determination must be made if the use of the
Multi-channel/DDC scope or any electronic breakout-box system is called for.
It may be possible to simply use regular diagnostic tools like a test light
and DVOM to do the job in much less time. The use of oscilloscopes and more
hard to connect equipment always adds hook-up time to the diagnostic
routine. Using such equipment must out-weight the extra time drawback , as
when dealing with circuit problems on complicated automotive system that can
not be detected through simpler means. The following steps must be taken to
determine if there is a need for the use of these tools.
1. A thorough visual inspection must be made to determine
the condition of the following components: Battery posts, ignition parts,
connectors, wiring harness, engine odors, ignition arching KV, ground strap
between engine and block, chassis grounds, sensors and relays, oil & fuel
2. Warm the engine to operating temperature and determine
that there are no cooling or major mechanical problems.
3. Connect a scanner and decode the ECM. Make sure that all
codes are recorded for future reference. See next Fig.
NOTE: The importance
of a basic visual inspection can not be overstated. A large percentage of
all electronic faults are resolved by simply doing a very thorough visual
inspection. Remember the faster you find the problem the better off you will
be. After assessing and analyzing the scanner codes and troubleshooting
information, determine whether to continue with these steps or go to the
Multi-channel scope/DDC hookup to continue the diagnostics. Scanner
diagnostics using graphing software linked to a desktop computer can be very
useful in diagnostics. It is important to correlate signals together so as
to establish a relationship between signals.
NOTE: By choosing the RPM, MAF or MAP
signal, LT fuel trims, and O2 sensor signals, it is possible to see the
reaction of the O2 sensor while power breaking or preloading the vehicle. A
momentary rich signal should be observed followed by a normal cycling O2
sensor signal, indicating that the ECM is in constant fuel control. The ECM should be in fuel control at
all times except WOT and Deceleration. So long as the O2 sensor is cycling,
the ECM is in control.
4. Perform a 4 or 5 gas analysis of the vehicle’s tail pipe
emissions, so as to ascertain proper air/fuel ratio on combustion. A gas
analysis can reveal the overall condition of all the different systems in
the engine in a very fast and efficient manner.
High HC indicates degraded mechanical or ignition
High CO indicates a rich mixture, from either lack of air or
High O2 indicates a lean mixture, from either lack of fuel
or excess air. High O2 can also be due to a continuously running secondary
air injection system.
High NOx indicates EGR problems, high combustion chamber
temperatures or an overheating engine.
High CO2 indicates good combustion. It is the only gas that
should always be high (14 % or higher).
It is important to understand that as far as gases are
concerned, CO is a determinant of excessive fuel and O2 is a determinant of
excessive air in the mixture, CO2 is the result of any carbon based
combustion and should always be high, NOx is the binding of nitrogen and
oxygen molecules at high temperatures and is a measure of EGR or lean (hot
mixture) operation, and HC is raw fuel exiting the exhaust (due to a misfire
or any other mechanical fault).
Typical readings for all gases should be as follows. (Newer
ULEV systems have much lower specifications). HC-under 100ppm CO-under 0.50%
O2-under 0.5% CO2-over 14% Nox-usually under 850ppm.
NOTE: These numbers are to be used only as
guidelines. LEV and ULEV vehicles have much lower readings.
5. Check fuel pressure and volume. Fuel pressure and
volume should meet specifications and not be slightly over or under.
NOTE: • When using a volumetric fuel
pressure gauge, the following rule of thumb could be used. Generally, with
engine running, SFI and PFI systems should have a 0.3 to 0.5 gal per minute
of fuel flow, at a minimum, and TBI should have 0.4 to 0.7 gal per minutes
of fuel flow. There are system variations and this is only meant to be used
as general minimum rule.
oscilloscope should be used to check the integrity of the commutator
windings inside the fuel pump motor, by using a low clamp-on amp probe to
determine current load ( Amps ) and fuel pump RPM. A 3000 to 7000 RPM and a
4 to 6 Amps load is acceptable.
If the fuel system is found to be within normal parameters
then again decide whether the use of the Multi-channel scope/DDC is called
for. If fuel mixture problems are suspected, then there might be a sensor
causing the problem. If not then go on to the next step.
6. Perform a
full ignition check using an oscilloscope. Determine that there is a
proper KV firing line, spark line KV and spark duration. ( normal coil
saturation (dwell) should be at around 4 mS to 5 mS, with as much as 8 mS on
7. Perform a back pressure check using a back
pressure gauge to determine if a clogged catalytic converter is the culprit.
8. Determine that ignition timing is up to
specifications. Older engines may require a slight off specs adjustments to
make up for slack in the timing chain or belt. Use your judgment.
9. Determine proper valve timing and/or timing-belt/chain
DDC/MULTI-CHANNEL OSCILLOSCOPE OPERATION
The DDC is a Multi-Channel oscilloscope using software
driven electronic break out box technology. The following steps must
be performed when using the DDC or any other multi-channel scope/breakout
box. However, this technique is applicable in using any multi-channel
Perform a sweep test of the entire wiring
harness. Start by checking all sensors and activate all solenoids and
actuators to determine proper operation. ( The conditional tests on
the DDC should not be performed under normal circumstances, since it would
only reveal a problem in extreme cases, as in open or shorted grounds).
NOTE: Pay particular attention to the fact
that sometimes unrelated components share the same ground or power feed
circuit. A short in the A/C high pressure switch may cause a MAP sensor
faulty code, due to the fact that they might share the same 5 volts
reference circuit from the ECM. This is the case of the 95 Plymouth Voyager.
You should never dismiss any out of specs sweep readings and assume nothing
when in comes to diagnosing modern vehicle systems.
the following signal waveform tests. Test all grounds while cranking the
vehicle, and while running at idle and at 2500 RPM. The ground signal should
not go above 300 mV in
cranking mode and should not go above 100 mV in idling mode.
Anything above those figures would constitute a ground signal problem.
However, certain vehicles exhibit a high amount of interference on the
ground lines. Such interference is normal, so long as the interference
voltage waveform returns to 0.00 volts.
• Power feed
test. Test all 12 Volts ECM feed lines including the 5 Volts reference
line to the sensors. Check for no sudden drop-outs of battery volts and
correct voltage levels.
signals. Start with the most important signals which appear at the
beginning of a pin out list. A general rule should be distributor or ignition signals
first ( pick up coil, crank, cam, ignition reference, ignition
primary if needed, knock sensor to determine ignition retardation, and
starter signal ), then
fuel related signals ( injectors, MAF, TPS, MAP, IAT, O2 sensor ).
NOTE: It is extremely important to arrive at a
correlation between signals to establish a relationship. Notice the two
Example 1- If a crank signal scope trace has a spike
that correlates with the ignition primary of cylinder # 1, then there is a
relationship between the two signals and the ignition components are
probably at fault. The use of PC-OVERLAY feature should be used to determine
a possible correlation and therefore establish a relationship between two
Example 2- If the vehicle is pre-loaded, the MAF signal
should indicate excess air coming into the engine and the O2 sensor signal
should show a momentary rich signal output and then start cycling again to
show that the ECM is in control. Lack of fuel control by the ECM points to a
fuel starvation problem or on the rich side an excess fuel problem. These
causes should be further analyzed.
When checking the fuel related signals, the ECM should be in
fuel control at all times with a few exceptions ( WOT & Deceleration ).
See figure 1.
check . Manual multi-meter or Graphing multi-meter tests should be used,
when necessary, to check sensors and generally slower moving signals like O2
sensor and MAF/MAP response.
Example 3- When checking MAF sensor response to a WOT
actuation, the resulting meter or GMM waveform time-base should be around ½
a second or 500 mS. O2 sensor response should be in the order of once every
second or 1 Hz. Also a TPS sweep should be performed, if needed. The manual
multi-meter tests can also be used to see signal correlation and establish
relationships between signals just as the scanner graphing or the
Proper diagnostics procedure is an integral part of good
troubleshooting practice. By strictly adhering to these procedures, you can
confidently arrive at the source of the problem at hand. It is important to
always avoid the pattern failure approach to diagnosing problems.
Over the years experience has shown the advantages and limitations of each
and proper procedure clearly will always lead you in the right direction.