Page 1434 of 1501

severe weakness that we will look at later). If an injector has a
fault where it occasionally skips a pulse, the meter registers it and
the reading changes accordingly.
Let's go back to figuring out dwell/duty readings by using
injector on-time specification. This is not generally practical, but
we will cover it for completeness. You NEED to know three things:
* Injector mS on-time specification.
* Engine RPM when specification is valid.
* How many times the injectors fire per crankshaft revolution.
The first two are self-explanatory. The last one may require
some research into whether it is a bank-fire type that injects every
360
of crankshaft rotation, a bank-fire that injects every 720 , or
an SFI that injects every 720 . Many manufacturers do not release this
data so you may have to figure it out yourself with a frequency meter.
Here are the four complete steps to convert millisecond on-
time:
1) Determine the injector pulse width and RPM it was obtained
at. Let's say the specification is for one millisecond of on-time at a
hot idle of 600 RPM.
2) Determine injector firing method for the complete 4 stroke
cycle. Let's say this is a 360
bank-fired, meaning an injector fires
each and every crankshaft revolution.
3) Determine how many times the injector will fire at the
specified engine speed (600 RPM) in a fixed time period. We will use
100 milliseconds because it is easy to use.
Six hundred crankshaft Revolutions Per Minute (RPM) divided
by 60 seconds equals 10 revolutions per second.
Multiplying 10 times .100 yields one; the crankshaft turns
one time in 100 milliseconds. With exactly one crankshaft rotation in
100 milliseconds, we know that the injector fires exactly one time.
4) Determine the ratio of injector on-time vs. off-time in
the fixed time period, then figure duty cycle and/or dwell. The
injector fires one time for a total of one millisecond in any given
100 millisecond period.
One hundred minus one equals 99. We have a 99% duty cycle. If
we wanted to know the dwell (on 6 cylinder scale), multiple 99% times
.6; this equals 59.4
dwell.
Weaknesses of Dwell/Duty Meter
The weaknesses are significant. First, there is no one-to-one
correspondence to actual mS on-time. No manufacturer releases
dwell/duty data, and it is time-consuming to convert the mS on-time
readings. Besides, there can be a large degree of error because the
conversion forces you to assume that the injector(s) are always firing\
at the same rate for the same period of time. This can be a dangerous
assumption.
Second, all level of detail is lost in the averaging process.
This is the primary weakness. You cannot see the details you need to
make a confident diagnosis.
Here is one example. Imagine a vehicle that has a faulty
injector driver that occasionally skips an injector pulse. Every
skipped pulse means that that cylinder does not fire, thus unburned O2
gets pushed into the exhaust and passes the O2 sensor. The O2 sensor
indicates lean, so the computer fattens up the mixture to compensate
for the supposed "lean" condition.
A connected dwell/duty meter would see the fattened pulse
width but would also see the skipped pulses. It would tally both and
likely come back with a reading that indicated the "pulse width" was
within specification because the rich mixture and missing pulses
offset each other.
This situation is not a far-fetched scenario. Some early GM
Page 1436 of 1501
times by increasing injector pulse width accordingly.
NOTE: Never apply battery voltage directly across a low resistance
injector. This will cause injector damage from solenoid coil
overheating.
Fig. 1: Injector Driver Types - Current and Voltage
CURRENT CONTROLLED CIRCUIT ("PEAK & HOLD")
The current controlled driver inside the computer is more
complex than a voltage controlled driver because as the name implies,
it has to limit current flow in addition to its ON-OFF switching
function. Recall, this driver typically requires injector circuits
with a total leg resistance of less than 12 ohms.
Once the driver is turned ON, it will not limit current flow
until enough time has passed for the injector pintle to open. This
period is preset by the particular manufacturer/system based on the
amount of current flow needed to open their injector. This is
typically between two and six amps. Some manufacturers refer to this
Page 1474 of 1501
Fig. 3: Determining Camber Angle
CASTER
1) Caster is tilting of front steering axis either forward or
backward from vertical, as viewed from side of vehicle. See Fig. 4.
2) When axis is tilted backward from vertical, caster is
positive. This creates a trailing action on front wheels. When axis is
tilted forward, caster is negative, causing a leading action on front
wheels.
Fig. 4: Determining Caster Angle
TOE-IN ADJUSTMENT
Toe-in is the width measured at the rear of the tires
Page 1475 of 1501

subtracted by the width measured at the front of the tires at about
spindle height. A positive figure would indicate toe-in and a negative
figure would indicate toe-out. If the distance between the front and
rear of the tires is the same, toe measurement would be zero. To
adjust:
1) Measure toe-in with front wheels in straight ahead
position and steering wheel centered. To adjust toe-in, loosen clamps
and turn adjusting sleeve or adjustable end on right and left tie
rods. See Figs. 2 and 5.
2) Turn equally and in opposite directions to maintain
steering wheel in centered position. Face of tie rod end must be
parallel with machined surface of steering rod end to prevent binding.
3) When tightening clamps, make certain that clamp bolts are
positioned so there will be no interference with other parts
throughout the entire travel of linkage.
Fig. 5: Wheel Toe-In (Dimension A Less Dimension B)
TOE-OUT ON TURNS
1) Toe-out on turns (turning radius) is a check for bent or
damaged parts, and not a service adjustment. With caster, camber, and
toe-in properly adjusted, check toe-out with weight of vehicle on
wheels.
2) Use a full floating turntable under each wheel, repeating
test with each wheel positioned for right and left turns. Incorrect
toe-out generally indicates a bent steering arm. Replace arm, if
necessary, and recheck wheel alignment.
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