display FORD FESTIVA 1991 Owner's Manual

Since DVOMs update their display roughly two to five times a second, all measurements in between are averaged. Because a potential voltage
drop is visible for such a small amount of time, it ge t s "a ve r a ge d o u t ", c a u sin g yo u t o miss it .
Only a DVOM that has a "min-max" function that checks EVERY MILLISECOND will catch this fault consistently (if used in that mode). The
Fluke 87 among others has this capability.
A "min-max" DVOM with a lower frequency of checking (100 millisecond) can miss the fault because it will probably check when the injector
is not on. This is especially true with current controlled driver circuits. The Fluke 88, among others fall into this category.
Outside of using a Fluke 87 (or equivalent) in the 1 mS "min-max" mode, the only way to catch a voltage drop fault is with a lab scope. You
will be able to see a voltage drop as it happens.
One final note. It is important to be aware that an injector circuit with a solenoid resistor will always show a voltage drop when the circuit is
energized. This is somewhat obvious and normal; it is a designed-in voltage drop. What can be unexpected is what we already covered--a
voltage drop disappears when the circuit is unloaded. The unloaded injector circuit will show normal battery voltage at the injector.
Remember this and do not get confused.
Checking Injector On-Time With Built-In Function
Several DVOMs have a feature that allows them to measure injector on-time (mS pulse width). While they are accurate and fast to hookup,
they have three limitations you should be aware of:
They only work on voltage controlled injector drivers (e.g "Saturated Switch"), NOT on current controlled injector drivers (e.g. "Peak &
Hold").
A few unusual conditions can cause inaccurate readings.
Varying engine speeds can result in inaccurate readings.
Regarding the first limitation, DVOMs need a well-defined injector pulse in order to determine when the injector turns ON and OFF. Voltage
controlled drivers provide this because of their simple switch-like operation. They completely close the circuit for the entire duration of the
pulse. This is easy for the DVOM to interpret.
The other type of driver, the current controlled type, start off well by completely closing the circuit (until the injector pintle opens), but then
they throttle back the voltage/current for the duration of the pulse. The DVOM understands the beginning of the pulse but it cannot figure out
the throttling action. In other words, it cannot distinguish the throttling from an open circuit (de-energized) condition.
Yet current controlled injectors will still yield a millisecond on-time reading on these DVOMs. You will find it is also always the same,
regardless of the operating conditions. This is because it is only measuring the initial completely-closed circuit on-time, which always takes the
same amount of time (to lift the injector pintle off its seat). So even though you get a reading, it is useless.
The second limitation is that a few erratic conditions can cause inaccurate readings. This is because of a DVOM's slow display rate; roughly
two to five times a second. As we covered earlier, measurements in between display updates get averaged. So conditions like skipped injector
pulses or intermittent long/short injector pulses tend to get "averaged out", which will cause you to miss important details.
The last limitation is that varying engine speeds can result in inaccurate readings. This is caused by the quickly shifting injector on-time as the
engine load varies, or the RPM moves from a state of acceleration to stabilization, or similar situations. It too is caused by the averaging of all
measurements in between DVOM display periods. You can avoid this by checking on-time when there are no RPM or load changes.
A lab scope allows you to overcome each one of these limitations.
Checking Injector On-Time With Dwell Or Duty
If no tool is available to directly measure injector millisecond on-time measurement, some techs use a simple DVOM dwell or duty cycle
functions as a replacement.
While this is an approach of last resort, it does provide benefits. We will discuss the strengths and weaknesses in a moment, but first we will
look at how a duty cycle meter and dwell meter work.
How A Duty Cycle Meter and Dwell Meter Work
All readings are obtained by comparing how long something has been OFF to how long it has been ON in a fixed time period. A dwell meter
and duty cycle meter actually come up with the same answers using different scales. You can convert freely between them. See
RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS TABLE
.
The DVOM display updates roughly one time a second, although some DVOMs can be a little faster or slower. All measurements during this
update period are tallied inside the DVOM as ON time or OFF time, and then the total ratio is displayed as either a percentage (duty cycle) or
degrees (dwell meter).
For example, let's say a DVOM had an update rate of exactly 1 second (1000 milliseconds). Let's also say that it has been measuring/tallying
an injector circuit that had been ON a total of 250 mS out of the 1000 mS. That is a ratio of one-quarter, which would be displayed as 25%
duty cycle or 15° dwell (six-cylinder scale). Note that most duty cycle meters can reverse the readings by selecting the positive o r n e ga t ive
slope to trigger on. If this reading were reversed, a duty cycle meter would display 75%.
Strengths of Dwell/Duty Meter
The obvious strength of a dwell/duty meter is that you can compare injector on-time against a known-good reading. This is the only practical
way to use a dwell/duty meter, but requires you to have known-good values to compare against.
Another strength is that you can roughly convert injector mS on-time into dwell reading with some computations.
A final strength is that because the meter averages everything together it does not miss anything (though this is also a 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.
Page 3 of 19 MITCHELL 1 ARTICLE - GENERAL INFORMATION Waveforms - Injector Pattern Tutorial
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BRAKE PAD WEAR INDICATOR
CATALYTIC CONVERTER
COOLANT (PROPYLENE-GLYCOL FORMULATIONS)
ELECTROSTATIC DISCHARGE SENSITIVE (ESD) PARTS
ENGINE OIL
FUEL PUMP SHUTOFF SWITCH
This switch stops flow of fuel to engine after a collision. The impact does not have to be great for switch to be triggered. Switch must be reset
after collision. Switch is located under left rear speaker in luggage compartment. Press button to reset switch.
FUEL SYSTEM SERVICE
HALOGEN BULBS
PASSIVE RESTRAINTS
RADIATOR CAP
RADIATOR FAN
WARRANTY INFORMATION
BASIC NEW CAR LIMITED WARRANTY
All parts of the vehicle, except tires, are covered against defects in factory-supplied materials and workmanship for 12 months or 12,000 miles, CAUT ION: Indicator will cause a squealing or scraping noise, warning that brake pads need replacem ent.
CAUT ION: Continued operation of vehicle with a severe m alfunction could cause converter to overheat, resulting
in possible dam age to converter and vehicle.
CAUT ION: T o avoid possible dam age to vehicle use only ethylene-glycol based coolants with a m ixture ratio from
44-68% anti-freeze. DO NOT use 100% anti-freeze as it will cause the form ation of cooling system
deposits. T his results in coolant tem peratures of over 300° F (149°C) which can m elt plastics. 100% anti-
freeze has a freeze point of only -8° F (-22°C).
CAUT ION: Propylene-Glycol Mixtures has a sm aller tem perature range than Ethylene-Glycol. T he tem perature
range (freeze-boil) of a 50/50 Anti-Freeze/Water Mix is as follows: Propylene-Glycol -26° F (-32°C) - 257° F
(125°C) Ethylene-Glycol -35° F (-37°C) - 263° F (128°C)
CAUT ION: Propylene-Glycol/Ethylene-Glycol Mixtures can cause the destabilization of various corrosion inhibitors.
Also Propylene-Glycol/Ethylene-Glycol has a different specific gravity than Ethylene-Glycol coolant,
which will result in inaccurate freeze point calculations.
WARNING:Many solid state electrical com ponents can be dam aged by static electricity (ESD). Som e will display a
warning label, but m any will not. Discharge personal static electricity by touching a m etal ground point
on the vehicle prior to servicing any ESD sensitive com ponent.
CAUT ION: Never use non-detergent or straight m ineral oil.
WARNING:Relieve fuel system pressure prior to servicing any fuel system com ponent (fuel injection m odels).
WARNING:Halogen bulbs contain pressurized gas which m ay explode if overheated. DO NOT touch glass portion
of bulb with bare hands. Eye protection should be worn when handling or working around halogen
bulbs.
CAUT ION: Before operating vehicle, securely fasten passive shoulder restraints to the em ergency release buckles.
T he buckle fits in only one way. Ensure to position it properly.
CAUT ION: Always disconnect the fan m otor when working near the radiator fan. T he fan is tem perature controlled
and could start at any tim e even when the ignition key is in the OFF position. DO NOT loosen or rem ove
radiator cap when cooling system is hot.
WARNING:Keep hands away from radiator fan. Fan is controlled by a therm ostatic switch which m ay com e on or
run for up to 15 m inutes even after engine is turned off.
CAUT ION: Due to the different warranties offered in various regions and the variety of after-m arket extended
warranties available, please refer to the warranty package that cam e with the vehicle to verify all
warranty options.
Page 6 of 9 MITCHELL 1 ARTICLE - MAINTENANCE INFORMATION 1988-93 MAINTENANCE Ford Motor Co. Maintenance Inform...
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