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like a poorly seating valve has a greater chance to be
diagnosed at low speed, typically cranking RPM. The reason
is simple, because it has more time to leak down. Example: A
burned valve may feel like a misfire at idle, but
performance wise there is very little negative effect at
high RPM, because the leakage is so minute at that speed. 2.
A vacuum leak drives the fuel trim positive at idle, but
equals out at high speed when the leak has very little
effect. That is how you know the difference between lean
injector and vacuum leak. 3. A leaky injector has more time
to drip at idle, while at high speed there is less time and
may be only a few drops. 4. Carbon build-up may keep the EGR
valve open causing a rough idle. At high speed the EGR valve
should be open anyway to control NOX. Therefore carbon
build-up at EGR has no negative effect at high RPM or load.

are some examples of fuel starvation: An 80% restricted fuel
filter is no problem at idle, but you won’t make it driving
uphill when volume and demand is far greater. 2. The same
holds true for a defective fuel pump. It may have a perfect
fuel pressure, but fails to supply enough volume at high
speed when high demand is critical, causing surging under
load, yet may pass every function at lower demand at idle.
3. Also a restricted exhaust has a greater negative effect
when the volume is high at wide open throttle and the
accumulation of inhaled air is at its peak. 4. A lean
injector may pass at idle, but fails at high speed,
typically at fast acceleration when the computer cannot keep
up with compensating for the lean condition. This can be
easily demonstrated with a snap-test (when the computer does
not respond fast enough to compensate), on the ignition
scope pattern.

FAILURE AT ANY SPEED: A dead hole at any speed can mean
almost anything from absence of fuel, or spark, or
compression. The fact that it is at any speed makes analysis
simpler because it does not fall under those above
categories. Reading the code tells which cylinder.

Final advice! Don’t memorize — analyze!

Mac   VandenBrink
Dynamic Auto Test Engineering Corp
Kalamazoo, Michigan, USA

Mac’s website is: www.datec,us

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“Ball Park” Voltage Drop Standards

A “ball park” value is useful as a starting point when testing for voltage drop. It is based on where the testing begins, and also on whether the load being tested is “low current” or “high current”. Low current loads are not controlled by a relay, and high current load are relay controlled. Low current loads includes any fuse terminal fed voltage, ignition switch, headlamp switch, and windshield wiper motor. High current loads include the rear window defogger, and power seats.
These “ball park” standards are based on the Engine Running.
Voltage Feed Side
Low current non-relay controlled loads:
No more than 1/2Volt drop between the battery positive (+) terminal and the input to any load.
No more than 100mV drop between the battery negative (-) terminal and the output pin from any load.
High current relay controlled loads:
No more than a 1 1/2Volt drop between the battery positive (+) terminal and the input to any load.
No more than a 100mV drop between the battery negative (-) terminal and the output pin from any load.
Ground Side
Low or high current loads:
No more than .100Volt (100mV) drop from the output terminal or case ground to the battery negative (-) terminal.
Generally, voltage drops should not exceed the following:
200millivolts for a wire or cable.
300millivolts for a switch.
100millivolts for a ground.
less than 50millivolts for any sensor connection.
Less than 50millivolts for any computer or control module ground.
zero millivolts for a connection between wire and its metal connector
The more conducting material, connections and contacts between the battery and the load, the higher the voltage drop. Since the ground path is usually a short piece of wire, or a case ground connected to the frame or sheet metal, the acceptable voltage drop on the groundside is always lower than the feed side. The Vehicle Voltage Drop Website

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About Generator / Alternator Testing:

What test can you do on an alternator?
How to quickly find out if the alternator is charging.
How to find out how much the alternator is producing.
How to test for shorted alternator diodes with alternator in the box (at the
parts store), on the bench, or in the vehicle with NO wires attached.
How to test for shorted alternator diodes with alternator in the vehicle with
the rotor feed wire attached and the B+ stud wire disconnected at the
How to test for AC riding on DC due to bad alternator diodes.
How to test for alternator undercharge.
How to test for alternator overcharge.
How to test for the alternator charging voltage.
How to test for intermittent overcharge or undercharge condition.
How to see the alternator ripple with Vat 40 or Oscilloscope
How to do test for voltage drop between the B+ stud of the alternator
and the battery positive (+) terminal.
How to do a Voltage drop test of the alternator ground side.
In 1996, the term “generator” replaced the term “alternator” in engineering jargon. I will use the terms interchangeably. I can remember polarizing a generator.
If you think you have problems with the alternator, don’t even consider looking at it until you have tested the battery. This will assure you that you will not be testing the alternator using a sulfated battery. If a battery is sulfated, the alternator cannot charge it.
When beginning to test the alternator, think basics first. Is the belt loose? Is the alternator mounting bracket loose?
Note: If you are working on a 1990 to 2007+ GM car or truck, refer to: GM bulletin #43-64-07A regarding low voltage readings or dim lights at idle. Don’t waste your time trying to fix something that, according to the manufacturer, is not really a problem.
The alternator cannot work as engineered if there is a significant voltage drop in the feed wire between the alternator and the battery positive (+) terminal, or between the alternator case, its ground back to the battery negative (-) terminal.
There have been cases where the alternator was replaced with a new one and the new one did not work. This is where voltage drop testing becomes very helpful in a diagnosis.
How can an alternator fail to produce electricity? Here are the most common ways:
1. Belt is not tight enough.
2. No voltage feed in to the rotor.
3. Voltage regulator is faulty.
4. Voltage regulator is good, but the grounding circuit for voltage
regulation is faulty.
5. Diodes are shorted – See “How components fail”.
The greatest percentage of problems with any alternator is with the diodes. An alternator with a faulty diode can put out enough current to supply the ignition system, but not enough to keep the battery charged. Diodes that go “bad”, (open or shorted) can be the result of improper test procedures, removing the alternator leads while the engine is running, reversing the battery connections, and from being mishandled.
Ninety-nine out of one-hundred diodes will short, rather than open, when they fail. Shorted diodes in the alternator provide a direct “short to ground” for
battery current. Shorted diodes can discharge a battery in a short period of time, whine or hum at idle or low speed. The whine or hum happens because the alternator, a three-phase device, is “out of phase”.
Shorted diodes can cause AC to ride on the DC. The vehicle electrical system relies on direct current. The vehicle electronic system relies on direct current, and also some very specific input of analog signals what can give an appearance of AC. If a diode does not rectify the AC in the alternator to DC, there will be an excessive amount of AC “riding on DC” throughout the entire electrical system. Voltage and current produced in the alternator is “analog” in nature, looking sometimes much like the signal produced by a crank sensor, wheel speed sensor, etc.
When AC rides on DC the AC “analog”, or constantly changing signal appears everywhere that voltage is present in the entire electrical/electronic system of the vehicle. This unruly signal has no defined target, like the wires from a permanent magnet generator, but rather is everywhere voltage exists.
Circuits that normally interpret and process analog signals can process the “AC riding on DC”. This causes electrical interference in the computer system. This could cause any control module to be triggered by an unwanted AC signal from the alternator rather than from a sensor engineered to send a specific signal at a specific time, on a dedicated wire. The end result could be drive-ability
problems, problems with cruise control, no start, etc. The Vehicle Voltage Drop Website

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