HOW TO DIAGNOSE THREAD

L337TurboZ

L337TurboZ

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This is going to be the start of a thread on how to perform diagnostics for all general automotive concerns.
I will try to break down issues by section. New threads will be made for each type of concern and the different ways on how to perform the test procedures.

The first subject will be on electronics/electricity. This is because modern cars rely so much on proper electrical operations many overlook performing the right steps and throw parts at their cars trying to fix an issue that may not be part related.

How does electricity work? Much like water flows. To properly diagnose a concern you must understand the fundamentals behind how it operates. Think of a battery as your well. Voltage is the total amount of electricity (water) available. Resistance is how much of a kink is in the wire (hose) and Amperage is how fast/how much the electricity can flow.

Here are some simple formulas for calculating Voltage, Amperage, and Resistance.

Voltage = Amperage x Resistance
Amperage = Voltage / Resistance
Resistance = Voltage / Amperage

You may not need to use formulas as many people don't when performing the diagnostics, however this is electrical 101 and it is important to have them.

A note about Amperage
Just like too much water pressure in a hose can cause swelling, too small of a wire can cause excessive heat when a lot of amperage is being pushed through it. This can be seen with high load components such as adding a cooling fan but selecting a wire not large enough to carry the current. The end result will be the fan will work, but the wire gets hot over time causing the wire to burn up and in a worst case scenario, a fire. When adding aftermarket electrical components please take this into consideration.

Next step is to understand the tests to perform. What they are, what they test for, and how to perform them. I will update the thread later on with specifics such as checking voltage drop, short to ground, short to voltage and so on.

As for the proper tools for testing electrical systems please have the following:

A good digital multimeter such as Fluke, Snap On, Matco etc. Having one with a MIN/MAX feature can be very helpful when performing tests on systems that only operate for a split second when the car is initially keyed on.

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A regular headlamp bulb NOT LED and socket. a 9004 is what I use for load testing. The socket with leads can be had for about 6 dollars and a bulb for about 10. Add some pins to the end of it or solder the stripped wire flat to allow it to be inserted into connectors. Polarity doesn't matter as current will flow through the bulb either way.

Power Probe. Probably the best tool you can use. These have digital displays, the ability to put power through a circuit or to ground a component for testing. Be careful with these because if you apply power to an ECU or other circuit board electronic on the wrong circuit you can burn the component up.

https://www.napaonline.com/en/p/BK_...bV36-BtNpLOHWcxZ0UbrJ_R9byNY60saAnGxEALw_wcB&

TO PREVENT DAMAGE TO CONNECTORS BUY THESE KITS!

These will allow you to properly check terminal tension and prevent spreading pins by shoving meter leads into terminals. I use these on all my electrical diagnostics as they connect right into my multimeter and power probe tool. Buy both of these and you will be able to properly probe every terminal out so far.

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TESTING FOR A SHORT TO GROUND

1.) Set your multimeter to the Resistance/Continuity function (Omega Symbol)
2.) Disconnect the load (your switch, solenoid, or other device activated by electrical current)
3.) Connect one lead to the wire being tested
4.) Connect the other lead to a ground point on the vehicles chassis or battery ground cable.
5.) If the meter reads anything other than OL or Infinite resistance then the wire has shorted to ground

TESTING FOR A SHORT TO VOLTAGE

1.) Set your multimeter to the Voltage function (V with solid lines/dots Symbol)
2.) Disconnect the load (your switch, solenoid, or other device activated by electrical current)
3.) Connect one lead to the wire being tested
4.) Connect the other lead to the negative cable or ground point
5.) If the meter reads anything over 1V then the wire has shorted to voltage

TESTING FOR VOLTAGE DROP

Sometimes resistance can be low in a circuit (the wires for example) but you may have a resistance concern within the component. To test for proper voltage drop connect the meter in this fashion. For example lets say the solenoid is operating intermittently. But you put power to it manually and it worked fine. There may be excessive resistance within the switch in this image preventing enough voltage to reach the solenoid. Remember that if all voltage being supplied on a circuit must equal zero at the end of the circuit.

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TESTING AMPERAGE OF A CIRCUIT/PARASITIC DRAW

This next test will be a little more involved. It is critical that you have spare fuses for your multimeter and that you also create an extra fuse inline with the meter. The purpose of the extra fuse is to prevent damage to your meter/meter fuse if there is a spike in amperage.

Get yourself one of these. It doesnt have to have the light function. Leave the wire ends bare so you can twist one end onto the lead and the other onto the wire.
LPM-04B-I_lg.jpg


Now here are the steps involved

1.) Set your positive lead of the DMM into the Amperage slot on your Multimeter (There are different lead locations on your meter and you will not get a reading if left in the normal Voltage/Resistance spot)
2.) The negative lead stays in the COMMON/GROUND
3.) The multimeter will have to be placed inline with the circuit being tested. Meaning that it cannot be on both power and ground. Power has to flow through the meter as if it was a part of the wire in the circuit. THIS MAY REQUIRE CUTTING A WIRE AND ADDING THE METER INTO THE CIRCUIT BE PREPARED TO PERFORM A WIRE REPAIR AFTER THIS TEST
4.) Activate the load and monitor the amperage flowing through the circuit.

TIP: FUSES ARE RATED FOR A SPIKE IN AMPERAGE NOT WHAT FLOWS THROUGH A CIRCUIT. EXAMPLE, COOLING FANS MAY HAVE A 30 AMP FUSE, BUT REALISTICALLY THEY PULL 20 AMPS. THERE IS USUALLY A SPIKE WHEN THE COMPONENT IS FIRST ACTIVATED.

PARASITIC LOAD

1.) Set your positive lead of the DMM into the Amperage slot on your Multimeter (There are different lead locations on your meter and you will not get a reading if left in the normal Voltage/Resistance spot)
2.) The negative lead stays in the COMMON/GROUND
3.) The multimeter will have to be placed inline with the BATTERY NEGATIVE CABLE. Ensure all loads are off (Headlamps, Radio, etc)
4.) Disconnect the negative cable from the battery
5.) Connect your inline fuse with the negative cable and the positive lead on the multimeter
6.) Connect your negative lead of the meter to the negative post.
7.) There will be an initial reading of amperage as modules such as Instrument Cluster, Body Control Module, ECM etc are "woken up" by a sudden change in voltage
8.) Wait 2 minutes with the leads connected. Amperage should drop to less than .02 mA
9.) Any reading over .02 mA indicates there is a component staying on. To find the component leave the leads connected as they are and begin to pull fuses while checking for the reading on the meter to drop to 0. Once you have found the circuit causing the draw, you can then isolate the issue or component in the circuit causing the concern
 
LOAD CIRCUIT TESTING

This test will be performed to test the ability of a circuit to carry amperage. You can do a resistance check of a wire and it "test good" however there's an issue down the line that it can't carry the load. For example one strand of wire from end to end can show less than 2 ohms resistance but the component won't be powered because not enough amperage can flow to it.

Things you will need:
A headlamp bulb and socket (I use a 9005 bulb)
Some extra wire connected to the socket
Alligator clip soldered to one of the socket wires.
A spare wire with an alligator clip

How to setup the socket

Only two wires are needed. One to connect to ground, and one to connect to the wire being tested. I recommend getting a small pin of some kind that can be inserted into the connector but not large enough to spread terminals

How to perform the test

For this example we will do a test of the Spill valve wires

1.) Disconnect the ECU connectors (ALL OF THEM) this is critical, you do not want to apply voltage to a module where it doesn't need to be applied

2.) Connect one wire of the socket to one of the ECU pins corresponding to the Spill valve, connect the other wire with alligator clip to ground

3.)take your spare wire with clip to battery positive, be careful not to touch it to ground

4.) Put spare wire to the Spill valve connector for the wire being tested currently. Only touch it momentarily! It should let the bulb flare up with light for a second.

HOLDING THE WIRE TO THE CIRCUIT FOR TOO LONG CAN BURN UP THE WIRE BEING TESTED, THE CONNECTOR, OR START A FIRE! All you need to do is just make sure the bulb can illuminate.

Perform this test for both Spill valve wires to their proper locations. If the lamp lights with both wires then circuit integrity is good. Begin to check for mechanical failure of components.

DIAGNOSING FUEL TRIM CONCERNS

What are fuel trims? The ECU monitors how much fuel is being used on average and tries to make sure the air to fuel ratio is as close to target as possible. Most manufacturers like to see fuel trims as close to +/- 10% Anything in that range could be considered acceptable and won't throw a check engine light.

Ideally within 5% is the goal. So what do the + and - actually mean? You have to think of this in an opposite way. Positive fuel trims are indicating that the ECU is trying to add fuel (lean) and negative means it is trying to remove fuel (rich). The ECU reads the air coming into the mass air flow sensor as a base reading, then calculates the air to fuel ratio to determine the fuel trims.

So how do we go about diagnosing fuel trims? This will be broken down into 3 sections. Oxygen Sensors, Lean Conditions, Rich Conditions.

OXYGEN SENSORS

The ECU uses these sensors to determine the amount of oxygen left in the exhaust gasses. If there is too much fuel there isn't enough oxygen and if there is too little fuel there is too much. Pretty simple right?

The ECU uses two sensors. One before the catalytic converter and one after. The one before reads your ACTUAL AFR and the one after is only for CATALYTIC CONVERTER HEALTH.

With this in mind only the front one plays a part in determining fuel trims. For both sensors the ECU has a voltage range each sensor should be in. For example if the range is 0-5 volts then at 2.5 volts it would be stoichiometric (14.7) anything below 2.5 volts could be rich and anything above it could be lean. It really depends on each manufacturer and how they have their ECU logic setup. Some sensors are only 0-1V. Usually narrowband sensors operate on a 0-1V scale where as wideband sensors operate on 0-5V.

How do you diagnose a faulty O2 sensor? Remove the sensor and use some brake/throttle cleaner soaked on a rag. Hold it around the sensor and see if it reads full lean. If it does then it indicates the sensor can sweep fuel lean one direction.

LEAN CONDITIONS

These are caused by a few factors. An air leak (most common), weak fuel pressure (faulty HPFP, in tank pump, clogged injectors), a skewed MAF sensor reading, poor tuning, too much boost pressure etc.

First things to check for a lean condition would be an air leak. This could be a bad gasket/seal, a failed evaporative emission purge solenoid, disconnected air pipe. Spraying an area with soapy water, throttle body/brake parts cleaner, or smoke testing can help locate a potential air leak.

If an air leak was not found then its time to start looking at fuel pressure readings and verifying mechanical fuel pressure ONLY ON THE SIDE GOING INTO THE HPFP. The HPFP has pressure far above what a normal gauge can read. DO NOT DO SOMETHING TO INJURE YOURSELF OR CAUSE A FIRE

RICH CONDITIONS

Rich conditions are fairly easy to figure out. The engine isn't properly burning all the fuel. Start by checking spark plug condition (gap, electrode health) next would be proper boost pressure, a leaking HPFP (it can leak into the crankcase causing the oil to thin out).

A neat trick for the possibility of a leaking HPFP into the crankcase is to remove the dipstick and run the engine while monitoring the fuel trims. If they begin to go back from negative to positive it indicates that the engine is burning oil thinned by the fuel contamination. When correcting this concern make sure you do an oil change so you have fresh oil that doesn't skew your verification results.


NOTE: I will come back later to finish up this post about fuel trim concerns. Hopefully this can help others and if anyone can add to this please do. I will update and make notes with references.
 
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