Troubleshooting ECU related short circuits

Question

I own a vehicle that has given me a lot of grief recently due to short circuits.

The benefit of this is that I have undertaken a class on professional automotive electronics diagnostics and repair and so I would say I am fairly good at fixing vehicular electrical/electronic faults, except of course this one that I am talking about.

The manufacturer provides two methods of identifying short circuits:

a. for circuits not connected to a control unit b. for circuits connected to a control unit

The circuit I am trying to troubleshoot is connected to a control unit.

The first set of instructions read as follows:

1. remove the fuse and main fuse of the circuit
2. disconnect all connectors of electrical components in the circuit
3. attach a voltmeter to the fuse box and reconnect each connector beginning nearest to the power source
4. check the voltmeter reading as the connections are reconnected

A short has occurred where the voltmeter reading changes

This will help to confirm or deny if there is a short that has resulted in/from a fuse melting, or a solenoid operating when the ignition switch is one or a CPU transistor burns out when the ignition switch is turned on.

This is why I am stupefied!

According to the wiring diagram for the problem circuit, there are several fuses! There is an engine fuse of 30A in the engine compartment which I think should qualify as the main fuse, and there are three more fuses in the left kick panel inside the vehicle, another engine fuse rated at 10A and two more meter fuses one rated at 15A and another rated at 10A.

My first question is - to which of these fuse connectors should I be taking the voltmeter reading at?

There second set of instructions for finding a short follows immediately below and it involves three steps:

1. Attach the voltmeter to the CPU connector
2. Connect to the Switch/sensor connector
3. Check the voltmeter reading

A short has occurred where the voltmeter reading is 0V

This will help to confirm or deny if there is a short that has resulted in/from the CPU thinking the switch is on because the same conditions exist as when the switch is on; as well as where the CPU sensing the sensor to be 0 Ohms because the same conditions exist as when the resistance value is 0 Ohms; or the CPU equipped with the self-diagnosis function outputs the code.

My second question is how exactly am I supposed to attach the voltmeter to the CPU connector?

I have attached a copy of the picture that came along with this instructions in case I can get some help to gain some clarity on what exactly I need to do!

NOTE: I will edited the part of the instructions that talk about using a test light because I prefer to use a multimeter!

Schematics of the circuit has 3 parts,

The voltage drop readings should be as follows ( manufacturer specifications)

IMRC TERMINAL 1 at idle = B+

IMRC TERMINAL 1 at 3,300RPM = Below 1.0V

IMRC TERMINAL 2 at Ignition ON = B+

IMRC TERMINAL 2 at Ignition OFF= Below 1.0V

IMRC TERMINAL 3 at all times = Below 1.0V

IMRC TERMINAL 5 at idle = 5V

IMRC TERMINAL 5 at 3,300RPM = Below 1.0V

IMRC TERMINAL 6 at all times = Below 1.0V

The voltage drop readings in my vehicle are as follows

IMRC TERMINAL 1 at idle = Below 1.0V (about 0.04V to 0.08V)

IMRC TERMINAL 1 at 3,300RPM = Below 1.0V (about 0.04V to 0.08V)

IMRC TERMINAL 2 at Ignition ON = B+

IMRC TERMINAL 2 at Ignition OFF= Below 1.0V (about 0.10V to 0.11V)

IMRC TERMINAL 3 at all times = Below 1.0V (about 0.03V to 0.04V)

IMRC TERMINAL 5 at idle = 5V

IMRC TERMINAL 5 at 3,300RPM = 5V

IMRC TERMINAL 6 at all times = Below 1.0V (about 0.05V to 0.08V)

These are the full diagrams for testing for short circuits in both circuits connected to a control unit and circuits without a control unit

POSTSCRIPT:

One of the reasons why I came here was persistent short circuits between IMRC Terminal 2 and body ground; IMRC Terminal 5 and body ground; IMRC Terminal 5 and power supply and finally IMRC Terminal 6 and power supply.

I finally fixed all these short circuits - it was from one of the oxygen sensors.

This leaves me with a faulty ECU. This diagram explains how the entire circuit works, it looks like. The problem appears to be between IMRC Terminal 1 and PCM Pin 42, which is now permanently closed.

Answer 1

First thing first. Who ever decided to call it a melted fuse needs to be shot. What they mean is a blown fuse. When a short happens a fuse is blown.

There are 2 kinds of shorts, ones that blow a fuse and ones that make something stop working.

When you have a blown fuse focus on diagnosing the circuit with the blown fuse. Don't bother with what they call the main fuse. The technique they are describing connects a volt meter across the removed blown fuse. If your digital volt meter is worth the plastic it's made out of it should have 20 mega ohms of resistance. This causes what i would call a choke point. Nothing that uses power can power up from such a resistance and a short would show up as a voltage on the meter. The short coming of this technique is that when something with capacitors inside is connected you will see the voltage on the meter twitch. Meaning that you will see it come up and then slowly decay. This is normal. What you will see when you find a short is the reading will come up and stay up, near or around battery voltage.

My favorite technique for finding blown fuse shorts involves a tool that has a buzzer and a thermal circuit breaker. Like your directions describe, unplug everything. Plug in this tool instead of the fuse. Plug things in until you hear the buzzer sound. The buzzer means the circuit breaker opened. The investigate the circuit that tripped the buzzer. If you are crafty enough you can make this tool with generic circuit breakers and by soldering wires and fuse pins to the circuit breaker.

With the second kind of short, the short bypasses some device. In the example given a switch that connects to ground is bypassed. This makes the switch appear as though its always closed. Connecting to the CPU connector is pretty easy. Take a pin such as a push pin or T pin and shove it into the connector from the wire side. This is called back probing. Do not, i repeat, do not front probe, unplug the connector and shove the pin in from the front.