X-Long Read, money saving Electrical Tips: Pics added, long download

[Doug99RS]

Electrical 101/201:
I've run across quite a few electrical problems where I got in way too deep too quick. From intermittant Check engine light codes, no start conditions, lights flickering to poor engine performance... there's alot to go wrong on Fuel injected cars today but diagnostics can be simplified with a basic understanding of the circuitry.

For most of these tests you will need a multi-meter (digital with auto-ranging really helps), a test light, and a decent wiring diagram of the circuit you are working with. The multimeter will be used to detect voltage on a circuit, resistance in a circuit and to provide a second path for current to flow in such a manor that you can measure it too.

Please note that alot of this information is from first hand experience and teachings from more experienced technicians. Nothing is 100% and I'm sure I will leave out some information here or there that you may want to ask me and fellow members about. Feel free to do so.

Starting out:
The quick and easy of all electrical tests starts with you check all fuses with a test light. If you have the kind with a site glass on the top be sure to touch the cover and wiggle it. You can't actually check it like a mini fuse but you can wiggle it to make sure the link inside isn't broken. It will be much like a filament in a light, you look at it and think it's good but then flick the bulb and the filament moves. Same logic with this fuse. Move it around. If you still don't trust the fuse replace it OR check resistance with a multi-meter.

No matter where you find the beginning and end of a wire keep in mind that you don't care about where it goes in between unless your readings indicate a problem. Don't get all caught up in tracing down the wire. Go straight to your end components and start there. Also, KISS. Keep It Simple, Stupid. Split the circuit in sections.

Example: I had a Neon last week that had no ASD voltage at the coil. I knew that the injectors, the coil, and several other components share the ASD circuit but didn't know which component was causing the voltage not to be there. On the 95's there's a large connector right next to the battery and junction block. I started out unplugging a couple of components on the engine but remembered I didn't have to do all that right now. I disconnected the large connector and checked my system again. The problem was still there, blowing fuse, and no voltage. That ruled out a TON of electrical components in the engine bay. I could have spent another hour checking each component but took 60 seconds and ruled out the entire engine harness. It was a bad computer for those interested.


Checking Voltage:
The first test uses the voltmeter section of a multi-meter. An easy example of this is putting the leads on the battery terminals. You should read in excess of 12 volts with the car running. Most of the time you will see 12.5 to 13.5 volts. Now you may ask which lead do I put where, there's a red and a black!?!?! It doesn't matter on most tests because the reading will have a negative or no indication next to it which only tells you the polarity (direction of current flow) of the circuit.

Where to use for a ground:
The best place for one of your leads here is directly to the battery negative. This is where all current leaving the battery wants to return to and should be viewed as being a constant known good ground. The next place you could use would be any bundle of wires at an actual chassis ground (stud, nut and wires with eyelets). Finally, you can use any clean surface of metal that is on the chassis. This should be free of rust and paint similar to a door or trunk striker (part that any given latch attatches to). I sometimes use the boths for the door check strap too, no paint and attatched to the frame of the car. In many situations it would be a good idea to actually use the same ground that the component you are testing uses.

Example: TPS uses a signal wire, a reference voltage wire and a ground wire. If the reference voltage is 8 volts use your meter and verify that it is there with one lead attatched to that wire and one to battery negative. Move the lead to the ground circuit pin in the connector instead of battery negative. You should see no change in your voltage reading if everything is good. A drop in voltage would mean you have high resistance in the path to ground.

Checking Resistance:
Doing this will test a circuit's ability to pass current. It used to be trusted 100% but as I will explain later you can still get screwed over by ONLY checking resistance. With your multimeter in Ohm-meter mode you will be checking a single piece of wire. Leads go at opposite ends from each other and you should see little to no resistance. A rule of thumb when doing this is that for each connection you should allow about 0.10 ohms of resistance so if your wire has been cut four times with four connectors installed you could read 0.30 to 0.50 ohms. Most of the time you will not see more than one or two connectors because 0.50 ohms will really mess up a sensative circuit such as TPS, ECT or MAP.

Here's why using just an ohm-meter sucks. It doesn't apply a load. If you do a resistance check on an 8 guage wire (new) then you will see you have no resistance. BUT would you use this wire to connect your house to the power pole at the street? No, because it can't carry the load the system requires. Same goes for a car.
Fact: As current flow goes up so does the temperature of the wires. As temperature goes up so does resistance. The higher the resistance the more amperage needed to push the voltage to the end component. Fuses get blown, wires get melted and components burn out.

Here's an alternate method for checking a circuit using a test light. The circuit again is the ground circuit of any given component but this time connector your test light clamp to battery POSITIVE and then touch the pin in the connector described as being the ground circuit. Your light should illuminate brightly. If it's dim or does not illuminate then you have high resistance and should fix it.

Note you should NOT use this when checking voltage coming out of the PCM because some test lights will draw too much current. This will cause the driver in the computer to burn up and now you need a new computer and still have the original problem you started with. Attention must be paid to determine if you need to disconnect the computer or not.

[Doug99RS]

Voltage Drop Test:
This test is VERY handy when you have flickering lights, charging problems or think your circuit is not up to par. Basically you will be introducing a second path for current to flow in addition to using the car's wiring.

Alot of people have experienced a battery that keeps going dead and you've replaced the battery and altenator. Then you find out your battery is dead again.

We start this test at an easy place, the battery. You're going to put the meter in volt-meter mode. Put one lead on the positive post of the battery itself and the other lead on the positive battery cable (car harness side). You don't want to see any voltage. If you see voltage then you have a bad connection right there at the battery. Remove the terminal, clean both contact surfaces and reinstall. Test again and you should see no voltage now.

Next leave one lead on the battery positive terminal and then put the other on the large red wire at the altenator. You may see 0.10 volts but you don't want more than that. If you have voltage then you can safely assume that there is resistance in this circuit which will prevent your battery from being charged and possibly hinder starter engagement.

Another service point frequently on this same circuit is the starter. Some vehicles have a heavy gauge wire going from the battery, to the starter and then on to the altenator. Consider the component in the middle to be a splitting section. If you have high resistance in the entire circuit, disconnect at the middle component and check from there back to the battery. The resistance should be found easily like this.

Checking Signal Voltage readings:
This is difficult using a mutli-meter on some circuits such as o2 sensor or MAP sensor because the signal varies rapidly while driving. You can apply some of this info to checking those components but really what we're going to look at here will be things like Engine Coolant Temp signal, Intake Air Temp Signal, Throttle Position Signal and a few others that use a three (or more) wire circuit.

The easy thing to remember is that the temperature sensor signals move fairly slowly and monitoring these can be simple. TPS depends on your foot and can be more difficult to get a good idea as to what's happening.

Fact: The engine coolant temperature sensor is a negative temperature coefficient thermistor type sensor. As coolant temp goes up, the resistance in the sensor goes down.
So for you with your meter hooked up to ground and the signal wire, your voltage should go up as temperature goes up.

TPS works a little differently but still varies based on throttle plate position. You can check resistance from the reference voltage pin to the signal pin. As you move the throttle plate with the TPS attatched your resistance should increase slowly as you slowly move the throttle plate. If you're checking the TPS using the volt-meter then your voltage should increase and decrease at the same speed you move the plate open and closed. An open circuit (resistance) or big difference in voltage readings mid-stream of the sweep would indicate a problem with the TPS.

Great tool to have:
Lab scope. I bought a PDA with an expansion slot on the back and the lab scope package from the tool truck. The scope package was $300.00 and Ebay will supply you with a good PDA for under $100.00. This will allow you to log a signal over a time frame of several minutes or seconds depending on the setting you want. Great for looking at o2 signals, watching for blips in TPS and coolant temp signals as well as a standard volt-meter option. This particular one does not have an ohm-meter option but can graph two different signals on the same screen at the same time.

[Motohead1]

Good stuff Doug.

[Redline165]

sticky

[Doug99RS]

Electrical: Reading Diagrams

Here's a bit more information since some folks are getting involved with wiring and less than obvious electrical problems. The diagram below is from a 93 talon AWD manual trans and deals with the powertrain electronics. I'll try and discuss a few key points of the wiring, connectors, grounds and such as well as some other things like relay operation and controller circuitry to help you better understand things.

Also, I'll open the thread back up so you can post questions and comments but please limit them to general questions about wiring diagrams. Please use other threads that already exist or start your own, referencing this if your like, to help diagnose problems you're having with your own car.

At the top of the diagram you'll see a triangle with an "A" in it. This means the circuit is coming from a different page, which I've left out, and you will need the other page to know how the rest of the circuit works, if it's a ground, a power source or where it leads. I'll save you some time and tell you that it leads back to the MPI relay and is powered ONLY when the relay is energized. More about an energized relay later.

Directly below that you'll see a 1.25R which indicates the gauge of the wire and the color, R is red. The Dot in the wire indicates a splice meaning that the wire is shared with other components.

I'm going to bypass the top half of the wiring diagram and go to the bottom half. It's a bit easier to understand and I'll try to explain Hall effect sensors and the rest another time.

Far left, lower corner is a ground. This is noted as ground number 7 and various ground locations can be found by using their numbers, some are at the battery, some near headlamps, others are at studs sticking out of various places. As you can see by this diagram, there are two different wires using ground #7 so expect to find multiple ground wires at that location.

To the right of that you'll find connectors. Remember Mitsubishi numbers their connectors as if you're reading the pins with the connector still plugged in to the component.

The big box in the middle you'll see is the MPI control unit (ECU). The first connector on the bottom, left side is connector C-64 and you'll see that all three wires come out seperately but eventually splice together and then lead to a common ground source. Note how the individual wires are all 1.25 guage but after the splice the the wire beefs up to 2.0 guage (I'm assuming this is probably in millimeters). This is because you've got to make sure that the computer has excellent grounds in addition to a good power source. Like a motor, You can cram all the fuel and air in to the combustion chamber but without a good exhaust... you're not going to move very far, fast.

Jumping over a little bit to the Engine Coolant Temperature sensor. This is a negative temperature coefficient thermistor type sensor. Basically, as temperature goes up, resistance goes down. As resistance goes down, the more current the sensor allows to pass through it. Well, where's the current coming from? I'm glad I asked.

Look at the ECU pin 20, yellow with a green tracer (YG) just above the ECT sensor. Inside the ECU the computer takes 12 volts from the battery (through fuses and wiring and such) and reduces it down to whatever voltage (usually 5 or 8 volts) it wants to. In this case it reduces it down to a low amperage 5 volts. Not a big push, not enough to really do jack shit but enough to use for sensors. Basically the computer sends 5 volts to the ECT sensor all the while it monitors how much of that voltage goes down and comes back. The colder the temperature sensor, the higher the resistance in the sensor and thus the less voltage the circuit allows to go to ground (back inside the ECU).

As the engine warms up, the resistance goes down on a sliding scale, HOPEFULLY not dramatically like steps, but like a gentle slope. So this is how ECT temps are monitored by sending 5 volts to a sensor that increases and decreases resistance based on it's temperature. The resistance dicatates how much of the computers signal voltage will go to ground. The balance of the signal voltage is used as a reference to indicate how hot the engine is and that value is used in conjunction with other sensor readings for correcting a/f ratios and emissions.

Next is the TPS sensor. This is a very good sensor to learn about because they are so picky on the 4g63 motors. Basically, this sensor has a reference voltage and a ground much like the ECT only it does not change based on temperature, it changes based on your foot . Refer to the second picture for a better description on this part.

Side note: A circuit that shows equal to it's reference volts +/- 0.3volts is usually unplugged or open i.e a TPS signal showing 5 volts probably has an unplugged tps sensor.

Basically, at throttle rest the resistance is highest in the circuit and probably around 4 volts or so. As you increase the throttle the voltage will lower proportional to the rate at which you open the plate. So unlike temperature changes which will happen gradually, you can open the throttle slowly and graph the signal to see a slow change or you can "pop" it and see a sudden change. Both techniques are useful because you will want to move a TPS slowly to check for dead spots in the TPS's range of motion as well as make sure the tps rests properly when popped and gently returned to it's "rest" position.

Now, look at the second TPS picture I've uploaded. This would represent a TPS that has a bad spot in it. Probably around the 25- 30% range?? Basically, this happens because you spend most of your time in this range of the throttle. It's kind of like a pair of pants that you constantly walk on the heels of. The more you do it, the more you're going to wear out JUST that part of the pants. The same goes for this sensor and others like it such as your fuel tank sending unit (gas gauge). So be aware of this when you're checking your TPS sensor. If you're moving the throttle plate slowly and you find that you keep opening it and your reading aren't changing... you've got a worn spot in the sensor.

[Doug99RS]

OK, there's been a number of help needed threads lately so I thought I'd add some more tech information that no one will ever read.

Some of the questions relate to the base idle set screw (BISS) and the throttle position sensor (TPS) so I grabbed a throttle body (TB), a multimeter (DVOM) and a camera and wasted my evening being a nerd.

This picture shows a TB with the throttle plate shut and what the resistance reading through the TPS sensor is for that given position:

The resistance is 4.68K ohms.

This next picture shows a partially open TB with a resistance of 4.31 K ohms:

Notice how the cell phone magically appears now. This is caused by an increase of airflow which is sucking more items in to the TB.


The next picture shows the resistance to be even lower (3.475 K Ohms) as the throttle plate is around 50% +/- open.



The last picture shows the TB plate fully open. Resistance has dropped to it's lowest point of 1.583 K Ohms.



What does all this mean? Well, the computer sends to the TPS sensor a voltage. Usually 5-8 volts. The resistance inside the sensor dictates how much of that voltage flows through the sensor and returns to the ECU/PCM. So at rest resistance is greatest and the TPS signal is going to be low. As you open the throttle plate the resistance decreases and the signal voltage will increase.

What does all this mean? Well, if you read all of the posts I had before this one then you'll now be able to understand the value of a multimeter when you're trying to see if your TPS (or any sensor) is bad. Additionally, you'll be able to TUNE your BISS and TPS sensors with the information above and below.

[Doug99RS]

Understanding the adjustments on your Throttle body:

Many people see something they can manipulate and they want to do so without understanding the effects. The BISS and TPS are two of those items and messing the them without knowing what you're doing can cause poor idle, poor part throttle performance, poor fuel economy, a hunting idle or other strange problems.

On the turbo and 2.4L models (according to the 99FSM) the resistance between pins 1 and 4 should be between 3.5 to 6.5 K Ohms. To check the sensor's range of operation like the pictures I just posted above you move your test lead to pins 2 and 4. As you increase throttle position the resistance should change proportionally and smoothly. Open the plate very slowly as you will increase your chances of finding a glitch if there is one. "If the resistance is outside the standard value, or if it doesn't change smoothly, replace the throttle position sensor. NOTE: Always adjust the TPS after replacement."

There is a second portion of the throttle position sensor called the "Closed throttle position switch". To check this put your leads on pin 3 and pin 4. With the throttle plate open you should have no continuity (Should show up a 0.L meaning open loop or infinite resistance). With the throttle plate shut you should show 0.00 ohms of resistance.

It's important to check this circuit when you're adjusting the BISS and TPS. The TPS sensor has a wide range of resistance BUT the computer typically only has 4 different modes of throttle range each with their own purpose.

Cruise/idle mode for little to no throttle input, acceleration mode for light to moderate acceleration, Deceleration which the ECU/PCM reduces or shuts off injectors, and WOT for heavy or all out acceleration.

If you have an improperly adjusted TPS sensor then you may be allowing in enough air for cruising but the signal from the TPS puts the ECU just beyond cruise and in to the acceleration mode. Likewise if you're sitting at idle and the TPS is adjusted too far then it may think you're trying to accelerate when you're really not thereby wasting gas. Also, if it adds fuel for an acceleration but you're just idling it's going to keep fighting o2 sensor feedback and TPS input.

There's several problems with having so many adjustments and they frequently show up when the car is in the hands of someone not fully versed in the nature of the beast. Things like vacuum leaks, improperly adjusted throttle cables and dirty throttle bodies typically get overlooked because they are never mentioned by the guy with a crystal ball helping you "Fix" your car.

A dirty throttle body like this one:



Is basically starving your engine for air. The carbon, oil, and dirty build up is blocking airflow required to maintain your idle quality. This is a big reason why people start messing with the BISS in the first place. They don't clean the throttle plate, it starts lowering the idle as it gets dirtier and because some Ass Hat on the web said "Twist this cool screw" first people create a bigger problem when they finally start learning how to do things right.

A cleaner throttle body like this one:

Will improve air flow thus improving idle quality.

This is a Base Idle Set Screw on a 420A TB:


While the TPS on the 420A is fixed and non-adjustable it's still possible to create a problem with it. When you have a dirty throttle body and you twist on the BISS not only do you increase airflow to bring idle back up but you change the throttle position sensor signal too. Now, every time you crank the car, you're starting out at a higher TPS signal then you should.

With an adjustable TPS sensor it's possible to increase airflow while changing the TPS sensor to show that it's even more closed than it was before you messed with the BISS. What this does is create a condition where the manifold pressure sensor sees more airflow than what the TPS is reporting it should have. It really gets crazy when you have so many adjustments and you don't know which is further out of adjustment.