Jeff's DIY

Advice on doing your own diagnosis and repair

Advice from the book Can I Do It Myself?

Automotive Electrical Basics

Modern car and truck electrical systems are based on a 12 VDC battery that is charged by an alternator and regulator. In most cases, the regulator is integral to the alternator but can be separate and may even be a part of the engine computer. The regulator produces 13.5-15 VDC to run everything in the vehicle and charge the battery. Aside from the engine computer, often called the powertrain control module, there may also be a body control module and a transmission computer. There may be additional logic in some components, such as the instrument cluster and climate control system. Most modern vehicles also use digital controls for many subsystems to reduce wiring. This can make chassis fault diagnosis much more difficult, unless logic diagrams are available (and they usually aren't available).


Relay--why do I have it and how do I check it?

Tips on how to check relays and the switches that operate them

Quick tip: If you don't care what a relay is and just want to try a quick test, you can swap the questionable relay with another one. If the relay has the same number of prongs, it is generally interchangeable. Note: when turning on the ignition switch to see if a relay is working, you may need to remove other relays that may also click with the key on to avoid confusion about which relay is clicking.

Also, the horn relay is a handy one. You don't need it for the engine to work, so you can always borrow it to fix a bad one and replace it later. Plus, if you suspect you have a relay that is shorted on, you can push it into the horn relay socket to see if it blows the horn without pushing the horn button. The horn relay and socket is a great troubleshooting tool!

What is a relay? A relay is used to turn something on that needs a substantial amount of power. It may be something that you use manually, like the horn, or something the car's computer turns on, like EGR and purge valves. Here are some typical components that use relays: A/C compressors, brake lights, radiator fans, some headlights, fuel pumps, and horns. Below is a typical relay out of the engine compartment relay/fuse box:


How does it work? A relay uses a smaller "command" signal to close a switch that can carry a larger current. The "command" is usually just a 12 volt signal from, say, the brake switch above your brake pedal. By pressing the brakes, you flip a small switch that connects to the relay. The relay then closes and makes a much larger current available to the brake lights. To work, a relay needs 4 terminals as shown below:

The command terminal on the left is connected to the switch (say your brake switch) that is turning the relay on. The current this causes in the coil creates a magnetic field that closes the contacts. The contacts thereby connect the hot input to the load terminal, which is connected to your brake lights or whatever is being turned on. The brakes lights, having their own ground connection to the chassis, will then light until the command signal is removed from the relay.

Note that the command signal can be negative or positive. Buttons that commonly use a negative (ground) signal are the horn and door switches. If the command signal is negative, the other coil terminal must be supplied with positive voltage and vice versa.

Now that you understand the relay's operation, you are ready to troubleshoot your circuit. There are two things to check for at the relay.
1. Is the relay receiving the command signal?
2. Is the relay working?

Note that if the command signal is positive, you don't have to know which terminal is which to troubleshoot. There is only one hot terminal when the command signal is absent (may be always hot or hot with the key on, depending on what the relay goes to--for example, the brake lights always have a hot input so they will work even when the key is off). When the command signal is present, there are 2 hot terminals. So by turning the switch on (by say holding the brake pedal down), you should have 2 hot terminals in the relay socket.

Using a test light (available for a few dollars at your parts store or local WALMART) with the clip connected to a ground point, test all terminals in the socket. The two photos below show a typical test light connected to a chassis ground and then being used to check a relay socket for the presence of a command signal.


If you find 2 hot terminals, the command switch is working.

In the case of a negative command signal, you actually have to verify that the ground is present when the switch is on. For example, to troubleshoot the horn switch, you would need to check each pair of terminals in the socket with the horn button on and off to see if the command signal is coming through. Another approach would be to use an ohmmeter to check for the negative command signal by verifying that the command terminal has no resistance to a ground point.

Next, with the command switch still on, push the relay back into the socket and listen for the contacts to click as the prongs make contact with the socket. If you hear a click, the relay is probably working.

Finally, you can check the wiring between the relay and the load by checking the connector terminals at the load. For example, remove the connector from the horn and have someone hold the horn button on while you use your test light to check the terminal(s) in the horn connector--there should be one hot terminal when the horn button on the steering wheel is pressed.

Failure modes: Now you know how to troubleshoot a relay circuit, but there are a few more things worth knowing, depending on what your car's issue is. The relay can fail open or closed. If any of the terminals becomes disconnected inside the relay, it will fail open--that is to say it will not supply voltage to the load. Other ways the relay can fail open are if the coil shorts itself and burns open or the contacts become damaged from arcing, thus no longer making a good contact.

If the return spring breaks and no longer pulls the contacts apart when the command signal is removed, the relay can fail closed, thus providing voltage to the load even in the absence of a command signal. Another way the relay can fail closed is if the contacts become fused due to extended arcing. A fail closed mode is very uncommon, and the condition of "un-commanded" voltage at the load is more often caused by a failed closed command switch (for example, your horn button being stuck on).

Trade Tricks: Note that the relay coil does not care about the polarity of the voltage applied--it will pull the contacts closed no matter which end of the coil has the positive voltage applied. The industry, therefore, has generally used this to design the sockets and relays such that the relays can be rotated 180 degrees without changing the function. See the figure below in which I have swapped the position of the Command and Load terminals to accomplish this purpose. Industry has also generally made all 4-pole relays that will fit the same sockets of the same power ratings, so that you can swap and rotate relays all you want without causing problems.

Diagram of a relay that can be rotated 180 degrees:


For circuits that are controlled by relays, the relay socket can really make troubleshooting a circuit a lot quicker, if you know what you are doing. I realize that, if you are new to electric circuits, all of this may be confusing. If you need help, please feel free to send me an email at If I can find your wiring diagram, I can provide step-by-step troubleshooting procedures.


Check Engine Light On--What Do I Check?

This light title is a bit of a misnomer. It is sometimes also called the "service engine" light, or "fault" light, among other names. The technical name is Malfunction Indicator Light or MIL. The light is indicating that the engine computer has detected a fault in the system. The computer uses a number of sensors to monitor and control what is happening with the engine and perhaps a few other systems in the car. In most cases, a scanner is used to hookup to the computer thru a connector, usually under the dash by the driver's knees. Here is a typical OBD II connector:


If you don't see it by the driver's knees, check behind the glovebox and also in the engine compartment near the relay box. It may be that your car is too old to have an OBD II connector (OBD II became a standard in 1996). Google OBD II and your car to see if you do. In some cases, there are procedures to extract the trouble codes without a scanner, especially prior to 1996. Also, many cars older than 1996 may have an OBD I connector. These systems may also be scanned, but the data may be less useful in many cases.

The scanner will read the fault codes stored by the computer. Many parts chains, such as Autozone, Advance, O'Reilly, or Pep Boys will provide a free scan of your car/truck. You can also buy a Bluetooth interface these days for about $20 that you can use with a free smartphone app to scan your computer. Your mechanic may have a more advanced scanner that can troubleshoot each code down to the component failure, but if you want to do it yourself, write down the codes and look them up online. The following page lists the standard codes and their probable causes: In addition to these standard codes, many cars have special codes or a different diagnostic system. If your codes do not match those in the above link, look online for a special set of codes for your vehicle. may have your specific codes in their free repair website.

The codes and causes only give you some ideas for troubleshooting. Let me know if you need help deciphering codes or troubleshooting your codes.

Once you have resolved the problem that set the code and the car has had time on the road to retest the affected system, the light will go out by itself. If it doesn't, a return trip to the parts store scanner can erase the codes.

Here are some tips and links for troubleshooting some common codes:

First, read the code carefully to identify troubleshooting steps. There is often quite a bit of data put into these codes that would help you to avoid making ineffective repairs. In addition, a few troubleshooting steps may help you isolate the problem to the bad component. If you have access to an advanced scanner, you may be able to run additional tests, including reading data directly from sensors and/or actuating electrical components to confirm their functionality.

P0130-P0175: These codes relate to data from the oxygen sensors. There is a tendency out there to lump these codes together and always change these expensive sensors when these codes appear. Many times, the sensor is indeed bad, but many other times it is not.

In particular, the codes P0171-P0175 point more to the air/fuel mixture and are rarely caused by a failed sensor. The codes P0171 and P0174 are usually due by a vacuum leak caused either by broken/loose vacuum hoses, cracked air intake hoses or leaking intake manifold gaskets. The codes P0172 and P0175 are usually due to a failed fuel pressure regulator or injectors stuck open. Please know that I do not propose here to replace or repair any items I identify without further troubleshooting.

Many codes in the range P0130-P0170 may be caused by wiring issues. Note that the oxygen sensor wiring sees a harsh environment, because it routes under the car and close to hot exhaust system components. Always check the wiring when you have a trouble code in this range.

P0300-P0308: See the tip on misfires.

P0420-P0423 and P0430-P0433: The downstream O2 sensors are seeing excess fuel in the exhaust while the upstream sensors are not. This usually indicates the catalytic converter is worn out, but may also be caused by a rich mixture that barely passes the limits that would trip the upstream sensor to identify a rich condition.

P0440-P0469 and P0496-P0499: These codes deal with the car's evaporation emission system, which is designed to minimize the release of fuel vapors into the atmosphere. The basic system seals the fuel tank and stores vapors in a charcoal canister to be released into the engine for burning at an appropriate time. Many modern systems are much more elaborate than what I describe here. A purge or vent valve/solenoid is used to allow the vapors to be drawn into the engine. It should be noted that these faults rarely affect the performance of the engine, except when the purge valve is stuck open or there is a leak between the purge valve and the engine, which will usually result in a slightly lean mixture.

Codes P0455-P0457 indicate leaks in the system. Always check your fuel cap first when these codes are present. If the cap has a good seal, check the evap hose on the fuel tank and follow it to the canister and on to the purge valve. Replace any suspect hoses, and make sure the purge valve isn't stuck open. After repairing an evap system leak, the car will need a day or two of operation to retest itself and clear the trouble code.

Sensors, sensors, and more sensors. Why do I need all these sensors?

While one can argue that designers have gone too far to add conveniences in the design of cars to the detriment of reliability, most of the sensors added to the engines over time have been needed to meet government imposed efficiency and pollution standards.

Mass Airflow (MAF) sensor: this sensor measures the flow of air into the engine. The computer uses this data to calculate the proper fuel flow for the injectors. This is equivalent to the function of the venturi in a carbureted engine. The same function can be performed by the combination of pressure sensors inside and outside the engine. These are known as the manifold absolute pressure (MAP) and barometric pressure sensors. While an engine may start with bad airflow data, it will not run effectively without it. Many cars will not even start without MAF data, while bad MAF data will cause lean or rich operation. Due to contamination of the MAF sensor, lean operation is more common as contaminants tend to reduce the signal from the sensor.

The MAP sensor is also useful even in engines that use a MAF sensor. A MAP sensor can be used to detect sudden throttle opening. A throttle position or accelerator position sensor can also be used for this purpose. The computer uses this data to richen the mixture for acceleration, much like the accelerator pump of a carburetor.

The Crankshaft Position sensor is used to tell the computer when piston #1 is approaching top dead center. The computer uses this data to time the ignition system. Data from additional sensors, such as coolant temperature and intake air temperature as well as knock sensor(s) are used to further optimize ignition timing. The knock sensor detects detonation and the computer compensates by retarding ignition timing to prevent engine damage. Crankshaft position data is required for engine operation. Most modern engines will not start without it. In fact, even a loose sensor will often prevent the engine from starting. This type of malfunction may not even set the MIL. Since the crankshaft rotates twice per engine cycle, additional data is needed on valve timing if a distributor is used for spark distribution. This data may be provided by a camshaft position sensor or other device placed in the distributor. In distributor-less engines, this extra data is not used, as the computer fires 2 cylinders at the same time, obviating the need to know if the cylinder is on the compression or exhaust stroke.

Camshaft Position Sensor: provides more precise information on valve timing to optimize fuel injection timing. Many engines will run with a failed cam sensor as the computer can use the crankshaft position data to fire the injectors. However, fuel efficiency can be gained by the proper use of camshaft position data.


Dead Battery Troubleshooting

So why did your battery die? Before assuming it's dead, make sure the terminals are clean and tight. OK, if that wasn't it and it's dead or at least at a lower charge than it should be, here are some possibilities:

1. Somebody left something on for a long time (radio, lights, something plugged into the lighter socket). In this case, you usually know why the battery went dead; and, if your battery isn't too old, it will probably recover from this incident. Another case worth mentioning is a dead battery after the car was parked a while out of earshot. If you jump the battery and your alarm goes off, it was probably set off a long time ago and ran the battery down. Reset the alarm and go about your business.

2. Your battery really died of natural causes (internal short). Any store that sells batteries can check yours to see if it's really dead and happily sell you a new one. If you want to make sure they aren't just selling you another battery, you can buy and use your own tester. To replace your battery, disconnect the negative terminal first, then the positive. Then remove the hold-down bracket. A typical hold-down bracket/bolt is shown below.


Note that many vehicles will lose codes and presets, etc. when a battery goes dead or is disconnected. If your battery still has power and you want to avoid losing codes and presets, use a second battery to jump the car while you replace your battery. Hook the jumper cables to terminals wherever your 2 main battery cables attach to the car and be careful not to bump them off until the new battery is connected.

3. Your alternator has failed and allowed you to drive on the battery until there was no juice left in it. Usually your battery/alt idiot light will provide an indication when your alternator is not charging. If you have a voltmeter in your dash, it should read between 14-15.5 volts when the alternator is working properly (engine running). If you have an ammeter in your dash, it should be on the positive side with the car running. If you didn't get an indication but want to check your alternator, you can either use a voltmeter to check the voltage at the battery with the engine running, or you can remove the alternator and have it tested at the parts store. If the alternator tests good, but you are not getting 15 volts to the battery, check for a blown alternator fusible link in the relay box.

4. You have an electrical short or draw in your car.
Before getting into a thorough testing of your circuits, check the most common cause of a short--the alternator. The alternator contains diodes to convert the AC power to DC. When a diode fails, it opens a path to drain the battery and can do it quickly.

Note: always unhook the negative battery cable before taking any hot leads off the alternator or starter and wrap these leads with electrical tape before reconnecting the battery.

If your battery is draining quickly, try unhooking the wires from alternator and leave the car with the battery connected overnight. If the battery survives the night but wouldn't before, replace the alternator. If it drained again as before, please continue reading.

This case requires a lot more work and know-how. I will now describe the first phase of troubleshooting. If you have a high current ammeter, you can check the circuits at the fuse boxes to see which ones are drawing current. If you don't know where your cabin fuse box is, check your owner's manual or common locations such as under the dash on either side, the outside corner of the dash, or behind the glovebox (open box fully and press tabs to open past the stops). There is also usually a relay box in the engine compartment that contains the larger fuses. A charged battery or a charger needs to be hooked up to the car to troubleshoot, but leave the ignition turned off. Your ammeter should be able to handle 25 amps for this chore. If you don't have an ammeter, use the "spark or test light test." Remove each fuse one at a time, and look/listen carefully for a spark when reinserted. If the circuit is drawing current, it should make a small spark when the fuse touches the fuse socket. Note each circuit that generates a spark. A better method, if you have or are willing to buy a 12 volt test light, is to test each fuse circuit with a test light. While not quantifiable like an ammeter, the test light will glow brighter when there is a bigger load/current and is easier to see than a spark. To use a test light, just push each of the leads into one side of the fuse socket until they both touch the terminals. Alternately, you can disconnect the negative battery terminal and hook the test light between the cable and the battery post. The light will stay on until you remove the fuse of the offending circuit.

There are some circuits that are supposed to draw current, even when the key is off. For example, the courtesy light may spark if you have the door open during the test. Any circuit that should not draw current when the key is off is now a candidate for further troubleshooting in phase 2. Complete the ammeter, test light, or spark test for all fuses in the cabin as well as in the engine compartment fuse box.

If none of your fuses generate a spark/light, either you don't have a short, the short is too small to make a spark, or the short is in the battery or something that doesn't have a fuse. In the second case, you can re-perform the test using a low-range ammeter.

For phase 2 troubleshooting, you will need to identify all of the wires and components that use the sparking/lighting fuse. Write down everything it says about those fuses in the guide on the fuse door or in your owner's manual. You can then start checking each of those components; but, before you do that, think about whether or not that circuit should be energized with the key off. If not, perhaps the problem is a stuck relay or bad ignition switch that is energizing circuits that should be open. It makes good sense to find the relay--if there is one--that energizes that circuit when the key is turned on. If you can pull the relay out of the fuse box and the spark/light goes away, you are holding a stuck relay--replace it. Check all of the sparking circuits in this way. An example would be a fuel pump that continues to run after the key is shut off--replace the relay!

Now, if once you have checked any associated relays and found them all good, it's time to check the components for the sparking fuse circuits. If you can find the connector attaching to those components, remove them and see if there is a spark at the connector terminals when the connector is reattached. If there is a spark at the connector, that component is using current and likely has an internal short. If there is no spark, leave the connector off and recheck the fuse for a spark. If the fuse still sparks, the short is in a different component that is powered thru that fuse, or it is in the wires that run to that component.

This is about as far as it makes any sense for me to coach you without getting some feedback from your tests. So, if you get to this point and have not found the short, hire a professional electrician to resolve your issue.


Stereo Says Protect or Clicks Off When Volume is Turned Up

Modern stereos and automotive head units have internal breakers that shut down the amplifier circuits when the output exceeds the rated wattage. They are designed this way to protect the circuits from burning up. If the stereo has a display panel, it may say "protect" or something similar when this happens. Assuming the breakers are working properly, there are at least two reasons for the system to protect itself. One is an inappropriately low impedance arrangement of speakers, and the other is a short, either internal or external. Once the problem has been resolved, the system will automatically restore the amplifier operation. Let's explore each of the possibilities to identify the problem for resolution.

1. Low impedance speaker setup.
In this case, the speakers attached to the output have an impedance lower than the system design specification. When designing or buying a system, you must ensure the impedance of the speakers is at least as high as that specified by the head unit or amplifier manufacturer. For example, if the spec is 4 ohms per channel, your speakers must have at least 4 ohms impedance. If you wanted to connect 2 speakers to such a channel, you could attach two 8 ohm speakers in parallel or two 2 ohm speakers in series. The latter case is called a bridge and is often used with subwoofers to bring up the impedance of the speaker system. If you do not know the impedance of your speakers, use an ohmmeter across the terminals to read it. Typical values are 8, 6, 4, and 2 ohms. Connecting two similar speakers in parallel halves the total impedance, while connecting them in series doubles the total impedance. Examples are shown below.



2. Short: Now, assuming you have an appropriate setup, let's consider a short.
Internal short: A short may be internal to the head unit or amplifier. To determine this, disconnect all speaker wire from the stereo and turn it on with the volume up. If the system goes into protect mode with no speaker wires attached, the short is internal. Due to the huge variety of systems, repair of an internal short is beyond the scope of this tip. If you have an internal short and you're good with electronics and your warranty has expired, feel free to open up the unit to look for burned transistors. Otherwise, consult a repair shop or the maker.
External Short: An external short may be in the speakers or the wiring. If one of the positive speaker wires is uninsulated at a point and touches the chassis of the vehicle, the unit will see zero impedance on that channel. Also, if the speaker on the channel has a melted coil, the impedance can be much lower than the design spec for the speaker. The first thing to do is to identify which channel is shorted. Do this by removing each pair of speaker wires and checking the impedance of the wires and speaker together. If it is lower than the spec for the speaker, there is a short. Check all channels in this way. Next, access the speakers for any case that has lower than spec impedance for the speaker. Remove the wires from the speaker and check the impedance of the speaker. If the impedance is lower than spec, replace the speaker. If the speaker impedance is good, check the impedance across the wires while the other end is disconnected. The impedance should be infinite unless there is a short. If the impedance is less than infinite repair or replace the wires.

Well that's about all there is to figuring out why your stereo is going into protect mode.

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