Rick's Free Auto Repair Advice

Car won’t start

How to get help and advice for a car that won’t start

Every day I see car owners write into automotive forms and ask for help by stating; “My car won’t start.” You should know that asking the question that way isn’t going to get you good answers. You’ve provided no useful information to the volunteers on the forum. I’ve listed several other was to ask the question that includes a bit more useful information

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So let’s start by defining some terms to get you thinking about what constitutes useful information.

What does CRANKING mean?

When you turn the key to start and the starter engages, it rotates the engine fast enough for it to fire up and run. This is called CRANKING. If the engine doesn’t rotate at a fast speed, it’s called SLOW CRANK. If you turn the key and get no response at all, that’s called NO CRANK. If you turn the key and hear a click but the starter doesn’t crank the engine, that’s called CLICK, NO CRANK. If you turn the key and hear rapid clicking but the starter doesn’t crank the engine, that’s called CLICKS, NO CRANK.

What does CRANK, NO START mean?

If the starter cranks the engine but the engine won’t fire up and run, that’s called a CRANK, NO START, or CRANKS BUT WON’T FIRE.

Now that we’ve defined some terms, let’s look at a diagnostic approach to find out what’s wrong. Owning a scan tool with live data helps immensely, but you can diagnose most no start issues with an inexpensive multimeter or a computer-safe test light. Here are the steps:

If you have a NO CRANK

1) Wiggle the shift lever and try again. If the park/neutral switch is worn, cranks but won't start, no star, no crankthis may allow it to make better contact and allow cranking. If that doesn’t work, move the shift lever to the neutral and try starting. If it still won’t crank try these steps in order

 

2) Check the battery: Battery voltage must be above 12.2 volts. Use your meter to check battery voltage. Set the

check battery voltage

Check battery voltage with a multimeter

 

 

 

 

 

 

 

meter to DC volts and attach the positive (red) lead to the positive battery terminal and the negative (black) lead to the negative battery terminal. If the meter shows less than 12.2, have the battery tested at an auto parts store.

Next, clean the battery posts and terminals and ground connections with a wire brush.

clean car battery terminals, clean car battery posts

Clean battery posts and terminals with a special wire brush

Remove the battery terminals and clean the posts on a top terminal battery and clean the side attachment points on a side terminal battery. Then clean the battery cable terminals. On a side terminal battery, check the condition of the bolt. The threads must be sharp with no corrosion. If they’re in bad shape, replace the bolt. You must have solind contact to the side terminal.

Then clean the ground connections. Clean the ground cable connection where it attaches to the engine. Clean the ground cable connection where it attaches to the fender or radiator support. And clean the ground cable connection at the firewall and where it attaches to the back of the engine.

 

Check for power at the starter and starter motor relay:

The voltage at battery cable on the starter and the starter “S” terminal can tell you a lot. If power isn’t getting to those terminals, there’s no way the starter will run. Here are the checks to perform:

Check for battery voltage on the heavy cable bolted to the starter.

starter terminals, car won't start

View of terminals on starter motor

Connect the positive lead to the large battery cable and the negative lead to ground. You must see battery voltage. If no battery voltage, repair or replace the cable/terminal.

4) Check for voltage at the “S” terminal on the starter when the key is turned to the START position. Connect the positive lead to the “S” terminal and the negative lead to ground. If you don’t see battery voltage while trying to start, get a wiring diagram and trace the power routing to the “S” terminal.

On older cars without computer control or antitheft, battery power to the starter motor usually flows through a fuse to the IGN switch. From there it flows to the park/neutral switch before going to the “S” terminal. So check for battery power coming into the IGN switch and going out of the IGN switch. That confirms the fuse is good and the IGN switch is good. Next, for battery voltage into and out of the park/neutral switch while trying to start the engine. If power comes out of the park/neutral switch but isn’t present at the “S” terminal, check for power at any connectors splices in between or look for an open in the “S” wire.

On newer cars the IGN switch doesn’t always switch power to the starter.

car won't start

Click on image to view a PDF of a typical starting system on a late model vehicle

Instead the IGN switch simply acts as a signal input to the ECM/PCM. When the ECM/PCM sees the IGN switch turned to START, it checks the state of the park/neutral switch or communicates digitally with the transmission control module to make sure the transmission is in park or neutral.

If the vehicle is equipped with an antitheft system, the ECM/PCM may check to see if an authorized key is in the IGN. Car makers employ several different strategies to prevent theft. They may allow cranking and starting, but cut off fuel within a few seconds if the wrong key is detected. Or, the car maker may prevent starter operation completely. If the antitheft system has been activated by using the wrong key, a defective key, or there’s a problem with the antitheft circuit, all cars equipped with antitheft will light the antitheft warning light. So check the dash for a solid or flashing SECURITY light. If you see that, diagnose and correct the antitheft problem first.

If the correct key is used, the ECM/PCM energizes or grounds a starter relay usually located in the underhood fuse box. At least one fuse provides power to the starter relay contacts and sometimes to the relay coil as well. So check the fuses to the starter motor relay.

Most car makers ground to the starter motor relay coil to move the relay contacts and provide power to the “S” terminal on the starter (although in some brands, the ground to the control coil is always present and the ECM/PCM switches power to the control coil). So check the fuse(s) to the ECM/PCM, starter relay control coil and relay contacts.

Next, remove the relay and swap in another relay with the same part number and try starting. If the car still doesn’t start, pull the relay and check for battery voltage on two of the terminals in the socket. If you get battery voltage on two terminals, check for good ground on the other two terminals with the key in the START position. If the control coil doesn’t receive ground and the ECM/PCM is designed to trigger ground that might indicate a problem with the park/neutral switch, transmission range selector, transmission control module (TCM), or a problem with the data lines between the ECM/PCM and the TCM.

 

Starter Clicks once but won’t crank

A single click sound can be coming from the starter motor relay in the underhood fuse box or it can come from the starter motor solenoid. If the single click is coming from the relay under the hood, that confirms the ECM/PCM has activated the relay and the relay contacts are closing. Power may be flowing to the starter solenoid “S” terminal or there may be an open between the relay and the “S” terminal. If you’ve checked the “S” terminal and find battery voltage as described above, and the battery has tested above 12.2 volts, then the problem is in the starter solenoid or starter motor.

The starter solenoid is really just another relay used to switch high amps from the battery to the starter motor. When energized, the magnetic field in the solenoid coil pulls a plunger that moves a copper disc to connect the battery power to the starter motor. That’s what makes the single click sound. If the starter solenoid doesn’t fully operate the plunger or the plunger and disc don’t make good contact, the starter won’t operate.

car won't start

PLunger and copper disc.
On left: pitted On right: new

Starter solenoids can fail in many ways. The wire windings can short or form an open. The copper disc on the plunger can develop pits or corrosion, preventing good electrical contact between the battery cable and the starter motor.

In some starters, you can replace just the solenoid. In other designs, you can remove a cover and replace the plunger and copper disc, along with the battery and motor contacts. And, in some designs you must replace the entire starter.

The other possibility is that the starter solenoid is working properly and conducting power to the starter motor and the starter motor windings have shorted and the starter motor won’t turn. That would sound like a loud single click. The easiest way to check for that condition (assuming you’ve already checked the battery and it’s in good shape) is to turn on the dome light and try starting the engine. If you hear a single click and the dome light dims way down, that’s a good indication the starter motor is bad due to a short in the windings. If you turn the key several more times and smell burning, that’s insulation melts off the starter motor windings. At that point you have no choice but to call a tow truck.

 

Starter clicks rapidly

The starter motor solenoid has two magnetic coils. The first coil pulls the plunger into position to engage the copper disc and flow power to the starter motor. It’s called the “Pull in” coil. The second coil holds the plunger in place. It’s called the “Hold in” coil. When a car battery is low on charge, it can provide enough amperage to activate the pull in coil, but not enough power to operate the hold in coil. In that case, the pull in coil will pull in the plunger but with no hold in coil, the internal spring will bounce it back out. This will cause a rapid clicking sound similar to a machine gun sound. A discharged battery is the number one cause of this condition. However, if the battery is good, high resistance between the battery and starter can also cause this condition. Finally, a bad hold in winding can be the cause.

 

Cranks but won’t start

If the starter cranks the engine but it won’t fire up, your job is much harder. To make it a bit easier, first understand what inputs ECM/PCM needs in order to provide spark and fuel and the outputs it provides to make that happen.

 

ECM/PCM inputs

Engine coolant temperature sensor (ECT). In order to calculate the proper air/fuel mixture the computer must know the temperature of the engine. The ECM/PCM uses the readings from the ECT. If the engine is cold, the ECM/PCM will command a richer mixture. You can check the ECT with an ohmmeter, voltmeter, or by performing a simple test. If the engine cranks but won’t fire up, depress the gas pedal halfway. If it then starts, suspect a bad ECT. What you’ve done is told the computer to bypass the ECT and provide more gas.

 

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Intake Air Temperature Sensor

Intake air temperature sensor (IAT)

Just like the ECT, the IAT provides critical outside air temperature to help the ECM/PCM calculate the proper air fuel mixture.

 

 

 

 

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Manifold Absolute Pressure Sensor

Manifold absolute pressure (MAP)

The computer uses the input from the MAP sensor to get a rough calculation of barometric pressure. Barometric pressure has a direct bearing on how many air molecules enter the engine and thus is critical to determining air fuel mixtures. The MAP is connected to the intake manifold with a rubber vacuum hose. When you first turn the key to start, the ECM/PCM takes a quick reading of pressure. Then, as the engine cranks and creates vacuum, the computer calculates the difference between atmospheric barometric pressure and engine vacuum to obtain manifold absolute pressure. If the vacuum hose is missing, cracked, or the engine has a major vacuum leak, that will affect the computer’s ability to calculate proper air fuel mixture. It will resort to

factory programming and attempt to figure out barometric pressure based only on ECT and IAC. That may cause longer cranking times and rough idling.

 

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Mass Airflow Sensor

Mass Airflow (MAF)

The MAF sensor may contain a single hot wire, hot plate, or a combination of hot wire and IAT. The MAF heats up the wire with battery voltage and measures how much current is required to keep the wire at a set temperature while air flows across it. Based on the current draw, the ECM/PCM can calculate the grams of air entering the throttle body. A bad MAF will not prevent the engine from starting (the computer will use IAT and ECT to infer grams of airflow), but it will affect engine idle, power, and MPG.

 

car won't start

Crankshaft Position Sensor

Crankshaft position sensor (CKP)

 

 

 

 

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Camshaft Position Sensor

Camshaft positon sensor (CMP)

The computer must know the exact position of the crankshaft and camshaft in order to know when to fire the spark plugs and fuel injectors. If the sensors are broken or intermittent, the engine won’t start.

 

Throttle position sensor (TPS). The computer must know how much you’re pressing on the gas pedal. The TPS is like the volume knob on your radio.

 

Outputs

 

no crank, no start

Fuel pump module

Fuel pump

In some vehicles the IGN switch activates the fuel pump relay, but in others it’s the ECM/PCM. Just like the starter motor relay mentioned above, the relay requires to battery voltage inputs and a control coil ground.

 

 

 

no start, no crank

Ignition coil

Ignition

On some vehicles the spark is commanded by the ignition module, but in others it’s commanded by the ECM/PCM. When spark is required the ignition module or ECM/PCM interrupts ground to the ignition coil. That causes the magnetic field to collapse and creates at least 25,000 volts for the spark plug to fire. Some cars are equipped with a coil pack that services all the spark plugs. Newer vehicles have one coil per plug.

 

no start, no crank

Fuel Injector

Fuel injector pulse

The fuel pump pressurizes the fuel rail, but the fuel injectors won’t allow fuel into the cylinder until it receives a timing pulse from the ECM/PCM. Most cars provide power to the fuel injectors and toggle the ground on/off up to 10-times per second depending on the demand for fuel. If the fuse to the injectors is blown, they won’t pulse.

 

Here’s how to check these inputs and outputs

 

1) Check for fuel pump operation. All fuel injected vehicles are designed to operate the fuel pump for 2-seconds when you first turn the key to the RUN position. Turn off the radio and close all doors. Turn the key to the RUN position and listen for a slight 2-second hum coming from the fuel tank. That confirms the pump is getting power and priming the fuel system. If you don’t hear the pump run, check the fuel pump fuse. If it’s good, find the fuel pump relay in the underhood fuse box and swap it with another relay with the same part number. Finally, tap the fuel tank with a soft hammer or your shoe. A dead spot on the fuel pump commutator can sometimes be jiggled enough by tapping that it’ll then engage. If that works, you’re due for a new fuel pump. It will fail again and get progressively worse each time.

 

2) Check CMP and CKP operation: On Ford vehicles, turn the key to the Run position and make sure the check engine light turns on. Then try to start the engine. If the check engine light turns off, that confirms the ECM/PCM is receiving good signals from the CMP and CKP. If the light stays on, wiggle the wires and connectors to those sensors and try again.

If you don’t have a Ford, refer to a shop manual for CMP and CKP testing procedures.

Posted on by Rick Muscoplat


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