car randomly wont crank. alternator, starter and battery good
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There are a number of reasons this could happen. there are neutral safety switches in both the automatic and stick shift versions of most vehicles. There is also a starter relay in the fuse box on some vehicles. check the battery cables to see if they are clean. I have seen people clean the outside of the cables but there needs to be a good connection between the post and cable or side terminal. they need to be removed to be cleaned. You say the starter is good but a bad starter solenoid could cause this problem. If this is the case I recommend replacing the whole starter. Ford cars have a separate solenoid and can be replaced separately.
Check if battery is charged o rule out bad battery. A good voltage while the car is off and has not been running for a few hours (idle voltage) is ~>=12.8V. Anything above 12.3 V is okay. Does any other high current load work well (headlights, rear defrost, fans .. while the engine is not running? What happens when you turn the key? Do the dashboard lights stay bright ? Is there any noise indicating the solenoid engages or starter try to turn ?
Rule out faulty relais, fuse, voltage drop on cables, conenctors etc.: Check the voltage on the solenoid itself while you turn the key to crank. If the voltage is close to the idle voltage the solenoid does not engage.
- suggestions to remove, clean and tighten all cables are always good especially in these high current connections.
- sometimes a trick to check if it is the solenoid is to have one person be in car and turn the key and then hit the solenoid with a heavy tool like a hammer- not too hard. This can loosen it. Sometimes it is not moving its core due to rust and dirt or its actually too weak because of too much voltage drop (see 1*)
or another trick to see if it is the solenoid/cable is to bypass the ignition:
shorten the connection from the positive cable of the solenoid to the starter. This can be dangerous because there is no fuse on the starter wire and a short to ground (any metal part in the engine compartment ) can cause high current to melt your tool. Also this bypasses the safety in newer cars that when not in neutral or parking the starter will move the car.
Assuming the starter has a good connection to the battery you will bypass any relais, loose wires, ignition switches (My dad did this with a large screwdriver or other metal tool.
In most cases if you found out that it is the solenoid try to replace only that one if possible.
Parasitic drain? Turn key off. Take oft positive battery cable. Take a DVOM and use the Amps setting. Plug the red lead into the meter's Amps terminal and the black lead in the common ( or ground ) terminal. Clip the black lead to the disconnected positive battery cable and the red lead to the positive battery post. I would hope not to have more than 300mA of current present. I think a half of an Amp is too much.
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