Rick's Free Auto Repair Advice

car wont start after jump

My car wont start after I jumped it

Every day I see people say their car won’t start after jump. They’ve tried jumping the battery and still the car won’t start after jump. First off, we have to understand that car won’t start is pretty useless information. Do you mean it won’t crank? Will it crank but not fire? Does it try to fire up but can’t keep running? Does it start and then stall? See, it’s just not that simple to get an answer to why your car won’t start.

I’m going to devote this article to a condition where you can’t get the starter to crank, even with a known good battery or a jump.

If you don’t know if the problem is a dead battery or bad alternator, read these articles:

Test your alternator

Find what’s draining your battery

Charge your battery

Ok, now on to your problem. In the old days the ignition switch actually switched power right to the starter motor, or at least to a starter relay mounted on the inner fender. Sorry guys, those days are LONG gone. These days, the ignition switch may switch power to a starter relay located in the underhood fuse box, or it may just provide a start request signal to the PCM. In other words, it’s not that simple. You MUST know how your vehicle is wired. I’ve included a PDF in this story showing you four common wiring diagrams. But there are 20 more.

wiring diagram, car won't start, charge battery,

Click on the image to download a 4 page PDF of typical starter system wiring diagrams

Some car makers want the PCM to control the conditions under which the starter motor can operate. For example, the PCM can check the status of the transmission state from a simple park/neutral switch or from the transmission control module. The signal can be a simple switch to ground, a completed circuit switch, or it can be a DIGITAL signal. Next, the PCM may check to make sure the anti-theft system is seeing the correct key or start condition. And, since the ignition switches in these highly computerized vehicles usually contains a bed of resistors, any corrosion in the connectors can result in an incorrect start request.

Plus, many of these computerized systems transmit start/no start commands via a data bus inside the vehicle. So, if you really want help with a no crank situation on your vehicle, at least tell us the year, make, model and engine size. Without that, we can’t help you.

And, for goodness sake, STOP using jumper cables to start a dead battery. That is one great way to fry the computers in your car—and there are plenty of them.

Posted on by Rick Muscoplat

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