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The most common reason for a car failing to start is simply that the battery cannot provide enough power to turn the starter motor. While the easy repair seems like a battery replacement, there's a chance the problem is elsewhere within the battery and electrical charging system. Before you go spending needless money on a new battery, perform a few simple tests to determine what you really need.

Check The Battery...Again!

First off - check the battery again! It really is the most common reason, and it's not enough just to check the voltage when resting or being charged by the alternator. Cold cranking amperage is the most important piece for starting a vehicle as it directly measures the power sent to the starter when turning the key. Unfortunately, you can't check this with just a volt-meter. Instead, you need a load tester that essentially short-circuits the battery and measures the absolute maximum power that can be delivered in that instant. If you don't have a load tester, many auto parts retailers do, and they'll test your battery for free in the hopes you'll buy one from them.

Cold-cranking amps vary by battery model and size, and there will be a specific threshold a new one needs for your specific model or engine. Check the CCA value with whatever is in the owner's manual for your car (don't assume the previous owner fitted the appropriate battery). Something like a BMW E38 7-Series will want at least 650CCA, though the minimum value depends a lot on your ambient temperature (a lower CCA may work for you in summer but then not be enough in winter).


Check The Starter

Once you rule out the battery, the starter is next for a look-over. There will be a thick red cable going to the starter that connects to the battery itself and a couple of thinner wires that come from the ignition. The biggest issue with the battery-to-starter cable and the big ground strap that completes the circuit is corrosion. When severe enough, the corrosion interrupts the metallic contact, preventing enough power from transferring between the connection or preventing a strong enough ground. The starter is also earthed to the chassis via the engine, so this means corrosion on the engine ground strap(s) can cause the same problem. Check for physical damage, then use a continuity meter to check that the bare metal on the engine block is properly earthed to the chassis. 

The thinner wires ultimately come from the ignition switch, though in practice, they are tied to security features on the car, such as an electronic immobilizer. On BMWs, the EWS module (Electronische Wegfahrsperre or electronic immobilizer) sits in between the ignition switch and the solenoid starter. In practice, this gives you two more failure points to look at - the switch sending a signal to EWS and EWS sending a signal to the starter. The switch is actually a common failure point in well-used vehicles - it's a moving part and can simply wear out and give intermittent contacts. Luckily, it's easy to replace yourself as it's just a remove-and-replace part. The EWS is more of a black box - but any attempt to start the car that the EWS has suppressed is logged internally, so you can check this with BMW-specific diagnostic software.

If everything above checks out and the starter is still not turning, then yes, it may simply be the starter has failed and needs to be replaced. Before you go down the route of removing it, though, do what many professionals do and give it a few sharp taps with a wooden broom handle! Sometimes, this can free up a sticky solenoid or re-seat worn brushes. If it works, it's a sure sign your starter will need to be replaced soon, but it'll at least get you where you need to go today.

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Written by :
Bryan McPhail

Bryan is a longtime BMW enthusiast in Florida.

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