Charging system issues and fixes

April 2, 2018
Here are a few examples and solutions to common and uncommon problems  to make diagnosis as straightforward as problem-free as possible.

It’s difficult to name a system on a newer vehicle that isn’t electronically regulated or controlled and that means, more than ever, that a malfunctioning charging system can affect the entire vehicle and not just affect the initial start-up. So when they do go wrong, customers tend to notice and bring the vehicles in right away for repairs – great, but diagnosing and repairing those newer charging systems can be quite a challenge. 

In fact, on newer vehicles, a faulty charging system can cause problems ranging from no-start conditions to MIL lights coming on intermittently and even cause accessories to occasionally stop working.  

This was an easy fix for a MIL on condition and replacing this battery in a 2000 Chevrolet truck was fairly straightforward – but it’s critical to avoid over-torqueing the bolt to the battery post so that it doesn’t crack, leak, or just plain seize up (something to maybe share with any apprentices or students before they install any batteries).

Additionally, charging systems on newer vehicles are much less tolerant of any problems than they ever used to be, and diagnosing them successfully means being aware of how seemingly small variations between measurements and specifications can actually mean the difference between normal vehicle operation and a drivability nightmare – even on vehicles designed for severe-duty conditions and very rough service. The basics are still there – battery, generator, connections and control – but how each system is diagnosed has definitely changed. And it’s important to know how to figure out what’s wrong. 

No worries. Here are a few examples and solutions to common and uncommon problems  to make diagnosis as straightforward as problem-free as possible. 

Getting started 

As ever, successfully diagnosing charging system problems begins by inspecting the battery – because if the battery isn’t within manufacturer specifications the rest of the charging system cannot be reliably tested (which can be a challenge when a vehicle is towed to the shop with a dead or drained battery).  

If there’s ever a question, inspect and charge the battery according to service information – including reviewing tech tips and service bulletins – and then when the battery is known to be good and within specifications, proceed to test the rest of the charging system. This is so important that even if the battery looks new or has been recently replaced, it’s still critical to ensure that the battery is OK before proceeding further with any diagnosis – charge it according to specifications if needed, check quickly for things that would likely drain the battery like the block heater or cord not working, a missing serpentine belt, or even that something was left on and only then continue with charging system diagnosis. 

This step is important and not to be skipped for two reasons.  

The first reason is that battery problems can easily be mistaken for drivability concerns – and often are – and there’s just no point wasting time chasing something that can easily be repaired. 

For example, one of our clients recently texted us because their Chevrolet 4x4 truck needed to be boosted after the door was left ajar and the MIL light then came on while driving. Replacing the battery fixed both the no-crank concern and the MIL problems, which took him completely by surprise because he couldn’t make the connection between the truck’s older, weak battery finally failing and the MIL coming on while driving. (I told him he was free to replace more parts if he wanted to.) 

In another recent instance a customer’s six-year-old Toyota Corolla was boosted in an underground parking space and then driven to the shop for further service (to save the cost of an expensive tow). The vehicle drove well for about 15 minutes after being boosted, and then the charging light came on and it stalled in the middle of an intersection (so much for saving the customer money). A new battery (and tow) fixed that problem as well.  

Top post batteries, like the ones in the Ford 4x4 truck are fairly easy to inspect and check for damage, loose connections and cables, and general signs that something’s wrong. That's a good thing, because the system is incredibly sensitive and malfunctions and no-starts can occur even if the battery indicates “12.0 volts.”

In other words, any time the charging system shows signs of problems, it’s critical to ensure the battery is within specification before beginning diagnosis and if it isn’t it must be successfully charged or replaced before any further diagnosis can take place. Time-consuming indeed, but it needs to be done — the charging system can’t function properly with a bad battery and unfortunately it wastes time and money to proceed otherwise.  

The second reason ensuring that the battery is within specifications before beginning any charging system diagnosis is so critical is because the tolerances used for modern charging systems are so exacting that even a slightly discharged battery can cause serious problems – and in the real world of freezing temperatures and harsh operating conditions, it’s fairly easy for a battery to discharge slightly.  

In other words, “Close enough to 12.0 volts… ” or, “One battery at 12.0 volts but the other a bit lower… ” is not the same as, “More than 12.0 volts…,” which is usually the specification required for testing to proceed (but always check and be sure). From experience, the voltage of a fully charged battery actually isn’t 12.0 volts – it’s usually 12.6 volts or slightly higher so a reading of 12.0 volts is already indicative of a severely discharged battery and worth investigating further. 

For example, newer Ford trucks with slightly discharged batteries that still measure 12.0 volts at the battery posts cannot start, and they can make a frightening clanking noise – even though their batteries measure at 12.0 volts. (Charging the battery for a few minutes can make all the difference, as can plugging in the block heater – the truck starts and there’s no noise, something to keep in mind if you encounter this problem during the colder weather). 

It’s also worth mentioning that the battery cables should be clean and make secure contact with the posts, and that if they’re loosened – for cleaning during diagnosis or to replace the battery – they should be re-secured without over-torqueing them. This is especially important for side-post batteries and it’s worth mentioning to any apprentices or helpers who install batteries. Over-torqueing side-post batteries can actually crack the case and cause a messy, corrosive leak or simply cause the post and bolt to seize together, which creates a future nightmare for the next tech who has to undo the bolt when there’s very little clearance. Torqueing fasteners and components correctly certainly hasn’t changed even though so many other things about the charging system have. 

Quickly inspecting the condition of connections, fuses and fusible links can save headaches later on.

Also, as ever, identifying and repairing a charging system problem early, before a no-crank or no-start condition develops, is much easier and more profitable than fixing it afterwards. If your shop has a small, handheld battery tester, using it to test vehicle batteries during routine oil changes or service is time well spent indeed since it can quickly identify batteries on the verge of failing (OK, it’s not foolproof, but it’s pretty good) and faulty parts can then be replaced while it’s easy and convenient to do so (in the service bay surrounded by tools), rather than when it’s a much more difficult and inconvenient job (such as in underground parking spaces).  

This is especially good practice before the winter weather arrives and also before summer road trip season starts. It can help pay for the tool and also keep customers happy – win, win indeed. 

Bad batteries causing problems is nothing new, but what is relatively new is the extremely strict adherence to manufacturer specifications. It’s essential to understanding that the small fluctuations that were acceptable even a few years ago can now cause major problems and can indicate where the problem lies. Keeping this in mind while taking measurements for diagnosis is critical because, as one excellent tech recently told me, “Close enough actually isn’t.” 


Another important thing to be aware of is that many manufacturers use one-way clutches and pulleys on many of their alternators, and not checking for this can result in an embarrassing and costly come-back.  

The one-way clutches are used for various reasons, notably so that the serpentine belt needs less tension and is therefore easier on everything in the system that is driven by the belt (or drives the belt), and also so that there are fewer noises and vibrations from the system. This is indeed quite a difference from the days of getting as much tension on a belt as possible to eliminate noisy chirps under high load (I remember techs using shims under the A/C compressor mounts of a 2000-era GMC truck used for racing to raise the pulley up slightly, increase tension on the belt, and thus get rid of a brief squealing noise at very high rpm). 

Four things to check before replacing an alternator

1. Verify all batteries in the system are OK — fully charged and capable of accepting and delivering a charge.

2. Check if the alternator uses a one-way pulley.

3. Verify that the control module isn't directing the alternator to shed loads or reduce output.

4. Verify that the cables, connections and fuses are all OK.

It’s certainly easy enough to check if an alternator pulley has a one-way clutch or not – with the belt off, check if the pulley rotates freely in one direction but not the other. Not hard at all. 

However, if you’re still unsure, check service information and find out because it’s important to verify that the replacement part being used also has a one-way clutch installed, and not just to prevent noise and vibration, but also to prevent the vehicle from coming back with the charging light on. 

This is important to do even if the replacement part came from the dealer and is handed to you still sealed in the box. 

One dealer tech I worked with was in a rush on a Saturday morning, and he didn’t check. He just installed the component the parts department handed to him and sure enough the replacement part didn’t have the one-way clutch on the pulley (even though it looked the same and fit perfectly). 

The vehicle road tested OK, and the customer drove away, but unfortunately the vehicle returned less than a week later with the charging system light on. And this particular alternator was downright ugly to replace once, let alone twice – it was way down in the engine compartment and underneath everything, and the engine had to be raised and various components removed to access it. However, difficult as it was to get the alternator on and off, installing an alternator with the clutch in the pulley did indeed fix the problem. (And of course, ensuring that the spacer bushing was pushed back enough so that the new alternator easily fit on to the mount made it a bit easier, though it still wasn’t a fun task at all.) 

The sealed pulley on this Toyota alternator indicates that a one-way clutch is used in the system, and the replacement part needs to also have this clutch to prevent problems.

And if you’re thinking you could just quickly swap over the pulleys if you’re faced with this problem, think again. Removing the pulley is not just a matter of undoing the nut, popping it off and then removing and swapping it out. Pulley removal and installation requires a special tool and is not a simple task at all – trying to do it isn’t recommended. Perhaps save a headache by ensuring that the replacement pulley is the correct style and size and that the one-way clutch on the new unit is indeed operating correctly before installing it (if you’re not sure, check service information and find out for sure). If the pulley isn’t correct, think twice before installing it anyway – the one-way clutch is important and leaving it out is not a good choice. 

Finally, it’s not uncommon to re-use the belt after replacing the alternator, but it is important to note the direction of rotation and reinstall the belt so that it continues to rotate in that same direction to prevent underhood chirps and other noises from developing. Never use belt dressing or the like to eliminate a noise as it can ruin the belt and make a mess on every pulley in the system. A little bit of care can prevent problems from developing and returning. 

Checking pulley condition has always been a good idea, especially if there’s belt noise, but now there’s just a bit more to inspect — that the one-way clutch is working OK and that the replacement unit does indeed come with the clutch installed — and that it works. All quite simple once you get in the habit. 

ECM control 

Finally, since the alternator’s output is so carefully monitored and controlled, it’s also critical to ensure that the control module isn’t reducing alternator output for any reason and that the actual connections, cables, fuses and connectors are in excellent condition before replacing the component – it doesn’t take much to cause a problem, and alternators are often wrongly accused of failing when the actual problem is in the control system.  

Even a fraction of a volt or a tiny bit of resistance makes a big difference on newer vehicles and shouldn’t be dismissed as “close enough.” Those tiny variations that were acceptable before are now the difference between the system working correctly and malfunctioning horribly. 

To prevent problems and save time, it’s wise to look up the specifications in service information and verify voltage at the control module with a scan tool, not just by placing a multimeter across the battery terminals before and after the vehicle starts and check that the voltage is 12.0 and goes up to 14.0 when it’s running. It’s important to verify that the control module sees correct battery voltage and isn’t commanding loads be shed or reduced.  

It’s also still important to check voltage drop across the heavy cables and also pin grip at the smaller terminals in the system when diagnosing charging system faults – female connectors can lose grip after being disconnected and reconnected as few as three times. In other words, it doesn’t take much to cause a problem and paying careful attention to details is the best way to quickly and accurately find the cause. It’s that attention to detail that makes the difference now. 

Also be sure to check fuses and fusible links for problems if something seems strange – all of them. We had a Honda Civic towed to the shop completely unresponsive, and it wouldn’t be boosted or power up. Turns out the main fusible link was blown. The customer tried to boost the vehicle themselves – backwards, which they didn’t mention – and the vehicle needed not only a battery but the fusible link replaced as well (the fusible link did its job and did indeed protect the rest of the system from damage). Sadly, customer behavior just doesn’t seem to change even though their vehicles sure do. 


Despite what’s currently posted online, the days of removing the negative battery cable while the vehicle is running to diagnose a faulty alternator have been over for a very long while (and let’s be honest, that was never a good test or a good idea to start with) and the days of accepting 12 volts across the battery terminals before cranking and 14 volts at the battery with the vehicle is running (measured with a voltmeter) as proof that the charging system was problem-free are gone as well. Diagnosing charging system problems on newer vehicles requires service information and the proper diagnostic tools – and a bit of experience knowing what to look for and where. But if you’re ready for it, diagnosing and repairing charging systems can help those diagnostic tools pay for themselves quickly. 

It’s definitely worth learning about what’s new with charging systems over the past few years to prevent headaches and reduce diagnostic time – and maybe even prevent some embarrassing situations from ever occurring. Charging systems are more critical than ever, and there’s certainly money to be made repairing them – which is still important, indeed.

About the Author

Vanessa Attwell

Vanessa Attwell is a Master Technician for two major manufacturers and has also worked on the bench of an independent shop. She has developed and delivered training for both vehicle manufacturers and independents, and helped develop government training and regulations standards. She drinks too much coffee and spends her spare time sitting in traffic.

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