Honda Electrical System Diagnostics -

Honda Electrical System Diagnostics

Honda charging systems won't present many problems for the experienced tech, but there are a couple of things to keep in mind. It's our experience that the charge indicator lamp on the dash does a good job of keeping the system in check - of course, that assumes the light is working as it should.


This month, we’ll be taking a look at the charging and starting system on the Honda line of vehicles. Hondas are always ­welcome at our shop, as they are good, reliable cars that fix well and for which there is good service information and parts availability.

Honda charging systems won’t present many problems for the experienced tech, but there are a couple of things to keep in mind. It’s our experience that the charge indicator lamp on the dash does a good job of keeping the system in check — of course, that assumes the light is working as it should. I stress to my techs the importance of checking that all the warning lamps are lit with the key on and engine off, and that they go off when the engine is started. That ­applies to every vehicle we work on.

Speaking of charge indicator lamps, Honda uses different strategies for operation depending on the model and year, so a look at the service information is in order if you’re having a problem.

One of the more common and challenging problems you may encounter is a vehicle that needs an alternator, and after the replacement unit is installed, the charge indicator lamp remains lit even though it seems to be charging fine. If you look at the wiring diagram for a 2000 Civic DX, for example, you’ll see that the indicator light circuit goes through the underdash integrated control unit, and, on a U.S. model, there is a device called the electric load detector (ELD). So, it’s easy to see how some techs could get lost after they install a quality remanufactured alternator, only to have the light stay on (more on this later).

Looking at a later-model wiring diagram may be even more daunting. Using a 2005 Civic DX as an example, while all the same components are included, all the alternator control wiring is now passing through the ECM for better control and diagnosis.

Getting back to our charge indicator lamp, there are numerous reports (and based on our experience) that the best route is to use only an OEM rebuilt alternator. In recent years, we’ve had good luck with high-quality aftermarket units on most models, except for on the CRV and the Element.


Back to the ELD we mentioned earlier, this simple device located in the fuse box monitors the vehicle’s electrical load and improves fuel mileage by turning off the alternator when loads are low. Don’t confuse it with the internal voltage regulator in the ­alternator.

The ECM on late-model computer-controlled cars will send reference voltage of approximately 4.5 volts to the ELD unit, then the ELD will pull this voltage down depending on the load. With no load, there will be about 3.5 volts on the signal wire telling the ECM to ground the control wire (terminal 2 white/green wire) at the ­alternator/regulator, and turn off the alternator without turning on the dash charge indicator. As the load increases, the reference signal is pulled lower toward one volt, informing the ECM to stop grounding the control wire and let the alternator charge as needed.

The ELD system will set a P1297 or P1298 code, “ELD ­circuit low or high voltage,” pointing to the reference voltage. There have been TSBs regarding these codes, discussing reflash cures and some rare ECM problems, but our experience has been with the ELD units going bad. They are inexpensive and simple to replace, so we always start by replacing the ELD unit.

If all else fails, and you still suspect a problem with the ELD, you can remove the control wire from the alternator plug, taking the ELD out of the system, which will allow the charging system to function as normal. Be sure to clear the codes after the test.

Keep in mind how the ELD ­system works as you’re checking the voltage output of the alternator. The first step for many of us when a car is brought in with a charging problem is to charge and test the battery. On Hondas with a fully charged battery, you won’t see any alternator output unless you introduce a load by turning on the headlamps, blower motor and rear defrost. On some models, charging is delayed for a few minutes after startup as the idle stabilizes; just be patient and remember to create a load.

Also know that the ELD won’t detect loads that are attached ­directly to the battery. With ­Hondas being so popular with the ­enthusiast tuner crowd, check for aftermarket sound equipment and video screens that may be overloading the stock alternator, causing it to overheat. An alternator failure will cause the battery to run down and go dead, so be aware that these ­accessories will affect the ­charging system.

If you have access to an enhanced scanner, it should be hooked up already, so you can see the commanded charge rate versus actual output, along with additional codes and information that will help with your diagnosis.

You shouldn’t have any problems with the nuts and bolts of alternator replacement. Do not guess on the labor; rather, always refer to your service information, as some models will require more time to access the unit.

While these jobs are well suited for less-experienced techs, good work habits need to be ­emphasized. This is certainly the time to replace a worn serpentine belt, as well as to check tensioners and pulleys for function and noise, and take a close look as the crank pulley for separation on high-mileage cars.

Last, but not least, be sure the alternator mounting brackets are tight and the mounting surfaces are clean in order to provide a good path to ground. While not a common problem, there are reports of ECMs being damaged by surges created when the ­alternator grounds are lost.

The ELD unit is easy to acess in the underhood fuse box.


When it comes to the starting system, Honda uses a simple and very reliable strategy. Still using the 2005 Civic DX as an example, the ignition switch gets its power from the load circuit of the previously mentioned ELD, protected by fuses 19 and 20. When the key is turned to the start position, current is sent to the control (and currently open) switch circuit in the underdash fusebox-mounted starter cut relay. The relay control winding is grounded by either the automatic transaxle-mounted range switch or the clutch pedal interlock switch on manual transmission cars. With a complete ­circuit on the control side, the switch side is closed, sending ­current to the solenoid. The GX models use an additional relay after the cut relay, functioning the same as the cut relay with the control circuit grounded by the ECM.

After working on thousands of Hondas over the years, I can’t say we’ve ever seen a problem related to the starter control circuit. We have seen problems with ignition switches where the car will start and stall, but it’s not a no-crank issue.

The most common starter problem is bad starter contacts. We should all be familiar with the symptoms — turn the key and you’ll hear one heavy click as the solenoid pulls in, but the worn contacts prevent the starter from cranking the engine. ­Depending on the condition of the contacts, the starter may engage after a few repeated key cycles. Contacts are readily available, but we always recommend a quality, ­rebuilt unit to ensure a quality repair we can stand behind. And if the contacts are worn, the brushes can’t be far ­behind.

The ELD unit is easy to identify by its nondescript ­reddish-brown color.

Diagnosing a no-start problem is as simple as listening for the solenoid tapping on the dead unit while an assistant works the key. A higher-tech solution is to confirm there is solid battery voltage at the starter wire with the key in the start position, that battery voltage is available at the battery cable at the starter, and that the ­battery grounds are intact and clean.

Like the alternator, starter replacement is straightforward and should be no problem for a tech at any experience level, but be sure to check your labor guide when pricing the job. As an example, on the Element SUV, it’s recommended that the intake manifold be removed to gain access to the starter. While it’s open to debate whether it’s necessary to remove the manifold, there is no ­debate that the tech has to be careful with the knock sensor if the ­manifold is left in place. Plus, it’s not a big deal to remove the manifold, which will provide easier access to the starter.

This last issue involves both the starting and charging systems. If you have a Civic towed in that acts like the engine is locked up, before you declare the engine to be seized, ­remove the ­alternator belt to confirm it is not locked up. The amount of grip that the serpentine belt provides is surprising.

I hope this illustrates that charging and starting system problems on the Honda line of vehicles are not something you should shy away from. If you do encounter a tough issue, there is plenty of helpful information available on the various technician websites that will guide you through. I’m sure they’ll provide good, profitable repairs with few problems; something any shop would welcome.

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