Charging System Diagnostics – Honda ELD – Electrical Load Detector

Engine emission controls have become involved in every aspect of today’s cars, including the charging system.  As with any device that is driven by the engine some level of load will be exerted, which will cause some change in the overall emission level out the tail pipe. Now with the PCM controlling things a more precise level of control can be maintained and help reduce those emissions.

We’ve all heard the groan from an alternator dragging down the engine while it was doing its best to keep up with a poor battery or an extreme load on the vehicle. The alternators in those days were always on maintaining a constant level of output whether it was needed or not. These days, well, the cars have gotten a lot smarter.  They know when you need that extra help and when it’s not needed.

Honda’s answer to this is the ELD (Electrical Load Detector). Since the early ’90s, Honda vehicles have used an electrical load detector (ELD) unit in the under-hood fuse/relay box. This unit reads the amperage load directly from the battery and in-turn sends a varied voltage signal to the PCM which regulates the field signal to the alternator. The ELD is a three wire unit mounted in the fusebox with a primary voltage lead, a primary ground, and a load output lead which is used by the PCM.  The PCM is connected to the alternator, not the ELD.  The ELD monitors the amperage requirements and informs the PCM as to what to do.
The theory behind this is to improve fuel economy by reducing the load on the engine during certain conditions.  These conditions can vary from vehicle to vehicle.  Such as; an electrical load, (usually below 15 amps), vehicle speed,  (between 10-45 mph or at idle while in drive), engine speeds below 3,000 rpm, coolant temperature above 167°F (75°C), the A/C system is off, or the intake air temperature is above 68°F (20°C).

The big complaint from Honda owners these days is the flickering of the headlights or parklights.  It’s a rather common issue that I see quite often.  After ruling out the battery, battery connections, and any “add-ons” that could contribute to the problem the best place to look for information about the problem is the TSB’s.
The Honda service bulletin explains it this way:  SYMPTOM: The headlights dim with the engine running and the headlights on, or DTC P1298 [electronic load detector (ELD) circuit high voltage] is stored in the ECM/PCM (but the headlights do not dim). PROBABLE CAUSE: The ELD has a faulty solder joint. SOLUTION: - replace the under hood fuse/relay box. 
On some of the older models the ELD is replaceable, and on some of the newer models it’s not.  Even though I might be able to remove the ELD from the fusebox it’s not a serviceable part.  A lot of times I’ve called the dealer and found the part was not available unless I purchased the entire fusebox.  Of course this adds to the even more things to take care of besides the charging system or the flickering headlights.

Everything from idle relearn, clock reset, radio theft codes, and the auto feature for the driver’s window needs reset.  Auto window feature procedure: (Push down the driver’s power window switch to the second detent (AUTO down) to lower the driver’s window all the way down. When the window reaches the bottom, hold the switch in the AUTO down for 2 more seconds. Pull up the driver’s power window switch to raise the driver’s window all the way up without stopping. When the window reaches the top, hold the switch in the up position for 2 more seconds. If the driver’s window AUTO function does not work, repeat this power window control unit reset procedure.)  (This is something to keep in mind when you’re working up that estimate for your customer.)

SO HOW DOES IT WORK
The ELD is essentially a current transformer that monitors the amount of current draw the car is pulling from the battery. This amount varies from time to time depending on what you have turned on (various electrical devices). The ELD will vary the output between .1 to 4.8 volts to the ECU. This reference voltage is what tells the ECU to increase or decrease the field strength in the alternator. Even though the voltage level is still very important in today’s cars, the amperage draw throughout the different systems is monitored to a greater degree than in years past.  As the current is ramped up or down the ELD ratchets the output voltage to the PCM to compensate for the load. 

Take the headlight flickering condition.  This is usually associated with low idle/near idle conditions.  This is where the ELD has detected a lower need for any alternator output increase so the headlights are running primarily off the battery.  As the current is increasing the ELD starts to send the corresponding signal to the PCM which will then increase the field signal to the alternator, however, if the vehicle isn’t under any additional load the ELD will also sense this and decrease the need for alternator output.  At that near idle condition the ELD is working overtime observing the current draw because of the headlights, thus, the flicker… on and off, and on and off.  (In some cases this is considered “normal operating conditions”)… yea right… explain that to “Mr./Mrs. Customer”… (Good luck with that one.)

I’ve found that I can fake out the ELD with a resistor between 1k and 820 ohms (for purposes of checking the wiring, alternator output, etc… not as a replacement of the ELD.)  This can be accomplished by pulling the fusebox up and removing the lower cover.  Once the lower cover is removed find the three leads that make up the ELD unit.  You’ll have to cut the lead to the PCM and install the resistor between it and the ground lead.  Of course this is a last resort method, but effective.  Obviously the better method to avoid cutting any leads is to use a scanner that is capable of accomplishing the same thing.
There is something else to think about when you’re dealing with a customer’s Honda with a battery charging issue.  Since aftermarket “add-ons” are usually attached to the positive terminal of the battery, their current path is not flowing through the ELD. Things like the amplifiers, LCD monitors, and anything else that isn't fed through the factory wiring is drawing an additional current level that isn’t being monitored.  Since the ELD isn't detecting any additional current draw from the battery, it is going to keep the alternator's output at the minimum level under the conditions it is designed for. (12.3 volts - just enough to keep the battery charged, plus enough to run the systems of the car.)

There’s always more ways to solve a problem and even more ways to diagnose these problems.  These are just some of the ways I deal with the issues I’ve seen.  Always check with the latest information and diagnostic procedures before starting any new project.  Keep those amps flowing!