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Discussion Starter · #1 ·
The following PDF shows how to monitor the alternator signals and how they should vary with electrical load. This might be useful for anyone trying to diagnose charging system issues:

https://drive.google.com/open?id=0B023spaMkQOldGNTcDB4Ty1SVnc

Here is a brief overview of the alternator connections and known good voltages:



IG = Keyed battery power supplied to the voltage regulator. Should be equal to battery voltage when Key is in the ON position and when engine is running.

L = Lamp signal – When the alternator/voltage regulator detects a problem (no charging) the alternator forces this line to a low voltage. When the PCM sees a low voltage on this line, it lights the Charging System Indicator on the dash. With Key On Engine Off, the voltage on L should be low (near zero). With the engine running, the voltage should be at the battery voltage (if there is no fault condition in the alternator).

FR = Field Return signal. This signal provides information from the alternator to the PCM to indicate how hard the alternator is working (what percentage of capacity it is operating at). This signal is a 5V square wave output from the alternator that is low when the field/rotor winding (armature) is energized (current being driven through it). The more time the FR is low, the harder the alternator is working (the more output current). If the FR is continuously low, then the alternator is at its highest output. If the FR is continuously high, then the alternator is very lightly loaded. If you monitor the AVERAGE voltage on this pin with a multi-meter, it should vary from about 5V when the alternator is operating at 0% of its capacity to about 0.36V when the alternator is working at 100% of its capacity.

C = Alternator Control signal From PCM to Alternator – The voltage regulator in the alternator has two modes. If the control wire is high (~8V) then the alternator is placed in high output mode and maintains ~14.5V output. If the control wire is low (near 0V) then the alternator is placed in a low “maintenance” mode and maintains ~12.5V output. The low output mode is only commanded when a set of conditions is met (engine up to temperature, under a certain amount of electrical load, certain speeds, etc.) to increase efficiency. I never saw the PCM command “Low” mode when the engine was at idle, but “Low Output” mode was commanded during driving conditions. Under normal idle conditions, this pin should be at ~8V (I measured 7.6V typically). It should go low (near zero) when “Low Output Mode” is commanded.

The "Low" output mode was a new one for me, and sounds like it might be a Honda-specific functionality. The following link has the best information about it that I was able to find online:

http://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1011&context=auto_pres
 

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Discussion Starter · #2 ·
To add a little more information on the HIGH/LOW alternator output modes...

The following image shows the electrical system PIDs on a scan tool while driving around when the PCM was commanding the "normal" high alternator output mode. The "Alt Ctrl = Alternator Control" reads out as "12.5V" for low mode (12.5V nominal output) and "14.5V" for high mode (14.5V nominal output). The "Battery Voltage" reads out the battery voltage, in this case charging at 13.9V. This looks like you would expect in most vehicles, a charging battery voltage of about 14V:



However, here is another case, while still driving around where the PCM has switched the alternator into "Low" output voltage mode and is maintaining the battery at only 12.3V. It is good to be aware of this "Low" output mode, since in most vehicles an observation of 12.3V battery voltage with the engine running might be interpreted as a failing alternator, but in this case it is an perfectly normal operating mode:




According to the reference given above:

"The ECM uses the low output mode
when the engine is starting or if all of the following parameters are met:
1. electrical Load below 15 Amps (varies with vehicle),
2. vehicle speed between 10-45 mph or at idle while in drive,
3. engine speed below 3,000 rpm,
4. coolant temperature above 167°F (75°C),
5. A/C Switch Off
6.intake air temperature above 68°F (20°C). "

In my vehicle, I never saw a case where the alternator was put in Low mode when at idle in drive, but the vehicle would switch back and forth between Low and High Alternator modes while driving around the neighborhood. Also, whenever the alternator was in Low mode, switching on the headlights or blower motor always made the PCM revert to High output mode.
 

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Elvira
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As always @JohnNH, a very informative writeup.
 

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What is the best method to monitor the ELD output with a DIY scan tool, such as a ELM327 based dongle with the OBD Fusion iOS APP? I don't see "Alt Ctrl = Alternator Control" listed as a SAE PID.

Reason is ever since I replaced the OEM DENSO with an AutoZone new aftermarket on an '03 EX-L 2.4L, I get RPM oscillations at idle if there is a sudden high electrical load (AC blower kicks on, brights, etc.) which causes body rattles due to RPMs dropping to as low as ~ 550 before spiking back up to ~ 1000 RPMs. This oscillating pattern goes on for several seconds until it reaches normal idle RPM and output voltage is steady 14.1 V. Absent of this condition, idle is smooth all the time.

Would appreciate any advice. Thanks.
 

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What is the best method to monitor the ELD output with a DIY scan tool, such as a ELM327 based dongle with the OBD Fusion iOS APP? I don't see "Alt Ctrl = Alternator Control" listed as a SAE PID.

Reason is ever since I replaced the OEM DENSO with an AutoZone new aftermarket on an '03 EX-L 2.4L, I get RPM oscillations at idle if there is a sudden high electrical load (AC blower kicks on, brights, etc.) which causes body rattles due to RPMs dropping to as low as ~ 550 before spiking back up to ~ 1000 RPMs. This oscillating pattern goes on for several seconds until it reaches normal idle RPM and output voltage is steady 14.1 V. Absent of this condition, idle is smooth all the time.

Would appreciate any advice. Thanks.
There is no standard OBD II Parameter IDs for ELD or alt-control. OBD-II PIDs - Wikipedia

There may be a Honda specific ones but not likely. Sounds like the Alt Control signal is just a wire that runs from the PCM to the alternator. You can probably measure it directly with a muleteer if you really really need to.
 

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Ok thanks for confirming on the PID. Thought that may be the case. I’ll look into probing the wire with my DMM.


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