Auto Service World
Feature   February 1, 2002   by Peter McArdle -- Asian Specialist

Honda charging systems

If you ever have worked on a Honda charging system, you may have noticed that this system operates a little bit differently from other systems as far as strategy is concerned. On startup, the alternat...


If you ever have worked on a Honda charging system, you may have noticed that this system operates a little bit differently from other systems as far as strategy is concerned. On startup, the alternator does not appear to be charging, but after several seconds it suddenly comes alive and begins to charge. If you raise the engine RPM to 2000, you may notice the charging system voltage drop unexpectedly. Being unfamiliar with this system will usually lead to unnecessary parts replacement.

From approximately 1988, the vehicle ‘s computer plays an important role in the charging system, and we must take its actions into account when diagnosing a charging system problem. For this particular tech tip, we will be using a 1989 CRX with a 1.6l engine as an example.

There are other Honda models that use similar systems, so that you can use this information when doing diagnostics by referring to the appropriate wiring diagram. Refer to diagram #1. There are five wires connected to the alternator. One wire is power from the fuse box (the solid white wire).This wire carries the alternator output current to charge the battery. The second is power while the ignition switch is on (the black wire with a yellow stripe). This circuit powers up the voltage regulator. The third controls the warning light (white wire with a blue stripe). The fourth is the charging rate input to the ECM (the blue or yellow wire).The fifth is an output from the ECM (white with a yellow stripe).This is the control circuit that turns the alternator on through the voltage regulator. There are a total of three wires to the computer that are involved in the charging system. It is the function of these three circuits that we will discuss.

These three circuits are: The ELD input (Electric Load Detection),the FR input (Charging Rate Signal),and the C circuit output (The wire that the computer uses to turn the alternator on.). The first of the inputs is the ELD or the electric load detection sensor. This sensor monitors the electrical load on certain circuits. It actually functions as an ammeter. The computer uses this information in conjunction with the FR or charging rate signal in order to control idle speed and whether or not to turn the alternator on or off.

The ELD sensor has three wires: battery power from the ignition switch, ground, and the ELD signal wire. The computer supplies a 5 volt reference signal on the ELD signal wire (green wire with a red stripe). This reference voltage can be checked with the sensor unplugged and the ignition key on-engine running or engine off. When you plug the sensor in, the signal voltage will be pulled down depending on how much current is passing through the ELD sensor. You can check this by monitoring the voltage on the ELD wire as loads are turned on or off. For example, with a warmed up engine at idle with all accessories off, there is approximately a 5 ampere current draw. With this amount of current, you should see approximately a 3 volt signal on the ELD signal wire. If you were to turn the heater blower on high, this would create a current draw of approximately 17 amperes.

With this type of load, you should see a signal voltage of about 1.8 volts. If the computer detects a problem with this sensor, it will set a Code 20. Not all of the electrical loads are monitored by the ELD sensor. Battery charging current, the warning flashers, and the computer memory keep-alive circuit are instead taken into account by the FR signal which we will now discuss. The second of the inputs is the FR signal (blue or yellow wire, depending on the vehicle), which is the charging rate signal.

This signal communicates to the computer on how hard the alternator is working to meet the total electrical needs of the vehicle, including the state of charge of the battery and any loads not directly monitored by the ELD sensor. This signal is a digital signal that varies in pulse width according to the total load on the alternator. The computer supplies approximately a 5 volt reference signal through the FR circuit to the voltage regulator. The voltage regulator will cycle this signal from 5 volts to approximately 1.2 volts in proportion to the alternator load. For example, with a warm engine at idle and all accessories off, duty cycle should be 30-35% (negative trigger on your meter). Turn your headlights on, and your reading should be 55-60%. Add the rear defroster, and it should be 80-90%. There are two situations when the FR signal will not be digital. One will occur when the load on the alternator is very high. At this point the signal will be a flat line, non pulsed voltage at approximately 1.2 volts (that is,100% duty cycle).That should be the lowest reading you should ever have if the system is functional. This signals the computer that the alternator is working at close to maximum output.

The second situation will occur if the total electrical load is very low. At this point the FR signal will be a flat line (non pulsed signal) at approximately 5 volts. This signals the computer that the battery is fully charged and that overall electrical demand is low. When this happens, the computer can turn off the alternator through the one computer output, the C circuit (white wire with a yellow stripe).This is the one circuit where the computer turns the alternator on and off. The voltage regulator supplies an 8.5 volt (approximate) reference signal to the computer. When the computer grounds this signal, the alternator will stop charging, as in the case of the battery being fully charged and vehicle load demand is low. If this wire is shorted to ground, the alternator will not charge.

In closing, the computer monitors the ELD and the FR signal and gets a good idea of what the electrical demand is like. Using this information, the computer controls the charging system through the C circuit. It also uses this information to regulate idle speed. By turning off the alternator, the computer reduces the load on the engine, and thus saves fuel and increases power output when needed. In diagnosing some idle speed problems, these charging system inputs or outputs, being out of specification, can be the source of the problem and need to be checked for proper operation.