As designed, the 1980 and 1981 Honda C70 Passport 6 volt starter circuit is marginal under ideal conditions. The culprit is voltage drop at the starter solenoid coil. The symptoms associated with this problem are typically a neutral indicator light that dims when pushing the starter button and no click heard from the solenoid. Sometimes the solenoid (and starter) will engage by gently moving the gearshift lever, or by kicking the side of the engine or frame. This does two things. Moving the gearshift lever exposes an unworn contact surface at the neutral switch reducing resistance in the circuit. Kicking or striking the side of the engine or frame sends a shock wave through the starter solenoid giving it a little "kick" to start moving and engage the contacts of the solenoid and make the starter spin.
If you test for voltage between the black wire and the yellow wire with red stripe (disconnected) at the solenoid, you will typically see adequate voltage. But when connected to the solenoid, it may not work or even click. This is primarily because of the voltage drop on the negative side of the starter solenoid circuit (yellow wire with red stripe) when under load (connected). This voltage drop does not always supply the "kick" needed to get the solenoid to move internally and engage the starter reliably.
Reasons for voltage drop:
There are simply too many connection points, too small of wire size and too much distance for the current to travel in the starting circuit ground path as designed. The current must pass through the following:
• Battery negative terminal to battery cable
• Battery cable to chassis ground
• Chassis to engine case through mounting points
• Engine cases to shift drum through oil-covered bronze bushings
• Shift drum to oil-covered neutral switch contact
• Neutral switch spring-loaded terminal to neutral light wire
• Neutral wire at engine through main harness to connector in headlamp housing
• Connector in headlamp housing to right handlebar switch starter button contact
• Starter button contact to other starter button contact through button disk
• Starter button contact to headlamp housing connector
• Headlamp housing connector through main harness to starter solenoid connector
• Starter solenoid connector to starter solenoid coil
All these connections create resistance to current flow and thus, voltage drop (and heat) across each connection, with each one adding to the total loss until there is just not enough amperage available to operate the solenoid electromagnetic coil to overcome the force of the internal spring. Other considerations are Environmental factors such as:
• Moisture
• Oxidation and rust
• Motor oil with contaminants
• Heat
• Vibration
• Age
• Copper wire work-hardening
• Insects (in switch housing)
• Improper repairs and maintenance
• Battery condition and state of charge
Honda solved the unreliable circuit problem in the same way the automobile industry overcame the problems of voltage drop, by redesigning the electrical system using a 12 volt battery and generator as a basis for their vehicles. As Ohm's law dictates, when voltage is increased, resistance decreases (given the same amperage draw), thereby reducing voltage drop in the circuit. 1982 and newer C70 Passports have 12 volt systems for this reason.
All hope for a reliable 6 volt starting system is not lost however. By simply adding a relay of the proper capacity into the starting circuit near the solenoid and battery, much of the inherent voltage loss can be prevented by reducing the large solenoid coil amperage requirements to that of the small relay coil amperage requirements, and allowing the relay to provide a direct connection of the solenoid coil wire to battery ground. Many automobiles and trucks use this method. The circuit will have a relay inserted between the key switch and the starter solenoid. On advantage of this is that smaller, lighter, less expensive wires and switches can be used in the circuit between the battery and the relay.
Replacing one or more components such as the starter solenoid and/or neutral switch may help temporarily but does nothing to correct the root cause. Even if your 6 volt Honda C70 Passport starter is functioning properly today, given the design limitations, it will likely experience intermittent starter solenoid operational failures. It is therefore advisable to install a relay to insure reliable operation even if no symptoms are present.
Reducing voltage drop by adding a relay will extend the service life of the starter and starter solenoid by reducing the amperage requirements (remember OHM's Law?) as battery voltage goes down, amperage goes up. This increase in amperage creates heat and arcing at the solenoid contacts and starter motor brushes. the starter windings will also build more heat, all of which is not a good thing. Low voltage supplied to the starter solenoid will promote premature burning of the internal contacts because the plunger moves more slowly (if at all) creating arcing and pitting of the contact disk. This disk is designed to rotate slightly each time the solenoid is operated which has a self-cleaning action at the contact area (ever wonder why solenoids have round bodies?). If the plunger action is weak or slow, the disk may not rotate properly and will eventually burn the contact area until it no longer functions. Have you ever had a car that would just click several times before the starter would finally engage? This is usually a burned spot or the disk that eventually rotates out of the way each time you turn the key.
It is therefore very important to minimize voltage drop to the solenoid coil to provide a strong magnetic field and maximum engagement force to minimize arcing and burning of the contacts.
The bottom line is that you should install a relay on these bikes even if the starter seems to be working properly. It is cheap insurance against starting system problems.
"Radical" Rich
1970-73 C70 Honda Service Manual: http://tinyurl.com/6ebwtw
1970-73 C70 Troubleshooting Guide:
http://tinyurl.com/6ebwtw (scroll to sec.7.1 page 101.jpg)
1980-83 C70 Honda Service Manual: http://tinyurl.com/hu42c
1980-81 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec.18-1 page 170.jpg)
1982-83 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec. 20-28 page 205.jpg)
1980-81 C70 Maintenance Schedule: http://tinyurl.com/z4zn6
1982-83 C70 Maintenance Schedule: http://tinyurl.com/hw35c
More resources for C70 Passports and Cubs:
http://www.shlaes.com/Vehicles/Scooter.htm
http://www.velodrome.com/HondaC70/HondaC70.html
