6V Voltage Regulator/Rectifier for Honda CT90 / Trail 90
The Honda CT90 "Trail 90" uses a 6-volt single-phase permanent magnet charging system. The factory design includes only a rectifier to convert AC from the stator to DC for the battery — there is no voltage regulation. This means the charging system continuously pushes voltage to the battery with no cap, which can overcharge and damage the battery over time. The original rectifiers are also failure-prone after decades of use, leaving many CT90s with weak or no charging.
This modern solid-state combination unit replaces the factory rectifier and adds voltage regulation — something Honda never included in the original CT90 charging system design. The regulator monitors battery voltage via a feedback wire and caps charging output to prevent overcharging, while the updated rectifier converts AC to DC more efficiently than the original. It is the single most effective electrical upgrade you can make to a CT90.
Specifications
| Spec | Detail |
|---|
| Replaces OEM Part Numbers | 31700-033-008, 31700-033-000, 31700-102-701, 31700-128-008, 31700-102-731 |
| Fits Models | CT90 K0–K9 / Trail 90 (1966–1979) |
| Also Compatible With | S90, C90, CL90, CB100, CL100, CB125, CL125 — any Honda with a 6V single-phase permanent magnet charging system |
| Type | Solid-state combination voltage regulator + rectifier |
| Voltage | 6V DC output |
| Sold As | Individual unit with complete install hardware |
| Battery Compatibility | Lead acid, GEL, SLA, AGM (not compatible with lithium-ion) |
What's in the Kit
- 6V regulator/rectifier unit with male 4-pin connector
- Female 4-pin connector with crimp terminals
- Posi-Tap splice connector for feedback wire
- M6 x 14 mounting bolt
About the Factory Rectifier
Honda used two different rectifier designs across the CT90 production run:
- Early style (K0–K2 era): An orange-finned rectifier mounted on a V-shaped bracket with three individual bullet connectors (no plastic plug). Some early models also lack a green ground wire on the rectifier.
- Later style (K3–K9 era): A smaller cast-aluminum finned rectifier on a single-bolt mount with a 4-pin plastic connector.
This regulator/rectifier replaces either style, regardless of what was originally installed on your bike. The included female connector and crimp terminals let you adapt to whichever wiring configuration your CT90 has.
Installation
Difficulty: Intermediate — requires removing several components to access the rectifier inside the frame.
- Access the rectifier: Remove the spine cover, air filter box, brush guard/crash guard, and muffler. Unbolt the gas tank and tie it up out of the way. Disconnect and remove the battery and the battery tray front bracket.
- Remove the old rectifier: Unplug the factory connector (or disconnect bullet connectors on early models) and unbolt the rectifier from inside the frame cavity.
- Mount the new unit: Bolt the regulator/rectifier into the frame using the included M6 bolt through the forward-most mounting hole (where the old rectifier bracket attached). Orient the unit with the wires facing toward the front of the bike and the mounting holes aligned vertically. Use the lower hole for the bolt and reuse the nut from the old rectifier. Do not use the upper hole — the air filter box sits flush against that surface.
- Connect the harness plug: Plug the 4-pin connector into the matching harness connector. If your bike has different connectors, crimp the included terminals onto your harness wires using the supplied female connector. Match wire colors: red to red, yellow to yellow, green to green, pink to pink.
- Connect the feedback wire: The black wire provides voltage feedback to the regulator — this wire is new to the system and was never part of the original CT90 design. Tap it into any black 6V-positive wire in the harness using the included Posi-Tap. A convenient tap point is the black wire at the rear brake light switch. Without this wire connected, the voltage regulation function will not work.
- Reassemble: Reinstall the battery (positive terminal first, then negative), battery tray bracket, gas tank, air filter box, muffler, brush guard, and spine cover.
Our complete CT90 regulator/rectifier installation video walks through every step — removing components for access, swapping the old rectifier, mounting the new unit, connecting the feedback wire with the Posi-Tap, and reassembly.
Frequently Asked Questions
Will this work on my early CT90 that has bullet connectors instead of a plastic plug?
Yes. Early CT90s (particularly K0 models) used individual bullet connectors instead of the 4-pin plastic plug. The kit includes a female connector with crimp terminals that you can install onto your existing harness wires. Match wire colors — red to red, yellow to yellow, green to green — and you are set. Some very early models may not have a green ground wire on the original rectifier.
What does the black feedback wire do?
The black wire provides voltage feedback to the regulator so it knows when to cap charging voltage. The original CT90 charging system had no regulation — it just pushed whatever voltage the stator produced straight to the battery. The feedback wire is what enables the new voltage regulation function. If you leave it disconnected, the unit will still work as a rectifier but will not regulate voltage.
Can I use this with a lithium-ion battery?
No. This regulator/rectifier is designed for use with lead acid, GEL, SLA, and AGM batteries. It is not compatible with lithium-ion batteries.
Does this fit other Honda models besides the CT90?
Yes. This unit works with any Honda motorcycle that uses a 6V single-phase permanent magnet charging system, including the S90, C90, CL90, CB100, CL100, CB125, and CL125. Connector configurations vary by model, but the included connector and terminals allow adaptation.
How do I verify the charging system is working after installation?
Connect a voltmeter across the battery terminals with the engine running. Per the Honda service manual, at 2,800 RPM charging should start (minimum 6.8V). At 5,000 RPM you should see a minimum of 7.2V and at least 1A of charging current.
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Last updated: March 2026
