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1977 Suzuki GS750 Charging System Upgrade

Charging systems improved markedly in the 1970s, both in terms of output and component quality. But age isn’t always kind, and 40 years later, many of those systems are experiencing their share of issues. Burnt or corroded contacts in the charging system wiring are something of a silent killer, creating resistance that heats up the wiring, often leading to stator failure.

That was the case with our subject 1977 Suzuki GS750, which was suffering from low voltage output from the stator to the rectifier/regulator. One of the stator’s three output lines was badly burnt at its connector. The rectifier and voltage regulator were working properly, but there wasn’t enough voltage output to keep the battery properly charged. The problem most likely started with mild corrosion, then as resistance built and the wires heated up the connection started burning, to the point where voltage output from that phase of the stator was blocked, leading to more problems with the stator itself.

At the same time we were looking into the Suzuki’s charging problems, we learned that Rick’s Motorsport Electrics was introducing a new line of lithium-ion friendly rectifier/regulators to pair with lithium-ion batteries in vintage bikes. Incredibly light and with no corrosive acid — you can mount them upside down if you want — lithium-ion batteries make great sense, but their use in vintage bikes has been somewhat limited due to the high charging voltage output on many older machines.

In a 12-volt application, a typical lead-acid battery works well with charge rates up to around 14.5 volts, but lithium-ion batteries work best with a lower voltage charge and within a narrower range as they don’t like large swings in voltage, something lead-acid batteries tolerate well. Working with lithium-ion battery specialists Ballistic Performance Components, Rick’s has developed a line of rectifier/regulators specifically tailored to the voltage output required for lithium-ion batteries.

We wanted to try the new regulator/lithium-ion battery setup, but first we sent our stator to Rick’s for refurbishment, getting back a freshly rewound stator with new output leads. The stator rebuild for the Suzuki was $175.95 (an off-the-shelf rebuilt stator is $200.95, with a refundable $50 core charge). The upgraded rectifier/ regulator was $129.95, and the Ballistic EVO2 8-cell lithiumion battery $169.95. You’ll also need a side cover gasket, which we got from Z1 Enterprises for $4.95. Rick’s one-piece rectifier/regulator replaces the stock separate items, and it also takes all three phases of the Suzuki’s stator instead of one of them powering the headlight circuit, a common setup back in the day to provide dedicated voltage to the headlight.

This is a straightforward job, easily within the capabilities of the average weekend warrior. No special tools are required, although we highly recommend having the correct JIS “Phillips” screwdriver for removing and installing the side cover. We used a JIS impact driver as the original side cover screws are notoriously stubborn to remove. We still ended up with several damaged screw heads, so we replaced all the sidecover screws with stainless steel Allen-head screws.

We did experience one unexpected issue that had nothing to do with the install: A corrupted memory card resulted in the loss of all of our photos of the disassembly phase. However, owing to this job’s relatively simple nature, we’re confident that if you read through our assembly, you’ll have a clear idea of how it all comes apart. Remember those manuals that always said, “Assembly is the reverse of disassembly”? Well, this time, just turn that phrase around!

With the new system installed our Suzuki started on the button, which it didn’t before as the system wasn’t supplying enough voltage to maintain the battery. A check with the engine running, with the headlight on, showed a charging output of 12.8 volts at idle, 13 volts at 2,000rpm and 13.5 volts at 4,000rpm, just where it should be. As always, we recommend having a good shop manual on hand for parts identification and proper torque specs.

  1. The original stator from our Suzuki. Note the burned connector on the white/blue output lead, the source of our problems. As always with any electrical work, the first step is to disconnect the battery, and in this case remove it.
  2. The stator cover removed, revealing the alternator rotor and starter drive. The stator is contained in the cover. During removal, we disconnected the stator leads at the rectifier, then pulled them gently with the stator and cover, not appreciating you should first remove the sprocket cover, as you’ll see.
  3. The stator is held to the cover by three screws. The stator output leads are secured in their routing in the cover by three metal plates. Note the relief in the stator; it must point down for the stator to fit its mount in the stator cover.
  4. The stator loosely positioned in the stator cover. Note the orientation of the output lead harness, which tucks into a channel cast into the cover. It follows a clear routing path. The molded wiring grommet should be pressed firmly in place in its recess in the cover.
  5. The stator firmly screwed into place. Note the metal bracket at roughly 12 o’clock securing the stator leads and keeping them clear of the alternator rotor, and the two additional brackets at roughly 2 o’clock and 3 o’clock holding the stator leads in place.
  6. Next, we installed the new gasket, with just a dab of sealant to hold it in place. We then fished the stator leads through the opening in the rear cover up into the recess for the starter. We had removed the starter cover earlier, but at this stage we still didn’t appreciate that we’d have to remove the sprocket cover.
  7. The stator leads pass from the starter recess along a channel cast into the engine case before exiting on their way to the rectifier. This is why the cover should be removed first, a point we didn’t appreciate during teardown.
  8. With the stator leads properly routed we removed the stock rectifier, which is screwed to the side of the battery case. We had already disconnected its leads: yellow=stator; white/blue=stator; white/red=headlight circuit; red=power.
  9. The stock rectifier (right) and the new combined rectifier/regulator. The stock setup sent one leg of the stator’s output (white/green) directly to the headlight circuit and then to the rectifier (white/red). The new setup directs all three phases of the stator’s output directly to the rectifier/regulator. The white/green and white/red in the wiring harness are simply left unused.
  10. The stock voltage regulator is mounted under the battery box, which must be removed to access the regulator. The yellow lead went to the old regulator and the black ground lead is secured by the front battery box screw.
  11. To remove the battery box, first unscrew the starter solenoid; the right screw secures the solenoid mounting panel to the battery box. Lift the mounting panel up to release it from the battery box and leave it free.
  12. Remove the three battery box mounting screws, two at the rear and one at the front. Note the two ground straps secured at the front screw, one of which goes to the regulator. Pull the battery box straight up and out.
  13. The battery box removed, with the stock regulator clearly visible. Remove the two screws securing the regulator and remove the regulator.
  14. Using the two Allen-head screws supplied with the new regulator, secure the new regulator in place in the same location as the original, ensuring the leads will exit to the left side once the battery box is installed and with the regulator’s green ground wire securely grounded at one of the mounting screws.
  15. Reinstall the battery box, making sure to secure the ground lead at the front mount. Slip the starter solenoid mounting panel back in place and secure the solenoid and panel to the battery box.
  16. Connect the three leads from the stator to the three yellow leads to the rectifier/regulator. It doesn’t matter which goes where. Connect the red lead from the rectifier/regulator to the red lead from the bike’s wiring harness.
  17. With the new combined rectifier/ regulator installed the white/blue and white/red leads for the lighting circuit are no longer used. We put heatshrink tubing on the exposed bullet connector of the white/blue wire to make sure it was isolated.
  18. Install the Ballistic lithium-ion battery. It’s much smaller and lighter than the original. We used foam blocks to take up space and keep the battery secure. A plastic tool tray sits over the battery, so there’s little risk of it moving vertically. Reconnect the battery.
Published on Aug 8, 2018

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