Browse Category: Suzuki GT 380

Front brake switch

Don’t even think about it, assembling the brake switch from below. The the spring will jump away along with rest of the small parts.

Turn the brake lever upside down. This is valid for GT750 as well.

I know, the nut and the bolt for the brake lever is wrong. Will be replaced later on when I get the parts. I have to use what I have to get going.

Make sure the switch moves freely to both positions.

Add the contact disk of the switch.

Mount the plastic enclosure and make sure you have a good strain relief.

Zip ties

I use reusable zip ties at this stage for an easy adjustments if needed.

GT380 Side Cover Clip


Two of the old clips were broken and new parts are still available for buy. Will cost time and money to buy and since it’s fun to make my own parts, why not give it a try? The model was done in Fusion 360 and the first set of six parts were 3D printed with black resin.

The parts looks stunning, but the function of the flexible clips were not good enough. Too fragile and got easily broken. Another type of resin must be used for this type of parts. I can use my rubber resin, but I will order some ABS like resin and give it another try.

The model drawing in Fusion 360

The STL file of the part can be downloaded for free for anyone:

This post will be updated with the result of the ABS flexible resin print when it’s done. The same if I do a rubber resin print of the part.
The ones I made worked fine, but as mention above they all looked fragile and some broke when mounted into the frame.

New updates: The file above is now updated to V2 release and will fit better into the frame. The new printed part using ABS like resin was also much better.


Before and after refurbishing

The coil wiring was in a bad shape like the rest of the electronics. Wrong colors on the wires and missing connector.

If you don’t know how the coils and ignition is working, please have a look at my post i did 8 years ago when working on my GT750:

How to start: All parts were dismounted and cleaned. I was not able to clean the coils properly so I decided to paint them white.

The metal brackets holding the cables were sandblasted and nickel plated.

I had to use impact driver to loosen the screws. Don’t forget to use JIS tools ( Japanese Industrial Standards)

The bracket was sandblasted and polished.

All of the wires had to be extended or replaced. Each individual strand in the wire must be sanded before soldering. After soldering the joint was covered in epoxy and painted.

The other end of the wire got new terminal and was crimped.

This is the color coding and placement of the wires into the connector.

The orange wires are all connected to a common +12 supply and the placement on the upper row does not matter. On the lower row, it’s very important to do it right, if not the bike will misfire on wrong cylinders.

Mounted with new wires and connector.

Before mounting I also did a spark plug test to verify all of the coils. Connect 12V to the orange wire and short the other wire to GND( 0v). The body of the spark plug must also be grounded. When you release the wire from ground the park plug will fire. ( Just tap the wire on/off to GND and you will see the spark) Do the same on all coils to verify the function.

The resistance in the coil winding should be around 4,5 ohm. The current in each coil when grounded will therefore be 12V/ 4,5 ohm= 2,7 Amp. That’s the reason why you drain the battery very fast when leaving the ignion on and the bike is not running.

Wiring harness GT380

Hmm, a lot of questions but few answers.

According the the parts manual it looks like the wiring harness should be installed on the right side of the bike, or… not, right side must be wrong. I’m told it right is right, but based on feedback with images I have only got photos from GT380 owners with the wiring harness on the left side of the frame, and it’s the same on my GT750 too. The drawing above must a guidance of how the wiring are connected, not which side it’s fitted.

Based on photos I have got I have some idea about how it should be done. Some advice can also be found on YouTube, but you can’t trust them. Some might be right and some are wrong, and others are horrible wrong.

So, to summarize: I’m not sure, and in some case I don’t have a clue, but I have to start and will use common sense when I have to make decisions. I think a look at mye GT570 will be at a good help. As long as I make it all in compliance with the wiring diagram it should be fine.

Connectors and tools:

I bought a kit from China with a lot of different type of connectors + the crimping tool needed.

In addition I got hold of the upper and lower wiring harness and only need to make some few extra cables to fit into the harness.

The cable from the alternator is already done, please see my previous post. The same for the cable from the ignition electronics (the points )

Next up was the battery cable. That became a bit tricky. Was not able to find the old cable anywhere and how to make a new one. The corresponding connector is made of rubber and how can I make that ? Same procedure as before. I drew the part in Fusion360 and fired up my 3D printer with rubber resin. And here is the result:

I found a picture of the cable on internet and did the measurements on the mating connector from the wiring harness. After printing the part I glued the terminals and added heat shrink to the wires.

The STL file for printing can be downloaded for free:

Next step:

I inserted the main cable for the upper harness through the lower hole in the headlamp and the upper hole will go to the clocks and and switches on the handlebar. I will use adjustable cable ties for clamping the cable onto the frame so I can easy move and do changes as I continues with the wiring.

That’s enough for today. Will continue another day πŸ™‚

Another day:

Still winter in Norway and I will spend some time in the Man Cave to do some wiring:

Still not sure how to lay down the wiring, but I give it a try and use reusable zip ties for an easy adjustments.

Head lamp wiring loom:

Pushing all the wires back into the headlight house to free up as much space as possible. Will be a tight fit later on when the headlight shall be mounted.

Hmm, something is wrong… The blue connector is not part of the schematics. Can it be for a later model with gear indicator ? No, the color code is different.

Some minutes later: Mystery solved, see the picture below:

The blue connector is for all of the small lamps in the clocks. According the the schematic it should be individual connectors, not a common one for all of the lamps. Anyhow, now I know how to do the wiring. It’s more easy too with one 6-pins connector, but why is it not part of the wring ?

Phuu, for a while I thought the main wiring loom was wrong. Well, a bit wrong. It’s not the wiring loom for the J-model and not for the latest ones either. Not a big deal since I know how to wire it and have the tools and parts to make the mating connectors.

Instruments bulbs:

One of the bulbs sockets looks horrible, and the wiring is in a bad shape as well.

I think I order new sockets and make a complete new wiring. If so,I have to 3D print the rubber cover too.

When it’s done, it will be a separate post about how I did it. I can’t put this old and ugly bulb loom back into the lovely bike.

Clamping to the frame:

Photos from the wiring layout:

Tried to lay the wiring close to the frame away from the air filter and other parts to be mounted later.

The wiring loom is quite stiff and to avoid too much bend I ended up laying down the cable as the picture shows. Probably not like the original, but I think this will work fine. But doing so, the fuse holder will fit better on the right side, close to the tool box, but why not?

I will 3D print a bracket for the fuse holder. The fuse holder is not the original, but close to. A 20A fuse.


Make sure the threads are clean without any paint to ensure a proper grounding.

Less questions, all the wiring are now sorted out :

The upper wiring loom I bought deviate from any standard GT380 wiring, it has some extra connectors.

In addition to the 6-pins blue instrument lamps connectors there are two connectors with Gray and Black/white wires. for instruments lights ( tachometer and speedometer light)

One red wire with +12V, direct from the battery + pole and one extra GND wire. Can be used for alarm system or a charge port to the battery.

One green/yellow wire in the same connector going to the ON/OFF switch. Not in used for my J-model.

One brown wire. The same wire as going to the tail lamp. This wire will also be at power it the ignition switch is turned to parked position.

On/off switch:

The on /off switch is a bit difficult to spot on the wiring diagram, except from the GT380B wiring. The color code is quite simple. Orange is the +12V after the ignition switch (red before the ingnition switch). The orange/white is the +12V to the coils and the bike will of course never run if this switch is turned off. According to the GT380J wiring it’s the orange/white wire changes to solid organge from the switch connector and down to the coils, but not on later models. A bit annoying and I’ve seen several variants of the J-wiring diagrams. Not easy to know what is correct.

A summary of the color codes:

Red (R): The +12V from the battery through the 20A fuse.

Orange (O): +12V after the ignition switch

Orange / white (O/W) 12V + after the ON/OFF switch. Turn on /off the voltage to the coils. Note! only orange wire on earlier models (According to the schematics)

White (W) : Brake lamp wire.

Brown (BR) Tail lamp.

Gray (GR) Instruments lamps

Blue (BL) Neutral position switch. The indicator lamp will light if the wired is shorted to ground.

Light Green (LG) Right indicator lamp.

Black (BK) Left indicator lamp.

Black/white (BK/W) GND (Ground, connected to the frame)

Light Blue (LBL) Turn signal relay wire.

Yellow in the rear wiring (Y) 3 phase AC voltage

Yellow in the front wiring (Y) High Beam Head Lamp.

Note: The Gray and Brown wires will be shorted by the ignition switch in run position and both the indicators lamps and tail lamp will light (if the main light switch is on). If the ignition switch is left in parked position the orange and brown wires are shorted and the tail lamp will light (regardless of the main light switch).

Mounting the oil pump

For about two months ago I tested several oil pumps in the test jig I made. I decided to go for the early version of oil pumps with suction. That was the one giving best result when idling. I got scared about the others giving little or noting at low throttle.

All o-rings were replaced and the same with the nylon spacers.

The 50 years old plastic and the oil-lines are very fragile, and I didn’t do much to clean or make it look nice. I knew it all worked since I had it in the test jig with good results. To drive the plastic base all the way down I prefitted the screws for holding the oil pump.

Torque settings for the banjo bolts is difficult to find. I did some investigation back in 2016 when rebuilding my GT750A and found a number of 2,5Nm. That’s lower than I today have on my smallest wrench. At that time I stole a key from my work, but this time I used common sense and was gentle to the bolts.

By using a syringe I was able to inject the oil all the way to the end of the pipes.

I have the original screws for mounting the oil pump, but I went for 5mm bolts with hex head instead. Much more easy to mount and the screw are not visible when the covers are on.


Clutch Release Assembly

The parts were cleaned, greased and refitted.

Mounted in the cover and fitted onto the bike:

See the service manual for the procedure how to hook up the clutch cable and do the adjustments. I will also update this post later on when I do my own clutch adjustments.

Side cover:

The bike so far:

Neutral Switch GT380J-K

The picture above shows the function of the neutral switch. The shifting switch (Part 44) is attached to the gear shifting cam. In neutral position the switch will reach the contact point inside the housing (part 39), this will lead to shortening the blue wire down to ground. Since the lamp in connected from the blue wire to +12V, the bulb will light up in neutral position.

Make sure the contact points are cleaned and polished at the inside and outside of the housing. I added a new washer and screw too.

Before mounting, put the engine into neutral position. Make sure the switch is mounted with the lip into the groove at the end of the shifting cam. If so, the contact point in the switch will meet the contact point in the housing.

To verify the function, measure the conductivity from the witch down to GND. Should be a short (close to 0 ohm) when the engine is in neutral position.

When you add the wiring harness, the blue wire from the alternator cable should be fastened to the switch.


GT380 schematics

The figure above is a copy from the wiring diagram. It shows the 3-phase rectifier with three yellow wires as AC input and the plus (red wire) and GND(ground 0V black and white wire) as DC output.
The wiring diagram also shows the housing (one of the heatsink) is grounded.

Note! I had a look on my GT750A model. An other type of rectifier (the smaller one) and the black and white GND wire is missing. All the grounding is done direct through the mounting bolt. Only four wires, three yellow and one red to the connector. A bit confusing, because the detailed wiring diagram for GT750 shows the GND wire, but not in the owners manual for GT750A. I assume it’s done different from 72-77 models.

GT750A version, no black & white GND wire.

If you are uncertain how the rectifier works with all 6 diodes, please review my previous post about the alternator for GT750 and GT380. There you will also find link to YouTube explaning how it all works.

The rectifier in the image above is from the J model. You can might find a different package on later models with smaller heatsink, but they all works in the same way.

You have six diodes, three with the anode connected to the case and three diodes with the cathode connected to the case.

Please note, the rectifier has the heatsink splitted in two parts. One is the pluss output ond the other one the minus output (GND) . If you scratch off the pain from the positive heatsink and by accident short the heatsink to GND you short all of the voltage to ground on your bike. Note! the heatsink is connected to plus, not the bolt. The image shows how the bolt going thrugh is insulated. I did a small mod using making the insulator in peek material to improve the design a little bit.
The other part of the heatsink shall be grounded to GND according to the schematics. Mine was not and I’m not sure if it was wrong mounted or it is common not to do do. Anyhow, both options will work, but if not grounded all of the current to GND has to go through the black and wire to GND. I did it in my way and made better connection to GND from the heatsink.

Remember to remove paint to achieve a proper connection to GND.

All of the connectors where polished and the red wire who was broken got replaced with a new wire and connector.

To remove the terminal from the connector, use a flat screw driver to bend the lip down and pull the terminal out from the rear side. Remember to bend the lip back to normal position when inserted.

Diode testing:

When using a multimeter for testing, switch to diode testing and do the measurements. Positive terminal(red test lead) on the anode and negtive terminal (black test lead) on the cathode. Measure all six diodes, but keep track of anode and cathode. Since the three of the diodes have the anode to GND you can also measure to the heatsink insted of the black and white GND wire/treminal

The instruments shall give you a value for about 0,45V, and no voltage if you swap the terminals ( positive on cathode and negative on anode).

Measure all six diodes, but keep track of anode and cathode. Since the three of the diodes have the anode to GND you can also measure to the heatsink insted of the black and white GND wire/treminal.

If you don’t have any multimeter you can use a small 9V battery as input and and a test lamp at the output. When you swap the polarity on the 9V battery applying power to the yellow 3-phase input the lamp shuld still light, and do the same betwen all phases.

Done !

Oil pump

This post will be about the oil pump, for both the GT750 and GT380, old and new models.

The information below is my understanding based on testing and iformation given to me. If something is wrong, please let me know and I will updatate my blog.

Some basic informations:

As the picture above shows we have an early model of the pump using one extra piston for suction. On later models the gravity from the oil tank above the thank do the job, and Suzuki decided to make it more simple with only two pistons.

Note! Never,ever go away from using the oil pump and only mix oil into the fuel, if so the crankshaft will not be lubericated.

Are all pumps interchangeable ? Yes, all of the GT380 to GT750, old and later models, but there are some differences. Later models for GT750 using the vacuum carbs have a different arm connecting the throttle cable, see the picture below:

The punch mark and alignment marks are at different location (order) on J-K (GT750 models and all GT380 models) compare to the latest GT750 models. The alignment procedures are also differnt, see the picture above and the service manual.

Test setup:

The speed of the pump is controlled by a DC motor and most of the parts are 3D-printed.

The shaft running the pump is made of brass and locked in position by two ball bearings.

Lesson learned after the testing:

At normal operation the cylinders get more oil compare to the crankshaft, but one important observation:

At the first punch mark there are almost no oil coming to the cylinders, only to the crank. On my test with two later models with only two pistons and no suctions, this was the case. A bit scary to watch. On the early model with three pistons and suctions I got a bit of oil to the pistons at the first punch mark as well, but no much. As soon as I increased the throttle it all worked as normal. Most of the oil to the cylinders and less to the crankshaft.

In the video below the pump is running between the first and second punch mark.

On my GT750 the oil consumption is close to 100ml per. 100 km, as it should be. In test above the rpm of the pump is 1hz ( 60 rpm) and refer to the engine that will be about 4000 rpm. The consumption from the test looks very much the same as I see on my GT750, but the 3-piston pump with suction gives a bit more.


I ordered new gasket from E-bay, but to start the testing I made my own by scanning the old one and cut new with my laser cutter:

Seems to work fine. Since I have paid the money for new ones, I might swap the gaskets when they arrives.

Copy from the manual:

Contact Breaker Assembly

(Denso type)

Note: Points or breakers are the same. I usually refer to the breakers calling them points.

As mention before, my GT380J project was in a horrible state when I bought it. No exceptions when it comes to the electronics. Wrong connectors and terminals. Wrong color codes on many of the wires. The wiring for the contact breaker was a mess too and I had to make it all new according to the wiring diragram.

This is how it was, wrong wire colors, wrong type of screws, and missing connector.

By the way, be aware of the difference from GT750 to GT380. Since the points are located on the right side of the bike on GT380 and left side on GT750, they have to be mirrored. Don’t buy the wrong ones.

The left point is locked direct to the main plate and has therefore a different shape compare to the center and right point.

Where to start ? I found this picture below on E-bay and it looks alright compare to the wiring diagram.

The picture also gave me some idea of the cable length.


I bought a kit from China with a lot of connectors and terminals making it possible to fit the corresponding connector in the main wiring harness.

Connection to main wiring:

Note, on the GT380 the connector goes directly to the main wiring, not through the connector plate as you see on the GT750.

I was not able to find a black wire with yellow stripe, and as you see, I painted the yellow stripe to get the color code correct.

Left point: White

Center point: Black and yellow

Right point: Black

How to fit the contact breaker and adjust the timing will come in a later post.