Suzuki GSX-R Secondary Throttle Valve Actuator repair

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Repair on the 2004-2005 GSX-R 600 and 750

"Why do you only repair the 2004-2005 600 and 750?"

Good question.  The simple answer is it's the one that needs to be fixed.  This type of STVA was first used on the 2002 750 and 1000.  Suzuki uses either Keihin or Mikuni throttle bodies on their motorcycles.  Typically, the Keihin components are used on the 1000cc and Mikuni on the 600/750.  The Keihin is also used for the 2008 and up GSX-R middleweight bikes.

In 2004-2005, Suzuki used a throttle body manufactured by Mikuni on the 600/750.  The initial design was good, but had a flaw I believe they didn't anticipate.  So the STVA was built with a stepper motor that was doomed to fail.  It was redesigned for 2006 and eliminated this flaw.  Of course, this is only my conclusion since neither Suzuki or Mikuni will acknowledge me.  I only repair the 04-05 600 & 750, because they're the only ones with this design weakness.  The redesigned 06-07 units do fail, but far less frequently and for a different reason.  The direct testing below works for all the similar designs for the STVA, but the wiring tests are specific to the 2004-2005 models.  The cool thing about the 06-07 is that the STVA doesn't have to be removed to do it.

"How do I know if my STVA has failed?"

The simple answer is that the bike will tell you.  All of the fuel injected bikes have a self diagnostic system that will detect and identify the problem.  Under normal conditions, the fault indicator on the dash will come on (just a red light), and the LCD will display "FI" in the coolant temperature area of the LCD.  There's some disagreement among those in the GSX-R community about the meaning.  Personally, I believe the light is the fault indicator and FI is to mean that the fault is a fuel injection fault.  You can lose oil pressure and the light will come on, but it won't display FI.  So that's my opinion and I'm sticking to it.

Just because this condition pops up doesn't mean that your STVA has failed.  There are a number of different codes and sensors on the bike.  To take the next step, we need the bike to tell us what it's found.  The frustrating part here is if you turn off power, the code clears until it's detected again.  So you have to leave the key on and kill the engine with the stop switch, or just wait for it to happen again.  The Suzuki manual describes a "dealer mode" switch that can be connected to the bike.  When this switch is activated, the bike will give you a couple things.  First is the throttle position sensor calibration (TPS) and second is the error codes detected.

"How do I access dealer mode and what can I do with it?"

Accessing dealer mode is very simple.  For this example, I'm going to use a small piece of 24AWG single filament wire.  If you look under the drivers seat, back behind the fuses and relays is a white plug with 6 ports.  Only 4 of the 6 are actually used.

Let's think about this plug, with the retainer on top like the picture, as having two rows of three ports.  The port furthest to the right in each row are for the dealer mode switch.  If you short these two, you end up with the bike in dealer mode.  Since you're here, my example below is the bike in dealer mode with the C28 error.  Some people have nice write-ups on how to make your own dealer mode switch and have mounted toggles under their tail and other cool things.  I think that's just overkill for the task at hand.

Along with the code, you see a dash to the left "-C28".  When everything is working good, the code reads "-C00".  The dash is actually used to easily calibrate the throttle position sensor (TPS).  In the middle, it's just right.  If it's below " _C00 " then the TPS is adjusted too low.  If it's high " -C00 ", then the TPS is set too high.  The picture below is what a healthy bike will display.  The light and oil indicator are on because the engine is not running and therefore has no oil pressure.  On a running engine, both the red LED and oil can icon will not be present.  

A note about the TPS adjustment.  It needs to be done with the bike fully warmed up and running at idle.  Also, a lot of people mistake the sensor mounted to the STVA as the TPS.  It's the secondary TPS.  The TPS is mounted below the STVA and has a grey plug.  Adjusting the wrong sensor will just cause you more headaches.  Slow down and make sure you know what you're adjusting before you put tools on it.

"My bike says it has C28.  What do I do now?"

On the 04-05 models of the 600 and 750, it's a pretty safe bet that it is the STVA.  If you've had regulator/rectifier problems, or your bike has been taken apart and put back together like a set of legos, it could be something else.  Let's make sure.

There are three basic parts to the SDTV system.  The STVA assembly (sensor and actuator), the wiring harness, and the ECU.  The ECU is impossible to test for the home mechanic.  The STVA and the wiring is very easy to test.  If both of those are good, the only thing left is the ECU, so we start at the STVA and work our way back.

Before we start, it's important to know that the sensor is spring loaded and snaps to the closed position when the STVA is removed.  To make sure you're ready for this, anytime the STVA is removed or reinstalled, it should be done with the secondaries in the fully closed position.  With the STVA installed, there's only one option.  Without, things can get reversed.  The easiest way to keep things straight is to use the high idle cam on the left side of the throttle body as your guide.  The images below show the secondaries at full open on the left, and fully closed on the right.


To remove the STVA, you'll want to first remove the driver's seat and prop up the tank.  The last thing we want to do is cause more damage to our bike, so we're going to tape over the frame to protect it.  I'm using 2 layers of regular masking tape to do the job.

The STVA is located on the right side of the throttle body.  First turn it closed by hand as described above.  Then disconnect the STVA main connector and the connector for the STPS.  Both have retainers that you'll need to squeeze to release the plugs.  A screwdriver can help you do this.


The STVA is mounted to the throttle body using two Phillips head screws.  The rearward screw is pretty easy.  The front screw is blocked by the frame.  Push hard and twist the rear screw out .  If it feels like you are starting to strip it, STOP.  The screws are steel and the throttle body is aluminum.  This causes a reaction between the metals and causes galling.  To help break things free, while twisting, tap the screwdriver with a hammer repeatedly.  This shock will help break it loose.  The front screw usually benefits from this technique because it's obscured by the frame.  Stripping the screw heads is not a fun thing to deal with.


Now, just pull the STVA toward the frame.  It fits tight to the throttle body, so give it a wiggle and it will pop free.  There's just enough space to get the STVA out from between the secondary throttle shaft and the frame.

Direct testing of the STVA is pretty simple.  All you need is a digital multimeter.  We start by measuring the resistance of the STVA itself.  There are two distinct circuits.  Let's call them left and right.  The arrows in the picture below point to the two pins for the left circuit.

The reading here should be close to 7 Ohms.  The image on the left is a good reading.  The image on the right is a bad reading.  If you've got a bad reading, your STVA has the failure.  At this point, you should send it to me for repair.  A bad reading on my meter may be different than yours.  Whatever your meter displays when the probes are not touching anything, that is your bad reading.


"My STVA is good, now what?"

The way the failure occurs, it could still be the STVA.  Intermittent failure is one of the most frustrating things for the shadetree mechanic to deal with.  Sometimes, problems are conditional and it's hard to recreate those issues in the garage.  Dealing with these really becomes a matter of your skills as a mechanic and is beyond the scope of these instructions.  So, let's assume it's a hard failure somewhere.

(in all the images below, pay close attention to the retainers on the plugs for proper orientation)

The easiest way to accomplish testin
g the wiring harness is to cover as much ground as possible as fast as possible.  We're going to repeat the STVA tests, but we're going to do it from the ECU plug.  The ECU has two connectors.  We're going to use the larger one.  

Once it's removed, you can look into the wiring harness.  Again, with the plug on top, as pictured below, we're going to think of this as two rows of 17 ports.  Port 1 is the top row, far left.  On most bikes, it's marked like this one.  The ports go across the rows 1-17.  Port 18 is directly below port 1 and the bottom row is ports 18-34.

These ports are tiny, and the contact socket inside is thin.  If you're forceful here, you can make them loose and make things worse, so be very delicate in the next steps.  To duplicate the STVA tests, we'll test both circuits again.  The left circuit are ports 1 & 18.  The right circuit are ports 2 & 19.  The best way to do this is with sewing pins clipped in the gator clips of your meter.


Again, you should get readings close to 7 ohms.  If this is the case, you either have an intermittent failure, or an ECU problem.  The quick way to know is to swap ECUs with a known good bike.  If the ECU solves the issue, it's the ECU.  If not, you've got a gremlin to chase.  Open circuits (infinite resistance), or just high resistance will indicate a problem with the wiring harness.

For the purposes of these instructions, we'll assume you found infinite resistance on the left circuit.  There are two wires here, so we need to test both independently.  Similar to the previous test, we need to test port to port.  This time, we're going to test from the ECU connector port, to the STVA port.  The image below shows the port mapping from the ECU connector to the STVA connector.  

With our assumption of the failure on the left circuit, you'll test resistance from port 1 on the ECU connector to port 1 on the STVA connector, and then again on port 18 of both.  The two pictures below demonstrate a good reading on the port 1 wire.  You can repeat this for all four wires if you wish.


Dealing with your problem from here on out is really on you.  There are too many possibilities to cover here, so I'll briefly describe the two common issues.  I will say if you're not good with electronic diagnosis and repair, seek professional help.  Finding broken wires and correcting loose connections is a learned skill.  If you don't have it, this is not the place to learn. These are small and fragile wires and connectors.  They are easy to damage and making matters worse is not the goal.

1)  You found an open circuit.  This could mean a broken wire anywhere between the plugs, or you just didn't notice that the connector pulled free from the plug.

2)  You found both wires good.  In this case, it's usually a poor connection between the connector in the plug and the pin on either the STVA and ECU.  Either a connector is party pulled from the plug, or the connector has corroded or deformed.
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