How To: Diagnosing A Bad T P S

It is becoming more common for motorcycles to have electronic fuel injection, also known as EFI.  You will find EFI on all types of bikes, street, dirt, every manufacturer is converting to EFI.  There are several reasons for this, but mostly it’s for the performance improvement and to comply with ever increasing emissions regulations.  The components of motorcycle fuel injection systems are reliable, but parts can wear out, or even fail prematurely.  When a failure happens the process for diagnosis and repair can seem daunting, but in most cases it’s only a loose wire, or failed sensor.

The most common sensor to fail is the Throttle Position Sensor, or TPS for short.   The TPS tells the ECM (Engine Control Module) where the intake butterflies are in their travel from fully closed to wide open.  That information, combined with data from other sensors, allows the ECM to correctly meter the fuel being sprayed from the injectors.  The primary reason for TPS failure is simply wear, since it moves every time you twist the throttle, eventually it wears out.  Typically it will be in one specific place on the throttle, this comes from operating on that point repeatedly.  During my research on TPS failure I found out cars that sit and idle like limos and taxi cabs have failures just in the idle area of the TPS.  Even at idle, a contact inside the TPS is moving ever so slightly, so eventually it wears and fails.  Vibration and environmental conditions can also cause failure.  The natural vibration of a V-Twin, or an off road vehicle operating in dust, mud and water would be good examples.

Inside the TPS you have two main parts, a traveling contact, or “wiper” as it’s called in the industry, and the surface that it moves along.  This surface has a conductive base with an overlaid variable resistance coating that is applied in a linearly decreasing fashion from high resistance at the closed throttle point, to low where the throttle is fully open.  As the throttle opens the wiper moves along this coating sending a signal voltage to the ECM.  This voltage rises as the throttle opens, typically the signal voltage starts at less than a volt at completely closed, say .5 to .9 volts, and rises to a maximum of 5.0 volts.  Any non linear change in resistance can result in a dip or spike in the voltage going to the ECM.  This “tricks” the ECM into either over or under supplying fuel for that point in the throttle travel.

What are the signs of a faulty TPS?  The signs of a  failure can be mild, like stalling, hesitation, surging at certain throttle points, an uneven, or rough idle, and, in most cases, a drop in gas mileage.  The failure can also be dramatic, as it was with my Victory V92 C.  When the TPS on my V92 C croaked it went completely nuts!  My Vic was bucking, cutting out, backfiring, the “Check Eng.” light was flashing.  With that kind of behavior, I was almost sure my bike would see a tow truck before the day was out.  But it got better, and the check engine light had stopped flashing, so I thought I had gotten bad gas because I had just filled my tank.  But that wasn’t it, after cleaning my gas tank of the supposed “bad gas,” I was left with two annoying conditions, a surge at one point in my throttle travel, and a drop in gas mileage of about ten to twelve percent.  I think what happened was that a small bit of the contact surface had flaked off, then spent some “happy time” under the wiper, causing random voltage spikes, and driving my ECM nuts.  It most likely worked loose and fell into the TPS housing, leaving me with the more typical bad TPS symptoms.

Since all of my previous motorcycles were not fuel injected, diagnosing the condition was driving me crazy!  So if it wasn’t “bad gas,” what could it be?  I’ll admit, there was a lot of random guessing going on by myself and my friends, so we started replacing parts……

Was it the spark plugs?  Ok, put some new ones in……  nope, not it….

How about the plug wires?  Let’s replace them…… no…. not that either….

Could it be the coil?  Bought a used one from eBay for Twenty bucks, put it on…… and…… still not it!  Shit!

I think beating my head against the wall would have been less painful.  I was suffering, the surge was right where I usually kept my throttle when riding curves in the mountains, remember the part about a TPS wearing in one spot?  You got it, I had wore out my favorite spot.  Having a surge at this point on the throttle made for tiring riding, not fun at all!  This went on for weeks, lots of time and money spent, with only disappointing results to show for it.  In the process of replacing parts I had introduced a second issue that compounded the difficulty of diagnosis, and that’s a whole other story in itself.  But, I’ll just say this, never, ever, assume a brand new part is “good.”

What I should have been doing instead of guessing, cussing, suffering, and swapping parts, was thinking logically.  I should have been using the Victory service manual and testing, testing, testing until I found the problem.  It took me quite a while to work it all out and fix it, but in my defense, in the beginning, I didn’t even know my bike had a TPS!  At the end of that ordeal my Victory was fixed with a $30 part from the auto supply store.  In the process I also learned a lot about the EFI system, how to test, and repair it.

I’m hoping that with this article I can save you from the steep learning curve of guessing, parts swapping, cussing and the suffering I experienced in the process of diagnosing my TPS.  There is nothing mysterious or magical about fuel injection, at least not as far as sensors like the TPS is concerned.  As for the dark fairies that live inside the “black box” ECU…… well, just be glad we’re not dealing with that today!

First off, if your bike is fuel injected, it has a TPS.  Now that you know this, you are one up on me when I first started.  Second, you are going to need some essentials, there is no cutting corners on these.

You Will Need:

1. A service manual for your make and model motorcycle.  (Print or PDF)
2. A good digital multimeter.
3. A Micro-Scale test lead kit.
4. Patience

The motorcycle I’ll be demonstrating the diagnostic  techniques on in this article is a 2012 Victory Cross Country.  As we go along I will explain what I am testing and why.  The techniques can be applied to your make and model by using the test points and specifications from your service manual.

Helpful Tip One: I know many people bulk at getting a service manual, but in the long run, they can be pure gold in terms of information, so I suggest you buy one.  You can go with one from the manufacturer, or purchase a Haynes or Clymer service manual, both are excellent sources of information.

Helpful Tip Two:  None of this is hard to do, but, I would advise you to take your time and be careful, it is possible to damage the ECU connector or the ECU with improper handling.

The test subject and symptoms:

As mentioned earlier, the test subject is a 2012 Victory Cross Country.  At the point it started to show symptoms of a failed TPS it had around 12,000 miles on the clock.  When the symptom first came on the owner thought he had gotten a tank of bad gas at his last fill up.  (Does that sound familiar?)  There was a slight surge at the very first part of the throttle opening, or, as it’s also called, “Throttle tip in.”  The surging was present at any RPM, if you were backing off the throttle at 4,000 RPM in a curve and the throttle was almost fully closed, it would surge.  There was also a drop in fuel mileage from an average 42-45 MPG to 38-40, the second sign of a possible bad TPS.

A failed TPS on a motorcycle with only 12,000 miles on it is unusual, so it wasn’t suspected at first.  Since the warranty was still in effect, it seemed like a good idea to take it a Victory dealership for diagnosis.  At the dealership they found nothing using the Victory “Digital Wrench” software that reads the trouble codes stored in the ECM memory.  The only items the technician found were a drive belt he said needed adjustment, and a battery connection that might have been loose, both billable items, and not covered under warranty.  Fixing these two items had no effect other than the new squeak that was coming from the belt now being overly tight.

Editors Notes: It’s not uncommon for a bad TPS to not show a failure code, unless it’s big one as with my V92 C.  Neither the supposed out of adjustment belt, or a loose battery connection, would cause a surge at only one point in the throttle, or a drop in gas mileage.  You can check and adjust your own belt tension using a tool made by Motion Pro, no need to pay a shop to do it for you.

Time to test and see what’s really going on with the TPS:

Before you start you should have the service manual for your make/model, a good multi meter and a micro lead test kit.  The multimeter should be a full sized one, not one of those pocket deals.  They just do not give the accurate reading you will need.

This is a good multimeter.

This is not.

The micro leads kit is essential because the connections we will be testing are much smaller than the standard probes on your multimeter.

Helpful Tip Three: If you attempt to use the standard probes you could end up damaging your connections, you have been warned!

Micro Scale Test Leads Kit

If you do a search for how to test a TPS you will find many that recommend looking for spikes in voltage coming from the TPS.  That’s fine, if they are big enough to be noticeable, in many cases they’re not.  The better test is to check the resistance in the TPS, you will get a more accurate test.  I will be following the test outlined in my Victory 2013 Cross Roads/Cross Country service manual, and taking resistance readings at two points on the ECM connector.  There should be similar tests in your service manual, it would be a good idea to follow them exactly for an accurate test.

The first step is to disconnect the ECM connector from the ECM.  This is where you take your time, it is possible to damage the connector or the ECM, so be careful.  The ECM can be damaged by static electricity, so make sure you have discharged any buildup by touching a portion of the frame first.  Also, this is a power off test, your key should be in the “off” position.

To disconnect the ECM connector, I first had to free the wiring harness by cutting two zip ties.   You may have to do the same, take care when you cut, you don’t want to nick a wire in the process.  I use a pair of tiny side cut pliers, they work great for clipping zip ties.

Zip ties to be cut.

Once the zip ties are cut, the harness can be moved to make room to remove the connector.  On this model Victory used a locking connector, to unlock it you gently pull up on a sliding latch on the connector.  This dismounts the connector for you rather than having to pull it free.  The advantage is that it comes off straight, and there is less chance of bending the small connector pins inside the ECM, do not touch the ECM pins.  If your motorcycle does not have this type of sliding mechanism, be very careful, alignment is everything, make sure you are coming out straight so you don’t bend your ECM pins.  Even if you have the sliding lock, come out straight until you see a gap between the ECM and the connector.

Sliding lock on ECM.

Locking posts on ECM. (A) Pull tab. (B)

We are almost ready to test, you can easily damage the contacts inside the ECM connector by not using a correctly sized probe, that’s where the micro lead come in.  I selected a size from the kit that would slip easily into the connector, but would not fall out.

Micro Scale Leads

Warning: When you insert the probes into the connector, this is where you are gentle, gentle, gentle.  Do not force them in all the way if you meet resistance, you could possible dislodge the pin receptacle and wire from the connector.  Make sure there is no pressure, or tugging of any kind on the probes, it could permanently distort the pin receptacle inside of the connector.  I support the weight of the multimeter leads by resting them on a foot peg, if that is not available, secure the leads with tape.  Rest your multimeter where you can see it from your throttle position, do not hand hold your meter.

Multimeter leads resting on the foot peg.

Following the Victory manual, my first test was from pin #53 which is the throttle position signal, to #55, the five volt reference voltage.  According to the manual, the resistance should start high with the throttle closed, and go lower as the throttle is opened.  The range is 5000 K. Ohm, to 1000 K. Ohm +/- 1000 K. Ohm.  If you are testing a different make/model, check your manual to find where to take your readings, and what you should expect to see in resistance and range.

Testing from pin #55 (A) to pin #53. (B)

Right off the bat there was indications of the TPS being bad. With the throttle fully closed the reading started off as 3418 K Ohms.  Then, as I slowly opened the throttle, resistance went up to 3487-3500, something it’s not supposed to do, then the reading dropped suddenly to 3422, and from there, smoothly to 1621 K Ohms.  I did the test several times, and the readings varied slightly, but, the resistance spiked and fell at the same throttle point every time.  The resistance it not supposed to rise, then go down, it’s just supposed to go down, smoothly, no gaps, no spikes.

Helpful Tip Three: Roll the throttle on very slowly, if done correctly, you can see the resistance rise or fall in single digits, don’t rush or you could miss a spike.  Roll the throttle off slowly as well, checking resistance all the way.  It is not unusual to see a slightly different resistance reading when it comes to rest again.  What you are looking for is consistency in where the spikes occur.

Starting resistance at throttle fully closed, 3418.

Editor’s Note: Since I was trying to test and shoot these images at the same time, I could not catch the meter at 3500.  Creating content for a magazine is not as easy as you would think.

A reading of 3487, just before dropping to 3422.

At 3422.

At 3421, and heading down in single digits as it should.

At throttle fully open, 1616.

The second test is pin #53 the throttle position signal, to #46, which is the sensor ground.  The resistance should start low, then get higher, 1000 K Ohm to 5000 K Ohm +/- 1000 K Ohm.  And that’s exactly what it did, it went from 1733 K Ohms, to 3422, smooth, no spikes, no gaps.

Testing from pin #46 (C) to pin #53. (D)

Throttle closed, 1733.

At 1734, on the way up, single digits.

3422, throttle full open.

This test was repeated several times as well, but there were no variations in readings.  Even though the test from #53 to #46, went well, the rising spike in resistance found in test one gives the TPS a failing grade.

Bottom line, the TPS is bad, and must be replaced.  You might ask, “Why don’t you just clean the TPS instead of replacing it?”  I’m sure if you found this article in a Google search, you have also found articles or videos about TPS “cleaning.”  I found these myself, including videos of a guy “cleaning” the TPS on his Yamaha sport bike.  It took him three videos to finish, removing all the bodywork, taking the TPS off, then spraying contact cleaner in the TPS shaft opening.  In the end he claimed it was “fixed.”  I thought his bike still had an uneven idle, and sounded no better than in his first video.  To be honest, this so called TPS cleaning is a waste of time, it won’t replace the worn contact surface, which is the problem.  The only real fix for a failed TPS is to replace it, which is what we will cover in second half of this article.

Words By:  Terry Cavender

Images By:  Terry Cavender