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.

radio shack t-rms digital multimeter
This is a good multimeter.
a.w. sperry dm-2a
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!

radio shack micro scale leads kit
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.

victory cross roads/cross country wiring harness zip tie locations
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.

victory cross roads/cross country wiring harness ecm connector lock location
Sliding lock on ECM.
victory cross country ecm locking connector detail
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.

radio shack micro scale leads
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 during tps testing
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.

victory tps testing from pin #55, to pin #53.
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.

victory tps testing from pin #55, to pin #53, resistance 3418 k ohms
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.

victory tps testing from pin #55, to pin #53, resistance 3487 k ohms
A reading of 3487, just before dropping to 3422.
victory tps testing from pin #55, to pin #53, resistance 3422 k ohms
At 3422.
victory tps testing from pin #55, to pin #53, resistance 3421 k ohms
At 3421, and heading down in single digits as it should.
victory tps testing from pin #55, to pin #53, resistance 1616 k ohms
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.

victory tps testing from pin #46, to pin #53.
Testing from pin #46 (C) to pin #53. (D)
victory tps testing from pin #53, to pin #46, resistance 1733 k ohms
Throttle closed, 1733.
victory tps testing from pin #53, to pin #46, resistance 1734 k ohms
At 1734, on the way up, single digits.
victory tps testing from pin #53, to pin #46, resistance 3422 k ohms
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

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10 thoughts on “How To: Diagnosing A Bad T P S

  1. Hi guys, I just read your first article on diagnosis of a failing Throttle Positioning Sensor and thought it was great.

    I have 2 bikes, both Harley Davidson’s and both have the Evo motor. My Bagger is a 1996 Ultra Glide Classic which is fuel injected while my 1998 Softail Custom on a S&S carb. I have experienced some issues with the EFI on my Bagger….like no settling into an idle when stopping. Recently the RPM stayed at 4000 RPM when I came to stop. I reset the ECU and haven’t had an issue since (last occurrence was a month ago). Hopefully this may have cleared the issue, if not I may need to replace the TPS and will do so.
    Keep up the good work and I look forward to your 2nd article re the TPS.

    Best,

    Don

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    1. I am glad you like the article Don. If you were skilled enough to reset your ECU, you should have no trouble following the outlined steps in your Harley Davidson service manual to test your TPS. However, the symptom you describe does not sound like the TPS, there may be something else going on, you might want to perform other tests. I wish I could help you more, but the last Harley I owned was an Iron head Sportster, so I’m not up on the latest Harley tech.

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      1. Hi Terry,
        Thanks for getting back to me. I will explore some other testing for sure. Keep those good articles coming my way.

        Best Regards,

        Don

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      2. Thank you Don, I’m currently working on the second half, replacing a TPS, so make sure you are subscribed to the site, that way you’ll know when I publish.

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  2. I saw your article on V92 throttle, I have my service manual and can’t seem to find pages on TPS replacement, is it under the throttle body?

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    1. Hello Richard,

      The Victory service manual does not cover TPS replacement, just as they don’t cover throttle body cleaning. At least they did us the favor of fully documenting the TPS test procedures! LOL!

      Your Victory Sport Cruiser uses an earlier version of the engine in my 2003 V92 but, I am sure they are similar. If it is anything like my V92 you should be able to find your TPS on the right side of the throttle body, at the end of the intake butterfly shaft. If it is, your replacement TPS can be found at your local auto parts supply, it is used in many American cars, when I bought mine it cost a little over $30 USD. The TPS on my V92 was round with two allen head screws securing it to the throttle body. I had to use a “ball head” allen wrench to remove the screws, if I remember right, it was a 5mm wrench. The tank must come off, and I had to loosen the air box and move it out of the way a little. When time came to put the screws back in I had trouble keeping them on the end of the wrench to get them started, so I used a magnet stuck on the allen so it was magnetized, that way the screw stayed in place. Even then, it was a bit tricky, I would get close, bump the screw, it would fall off, I’d put it back on, bump it again, and so on. In the end I got it. Working on a motorcycle can test your patience, but if you take your time and be methodical, you can get the job done.

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  3. Hi TERRY, great post on covering TPS testing technics. I have a HD Softail fatboy ’03 EFI all stock, that suspected have e bad TPS, only because i’ve tried replacing CKP, engine temp sensor, i borrowed from a friend didn’t fix it and checked all the wirring and connectors seemed to be ok, no DTC code either which makes it hard to trace..

    it would sputter, backfire, spit thru throtle body just off idle, but once you let go the throttle it will always idle strong.. it only happens when the engine gets hot and then parked for 20mins or so.. other than that you can not reproduce the problem.. you can ride it hard until it gets really hot still it runs strong, stop a bit to get gas, still run strong.. you have to let it sit or park it for 20mins-30mins then it would happen, once it happen it will keep sputtering no power at all untill you let it cool all the way down say over night.. do u have a say about this strange phenomena.. i figure, it would not be sensors or ecu or wiring or coils, ’cause it’s like you have to let the heat to soaked in to whatever that’s causing the problem.. i’ve been chasing this ghost for almost a year.. thanks if you can give a positive input, be much appreciated..

    Hendrik

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    1. Hello Hendrik, to be honest I don’t have much background with late-model Harley-Davidson’s, now if you want to ask me how to set the timing on a 1978 shovel head, I’m your guy. This sounds like a complex issue that there is no way for me to diagnose from where I am sitting. You’re going to have to do what I did, figure out a way to test the TPS, and confirm that either is, or is not functioning correctly. That said, I’m not quite so sure it the TPS, have you done a complete inspection of the fuel injection system, including checking for pinched hoses and proper gas tank ventilation? Sometimes when people are dealing with fuel injection they never stop to consider that might be just something as simple as a pinched hose, or vacuum leak, they always suspect a sensor, or other electrical component. Good luck, I hope you get to the bottom of this issue while you still have some good riding weather left.

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      1. Thank you Terry for the quick reply. I have tested my TPS as per your instruction and i found something strange with HD pfc4 tps. I tested with ohm meter the signal pin in reference to +5v, at closed throttle reads 1.780k and at WOT is 1.253k, but as i sweep through the entire range slowly, it would go up steadily at the first 30% of throttle opening up to 1.895 then go down steadily to 1.253. i was thinking, this is exactly what TERRY was talking about a bad TPS. But i can recall someone say that you can not test a TPS usimg ohm meter, you have to test it with volt meter.
        Then just to make sure, i apply +5v and ground with an adaptor straight to the cps unplugged, and test for voltage at signal pin, I got 0.5v at closed and 4.5v at WOT and as i sweep very slowly thru the entire range, it has a very good linear, no spike, just smoothly rising up 0.5v to 4.5v . I guess on some tps we can not test by using ohm meter.

        My question is, do i need to change my TPS or do i have a good TPS?

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      2. Thanks for the kind words, but, I have a question. How does your Harley manual say to test the TPS? And, what is the range of readings that are acceptable? THIS is the only acceptable way to test. Yes, in my case I did have to do a workaround because I did not have the Polaris software, but I followed the testing procedure exactly, except where I ignored the typo giving the same reading for both opening AND closing of the throttle. So, read your manual if you have one and test again, or buy/borrow a manual and test. Your other option, and I don’t know how much it costs, buy a new TPS and put it on, but that would not be as satisfying as figuring it out would it?

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