How to fix a malfunctioning Throttle-Position Sensor.

[Nick V]

Should anyone be having, or have had, throttle-position sensor (TPS)-related issues on a hex-head or cam-head engine, I have some suggestions that may solve the problem.

The 'issues' I'm talking about include:
- Erratic throttle response (the engine may 'snatch' or 'jerk' at low revs).
- Misfires at low revs and under acceleration.
- The engine dies at idle, and can only be kept idling by using the throttle.
- Grey exhaust smoke (over-rich mixture) when starting from cold.
- Higher-than-normal fuel consumption which starts quite suddenly.

The TPS is basically a potentiometer (a type of rheostat, or variable resistor) which sends an electrical signal of varying current through to the engine ECU, depending on the position of the throttle.

To send the throttle-position signal, the TPS uses very fine steel-wire brushes which move on carbon 'tracks'. The greater the effective length of the track the brush is on, the more the resistance. Straightforward enough - but should the internals of the TPS be contaminated with dirt or water, it is likely that the resistance value being read by the ECU will be outside the parameters the ECU is programmed to accept. In this case, the ECU will become confused and the above problems will start to manifest.

An annoying characteristic of this issue is that the yellow triangle (Master caution light) on the digital display will not illuminate, and no ECU fault codes will be registered.

Below is the method to check for a faulty TPS.

Tools and consumables needed:
- Your bike's ignition key.
- Suitable tool(s) to remove and replace the battery terminal(s) - typically either a screwdriver or hex wrench.
- 1x T20 Torx wrench.
- 1x appropriate-sized rubber O-ring (if needed).
- Unopened bottle of methylated spirits.
- Clean, lint-free cloth.
- Rubber car floor-mat.
- 1x 5.5mm round dowel (in an emergency, an old 5mm metric bolt or screw can be used).
- Magnifying glass.
- Cotton-wool buds.
- High-quality metal polish (if needed).

If the bike exhibits most or all of the symptoms listed above:

1. Disconnect the battery negative terminal.

2. Disconnect the three-pin electrical connector from the TPS.

3. Un-fasten the two T20 screws holding the TPS to the left-hand throttle body, and remove the sensor from the throttle body.

4. CHECK POINT 1: Examine the rear side of the TPS. There should be a rubber O-ring nested in the cavity between the TPS and the aluminium throttle body.
If the O-ring was absent or is damaged, it is more likely than not that the symptoms are due to water contamination of the TPS.
Clean the O-ring well with methylated spirits and a clean, lint-free cloth.
Examine the O-ring carefully. If it is perished, cracked or damaged, discard it and replace it with a new one.

5. Prepare a clean, dry workspace approximately one metre square. Place a pliable but firm buffer (such as a clean rubber car floor mat) on the work surface.

6. Place the TPS on the workspace with it's outer side face-up.

7. Holding down the TPS with one hand, use a 5.5mm round dowel (in an emergency, an old 5mm metric bolt or screw can be used) to carefully press out the two steel fastening pins by pushing the pins in the direction of the work surface.

8. Holding the TPS face-up over a clean, dry container, carefully separate the two halves of the TPS.
The brush spindle - a white plastic part with two small steel brushes - will be exposed.

CAUTION: The brush spindle is exceptionally delicate and should under no circumstances be damaged.

9. Carefully remove the brush spindle from the TPS body.

10. CHECK POINT 2: Carefully examine the ends of the brush spindle's steel brushes. All brush points should terminate in a uniformly straight line without deviation or kinking, and should be free of discolouration and corrosion.
If either or both of the brush points are damaged or kinked, the TPS may have suffered impact or have been incompetently repaired at a previous stage.
If the brush points are discoloured or corroded, it is likely that they have been exposed to water and/or a foreign substance.

11. Place the brush spindle where it cannot be damaged or mislaid.

12. CHECK POINT 3: Examine the space between the outer TPS cover and the TPS body. There should be a labyrinthe rubber seal nested in the cavity between the TPS cover and TPS body.
Remove this seal, clean it well with methylated spirits and examine it carefully. If the seal is perished, cracked or damaged, it is not available as a separate part from BMW. In this case, two options exist: scavenge a serviceable labyrinthe seal from a non-functional TPS, or replace the entire TPS assembly with a new one.

13. Examine the interior of the TPS, paying special attention to the condition of the carbon tracks for damage and contamination by foreign matter and water. If the bike started exhibiting symptoms after being washed and/or ridden in the rain, you may well see water droplets on or near the carbon tracks.

NOTE: The following step should be performed only using a freshly-opened bottle of methylated spirits. Methylated spirits contain alcohol which absorbs airborne moisture, possibly resulting in condensation forming on the carbon tracks and/or being trapped by the steel brushes.

14. Open a fresh bottle of methylated spirits. Soak a clean, lint-free cloth in spirits and use this cloth to carefully clean the TPS interior, paying special attention to the carbon tracks.

15. Allow all alcohol and other fluids to evaporate completely.

16. Using a magnifying glass, carefully examine the carbon tracks. Paths along which steel brushes have travelled should be obvious, but if the carbon tracks are scored, worn though or have breaks or cracks in them, they are unserviceable and the TPS will have to be replaced or rebuilt using a new main body.

17. Gently touch a clean, dry cotton-wool bud to the tip of each steel brush. Apply minimal pressure to wick any moisture from the brush threads.

18. Using a magnifying glass, carefully examine the brushes for any corrosion or discolouration.
If there is corrosion and/or discolouration, it may be possible to remove it by dipping the brush ends in a high-quality metal polish, cleaning well with methylated spirits and drying by gently touching a clean, dry cotton-wool bud to the tip of each steel brush.

CAUTION: Do not apply water-repellent lubricant, silicone grease or any other substance to the inside of the TPS. Any foreign substance will change the resistance values of the potentiometer and cause the TPS to send 'corrupt' signals to the engine ECU.
Do not apply petroleum jelly or similar substances to the rubber seal and/or O-ring. Petroleum-based substances will destroy any rubber-containing parts.
Do not apply silicone sealants containing acids to the rubber seal and/or O-ring. The acid is released as the sealant cures, and will corrode the brushes and/or carbon tracks.

19. Replace the labyrinthe rubber seal in it's correct orientation between the TPS cover and TPS body.

20. Holding the TPS face-up over a clean, dry container, carefully replace the brush spindle into the main TPS body. The steel brushes should contact the carbon tracks, and the detent (see diagram) should face toward the sensor's terminals.

21. Carefully replace the upper half of the TPS over the lower half.

22. Turn the TPS so that the brush spindle interface (the white plastic part) is visible. Use a suitable tool to carefully rotate the brush spindle to full lock in both directions; the spindle should turn freely and easily. If not, do not force it - stop and investigate the problem.

23. Holding the two halves of the TPS together, press the two steel fastening pins into the body of the TPS from the rear side. The pins should 'snap' into place with only moderate pressure from a thumb.

24. Place the rubber O-ring into the cavity around the TPS's brush spindle interface. Ensure it does not fall out as the following step is performed.

25. Carefully replace the TPS on the left-hand throttle body with the electrical terminals facing down, ensuring that the TPS spindle protruding from the throttle body mates correctly with the TPS brush spindle interface.
When you are sure that the two parts are in perfect alignment, push firmly on the TPS to seat it against the throttle body.

26. Ensure that the two T20 screws holding the TPS to the left-hand throttle body are equipped with appropriate washers. Fasten the T20 screws to the correct torque value.

27. Re-connect the three-pin electrical connector to the TPS.

28. Re-connect the battery negative terminal.

29. Turn on the ignition switch and kill switch. Do not start the engine at this time. Turn the throttle to fully open, then release so that it comes back to idle position. Repeat this step once or twice, then turn off the ignition switch and remove the ignition key.

30. Leave the bike to stand for five minutes.

31. Turn on the ignition and start the bike.
If you have performed all the above steps correctly, the bike should start, idle and run normally, with no symptoms.

32. Be sure to re-set the on-board clock to the correct time.

 

[Greenman14]

And you don't get all that from the Haynes manual. A post to file away, just in case. Nice one Nick.

 

[Bendy toy]

@Nick V should get a free membership for this. Very helpful thanks .

 

[harry]

A very useful post, thanks, Nick

 

[Nick V]

Thank you all kindly for the compliments, gentlemen!
If I can help at least a handful of folks get around this annoying problem, I will know the effort was worthwhile!

And you don't get all that from the Haynes manual.

Many's been the time I've wanted to scream in frustration at the elementary level of what's told us in Haynes, Clymer and similar manuals. Have you noticed how often they seem to be communicating a message of "Now, now... this is way too complicated for a spanner like you to understand, and even if we thought you could, we couldn't be bothered to explain anyway."

A good example is bleeding of BMW ABS-equipped brake systems.
I didn't spend nearly R500 on a manual to be told: "This task should only be performed by an authorized BMW dealership".
As long as you've taken a bike to pieces to write the manual, why not apply some basic engineering skill, analyze the problem at hand, and find a way to safely improvise this (vital) task - and others like it?

And you're right... what you won't find in the manuals - aftermarket or otherwise - is a list of known product issues, along with possible countermeasures to be applied by the owner, and what's been done by the manufacturer to solve the issues permanently.

At least the IT field has this one right. Product-development requires an approach where problems are identified by name and eliminated, one by one.
Their existence should not be denied, and the customer should not simply be told "They all do that". (Can you imagine how total honesty on the sales droid's part might work? "...All their fuel-pump controllers fail, all their throttle-position sensors are vulnerable to water, all their fuel gauges are hopelessly inaccurate. Yes, Sir... they all do that.")

...As an aside, I composed a similar 'how-to' a couple of years ago, for installing a separate fuel filter on K-series bikes.
With tweaking, exactly the same procedure could be applied to any other BMW where the standard filter is integral with the pump. I'll try and post it soon - it's just got lots of piccies.

 

[Bendy toy]

I used to be a technical author whose job it was to decipher the convoluted and often incorrect stuff written by design engineers. I lost track of how often my text would be pronounced as wrong until I went through the language mangled tech specs to show that I had described EXACTLY what was originally written.

Only then would the tech team actually look at a problem they had created by not properly documenting their own work. All this was time consuming and often unpopular. Its amazing how personally attached some people are to what they write. Its even more amazing the management do not control the documentation standards properly. How can anyone design a widget when the information about widgets it works with and alongside are not documented properly?

So back to Haynes, et al - They avoid the complicated stuff either because the authors can't be bothered or because the company doesn't want to upset the big manufacturer's tech teams. I strongly suspect it's author's laziness and rubbish sub-editing, but Haynes is now a large outfit so who knows. It is interesting, the older versions (for such as a Pre Unit Triumph 500 or BL Mini) did include some handy workarounds. However, those bikes/cars were so much more simple to work with - and Haynes was smaller.

 

[Nick V]

I used to be a technical author whose job it was to decipher the convoluted and often incorrect stuff written by design engineers. I lost track of how often my text would be pronounced as wrong until I went through the language mangled tech specs to show that I had described EXACTLY what was originally written.

Ah, you know where I'm coming from...
I'm in almost exactly the same situation. Back in the old days of South Africa's Delta Motor Corporation, I started my working career as an automotive technician for Opel (Vauxhall), Isuzu and Suzuki. Later, I became a technical expert for Toyota. Still later, I moved to technical writing for BAE Systems, then for a couple of independent companies. I'm still a tech scribe, but now for a large software company.

People are quick to pronounce text 'wrong' until they realise that they didn't read it; they scanned it.

Only then would the tech team actually look at a problem they had created by not properly documenting their own work. All this was time consuming and often unpopular.

Oh, it still is...

Its amazing how personally attached some people are to what they write. Its even more amazing the management do not control the documentation standards properly. How can anyone design a widget when the information about widgets it works with and alongside are not documented properly?

There is a factor that we should be told about in high school, but aren't: internal company politics. Tech pubs departments are usually treated like lepers, because we are a 'retardant force'; it's something we have to accept, and fight our own corner if we want to get on with the job.
It's a consolation to know that without the documentation we create, most if not all developers would be even more lost than they currently are.

So back to Haynes, et al - They avoid the complicated stuff either because the authors can't be bothered or because the company doesn't want to upset the big manufacturer's tech teams. I strongly suspect it's author's laziness and rubbish sub-editing, but Haynes is now a large outfit so who knows. It is interesting, the older versions (for such as a Pre Unit Triumph 500 or BL Mini) did include some handy workarounds. However, those bikes/cars were so much more simple to work with - and Haynes was smaller.

I suppose so... corporations take their image extremely seriously. And in BMW's case, they have the 'German Reliability' paradigm to uphold - and nothing must be allowed to stand in the way of this, because it would mean lost sales (even if a basically good bike is partly spoiled by some frankly incompetent engineering due to what seems like rushed R&D...)

It's basically inability to admit mistakes. Or lack of effective communication within a company employing thousands. Or maybe simple marketing arrogance which leads to "who cares if bad TPSs strand two hundred customers? The other two hundred thousand aren't complaining..."

I can see how a company like Haynes lives in terror of the companies whose products it 'reports' on.
All BMW has to do is say: "Expose a flaw on our bikes, and the next time you release a manual on one of our bikes, we'll sink you for copyright infringement".

 

[sola_bristol]

Loss of throttle control

I'm wondering if I have a problem with the TPS. I have a 2008 Mk2 1200 GS. The bike has been functioning perfectly normally but I am about to sell it and spent yesterday cleaning it up and removing the "extras" for separate sale, transfer to new bike etc. Many of these bits were electrical involving disconnection and reconnection of the battery and I did remove the TPS cover I had bought to protect this rather vulnerable part. I didn't disconnect the TPS however. Having put the bike back together again I turned the throttle 3x (as I've been told to do to reset the throttle) and though the bike starts easily enough, it goes straight to 5000 revs which is exactly what happened to a friend of mine when he broke the TPS in a drop....................so I'm thinking it's likely connected but how will it have happened?

 

[DrFarkoff]

I take it you have had the tank off ??

You probably have caught the throttle cable out of its holder

Similar idea to the 1150 but its the one at the cable splitter that gets pulled out Opening the throttle

Happened to me and I fecked around wondering what had happened

Then I spotted the front section resting on the cable splitter at the left side just in front of the airbox

Hope that helps

I'm wondering if I have a problem with the TPS. I have a 2008 Mk2 1200 GS. The bike has been functioning perfectly normally but I am about to sell it and spent yesterday cleaning it up and removing the "extras" for separate sale, transfer to new bike etc. Many of these bits were electrical involving disconnection and reconnection of the battery and I did remove the TPS cover I had bought to protect this rather vulnerable part. I didn't disconnect the TPS however. Having put the bike back together again I turned the throttle 3x (as I've been told to do to reset the throttle) and though the bike starts easily enough, it goes straight to 5000 revs which is exactly what happened to a friend of mine when he broke the TPS in a drop....................so I'm thinking it's likely connected but how will it have happened?

 

[sola_bristol]

Thank you Doctor. I haven't taken the tank off but have been ferreting around the airbox retrieving wires from a set of auxiliary lights so it's possible and I'll check

 

[sola_bristol]

You are a gem Dr Farkoff. Took me about 20 seconds to fix and saved me a very embarrassed visit to my local dealer. Thank you!

 

[Nick V]

I've done that myself once or twice.

It also helps to check whether the same thing hasn't happened where the throttle cable meets it's adjuster on the way into the twistgrip pulley.