Throttle Position Sensor

[Paul Bowers]

All,

I am posting again in hopes that someone has had the same problem and has just not posted to this forum.

Since 2010 I have had a problem with my coach losing power and going into the limp mode.  You can read my original post here:  http://forum.bacrallies.com/m-1352178479/s-0/#num0

I have spent a considerable amount of money on troubleshooting the problem with local area mechanics with no solution being found.  Since I do not trust the coach, I don't drive it except to try and troubleshoot the problem.  I took the coach to have it serviced and the mechanic found himself having to limp it back to the shop.  A couple of hours of troubleshooting and no results.  You see, when it does this, you can cut the engine off...wait a few seconds and crank the engine and it will run normally.  This process also erases the fault...until the next time.  The mechanic had to limp it home because he was not told about turing the key off to reset.

It appears to me the problem occurs when you let completely off the accelerator for a few seconds and then begin to re-apply power. I have checked the connections to the accelerator pedal (TPS) and voltage and all appear to be ok.  Since it is intermittent, all the mechanics say they can't do anything until it totally breaks.

At present, it is costing a lot of money to have the MH just sit and not be able to use it.  Is it time to start throwing parts at the problem in hopes something will fix it?  Thinking of starting with a new accelerator pedal with TPS and see what happens.

I would love to sell/trade the coach, but would feel guilty in pushing this problem onto someone else.

I have spoken with CAT Hotline and local CAT dealer with no help.  In fact, the Throttle Sensor CLBRT code that shows up is not even in their book.  

Any and all suggestions welcome.

Thanks in advance.

[Joel Ashley]

I'm very surpirsed you didn't get some opinion from Brett Wolfe.  This problem should be right up the old CAT Club's alley, and should have been resolved by now.  My coach spends a lot of down time too, but for financial and logistical reasons, not engine problems.  I would think that if it were at Peterson CAT in the Willamette Valley or Redmond, or even at BCS in Bend, someone would have pulled out the stops to ferret out the cause and fix it.  But getting it there from Alabama would no doubt be no small feat, given the distance and the issue.  It's very puzzling that your sensor exhibits a code that not even CAT is familiar with.

My highly limited input here is to proffer that the throttle position sensor needs recalibrating, but you probably already have been there done that.  Whatever's haywire with the TPS system, I wouldn't think it so difficult for a knowledgeable CAT tech to run down;  the fault is obviously sending a signal to the engine ECM to revert to low idle.  I don't blame you for being tempted to throw a new TPS at it, but would hope someone here or at the CAT forum would offer alternatives - a new TPS may not fix it, and more $ down the tube, but then again it may be just the ticket.

Joel

[Edward Buker]

Paul,

There is information in this link about how the TPS works. It uses pulse width modulation to define the throttle position. Pulse width modulation is a means by which you can get an analog result out of a digital signal. The unit self calibrates and there is no adjustment. So you have the sensor, the coach wire in between the sensor and the ECM. The two easiest candidates to deal with are the TPS unit and the ECM. I would have them reflash the ECM with the latest program level and see if that helps. If you have done that then changing the TPS pedal unit would be next in my mind. The fact that it is an electronic unit that generates a pulse width modulated signal indicates that there is circuitry involved at the TPS and circuits can drift, heat up and change electrical characteristics, go into oscillation. The question is do I have good stable components in the TPS and there is no way of knowing without changing it out.

http://ww2.justanswer.com/uploads/ClaytonH729/2010-09-06_133816_3406timingsensorinstall.pdf

On the subject of PWM (Pulse Width Modulation) here is a little explanation. The graphs are very helpful in visualizing what is supposed to be happening at your TPS and for some reason is not. For the ECM to go into limp mode it must be getting a signal that it cannot interpret or it recalibrates on the fly as you drive and something signal wise from your unit goes out of range for calibration. I think the ECM could be at fault but that is less likely given no other type fails have been noted. Paul, I am no expert on  these units and I hope this info will be of some help. I certainly see no down side to changing out the TPS given you are at a point where you cannot trust the coach and drive it.

http://digital.ni.com/public.nsf/allkb/294E67623752656686256DB800508989

Other possibilities....The voltage at the sensor I understand should be 8V on the supply, 0 on ground, and the PWM wire would need a meter that could read duty cycle which is not common. I read one post where a wire had rubbed and shorted to ground at the pedal area so I am guessing you have already been there.

There is some chance that some electrical interference is generated within the coach that gets on the signal line but the reason for using PWM is that it is quite robust from an electrical noise type of immunity. The extra signal would have to affect the pulse width and have significant amplitude to interfere and that does not seem likely. I know this is a continuation of an older post and hopefully there is some new and useful information here. Good luck and keep us posted.

Later Ed

[Paul Bowers]

Joel and Ed...sorry for the late reply, but was hoping to have some news for you, but so far, nothing.

Thanks for taking the time to respond to my problem.  I think it makes good sense to have CAT re-flash the unit and see if there is any change.  If not, then replace the TPS, assuming no wiring is damaged and the voltage is correct coming to the pedal. I can have CAT check that out when it is in the shop for the re-flash.

I will keep you informed on what I find out.

Thanks again...