More TPS nonsense

[timflys]

Rather than piggyback on BP@9r's thread on tps woes, I have a simple question. How do you tell if your TPS is bad? Short version is that my tps is very very difficult to adjust and get a constant voltage. I can just place my hand on it with a miniscule amount of pressure while the screws are locked down and watch the voltage jump around. Earlier today I used Roger's method of setting tps to 250mv and then using throttle stop screw to achieve .350mv. Fired the bike up and it ran, though very rough. Attempted to use right throttle stop screw to balance it out a little. Still ran rough. Shut down and took a long lunch break. When I came back, just for a kicks I turned the key on and voltage was 580mv without touching anything. Decided to fire it up and it ran much better but at a high idle for a cold bike (1100rpm) Fast idle was not on and there was slack in that cable. I left the dvm on and as the bike warmed up, the voltage from the tps started dropping and eventually settled around 360-380mv. That is without any adjustments or input on my part. As the voltage dropped the bike seemed to run worse with more pops and sputters. I shut it down and came here seeking advice. The most annoying thing is this morning when I was trying to initially set the tps, it would take almost no movement of my hand and the voltage would jump from 160mv to 400+mv.
I have done the old Zero=Zero procedure on 93 rs I used to own with no issues.
BTW, the bike in question is a 96 R1100RSL w/ approx 51k miles. Yellow cat code plug.
Anyone want to take a guess?

 

[dieselyoda]

Take the TPS off, make sure that the mounting surfaces are clean. Then turn the centre of the tps by hand and make sure it's smooth all through.

 

[timflys]

Thanks DY. I'll try that tomorrow. I didn't think tps' went bad very often

 

[AntonLargiader]

I actually had a bike come through the shop within the last year with an internally broken and jammed TPS. Very strange failure.

 

[pauls1150]

While it’s certainly possible the TPS has failed, it is not a common occurrence. There are two styles of TPS, early and late. One uses a pair of carbon strips for the resistive elements, the other uses wirewound elements. In either case, the supporting substrate can crack, the resistive portions can crack (becoming open or intermittent), there may be dirt (carbon) buildup on the wiper, or the wiper may lose its tension against the resistors. As DieselYoda said, the travel should feel smooth, no bumps or snags.
There are also eight solder connections inside (two per wire) that may become intermittent if the factory assembly was not perfect, either a cold (improperly flowed) solder joint or over-heated and then partially broken.
Your case does sound like a bad TPS. These are sealed assembles and not meant to be repaired; if you're willing to potentially sacrifice a good one, see if you can pop off the outer cover. If it's fixable, JB Weld should make a good seal at reassembly.

In any case, before dropping the bucks for a new one, verify that:
1 - Your DVM has good batteries.
2 - You really are making good solid contact with Pin 1 (the white wire with the red runner) and Ground. For your ground, use the brown wire immediately adjacent to the throttle body – that’s what the computer will see, so this connect point gives your DVM the same measurement.
3 - Make sure that the connector on the bottom of the body is in good shape. Squeeze the clip and gently pull the plastic down, and closely inspect the pins inside, and the mating pins on the TPS, for straightness or dirt buildup. Put the connector back on and see if the DVM indication changes if you wiggle the connector a little bit.

Sometimes the connector may have been accidentally kicked and damaged that way.

 

[timflys]

Thanks for all the suggestions. Here's what I did today. Removed the tps from the tb, checked for smoothness through the range of operation. Verified that the butterly itself is not hanging up. The tps mating surface looked clean, but I wiped it down anyway. Don't know if there is supposed to be any resistance when the tps is moved, but felt no stutters, hangups, etc. In fact it seemed to have no resistance when I was turning it, but it is just a potentiometer. Verified the male and female sides of the connector were clean and mated properly. Checked the pins I was using for connections, pin #1 and pin#4. Did the old straight pin through the blue seal on the connector. To doublecheck, I moved the ground lead on the dvm from the straight pin to an engine bolt that I knew was a good ground and there was no change. Triple checked the dvm by connecting to the +lead on the battery and a frame ground and got what I expected, 12.8v. Before I did all the trouble shooting today, I connected the tps to the dvm and got a 667 mv reading which is a different reading than I got when I ended yesterday. And the dvm still showed the tps voltage slowly decreasing the more it sat there. Attempted to fine tune the tps again to 250mv and could never get close. When I would tighten the set screws the voltage would drop 100-200 mv and then just kind of jump around to from maybe 160 mv up to 460mv. This has got me scratching my head. Should I just bite the bullet and order a new one?https://forums.bmwmoa.org/images/smilies/banghead.gif

Thanks again for all the advice

Tim

 

[dieselyoda]

Something just occurred to me. Most really good DVOM's have very, very low(or is it high? Can't remember, regardless) impedance, meaning, they measure with very little current. Some of the one, like the ones I buy at Canadian Tire for $20, have very high (or is it low??? sucks to get older) and they pass a fair bit of current through the circuit you are testing.

Even though the Alzheimers seems to have kicked in today, (or is it I can't brain today because I have the dumb?), it's possible your meter is giving you fits. An old fashioned Analog meter, with a needle is what I use.

Just a thought as I can't recall the last time I changed a TPS on anything. They are pretty robust but, I have changed some over the years.

 

[nrpetersen]

DVOMs are high input impedance devices - think of it as sorta like high resistances with other complicating factors............

I've had industrial experience with conductive film pots & wouldn't be very surprised to hear one drift out of initial spec - especially with the linearity & wiper output impedance. In our application, all we really need is repeatability, but if the wiper is output is intermittent the processor optimization routine isn't gonna work very well.

 

[roger 04 rt]

Thanks for all the suggestions. Here's what I did today. Removed the tps from the tb, checked for smoothness through the range of operation. Verified that the butterly itself is not hanging up. The tps mating surface looked clean, but I wiped it down anyway. Don't know if there is supposed to be any resistance when the tps is moved, but felt no stutters, hangups, etc. In fact it seemed to have no resistance when I was turning it, but it is just a potentiometer. Verified the male and female sides of the connector were clean and mated properly. Checked the pins I was using for connections, pin #1 and pin#4. Did the old straight pin through the blue seal on the connector. To doublecheck, I moved the ground lead on the dvm from the straight pin to an engine bolt that I knew was a good ground and there was no change. Triple checked the dvm by connecting to the +lead on the battery and a frame ground and got what I expected, 12.8v. Before I did all the trouble shooting today, I connected the tps to the dvm and got a 667 mv reading which is a different reading than I got when I ended yesterday. And the dvm still showed the tps voltage slowly decreasing the more it sat there. Attempted to fine tune the tps again to 250mv and could never get close. When I would tighten the set screws the voltage would drop 100-200 mv and then just kind of jump around to from maybe 160 mv up to 460mv. This has got me scratching my head. Should I just bite the bullet and order a new one?https://forums.bmwmoa.org/images/smilies/banghead.gif

Thanks again for all the advice

Tim

Tim,
Inside the TPS package there are a pair of potentiometers. The fine pot is used for the first 20 degrees of rotation and is naturally very sensitive to movement--something like each 0.3 degrees of travel is 90 mV. Usually though, once set they stay put to within about 10 mV.

From your description you either have a failing pot (it does happen once in a while) or your TB shaft has a lot of play although the amount of voltage variation you're seeing doesn't make that explanation seem very likely.
RB