Can a 4-Valve K be made to run without the TPS?

[Sticklebrick]

Hi Guys

Has anyone ever tried to permanently disconnect (or otherwise adjust) the TPS on a K1100 (or other 4V K)?

My understanding of the TPS on the 4V bikes is that it's a potentiometer connected to the throttle butterfly spindle, that tells the Motronic ECU where the throttle is at any given point in time within its range, so that Motronic can vary the its settings across a range of calculations (unlike the 2V bikes, where it is a stepped position/switch arrangement).

I ask this because today I went for a ride on my K1100 LT (1992) with the TPS unplugged, and it was night-and-day a better bike to ride by a looong way.  Smoother, easier to ride, altogether a different machine. 

After a few blissful miles of smooth riding, I came to a set of traffic lights. Unfortunately, it "bogged down" when moving off from the lights at the just off-idle throttle opening and wouldn't accelerate in 2nd gear (it did this twice).  So I stopped and plugged the TPS in again.

But it got me thinking ...

Can a 4V K be made to run without the TPS?

The K75/K100 TPS can be adjusted out of range or even a switch fitted, but can anything be done on the K1100?  Is there any way to bypass/hot-wire the TPS/plug/etc so the bike doesn't lean the mixture so much at the lower revs? 

First idea ... as it was so good in all other parts of the rev range, could I just remove the TPS altogether and adjust the CO2 mixture pot/screw (the one above the coils) to a richer setting (ccw) in order to achieve the effect of stopping the lean condition at lower revs?

From what I can find here and elsewhere, the bottom line seems to be that the K1100 (and all 4V K bikes) can't run with a disconnected TPS - or at least no one has found a way of doing this (?).  If there was some way to make it do so, this bike would be as good as it could be.

Many thanks

Steve

PS:  MY BIKE is a UK-model (no exhaust sensor) K1100 LT (1989-1999), built March 1992 (K589, VIN 6457400).

[Sticklebrick]

I also found this, a write-up on the TPS and the relevant connections/pins on the Motronic Unit.


https://www.oocities.org/fwarner_au/mc_things/K_fuel_injection.html


Is is possible to "cheat" the Motronic at the lower revs by introducing a resistor (a wire connected to either the pins of the TPS connector or to the main Motronic Multi-Connector at the ECU under the seat?

[The Mighty Gryphon]

I am probably wrong, but I think the Motronic has a "limp home" mode that in theory is supposed to take over if a failure is detected in the system.  Possibly, disconnecting the TPS puts the ECU in this mode, and that is what you were riding around on.  I would think the ECU goes to some sort of setting that is appropriate for 1/4 throttle and 3-4k rpm so you can sort of ride the bike home or to a shop where it can be serviced. 

Just curious, have you set the TPS at the point where the throttle is just coming off closed?  There is a self test built in that is initiated at the diagnostic plug and makes it possible to set the TPS.  Since it sets the base point where the injection curve starts, it's important for having the proper mixture through the rpm and throttle opening range.

[Sticklebrick]

Yes I used the Diagnostic Plug and warning light to set the TPS, I have set it so the light just comes on when the throttle is closed (ie, I don't touch the throttle and rotate the TPS until the light just comes on and set it there).

Should I be setting the throttle just off idle and then moving the TPS until the light illuminates?

The Limp Home mode can also be activated by leaning out the CO2 setting (by turning the CO2 screw too far clockwise) can't it? 

[Sticklebrick]

FIXED!

As The Mighty Gryphon advised, I measured the voltage from Pin 1 and Pin 3 of the TPS and set the TPS so it read 0375 V and the result is a big improvement, the bike runs very smoothly, much better than using the"diagnostic plug" method to set the TPS.

In fact, as a comparison, I measured the TPS voltage where it was set by the plug/light method, and the diagnostic plug setting when the light came on was reading 5V, which is enough out of range to made a big difference (ie make the bike run badly). It pays to put thin wires into the TPS socket, plug the socket in, and test using a multi-meter from those wires - check the procedure on the forum if you haven't done it before. 

Worth noting for those who have a 4-Valve K.

many thanks!

Steve

[TommyT]

FIXED!

As The Mighty Gryphon advised, I measured the voltage from Pin 1 and Pin 3 of the TPS and set the TPS so it read 0375 V and the result is a big improvement, the bike runs very smoothly, much better than using the"diagnostic plug" method to set the TPS.

In fact, as a comparison, I measured the TPS voltage where it was set by the plug/light method, and the diagnostic plug setting when the light came on was reading 5V, which is enough out of range to made a big difference (ie make the bike run badly). It pays to put thin wires into the TPS socket, plug the socket in, and test using a multi-meter from those wires - check the procedure on the forum if you haven't done it before. 

Worth noting for those who have a 4-Valve K.

many thanks!

Steve

To be clear are you measuring 375 volts or 3.75 volts?

[Sticklebrick]

Yes 3.75 v thanks ...

EUREKA ... so far, for me, anyway ...

I wasn't going to mention this until I had tried it for a few days, but I think I may have discovered how to do the K75/K100 TPS mod on the K1100!

I looked at the schematic for the 2 circuits of the TPS. The one that governs the lower RPMS is fed (I think) by the middle 2 connectors on the TPS plug.

So I put insulation tape (cut to a narrow strip) over the centre 2 female sockets on the TPS plug. 

Wow!  The bike runs like a dream, no stop-go-stop of the TPS, very little if any backfiring, totally smooth and excellent higher range rpm response.  It is now a joy to ride.

I'm not suggesting anyone else should do it  , but I am running this as a trial and so far it has transformed the K1100 for me and I think I'm onto something!  This may work for all 4V Ks ... try it on a private road and do so at your on risk obviously!  YMMV

But if a lot of guys are doing a similar thing on the TPS of the K75/K100, I myself wanted to achieve the same effect on the K1100.  So far so great ... I won't be taking that tape off anytime soon ... now it really is the FLYING brick!!!

[The Mighty Gryphon]

Yes 3.75 v thanks ...

EUREKA ... so far, for me, anyway ...

I wasn't going to mention this until I had tried it for a few days, but I think I may have discovered how to do the K75/K100 TPS mod on the K1100!

I looked at the schematic for the 2 circuits of the TPS. The one that governs the lower RPMS is fed (I think) by the middle 2 connectors on the TPS plug.

So I put insulation tape (cut to a narrow strip) over the centre 2 female sockets on the TPS plug. 

Wow!  The bike runs like a dream, no stop-go-stop of the TPS, very little if any backfiring, totally smooth and excellent higher range rpm response.  It is now a joy to ride.

I'm not suggesting anyone else should do it  , but I am running this as a trial and so far it has transformed the K1100 for me and I think I'm onto something!  This may work for all 4V Ks ... try it on a private road and do so at your on risk obviously!  YMMV

But if a lot of guys are doing a similar thing on the TPS of the K75/K100, I myself wanted to achieve the same effect on the K1100.  So far so great ... I won't be taking that tape off anytime soon ... now it really is the FLYING brick!!!

None of this makes any sense. 

The Motronic on 4V engines uses the throttle position to determine the amount of fuel to inject.  The Jetronic throttle position output is a switch closure at closed and wide open throttle, while the Motronic throttle position is a proportional signal representing how much air is entering the cylinders.  By disabling the throttle position input the Motronic ECU has no idea of how much fuel is needed for a proper mixture.  I am amazed the engine even runs at all.

[Sticklebrick]

I know, that's what I read at first, then I went a little deeper into how the TPS works, specifically for the 4V.

My understanding is the TPS has 2 measures of resistance/circuits within its housing (on the 4V bikes), comprising of 2 physical sweeping arms that measure the position of the throttle via resistance - one arm for the lower and one for the higher rev ranges respectively.

There is a point where the lower one gives way to the higher one.  Therefore only the first of the 2 sweeping arms conveys a signal relating to the fuel cut-off at lower revs.  I am only disabling the one that deals with the lower revs.  It is working so I am assuming that in the absence of a signal from the lower rev range resistor, the Motronic defaults to a set mixture/value until the higher resistor kicks in.

This is all supposition on my part and I am perfectly prepared to be proved wrong - but the practical test shows that it works.  Idle, acceleration, starting, and running are better than ever.

If there is something I have done that explains this improvement other than this mod, I can't think what it is.  I tried running it without the TPS and it bogged down in 2nd gear and stalled exactly as you describe.  I plugged the TPS in again and it was back to normal TPS riding.

Then immediately after this, I pulled over on the side of the road, put the tape between the 2 centre sockets, and now I love the bike, it's smoother than my K75RT was.

I'm saying this respectfully people here are much more knowledgeable than I am, I've just found something that works for me (almost by accident) and after 15 years of riding Ks I can honestly say this has made a huge improvement.  But I'm the one taking the chance to see how the bike runs, and not advising anyone else to try it as such.  All I know is, I am keeping my K1100RT for life, whereas before I actually considered selling it as it was great at slowing down but not great around town (a bit of TPS poetry there) 

[TommyT]

This is interesting. I wonder what your CO measurement is with your mod versus what it is without it.

I have 2 new to me 16v k100, both seem to have a flat spot at 3500 and part throttle. I wonder if that is the point where the one tps circuit hands over the signal to the other circuit. It will try to see if I can do the 3.75 v setting on the TPS and see if that helps the bikes.

[The Mighty Gryphon]

Doing some research on this I found an article by Frank Warner about the Motronic TPS. 

It seems that one of the potentiometers in the TPS controls the ignition advance.  I suppose that disconnecting that function could add some advance that would help the performance.

https://web.archive.org/web/20080405214941/www.geocities.com/fwarner_au/mc_things/K_fuel_injection.html

[TommyT]

This is interesting. I wonder what your CO measurement is with your mod versus what it is without it.

I have 2 new to me 16v k100, both seem to have a flat spot at 3500 and part throttle. I wonder if that is the point where the one tps circuit hands over the signal to the other circuit. It will try to see if I can do the 3.75 v setting on the TPS and see if that helps the bikes.

Okay this is an old thread that I been looking to help solve the rideability of my 16v K100. I have now found a few errors in this thread and in case anyone uses this thread in a search for a solution to their TPS problems I feel the errors should be noted.

The correct voltage is .375v (not 3.75v)  between pins #1 & #4 (not 1 & 3)!

Please see from http://www.motobrick.com/index.php/topic,7996.0.html:

NOTE: Measuring the voltage between pins 1 and 4 at the TPS connector should indicate
0.375 Volts at the idle setting (closed throttle) with ignition ON engine OFF.
Snap the throttle open/closed a few times to ensure a consistent reading. To adjust the TPS, loosen the two screws
slightly and rotate the TPS until the temperature warning lamp (or the Test LED) 'just' comes ON at idle and goes OFF
just above idle. If you are using a voltmeter, adjust the voltage to 0.375 at idle (closed throttle). This voltage MUST
remain below 0.400 Volts at idle.

Setting TPS at idle to .375v fixed my 3500 rpm flat spot and greatly reduced the jerky nature of the bike going on and off the throttle. Yeah!!

[Laitch]

Okay this is an old thread that I been looking to help solve the rideability of my 16v K100. I have now found a few errors in this thread and in case anyone uses this thread in a search for a solution to their TPS problems I feel the errors should be noted.

The correct voltage is .375v (not 3.75v)  between pins #1 & #4 (not 1 & 3)!
Setting TPS at idle to .375v fixed my 3500 rpm flat spot and greatly reduced the jerky nature of the bike going on and off the throttle. Yeah!!

A more precise link to the source document with data supporting Tommy T's correction and his results is this one. Thanks to him for this followup, and to volador for supplying the document in the first place.