Potentiometer problem?

Boxertools · 09-13-2002, 11:50 AM

Can anyone help?
I have had a major surging problem with the "s" and it seems to have my dealer (Linds of Norwich) stumped!
I can be riding steadily at about 4000 rpm & the revs just cut back by maybe 1000rpm or more and then comes back up again. It's like backing off the gas.
I can sometimes sit in nuetral with the revs up and it occurs.
My dealer put it on their diagnostic unit & it came up with "potential potentiometer fault".
They replaced the part with one from another bike, but the fault occured again. They now tell me they don't know what to do next.
Any ideas?
Mike.

roger albert · 09-13-2002, 12:58 PM

Hmmm,

Tough one. The obvious place (though not necessarily the correct one) to look next is the entire wiring harness including all plugs and going right up to the motronic, that connect to the TPS. It follows that the diagnostics have no way to differentiate a bad pot from a bad connection to the pot.

They should also know what the limp-home mode is for a fully failed (not just intermittent) pot. Assuming that mode runs half decently, then I'd ride around with the tps disconnected as my next step, and then observe whether the problem goes away, or changes or just what.

Moisture in GAS? (not that that should show a tps fault)

Check ground wires near motronic, injectors, and tps.

hth
roger

JF · 09-13-2002, 01:15 PM

They should be contacting BMW directly or you should. They cant, even if they are being nice about it, say sheepishly "sorry, we don't know what is wrong" and hand you the keys back. "Give me a new bike then." or "swap out the entire computer/wiring/injectors for new" would be my direct reply. Try another dealer if there is one close.

Boxertools · 09-14-2002, 10:35 AM

Thank you for the above.
Having collected the bike from the dealers in my van & them telling me they cant get the fault to reappear, I have just been for a 150 mile trouble free ride. Spooky!

DavidSoine · 09-14-2002, 12:15 PM

Isn't the TPS a critical input for the motronic? I would think the bike wouldn't run at all without it...

best,

Dave

roger albert · 09-14-2002, 01:51 PM

Hi David,

I'm not sure. On a normal measured-air system it wouldn't be (just like the lambda sensor isn't really)

On an alpha-N system like this it is certainly much more critical, since airflow is inferred from throttle position, among other things.

Still, control systems folks don't like to leave any one input as absolutely critical, and fail-safe / limp-home modes are almost always designed in where at all feasible.

I don't actually know how this particular system handles it. It would certainly be possible to assume a midrange value and work using rpm as the primary determinant. It would leave you very lean at high throttle positions and very rich at low throttle, and be sluggish, but it could work.

I've had a couple classes on this stuff, but it's all theory. So hard to know what Bosch and BMW actually do on bikes (the car stuff is half decently documented, but still not well - bike stuff is horrible)

Brad or Lennie know?

regards
roger

roger albert · 09-14-2002, 01:58 PM

OK,

I was too curious to wait for an answer that might not come anyway. Plus, it's not like it's hard to check out. I can't say how it would perform under load, but it runs fine in the driveway with the tps disconnected. It _does_ run very very rich. My nose alone was enough to tell me that. I'm not sure how long it could sustain that. Would probably work ok as long as you spent most of the time between, say, 1/4 and 3/4 throttle. Man she smelled rich. Shut the bike off, plugged the tps back in, and ran it another 30 seconds. Started smelling normal again

regards
roger

Mark Smith · 09-14-2002, 01:58 PM

If the Battery is disconected, doesnt the 'fault logger' reset?

09-14-2002, 02:34 PM

You could check for air leaks. The little bungs on the bottom of the throttle bodies can perish which might cause a surge. Or possibly a duff lambda sensor if it happens at constant throttle. You could also check if there is a coding plug fitted in the left hand electrical box. I have done much experimenting with this, connecting in various configurations and by far the best setting is to not have it there at all. There were various degrees of surging with all other configurations.

tominator · 09-14-2002, 06:14 PM

Mark beat me to the punch!
The battery disconnect must have cleared it up.....

Weird!

Tomas

Boxertools · 09-15-2002, 09:12 AM

Yet another 100 miles or so with no problem. (been to the BMF tail end) (British Motorcycle Federation rally).
Always on the hunt for bargains!

Mike.

By the way, the bike is two tone blue, 1999 with Remus cans.
Can anyone recomend a chip to suit?

tonys · 09-15-2002, 10:11 AM

BBPower, no question!
As you're UK based, have a look at 'boxertrix', a Yahoo group