Speedometer woes 1990 K75s

[gmawby]

Hi,
my speedo is playing up, it understates the speed.   At 5000rpm in 5th gear I should going about 110kph but it reads around 90kph.  At 3000rpm in 4th gear I should be going about 60kph but it reads < 50kph.   After reading other posts I've tried the following.

Cleaned the sensor at the final drive.
Cleaned the connector by the fuel tank.
Cleaned the connectors at the instrument cluster.
Tested the sensor with a multi meter when the bike was running and got 120 millivolts (I think I was reading that correctly).

Anyway, all that made no difference.  Occasionally the needle jumps to where it should be when riding.

Regarding the sensor, does anyone know what it's tech specs are?  And anyone got any ideas?

Glenn.

[Elipten]

They are not accurate form the factory.  Then there is tire size variation.

There are articles on how to calibrate.  Search for them.

[Laitch]

Occasionally the needle jumps to where it should be when riding.
 And anyone got any ideas?

Have you been there and done that?

[gmawby]

Hi,
thanks Laitch, not tried that one.. it will have to wait 'till next weekend.

Glenn.

[jay1622]

This brought back memories. I had the same problem with my '86 K75 when I bought it used in 1998. The stealer knocked off $300 cause they couldn't fix it. Cleaning the contacts in the speedo as referred to here did just the trick!

[gmawby]

Hi,
an observation:  It has been chilly  here the last couple of mornings ( 8 dec C ) and when I ride off at 7:30am the speedo reads about the right value.   After about 2km when it's warmed up it understates.    When I leave work in the afternoon and the bike is warm (22 dec C)   the speedo is way off.

No idea what that means but interesting anyway.   

Glenn.

[Brahma]

found this article written years ago the other day......  Might be of help.

-------------------------------------------------

From: hawley@aries.scs.uiuc.edu (Chuck Hawley)
 Subject: Re: BMW: K75 Speedo Problems
 Date: Wed, 9 Aug 1995 13:27:43 -0500 (CDT)
I found the intermittent contacts to be where the speedo plugs into the printed circuit board inside the housing. Just moving the unit around seems to clean the contacts for awhile. I installed a small 3 pin plug and socket on both of our '86 K100's, and they seem to be cured. I introduced a problem on one of them in that when I soldered to the signal input on the circuit board on the speedo, the trace connected to it became unconnected. This caused the symptom where the speed needle goes full scale and wanders around in general. So I found that eventually and fixed it. Incidentally, the input to the speedo pins are signal, gnd, and +12 looking at the back of the speedo unit. The signal comes from the small circuit board strip attached to the back of the speedo (on the later units it's attached to the main circuit board, but is still in the same location). The signal that this board puts out is a 0 to 6 volt square wave, and is sent to the speedo unit signal input pin (it also goes to the turn signal unit via the main housing connector). The input to the amplifier strip is the pickup unit on the bevel gear housing. I found that a 90 Hertz square wave gives about 80 MPH. You could figure this out by counting pickup pulses for a wheel rotation...etc.

Does anyone know what the chip on the speedo PC board is? It is a stepping motor driver for the odometer, and puts out a current proportional to freq. for the meter (speedometer). I have never known one to go out, but it would be good to have a spare chip. The amplifier chip is an LM2904. This one sounds available.

I wish I had a few of the jillions of K instrument units that were thrown out. I feel that we could make all of them work. Oh. Also I fixed the trip odometer. Two of the wheels would not reset. They needed to have a couple of nibs glued back in place inside the wheels. It's tiny but able to be done.

Charles Jack Hawley Jr.
 Amateur Radio KE9UW (A.K.A. 'Chuck' in Ham Radio)
 BMW K100RS BMWMOA #224 (A.K.A. 'Jack' in Motorcycles)
 hawley@aries.scs.uiuc.edu
 Sr. Research Engineer Emeritus
 Univ of Ill, Urbana-Champaign

[gmawby]

Hi,
fixed!!!..  I followed the instructions Laitch referenced and cleaned up the pins.  I took it for a 70km ride this morning and the speedo works at highway speeds and around town.   When I pulled the connector out it looked clean (see the photo, it was taken before I cleaned the pins)  but I rubbed the pins with fine emery paper and washed them in alcohol. I gave the pins a twist with pliers before assembly.

All good.

Glenn.

[Laitch]

All good.

That's how it is here at the world famous MOTOBRICK.COM, mostly. Thanks for the update, Glenn.