Date: 9 Aug 1995 09:24:48 UTC
From: "Diaz Jon" <DIAZ_JON@MACMAIL1.SWINDON.RTSG.MOT.COM>
Subject: BMW: K75 Speedo Problems
Jim Colburn enters the world 'o K:
>The speedometer on the 1986 K75C that I just got has stopped working. The
>previous owner says "just tap it/honk the horn/wait a while" and it'll start
>working again. Is there a cheapish way to fix it? I see from Capital Cycle's
>catalog that they only stock the whole instrument cluster at $625..... is
>the speedo available separately? I tried calling speedo-fixer Irv Simon and
>he says he can't repair it.
Something I recently learned about the speedo is that it may be the pickup cable from the final drive to the head unit. When it craps out, try wiggling the wire down on the swingarm and see if anything comes of it. My speedo has shut itself off for about 40 miles on two different occasions this summer (both stinky hot days, FWIW), but since the last hiccup it has been fine for about 1200 miles. Weird.
If the turn signal stays on forever, then it is likely this cable. If it turns off, then the unit is still reading distance and you have the pin problem inside the speedo assy.
Just another thing to check......and this wire goes for $52 (dealer invoice $35 or so :)). I checked.
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)
Sr. Research Engineer Emeritus
Univ of Ill, Urbana-Champaign
Ah, the Motometer shuffle, again.
You probably don't have the updated instrument pod, identifiable by two Gore-Tex vents on the back cover (and internal electrical improvements).
Do the easy stuff first:
K Whiner MC#11
I have an '88 K75S. The speedo has failed 3 times. First two replacements were done under warranty at dealers stateside. The third Lone Star BMW sent me a unit to replace myself. Did so, same problem. Turned out that the sensor ring in the final drive had come loose. Fix is to take the final drive apart, epoxy the ring back in place, put back together.
Tom Yeager, AKA Rainman