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1987 K75S, wonky tach.

dieselyoda

Active member
I'm getting closer to getting this boat anchor back to form. I got all the wiring and electrical systems working and was ready for a start up. She fired up quite nicely without the high idle lever and stumbled with good blue smoke. I was fine with that.

Checked my tach, steady at 2000 RPM. I guessed I was about 1000 RPM. After the smoke cleared and she was running very nicely, tach at 2000 RPM.

A little throttle, no change. A bit more throttle, the needle jumps and won't settle.

Confirm the tach isn't anywhere close to real RPM. Confirm that my #1 coil isn't breaking frequency. Check my grounds, less than 0.1 ohms everywhere. Confirm my alternator light comes on, part of the circuit as I see it.

Now I'm thinking I have a bad tach head.

What do I do know?

She is a babe and a ton of fun but I'm getting a bit frustrated with all the ghosts.
 
Yoda,

Sounds kind of like a ground problem. First thing I would do is clean the main grounding bundle under the gas tank on the center frame rail. And obviously the ground cable from the battery to the transmission.

While I don't know offhand what might be causing your problem, bad/oxidized connections are common on these old bikes.

I would also look at the multi pin connectors at the back of the instrument cluster. These have square pins. Sometimes it helps to take some needle nosed pliers and give each pin a slight twist. This causes the corners of the pins to dig into a different spot in the socket and gives a tighter fit.

Also, the early K instrument clusters suffered from moisture/condensation problems. The tach (and other components) each plug into the internal cluster circuit board with similar square pins.


Flakey electrical/electronic problems are SO much fun to chase! :scratch



:dance:dance:dance
 
As one of my old bosses drilled into my head, "grounds, grounds and then check them again". I'm pretty confidant that I'm not dealing with a ground problem.

I see that Capital Cycle sells just a tach head, pricey though. Does that imply the tach head is serviceable to a point?

The whole harness has been apart once and cleaned up but I have to do it again because I don't have a brake light from the front brakes and the switch closes.
 
The new tach head lists for $556 from any BMW dealer. I would NEVER go that way. That's IF the problem is in the cluster. I would still open the cluster and check the tach head connecting pins (clean and twist) inside. Replace the gasket on the housing while it is apart.

Used complete clusters can be had for $100-200. Just get a guarantee that everything works or it can be returned before you buy.



:dance:dance:dance
 
The whole harness has been apart once and cleaned up but I have to do it again because I don't have a brake light from the front brakes and the switch closes.

You might want to check the connection at the bulb monitoring relay in the relay box under the tank. End pin E1. Other end E2 is from the rear switch.



:dance:dance:dance
 
The new tach head lists for $556 from any BMW dealer. I would NEVER go that way. That's IF the problem is in the cluster. I would still open the cluster and check the tach head connecting pins (clean and twist) inside. Replace the gasket on the housing while it is apart.

Used complete clusters can be had for $100-200. Just get a guarantee that everything works or it can be returned before you buy.



:dance:dance:dance


OOOOHHHHH, inside....................

Crap, the closer I get to finishing this thing, yet another thing.
 
OOOOHHHHH, inside....................

Crap, the closer I get to finishing this thing, yet another thing.

If you finished it, you wouldn't have anything to do. That could get boring. Nothing but riding.:stick

At least they're not Lucas electrics!



That's why Lucas invented automotive electrics, to keep the Brits from getting complacent.

Lucas, the Prince of Darkness

The Lucas motto: Get home before dark

Lucas, the inventor of the dead short

The three positions on a British headlight switch: Dim, Flicker, Off

Lucas, the inventor of the first intermittent wiper

Lucas, the inventor of the first self dimming headlight

The reason British drink warm beer: Lucas refrigerators

smoke.jpg




:dance:dance:dance
 
I know why the British don't make computers; they can't make them leak oil.

I gotta get me a jar of universal replacement smoke.
 
I had a positive experience with Rick at Overseas Speedometer in Austin TX - he is very familiar with the K cluster & honest & fair.
 
I'm 99% finished, almost ready for a test drive.

Sooooooooo, I followed the suggestions here. Took the head all apart, cleaned up everything, turned the green stuff to shiny and now, no tach.

What am I missing?
 
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