Tachometer question

[fbenach]

Hello everyone,
here´s another question for the gurus...
A few weeks ago, I bought a speedo and tach that came up from an european 1979 BMW R65... I got them because they are measured in KPH, as well as my bike, after checking the "W" number was the same as mine.

Now, speedo works just fine, my past unit was too jumpy on the speed needle, and the kilometer count was working every other day...

Now, the tach... I plugged it just to check if it worked (I´m actually keeping my old tach) and for my surprise, it only goes up to 2000RPM...

I have to ways to go: either I resell it for parts on eBay... or I get my tester and start dismanteling the unit to see if I can fix it...

Have anyone ever did that? any diagrams that can help me out on the task??

Will love to hear your thoughts!!
Take care!!

[Session101]

well the motor is going past 2000rpms right?
might be the gears on the tach then? if you can't get an answer here i would try to call a bmw gauge shop and ask them what they have experienced in the past, i doubt they would want to diagnose it without tearing into it or seeing it.  i don't know, its electric tach, and i only know of mechanical ones..

[Barry]

It's an electronic tach that counts ignition pulses so unless the needle is catching it's either a bad connection or there must be a component failure. I have a vague recollection that an input capacitor can go bad.

[bruce_launceston]

Does it have gears, they are electronic? I suppose something has to drive the needle around.

[Barry]

I doubt it has gears.  I would imagine the ignition pulses are counted and converted to an analogue signal which then drives a moving coil meter.

[montmil]

Ask wirespokes. He knows.

[nhmaf]

There is a potentiometer inside which is generally used for calibrating the tachometer.  I don't know if its range of adjustment would cover the amount of discrepancy that you are seeing, but certainly that potentiometer setting can shift the reading.   Possibly there are other issues with the tach, but it could be that component may be broken or needs to be adjusted with a tiny screwdriver.   I suppose it wouldn't hurt to try it (it is usually glued in position, so might have to remove old glue) - however if the potentiometer body is cracked, then it is broken and needs to be replaced - I don't have the part number/resistance value for it at the moment..

You have to take the back cover off the tach unit to see the potentiometer on the small circuit board inside.

[fbenach]

OK, thanks... I decided I will open it and see what´s inside... I´ll let you know if I come up to something positive!!

Thanks!!

[Bob_Roller]

The electrical signal for the tachometer, comes form one of the ignition coils, it's single black wire, may want to check both ends of the wire and see if there is corrosion, etc....., at the coil, or the tachometer connector, causing a faulty signal .

[georgesgiralt]

Hello !
If someone has the wiring diagram of the tacho, I'll be DeLiGtEd to get it !
Mine oscillate more and more around the position it should stick to ...

[Bob_Roller]

I've never seen a wiring schematic for the tachometer .

[tvrla]

The R65 Tachs are all electronic. No gears. And like Barry said, they're something like an AC volt meter - converting ignition pulses into something that drives the needle movement.

I'm no expert on the electronic tachs, and actually don't get much call for them - I guess they're pretty rugged and don't often need attention.

In this case, my guess is something died on the circuit board rather than the pot went out of adjustment.

Hope you got it cheap! I don't know of any repair shops around here that work on them. When I was searching I found a place near San Francisco that charged $180 to get one working.

[Barry]

This probably doesn't help much with the problem but you can see how it all goes together. I've deliberately left the image full size so it shows the detail.

[fbenach]

Oh waw!!! thanks Barry, that´ll be my bible opening the device!!!
Great!!