Crazy Tach!

[DuncanS]

Hi

I have a P1800ES '72. The Tachometer is driving me crazy. Sometimes it works fine. BUT then it goes mad. Jumps around and then is completely dead. Then all of a sudden Ill be driving along and it comes back to live as if nothing had happened. just to add a twist to the story when it is working and I use the indicator it goes all mad until the indicator is cancelled.

I've tried to trace the electrics and make sure there is no short or bad earth but electrics are not my strong point. Any one got any ideas/suggestions?

Thanks
Duncan

[TomG]

hey, mines knackered too. i've found it sorts its self out if i turn on the lights, the fan and the wipers.

[Mike Arnold]

Assuming the +12V and earth connections are sound you could have an internal problem. The 1960s components were not that brilliant. However, there is something else you can try. Assuming that you have the current loop type (where the ignition coil wire is wound through a little metal loop at the back of the tach - ie no direct electric connection) you can check that that pickup loop is properly in place. If still no good then you can see if you can add another turn through the pickup loop.

I have a Smiths tacho in my 122 and it needs a knock to start and then works fine.

Incidentally, adding electronic ignition can cause similar symptoms but there are ways round that.

If all else fails and you cannot find anyone else to fix it I might have a look at it.

[DuncanS]

Hi

Actually it seems to have the connections plugged in rather than looped through - therefore a direct electrical connection. I had pulled these and cleaned all connections and made sure a good earth but nothing seemed to work.

Last week I did a tune up - just cleaning and checking the points and plugs really. Ever since the tach has worked! Still gets a little odd when the indicators are on but basically it is working fine.

I guess there must have been a loose connection that is now fine?!

Thanks
Duncan

[rainflye]

Just as an update mounting 2 new radio noise suppression capacitors at the coil replacing the old original one has kept my tach stable and steady for months now.The effectiveness was verified when one of my new connector crimps pulled out and tach went crazy! I know its just a workaround but as a band aid it has worked great...very cheap which always gets me excited!




Have fun!

Craig

[Ron Kwas]

Reference: http://www.sw-em.com/Smith%27s%20Tachometer.htm

Duncan; You can try Craig's fix, and maybe you'll get lucky with your results, but your symptoms do suggest tach internal filtering capacitor (C3) is intermittent...electrolytic capacitors do dry out and loose their function due to age, and this can certainly result in whacky, and variable symptoms, all related to system noise.

There is no (galvanic) electrical connection to ignition wire at tachometer. System was intentionally designed that way to assure that NO failure of tach can cause a failure of the ignition system (very good design practice!)

From memory...I believe some tach versions may have had an internal pick-up loop, so it may appear that there is a connection...but it only looks that way...there is still no connection to the internal circuit other than magnetic (there should be no break in the insulation of sensing wire at the tach). If you can disprove me on this, I am willing to listen and would love to see pictures...

Cheers from Connecticut!