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Electrical problem: tachometer

P

pruyter

Donation Time
I have been experiencing something strange with the tachometer of my Sunbeam Rapier series IV. Last year the tachometer was professionally rebuilt by a company, the reason for this was both a wavering needle and the change over to a electronically distributor. The tachometer was not installed by myself but by a garage which deals with classic cars.
So far so good and the tachometer is still behaving as it should, but with one exception.....
When I switch on the contact, but without starting the engine the needle of the tachometer goes to about 800/900 RPM and stays there. When I at that point press the button of the horns the needle goes directly up to something like 2500 RPM and stays there while the horns are still activated.
My question is: what is wrong with it?
Thanks in advance for your help!

Regards,

Peter
 
T

Tom H

Platinum Level Sponsor
Peter, without knowing what the "company" did to rebuild your tach it's not possible to figure out what is happening. I suspect that the internal circuit has been totally replaced by a digital circuit of some sort and that the transient created when the Ign is turned on and when the horn is activated triggers or sets the circuit to some state, and with no pulses occurring on the coil lead there is no additional input signal to change the circuit status. Sounds like an odd behavior, but I assume it reads correctly once the engine starts. If so, I guess it's pretty easy to live with it. I suggest you call or write the company to see what they say. It may even be the norm that they expect, a way to confirm that the tach is working, just like a modern car where all the warning lights light up when you first turn the key.

Tom
 
P

pruyter

Donation Time
Hi Tom,
Thanks for your rapid reply and yes you are right I can easily live with it.
It was after more then half a year after the tacho conversion that I discovered the strange behaviour of the needle when activating the contact while last saturday a friend of mine pushed the button of the horns while the contact was activated and then I discovered the second strange behaviour.
The main reason to ask the forum for help was that I am very curious what cause could lead to this strange behaviour of the needle of the tachometer, but well you have suggested some causes and I accept this situation and will live with it. Again thank you very much for your help!

Regards,

Peter
 
R

RootesRacer

Donation Time
Check the wiring to the tach to be sure that it gets a dedicated ground.
On my setup I had a plethora of circuits connected to the same ground that fed the tacho and things like the blinker as well as other loads would generate a spike with the engine running or not. This is with an unmodified stock tacho BTW.
 
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