Tacho

1

I have purchased a cheap and cheerful tacho from SupaCheap (for the ignition set-up) but the accompanying instructions are a bit vague when it comes to finding an RPM signal.  (I haven’t fitted a tacho in 40+ years!)

It says “Tachometers can be wired through various sources – coil or RPM signal wire.”  Unfortunately, they don’t say what coil terminals to use.  The Hall sender would probably be the best source of signal but I will have to try to find which is the signal wire.

Does anyone have experience more recent than my 40/50 year old memories?

Les

 

2
Les if you want the tacho in front of the driver there is a tacho wire ready to go at the instrument cluster plug. If you want to hook it up temporarily at the engine bay you use the negative side of the coil.
 
From: "'Les Harris' leslieharris@optusnet.com.au [Syncro_T3_Australia]" <Syncro_T3_Australia@yahoogroups.com>
To: Syncro_T3_Australia@yahoogroups.com
Sent: Saturday, 4 October 2014, 9:06
Subject: [Syncro_T3_Australia] Tacho

 
I have purchased a cheap and cheerful tacho from SupaCheap (for the ignition set-up) but the accompanying instructions are a bit vague when it comes to finding an RPM signal.  (I haven’t fitted a tacho in 40+ years!)
It says “Tachometers can be wired through various sources – coil or RPM signal wire.”  Unfortunately, they don’t say what coil terminals to use.  The Hall sender would probably be the best source of signal but I will have to try to find which is the signal wire.
Does anyone have experience more recent than my 40/50 year old memories?
Les
 


3

Thanks Greg.

Question: how is the RPM signal extracted from what amounts to the 12V supply to the coil?

I want to use the tacho at the rear of the vehicle primarily so that I can read it when standing behind the engine bay.

 

From: Syncro_T3_Australia@yahoogroups.com [mailto:Syncro_T3_Australia@yahoogroups.com]
Sent: 04 October 2014 09:11
To: Syncro_T3_Australia@yahoogroups.com
Subject: Re: [Syncro_T3_Australia] Tacho

 

Les if you want the tacho in front of the driver there is a tacho wire ready to go at the instrument cluster plug. If you want to hook it up temporarily at the engine bay you use the negative side of the coil.
 

4
Les, forgot to mention it is the green wire at cluster.
From: "Greg Esposito gregespo73@yahoo.com [Syncro_T3_Australia]" <Syncro_T3_Australia@yahoogroups.com>
To: "Syncro_T3_Australia@yahoogroups.com" <Syncro_T3_Australia@yahoogroups.com>
Sent: Saturday, 4 October 2014, 9:10
Subject: Re: [Syncro_T3_Australia] Tacho

 
Les if you want the tacho in front of the driver there is a tacho wire ready to go at the instrument cluster plug. If you want to hook it up temporarily at the engine bay you use the negative side of the coil.
 


From: "'Les Harris' leslieharris@optusnet.com.au [Syncro_T3_Australia]" <Syncro_T3_Australia@yahoogroups.com>
To: Syncro_T3_Australia@yahoogroups.com
Sent: Saturday, 4 October 2014, 9:06
Subject: [Syncro_T3_Australia] Tacho

 
I have purchased a cheap and cheerful tacho from SupaCheap (for the ignition set-up) but the accompanying instructions are a bit vague when it comes to finding an RPM signal.  (I haven’t fitted a tacho in 40+ years!)
It says “Tachometers can be wired through various sources – coil or RPM signal wire.”  Unfortunately, they don’t say what coil terminals to use.  The Hall sender would probably be the best source of signal but I will have to try to find which is the signal wire.
Does anyone have experience more recent than my 40/50 year old memories?
Les
 




5
The easiest way to explain it is to think of a points ignition system. The positive side of the coil is supplied when the switch is turned on. The negative side is switched or pulsed to ground in time with the engine via the points. This is why we use the NEGATIVE side of the coil. There is more to it re the condenser but you can see how a signal can be derived from the neg terminal.
 
From: "'Les Harris' leslieharris@optusnet.com.au [Syncro_T3_Australia]" <Syncro_T3_Australia@yahoogroups.com>
To: Syncro_T3_Australia@yahoogroups.com
Sent: Saturday, 4 October 2014, 9:15
Subject: RE: [Syncro_T3_Australia] Tacho

 
Thanks Greg.
Question: how is the RPM signal extracted from what amounts to the 12V supply to the coil?
I want to use the tacho at the rear of the vehicle primarily so that I can read it when standing behind the engine bay.
 
From: Syncro_T3_Australia@yahoogroups.com [mailto:Syncro_T3_Australia@yahoogroups.com]
Sent: 04 October 2014 09:11
To: Syncro_T3_Australia@yahoogroups.com
Subject: Re: [Syncro_T3_Australia] Tacho
 
Les if you want the tacho in front of the driver there is a tacho wire ready to go at the instrument cluster plug. If you want to hook it up temporarily at the engine bay you use the negative side of the coil.
 


6

Ah!  So the tacho is counting every firing pulse on the coil, not just #1 cylinder pulse.

This is a bit cumbersome and I imagine that the Hall sender would send only one pulse per revolution of the distributor

From: Syncro_T3_Australia@yahoogroups.com [mailto:Syncro_T3_Australia@yahoogroups.com]
Sent: 04 October 2014 09:27
To: Syncro_T3_Australia@yahoogroups.com
Subject: Re: [Syncro_T3_Australia] Tacho

 

The easiest way to explain it is to think of a points ignition system. The positive side of the coil is supplied when the switch is turned on. The negative side is switched or pulsed to ground in time with the engine via the points. This is why we use the NEGATIVE side of the coil. There is more to it re the condenser but you can see how a signal can be derived from the neg terminal.
 

From: "'Les Harris' leslieharris@optusnet.com.au [Syncro_T3_Australia]" <Syncro_T3_Australia@yahoogroups.com>
To: Syncro_T3_Australia@yahoogroups.com
Sent: Saturday, 4 October 2014, 9:15
Subject: RE: [Syncro_T3_Australia] Tacho

 

 

Thanks Greg.

Question: how is the RPM signal extracted from what amounts to the 12V supply to the coil?

I want to use the tacho at the rear of the vehicle primarily so that I can read it when standing behind the engine bay.

 

From: Syncro_T3_Australia@yahoogroups.com [mailto:Syncro_T3_Australia@yahoogroups.com]
Sent: 04 October 2014 09:11
To: Syncro_T3_Australia@yahoogroups.com
Subject: Re: [Syncro_T3_Australia] Tacho

 

Les if you want the tacho in front of the driver there is a tacho wire ready to go at the instrument cluster plug. If you want to hook it up temporarily at the engine bay you use the negative side of the coil.
 

 

7
No the hall sends four pulses. You will see four "windows" in the dissy. What it does not do is discriminate which cylinder is which. That is up to you getting the rotor button and leads in the right spot. Modern systems use a crank angle sensor with lots of increments and tdc identifying increment.  The efi system will also use this signal for the sequential injection.
 
From: "'Les Harris' leslieharris@optusnet.com.au [Syncro_T3_Australia]" <Syncro_T3_Australia@yahoogroups.com>
To: Syncro_T3_Australia@yahoogroups.com
Sent: Saturday, 4 October 2014, 9:54
Subject: RE: [Syncro_T3_Australia] Tacho

 
Ah!  So the tacho is counting every firing pulse on the coil, not just #1 cylinder pulse.
This is a bit cumbersome and I imagine that the Hall sender would send only one pulse per revolution of the distributor


From: Syncro_T3_Australia@yahoogroups.com [mailto:Syncro_T3_Australia@yahoogroups.com]
Sent: 04 October 2014 09:27
To: Syncro_T3_Australia@yahoogroups.com
Subject: Re: [Syncro_T3_Australia] Tacho
 
The easiest way to explain it is to think of a points ignition system. The positive side of the coil is supplied when the switch is turned on. The negative side is switched or pulsed to ground in time with the engine via the points. This is why we use the NEGATIVE side of the coil. There is more to it re the condenser but you can see how a signal can be derived from the neg terminal.
 
From: "'Les Harris' leslieharris@optusnet.com.au [Syncro_T3_Australia]" <Syncro_T3_Australia@yahoogroups.com>
To: Syncro_T3_Australia@yahoogroups.com
Sent: Saturday, 4 October 2014, 9:15
Subject: RE: [Syncro_T3_Australia] Tacho
 
 
Thanks Greg.
Question: how is the RPM signal extracted from what amounts to the 12V supply to the coil?
I want to use the tacho at the rear of the vehicle primarily so that I can read it when standing behind the engine bay.
 
From: Syncro_T3_Australia@yahoogroups.com [mailto:Syncro_T3_Australia@yahoogroups.com]
Sent: 04 October 2014 09:11
To: Syncro_T3_Australia@yahoogroups.com
Subject: Re: [Syncro_T3_Australia] Tacho
 
Les if you want the tacho in front of the driver there is a tacho wire ready to go at the instrument cluster plug. If you want to hook it up temporarily at the engine bay you use the negative side of the coil.
 
 


8

Thanks Greg.  That makes sense out of it all. 

From: Syncro_T3_Australia@yahoogroups.com [mailto:Syncro_T3_Australia@yahoogroups.com]
Sent: 04 October 2014 10:18
To: Syncro_T3_Australia@yahoogroups.com
Subject: Re: [Syncro_T3_Australia] Tacho

 

No the hall sends four pulses. You will see four "windows" in the dissy. What it does not do is discriminate which cylinder is which. That is up to you getting the rotor button and leads in the right spot. Modern systems use a crank angle sensor with lots of increments and tdc identifying increment.  The efi system will also use this signal for the sequential injection.