Dreaded Smiths Tacho

MKnight702 · Post by **MKnight702** » Sun Jan 21, 2007 5:11 pm

I am having trouble getting my Smiths Tacho (RVC 2415/00AF) to work with the Megajolt. I have tried both circuits posted on here to drive the gauge but both give the same result, 2,500 rpm when the engine is not running!

Has anyone else had this problem and what was the solution?

The gauge has 2 inputs, a male spade which I have as 12v in and a male post that I have as signal in, the case is grounded.

Any clues?

Matthew

Dave999 · Post by **Dave999** » Wed Jan 31, 2007 5:46 pm

The edis Module gives out a repeated 64uS pulse when the RPM is zero this was used by the ford ecu to indicate ignition on and functioning

so if you use the taco out wire from the Edis modul your smiths tacho is going to display that 64us pulse as an RPM

suggest that the little circuit shown at the end of the installation instructions is the way to go

David G
UK
VH Valiant charger 265 CID 6 cyl

MKnight702 · Post by **MKnight702** » Mon Mar 05, 2007 9:13 am

Still can't get the thing to work.

I have tested the tacho on a friends dizzy engine and it works. I have made up both versions of the Smiths tach driver, the output from the Megajolt and the flyback pulse but neither works. I have established that the wiring diagrams on the net are backwards though, the post is 12v in not signal in. What outputs should I expect from both circuits so I can test them?

Rolling road session booked for Thursday and SVA application posted last week so this is quite urgent now.

Post by **brentp** » Mon Mar 05, 2007 9:47 am

Have you tried the diode circuit at the bottom of the install instructions? http://www.autosportlabs.net/index.php? ... stallation

This circuit is designed to emulate the high voltage kick-back pulse normally seen by a single coil distributor setup. Many tachs need to see this high voltage pulse in order to operate correctly. If this is what the smiths tach needs, it *should* work.

On this circuit make sure the two legs are connected to the negative side of the coil, i.e. the side that connects the coils to the EDIS module.

Even before making this circuit, you can hook the tech signal wire to just one coil and observe your RPM changing. Of course, it will register a fraction of your real RPM : e.g. if you're running a 4 cylinder engine, it will show 1/2 RPM. If you can get it to work this way, then the diode circuit should work as well.

EDIT: try searching the forums for "Smiths" - there are previous discussions on the topic that may help you.

When you tested the tach on your friends engine, where did the signal wire connect- was it to the (-) post on his distributor? The equivalent of that on the EDIS are the (-) wires going between the coil pack and the EDIS module.

Ivan · Post by **Ivan** » Mon Mar 05, 2007 10:46 am

The same here with my tacho, I'v tryed the diode circuit and no result, on one side of the coil
it works and gives indeed 1/2 of the revs.

I used two fat round blak diodes and the zener is orange but smaler.
Could it be that the zener is blown from the high voltage?
anyway keep trying

T3tone · Post by **T3tone** » Mon Mar 05, 2007 6:17 pm

I use the zener/diode circuit and it works fine for my TIM guage. If i remember correctly the diodes and zener have polarity and only work one way?

Ivan · Post by **Ivan** » Mon Mar 05, 2007 10:00 pm

Ofcourse they have a polarity, diodes are a one way street and the zener is a limiter,
but what I don't know is are there also zeners for higer capacities?

Maybe some part numbers that work

MKnight702 · Post by **MKnight702** » Tue Mar 06, 2007 10:12 pm

Well, I've tried one of the wires from the coil and I do get 50% rev readings on the gauge. I have tested my diode circuit and that seems to be OK and not blown but it still does nothing on the tacho. I even tried the direct feed from the MJ which I measured at 13.2v at tickover but still nothing doing. What am I doing wrong?????

M

Conrad · Post by **Conrad** » Wed Mar 07, 2007 10:33 am

Try a 1k risistor between 12v and signel input fo your tacho 1/2 watt

MKnight702 · Post by **MKnight702** » Sun Mar 18, 2007 7:17 pm

After many trials and tribulations (see elsewhere) I have managed to get the tacho to take feed from both coil leads. In the end I just removed the Zener diode from the circuit and hey presto it works. Don't really see why it made such a difference but early 70's Smiths tachos must be slightly diffierent???

Matthew

Ivan · Post by **Ivan** » Sun Mar 18, 2007 8:36 pm

So you used only the 2 diodes and nothing else ?
I'll try it in the morning

fikus01 · Post by **fikus01** » Sun Mar 18, 2007 9:07 pm

the early 70's smiths tachos as far as im aware had 2 input terminals on the back, similar to bullet terminals, 1 male and 1 female, for coil signal! is this the type u have?

Ivan · Post by **Ivan** » Sun Mar 18, 2007 9:26 pm

Mine is the original opel tach with 3 connections
1 = the +12V
2 = the ground
3 = signal from coil

MKnight702 · Post by **MKnight702** » Mon Mar 19, 2007 9:18 am

fikus01 wrote:the early 70's smiths tachos as far as im aware had 2 input terminals on the back, similar to bullet terminals, 1 male and 1 female, for coil signal! is this the type u have?

There are two types of Smiths tacho, the RVI, current sensing, which is inductive and has a coil of wire either externally or internally and is in series with the coil feed wire.

The second type is the RVC, voltage sensing, which is fed from the negative side of the coil. This is the sort that I have, it has a bullet connector for 12v in and a spade connector for the signal, the case is grounded.

Autosport Labs

Dreaded Smiths Tacho

Dreaded Smiths Tacho

The edis Module gives out a

The thickun plots - URGENT

tacho

Better and better

Try this

Working tacho

RVC vs RVI