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  #1  
Old 01-16-2004, 07:18 AM
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Location: Brisbane, Australia
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6.3 Tacho into 3.5 (with pic)

I acquired a nice 6.3 tacho and want to replace my clock with it - have seen it in another 3.5 108 and it is pretty cool.

Has anyone wired one of these in?

For everyone's info, emerging out the back of the tacho is a little bundle of wires with a dual pronged plug on the end. In the 6.3's was there a coupling that attached to the plug that then fed off to the lighting, coil, etc? What do I need to do to make this work?

Any help, greatly appreciated.

Regards

Richard


Last edited by zambo; 01-17-2004 at 04:42 AM.
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  #2  
Old 01-17-2004, 05:02 AM
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That elusive pic

Here it is!

Sorry for the focus issue. Also, the plug looks like it has 2 long and 2 short silver prongs - it's just a trick of the flash I think. There are just 2 long silver electrical connectors at the back with the black plastic guide prong in the middle, though offset from the silver ones
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6.3 Tacho into 3.5-tacho.jpeg  
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  #3  
Old 01-24-2004, 11:32 PM
Tim's__Benz
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anyone? I am interested as well for my 4.5. I've heard the 4.5 has connections. Someone please comment on this.
Tim
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Old 01-25-2004, 04:58 AM
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Gidday Tim

As it turns out, there are quite a few people that would like to know how this can be done.

Essentially, I ran this past the folks at the M100 site and a few there would like to know as well as they have the smaller V8's as daily cars.

What I have discovered is that the transistorised ignition used in the 3.5 (and maybe the 4.5) was to a different spec than the 6.3. So if you have a tacho from a 6.3, you have a unit that would normally be able to draw say 9-10 volts from terminal 15 on the coil on the 6.3.

On a 3.5 (and 4.5?) the ignition at that point puts out only about 2.5-3.5 volts as part of it's low voltage design. I have had this verified by some MB documention relating to the measuring various outputs - volts, dwell, resistance, etc - which people (i.e. MB mechanics, etc) could use to service the vehicles. It clearly describes the various distributors and transistorised systems used for the various models of this era (early 70's) and the quite different outputs - electrically speaking.

So that's what I know for certain - now apparently you can get a voltage amplifier that theoretically would be put inline between the coil and tacho and raise the voltage to the required level. We are on a long weekend celebrating our national day on Monday, so I will have to wait until Tuesday to make some calls to electronics houses.

However if this rings any bells with you guys in the Northern Hemisphere and you think you might have a solution for the device required, please chime in.

Will report back when I know more.

Regards

Richard Zammit
Brisbane
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  #5  
Old 01-25-2004, 05:16 AM
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Location: Brisbane, Australia
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and to answer the question ...

My original post was about the plug/wires on the back of the tacho and what did what. Well if you have a tacho already, you'll have a green wire and a red wire out the back. Red is for connection to a continuous power supply and the green goes to the coil - once you lift the voltage coming from the coil, that is!

You'll need an earth connection to the body of the tacho that you can pick up anywhere on the loom.

As to the "connections" you refer that a 4.5 has that the tacho can connect to, well the connections people are usually referring to are are located just below ignition module itself, just on the side of the radiator and low down in the engine bay (passener side in Oz). It's like a little terminal block. However those points are for measuring the test values I mentioned in the previous post. If you put even a simple test light to the terminals (I have a two terminal arrangement) it may cause the engine to die - as the test light draws all the power out of the ignition system at that point!

As an alternative, if you have a 6.3 tacho you intend to install, you can do the test I did - in the engine bay, connect up a wire to the positive on the battery to the red wire, a wire to the negative on the battery and to tach body. And finally a connection between the green wire and terminal 15 on the coil.

It should idle nicely at under 1000 on the dial (on mine, anyway) and will rev up to 2000 (again on the dial) - but further revving will make the tacho fluctuate wildly as it's requirement for the higher voltage makes itself know.

Don't do this for too long as normally you would put in inline fuses to protect the unit, but as part of the learning process, this helped confirm what I had read and been told about the voltage issue.

Hope this helps.

Richard

Last edited by zambo; 01-25-2004 at 05:24 AM.
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  #6  
Old 01-25-2004, 11:08 AM
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I was going to suggest that one was a positive lead, another the coil pickup or point pickup, and the body ground - but I didn't know and didn't want you to fry the unit!

What I suggest is trying to connect the lead which would nomally go to the coil on the 6.3 to before the ballast resistor(s) on the 3.5/4.5. The worst that could happen is the tach drains too much current and the car won't run, in which case you can just disconnect it.

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