Hi All,

I'v been making my scuttle & dash and I thought I'd do a mock up of the dial positions to see what it would all look like.

That's when I noticed those little words on the tacho...... positive earth.

So, the question is, is there anyway I can use it on a neg earth car ?
The dash is made of wood, foam & vinyl.

Can anyone help, pleeeeeeez.

Paul G
Rescued attachment dash & tacho.jpg

hi mate I think you should be able to use it if the body of the instrument is insulated from all metal parts and you conect the active wire to said body and the wire that would normally conect to your distributor going to earth. How ever you will have to do somthing with the instrument light because as soon as you conect the light to power instant short circuit .I would use an instrument light out of a jap bike they are usually a neoprene housing with a seperate earth.

Altermatively you could open up the clock and see how it is put together You may find that it is possible to change the wireing of the field coil so that it is neg earth and all will be sweetness and light. think of electrical systems as plumbing with electrons and in DC systems direction of travel matters .Old brit cars and bikes were positive earth and all things ran the other way so if you hook up a pos earth tacho to a neg earth system it will try to go backwards.
I hope this helps but in a worst case scenario and you blow up your tacho you can always put the mechinism from a neg earth one into the existing case using the original face so it matches the other instruments
best wishes
Iain

Hmm - had a think about that one... and came to the conclusion, yes.

Providing the illumination doesn't use the case as it's earth you could reverse the polarity simply by switching the positive and negative wires to the tacho feed. The lamp will be ok as it doesn't matter which way around they go anyway. If there is only 1 positve feed to the tacho then it will mean that the whole casing should be negative. But in your instance it would be positive... which is not a good thing really!

I'd go round it a few times with a well aimed multimeter and if in doubt - bin it!

Hi again,

First of all thanks for the replies.

I'v been over it with the multi meter and and found the following....

red = bulb supply
black = case earth (on the clamp screw)

So bin the metal bulb holder & replace with an insulated one with two wires. OK?

This leaves 3 connections.

purple (spade) resistance detected between this & case. (fine ohm scale, x10K)

between white & the one with no wire, resistance detected (course scale x1) almost a dead short.

On the tacho white & no wire are female & male bullet connecters.
Can I reverse these two, and is the purple the wire from the points side of the coil ???

I would assume that no wire, would have been the live (the wire in the dash would have been an insulated female bullet) and therefore be neg, and white is pos.

Does any of this make sense ???
Bl**dy electrics..... give me a welder anyday.

Thanks again. Paul G

ps The reason I'd like to use this tacho is that it reads up to 7000 rpm. My engine is supposed to red line at 7000. Old Smiths tachos that read this high are hard to find.

[Edited on 22/8/04 by type 907]

Paul
SVA will not be happy with the insturment bezels
Regards
Steve @ sVc

Hi Steve

The SVA is the last of my worries at the moment. I'm more concerned with getting them working.
I did say that the dash was just a mock up. perhaps a pic from another angle will give the game away.
Any ideas anyone on the wiring ?

Paul G
Rescued attachment dash 004s.jpg

quote:

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Originally posted by Steve @ sVc
Paul
SVA will not be happy with the insturment bezels
Regards
Steve @ sVc

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Made me think of this................
Rescued attachment dali_header.jpg

Paul,

I don't want to dampen your enthusiasm, but I think you're brewing trouble for yourself with that positive-earth tacho.

Even if you isolate it totally from the chassis, the case is still going to be 'hot' and could cause unexpected flashes and bangs when you're working on the dash wiring. As a minimum, I would put a low-amp fuse in the lead that goes to the casing.

Also, a neg earth tacho is designed to react to alternating shorted-to-negative-earth / live-to-positive-12v-through-coil potentials. Your tacho will be designed for alternating shorted-to-positive-earth / live-to-negative-12v-through-coil potentials. It could be interesting, to say the least.

Might be worth trying it out before mounting it in the dash...

rgds,

David

Quote.
[Also, a neg earth tacho is designed to react to alternating shorted-to-negative-earth / live-to-positive-12v-through-coil potentials. Your tacho will be designed for alternating shorted-to-positive-earth / live-to-negative-12v-through-coil potentials.]

Hi David, This may have hit the nail on the head.

I'v fitted a two wire insulated bulb holder.
The case is no longer part of the curcuit.
Have fitted a wire on the male bullet (orange).
As you can see in the pic the white & orange are insulated from the case.

Have I got the wires right? Is the purple spade the one to connect to the coil?
If someone can confirm this I will give it a go.

thanks
Paul G
Rescued attachment dash 007s.jpg