Sunpro Super Tach II CP7901hooked to my 6 volt system

[39flyer]

I always liked the Sun Tach.Had one in my 65 Chevelle SS back in high school so I wanted to see if one would work with the 6 volt positive ground on my 39 tudor.Yes I know it is not original but it is a personal thing. Saw a lot of information on here and came up with, what looked like, a way to do it using a Vintageautoradio inverter. I called Steve and he indicated that it would not work with the 6 volt positive ground system.So I thought I was out of luck.After discussing it with Steve for a while he gave me an idea to try and see if it worked.I tried it today and yes it works.Basically I took a 12 volt battery and grounded the positive side to the frame, hooked the black tach wire to the negative side of the battery and hooked the green wire to the condenser mounting tab on the coil. Has anyone else tried this?

[Blu Dice]

You lost me - is the car 6v or 12v? You stated its a 6v positive car - are you going to drive the car around with a 12v battery too??

[41ford1]

I used one of these inverters to power a 12v neg ground radio in a 6v pos ground car.
One of these may be what you are looking for.
http://www.classiccarstereos.com/pro...ters/PGPI.html

[Blu Dice]

Wow would it be possible to run a tach AND a radio off one of these??

[tubman]

That looks like an interesting device. I would have to believe that the tach draws very little current, so you should be able to run both. BUT, my head hurts trying to figure out how the grounds would work and if the device being powered would have to be physically isolated from the vehicle. Any electronical geniuses out there that know?

[39flyer]

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Sorry for the confusion.My car is a stock 6 volt positive ground system.I did not want to change it so I was looking for options to run my 12 volt Sun tach..I saw a post on the Ford Barn from 2012 with good information and the name of vintageautoradio.com .They make invertors for adapting 12 volt radios to the 6 volt system.I called them and talked to Steve and he indicated that their invertor would not work with the Sun Tach.But he offered some advice on how to make it work.It did so now I will mount a small 12 volt battery in my trunk to run the tach.

[Blu Dice]

Well that makes more sense - you can put in an upgraded radio too while your at it huh!
Be sure to tie that battery down good - don't want no sparks near the gas tank huh?

[41ford1]

Think about how this inverter is hooked up on a 6volt neg ground system. The case is bolted to the car chassis. That becomes the "electrical common". 6 volt negative is the input to the inverter. The inverter output is 12 volts positive. The chassis or "electrical common" is the ground for the 12 volt devices. A tach should work just as well as a radio when powered with one of these. Keep the total current draw below the rated output of the inverter

[40 FourDrDlx]

Originally Posted by 39flyer

I always liked the Sun Tach.Had one in my 65 Chevelle SS back in high school so I wanted to see if one would work with the 6 volt positive ground on my 39 tudor.Yes I know it is not original but it is a personal thing. Saw a lot of information on here and came up with, what looked like, a way to do it using a Vintageautoradio inverter. I called Steve and he indicated that it would not work with the 6 volt positive ground system.So I thought I was out of luck.After discussing it with Steve for a while he gave me an idea to try and see if it worked.I tried it today and yes it works.Basically I took a 12 volt battery and grounded the positive side to the frame, hooked the black tach wire to the negative side of the battery and hooked the green wire to the condenser mounting tab on the coil. Has anyone else tried this?

Yes, I've tried it without luck so far. Did Steve indicate why the inverter wouldn't work with a tach? In your wiring with the 12v bat. where did you attach the red wire? Some have said their 12v tachs worked on 6v, but not mine. So unless I've blown up this new tach experimenting I'm at a loss. Any ideas? And yes I'm aware of the Westech(?) 6v unit, but I wanted the vintage Sun tach look. Thanks, Bob.

[flatheadmurre]

Problem you get is that you always need a ground reference.

Your dizzy has a reference to the 6+ground.

That adapter is probably supposed to be floating (the radio and the adapter is not supposed to be grounded to the chasssi.

To have a stepup driver that can be referenced to ground it need to have a transformer isolating primary and secondary side.

Now you short it out and the failsafe goes in and shuts it down.

Does it make any sence or am i only complicating things...

[flatheadmurre]

Second try at it...

Without an isolation transformer grounding your 12 neg gnd is shorting.

With an isolation transformer its like a stand alone unit same as your battery.

[4dFord/SC]

FWIW, I found an earlier Sun "football" tach with sending unit and had it converted to solid state for use with 6v positive ground.

[40 FourDrDlx]

Originally Posted by flatheadmurre

Problem you get is that you always need a ground reference.

Your dizzy has a reference to the 6+ground.

That adapter is probably supposed to be floating (the radio and the adapter is not supposed to be grounded to the chasssi.

To have a stepup driver that can be referenced to ground it need to have a transformer isolating primary and secondary side.

Now you short it out and the failsafe goes in and shuts it down.

Does it make any sence or am i only complicating things...

Flatheadmurre, thanks for the thoughts but the inverter instructions say "it uses a common ground, so isolation is not necessary" The metal case of this small unit must be grounded. Maybe I better recheck the ground.

[dean333]

How are you going to charge the 12v battery in the trunk??

[flatheadmurre]

Originally Posted by 40 FourDrDlx

Flatheadmurre, thanks for the thoughts but the inverter instructions say "it uses a common ground, so isolation is not necessary" The metal case of this small unit must be grounded. Maybe I better recheck the ground.

Doesnt mean that the 6v negativ and the 12 positive can work togetter which is what happens when your taking the signal of the dizzy sending it into the tach.

Have you tried the tach with a battery now to check if its alive ?

Im just thinking costwise and i doubt the manufacturer uses an isolated boostconverter over a cheap single chip device.

If it works with a stand alone battery and not with the adapter its still my bet.

[39flyer]

I have not got a whole lot of time on mine yet so can’t tell you how long the battery will last but it is working so here is what I have done.I can post pictures if it will help.

Battery is a Duracell Ultra “Dura 12-9F2” mounted in the trunk.
This is a stock 6 volt positive ground 39 Deluxe
I ran the red tach wire to the positive battery terminal and then ran a wire from the positive terminal grounding it to the frame.
I then ran the black tach wire, through a on off switch, to the negative battery terminal installing a 10 amp inline fuse at the battery terminal.
Next I ran the green tach wire to the bottom screw tab on the condenser.
Hope this helps.Like I said I have only run this for maybe 30 miles so there
is not a lot of time on it. Yes, I know you really don’t need the tach, like
others have said, but I have always liked the looks of that tach.It is a
personal thing for me.

[flatheadmurre]

It will run a long time the tiny batteres in the SUN/SW units lasted a long time.
You can get a rechargeable battery and charge it in the garage sometimes if you like the setup. Glad too hear your up and running !
Not needing it....you like it and thats the reason we all are into this

[wisbangman]

I wanted the reliability of a modern new 12 volt radio with cd capability, in my 39 Ford. I put a 12 volt Sony radio in my glove box. I mounted a 12 volt marine type battery in my trunk. To make it look kosher, I bolted a wicker type picnic basket in the trunk, and put the battery inside that. If I open the trunk,and folks say, " look at the cute period correct picnic basket ! " I throw a charger on the battery every once in a while. The radio has never let me down. I am presently using a " hidden antenna " but that does not work too well. I am planning to put an original 39 roof antenna in, and mating that to the 12 volt radio for better reception. I am thinking about getting a tach and an inverter like 39flyer

[tubman]

39flyer,

Let me know how this works out for you. I mentioned this to my next door neighbor (who is also a long time friend) who is an electrical engineer, and he says that making a small converter box (using parts that are less than a couple of dollars) would be easy to do. Normally, I would discount this as BS, but this guy has done a lot of work for Autometer over the last 20 years modifying tachs for snowmobiles. Given the number of 12 volt tachs out there, and the paucity of 6 volt units, he may be on to something. I know he was able to fix the tach in my boat that had burned components and a scorched circuit board.

He also says that polarity is not a problem. I believe him, but don't understand it.

[flatheadmurre]

What you need to make a driver for the tach meter is basicly a counter chip the 555 is dirt cheap and common and an operational amplifier to get a strong enough signal to drive the meter.

With that you can rip out whatever is inside the meter and just use the meter and you own board.

Your neighbour can design a generic board that will drive any tach out there.

You need 2 adjustments to make it work.

First you need to able to adjust fullscale output to adapt to the meter youre using.

Second is how many pulses it takes to give a fullscale reading ( 6cyl x 5000rpm or 8cyl x 10000rpm)

Chips accept 5 volt as working voltage so with the addition of a voltage regulator the board could accept 6 and 12volt.