12 volt tach on a 6 volt positive ground system
 

Here's a post I recently made on the "H.A.M.B." Do any of of you guys have any interest in something like this?

"I have been disappointed with the availability of tachometers for 6 volt positive ground systems. There is not a good variety, and what's out there are very expensive. I was discussing this with a friend of mine last year and he said that with today's electronic components, it should be doable. It was, but it wasn't as simple as we first thought. We not only had to reverse the polarity and boost the voltage of the tach control logic, but we also found we had to do the same thing to the signal from the ignition system. After a winters work, my friend was able to come up with a neatly packaged solution that is "wire-up and go".

I have one on the '51 Mercury engine on my test stand and we also have two out being field tested on old GM pickups. Everything works like it should with no problems. I have included a video of the unit running on my test stand. I would like to apologize in advance for the quality of the video, but I am not the world's best "one hand" video guy. Here is that video on "YouTube" : https://youtu.be/TaxpuDkWAqc.

The video starts with the engine off, then I start it and let it run at idle for a few seconds. After I rev it a couple of times, I pan down to a shot of the driver unit itself (the black box) and then down to the 6 volt Optima battery. You can trace the cables back to see it is positive ground.

I believe that there is a decent demand for a unit like this among old car enthusiasts of all kinds. If there is enough interest, we plan on producing these for general use. Our problem is that since this is quite a complicated device, it must be built on a commercially produced circuit board to allow us to package it properly. This, along with the quantity discounts available on the required components, will cause the price to vary greatly depending on how many we make. We would probably have to make at least 300 of these to get the price down to where it would be reasonable. We would expect this to be between $50 to $100, depending on quantity. So, what do you guys think? Is this something that people want and will buy? Let me know.

By the way, my friend says that we could easily add an option that would allow us to drive old "meter-type" tachs with this. You know, the old Sun and Stewart Warner units that have a separate control box that either missing or not working. I don't have one of these, so we haven't tried it yet. Let me know about this as well."

Re: 12 volt tach on a 6 volt positive ground system
 

Would this work on an earlier car? Example: 39/40 with a helmet oil, with resistor under dash only puts out 4 bolts?

Re: 12 volt tach on a 6 volt positive ground system
 

At $100 - $150 I would most likely be a customer.

I was considering a generator modified with a tractor tach adapter with a cable drive attachment. Seems like a lot of expense and work compared to a simple electronic unit.

Re: 12 volt tach on a 6 volt positive ground system
 

The short answer is "Yes". The power booster units we use are adjustable to use input voltage from 3.0 volts to 30 volts. You do raise an interesting point, though. We may need to know the coil voltage for each order. In the alternative, the output voltage is adjusted by turning a small screw that could be set by anyone with a modicum of skill and a multi-meter.

Re: 12 volt tach on a 6 volt positive ground system
 

Count me in for one that will work with 4 volt Helmet coil.

Re: 12 volt tach on a 6 volt positive ground system
 

I had a Sun "Football" tach converted to solid state for operation on 6V positive ground some years ago. It was not cheap. I'll be glad to lend another old Sun tach for testing, if it will help develop a less expensive alternative for other "old school" types like me.

Re: 12 volt tach on a 6 volt positive ground system
 

Thanks for the offer, "4dFord/SC". I may take you up on it. However, this project may go no further because of the lack of response I've been getting. A couple of guys here seem ready to step up, but the only response I got on the H.A.M.B. so far is from some clown who says it can "easily be done with less than $10 in parts". (He's wrong, of course.) I thought there would be a lot of people interested in this, but I guess I was wrong. I'll bump the thread occasionally for a couple of weeks and see what happens, but I'm quite disappointed in the results so far. I think the thread over there got off to a bad start because of the negative responses (the $10 guy and someone who says that you really don't need a tach.)

If I can't get something going by the end of the summer, we may revert to making these five or ten at a time. The price will probably be closer to the $150 mentioned above than our target of $50, which will severely limit the market..

Re: 12 volt tach on a 6 volt positive ground system
 

The larger market may be the "conversion" or replacement of the old Sun sender boxes. They are very traditional but the required Mercury batteries are no longer available. As mentioned above, the solid state conversions for the old Sun senders are quite expensive (but they do work well) A less expensive option would be appeal to a wider market if it could be offered for 6 Volt and 12 Volt and/or for 4, 6, or 8 cylinders. There are plenty of the old Sun tachs around which folks would like to use.

Re: 12 volt tach on a 6 volt positive ground system
 

There already are reasonably priced units available to run the old "meter" tachometers on 12 volts. For example, RT Engineering sells a board the goes into the old sender box for $75. I have also seen stand alone units. I surmise that the number of people out there that have a 6 volt positive ground vehicle and have one of these old tachs laying around is quite small. I would have thought that there would be a lot of folks out there that would want an alternative to a $125 Westtach 6 volt unit (which is only a 2" gauge). If we could get feedback that would justify building a few hundred of our units, we would most likely add this capability.

Re: 12 volt tach on a 6 volt positive ground system
 

Hello, Count me in, PM me how I could go about getting one. You tube video is great, I am sure if more folks knew about it the response would be much more

Re: 12 volt tach on a 6 volt positive ground system
 

Count me in also.

Re: 12 volt tach on a 6 volt positive ground system
 

I just had 10 pre-production boards built. I have one on my '51 Ford and it is working great. I also have 3 prototype boards out in field test, one on a GMC in Oregon, one on an 8BA in Sweden (thanks "flatheadmurre"), and one on my engine test stand. We had a problem with voltage spikes from generators and solid core wires on the prototype, but we solved that with an output filter.

If anyone is interested in one, I have them for sale at $120 each. This is my cost (with the 3 prototypes thrown in the mix), but I want to get them out there and into the right hands. By this I mean someone that will work with me if there is a problem (which I don't expect) down the line.

PM me if you want one.

Here are some pictures of the unit mounted on my '51. I "piggy-backed" the mounting for the board on the overdrive solenoid so I wouldn't have to drill any new mounting holes for it in the firewall (I ran the wires through the access hole in the firewall used by the overdrive ignition connection). The second picture shows the funky little "Sun-Pro" tach I am using (at idle). The third photo is of the engine. Notice the "Trash Can" condensor on the Mallory dual point distributor; I make those as well.

Re: 12 volt tach on a 6 volt positive ground system
 

I installed a 12 volt Sun Tach in my 39 with the original 6 volt system. I used a 12 volt Duracell batt and positive grounded it and it works fine. Check my post from a couple of years ago. I installed the batt in the trunk.

Re: 12 volt tach on a 6 volt positive ground system
 

"39flyer" - Is the Sun Tach you are running one of those with a separate control box?

Re: 12 volt tach on a 6 volt positive ground system
 

No It does not,

Re: 12 volt tach on a 6 volt positive ground system
 

Here is my post from2015.
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.

Re: 12 volt tach on a 6 volt positive ground system
 

An additional post for 2015:
https://www.fordbarn.com/forum/images/icons/icon1.gif Sunpro Super Tach II CP7901hooked to my 6 volt system
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?

Re: 12 volt tach on a 6 volt positive ground system
 

Glad it worked for you. If someone already has that particular tachometer and doesn't mind an extra battery in the trunk, it's probably the way to go.

Re: 12 volt tach on a 6 volt positive ground system
 

The battery measures 6x4x2 inches so doesn't take up much room. It go under the seat. I run the tach and blinkers off it.

Re: 12 volt tach on a 6 volt positive ground system
 

Not all tachs like to be run on negative ground power in a positive ground car....and you need to have a common ground to get impulse from the coil....some don´t even read a signal as low as 6v and a fair chance of frying a tach.
You must have found a pretty good tach that accepts it all without complaining.