Need ignition help

Goober · #1 (**permalink**) 07-22-2008, 10:33 AM

OK Guys. Time to pick some brains. Nothing to do with Cobras, but never the less tech stuff. I buy and sell old tachometers and gauges and want to build a system to duplicate a tach signal that normally the neg side of the ignition coil puts out. What I thinking is get an old distributor (point type), an ignition coil, and a 12 volt source. If I figured a way to wire this stuff up and spin the distributor, would that create a signal to test the tach.
Not sure as how to hook it up, but must be a way. I know the coil wire will have to be attached, but as the rotor spins (without plug wires) will this show anything? Doesn't have to be accurate, just produce a signal
If any bunch of guys can come up with the answer, this group on CC can
Any help appreciated.

Mark

Goober · #2 (**permalink**) 07-24-2008, 09:51 AM

Come on guys, 66 views and no replies. I know someone out there can shed some light.

Thanks
Mark

CHANMADD · #3 (**permalink**) 07-24-2008, 02:03 PM

Go to the Junk yard and get an old HEI distributor stick in a vice,hook it up to battery and spin it with an electric drill. Don't touch the wire terminals or you will be in for a shocking experience. The HEI has a connection for the Tach.One powwer wire and One for the tach...Oh and of coarse a ground.Good luck . Get someone with a camera for the pic when you do get a shock!!

Goober · #4 (**permalink**) 07-24-2008, 02:20 PM

Thanks. This is the kinda info I'm looking for. So I don't need a coil?
As I spin the distributor should I tell my wife "hold my beer and watch this" We tend to say this quite a bit in Texas. Normally just before a disaster.
Could make a good youtube clip

Thanks Again
Mark

CHANMADD · #5 (**permalink**) 07-24-2008, 02:34 PM

With an HEI the coil is in the distributor cap,make sure the one you buy has a coil.
They actually make an HEI for Ford motors today, sure beats all the fancy electronc ignitions that cost a fortune. If I was close enough I would come and hold the beer!!!

Goober · #6 (**permalink**) 07-24-2008, 04:42 PM

Just for that, I'm going to go to Slither Clothing and order something. Once I find a HEI, I'll let you know how it works out

Thanks

Mark

SantaFe66 · #7 (**permalink**) 07-24-2008, 10:17 PM

I took my Smith's tach to a lady in Albuquerque with a business that has a ton of customers. She restores old instruments for vintage cars as well as newer ones. They put my tach on the bench and hooked it up to a signal generator that simulates actual operation, no dizzy involved.

The name of the business is MO-MA, the owner is Margaret Lucas (wow, what's that about?), phone number is (505) 766-6661.

I have no idea whether she'll tell you what they use, but it's worth a try.

By the way, she fixed my Smith's tach and gave me a wiring diagram to use with my MSD so it would work in the car. It did.

Paul

Ronbo · #8 (**permalink**) 07-24-2008, 10:37 PM

Search the web (or ebay) for square wave generators or function generators.

You don't have to exactly use the current levels found in an actual coil/points system as tachs have high impedance inputs.

For frequency range RPMs x half the total cylinders / 60 = frequency in hertz. (in case of four stroke engines)

You need an output range of around 5~6 volts min. PtP. (peak to peak)

trularin · #9 (**permalink**) 07-25-2008, 06:50 AM

555 timer, 556 timer, a simple driver transistor with a 10K strapping it to Vcc. Set the duty cycle to 50%.

OC driver configuration and the 555 set to variable freq mode will allow you to crank it to mid audio range with out problems.

We build these all the time. We design half the world's instrument clusters.

Goober · #10 (**permalink**) 07-25-2008, 08:58 AM

Thanks for all the input. Boy, some of you guys are way up there in electronics. Different language to me. I have looked at some generators on Ebay and I guess if I find one that can generate sine/square/etc, it may work.
Any thing else I should know. Looking at a Heathkit currently on ebay for about $30.00. Are these any good

Mark

Ronbo · #11 (**permalink**) 07-25-2008, 12:18 PM

Actually the duty cycle should be about 20~35% (on or 12v time) to properly simulate an ignition system. Dwell for those that remember this...

Since ignition systems don't produce true square waves (sloping transitions and ringing from the coil) they usually utilize overdriven amp stages for wave shaping as well as low-pass filtering.

Look at a car on a scope sometime, you'll see what the waveform looks like.

Also I would advise using a linear power supply not a switcher supply as the switchers can produce noise at certain load levels. (Look for a supply that uses a transformer) Weight/size is a good indicator of a linear supply. (transformers are heavy/large) Or just use a battery.

The Heathkit stuff was pretty good usually, but since they were kits you never know how poorly they were assembled by the "hobbyist", the plus side is if it has all the manuals, schematics should it need repair.

Wavetek made some pretty decent affordable generators... Have some WD-40 handy to spray the scratchy pots and switches. Don't laugh, this stuff worked wonders on Motrac heads in concrete trucks. (Old Motorola radio tech here)