MSD Early(1969) Tach Problem
 

When I first put my engine (2.0 , webers) back in, the car had a Permatune Box, it started and ran and the tach. (then triggered at the coil) worked ok.

I currently have a MSD6 Box, MSD coil and 8920 Tach adapter installed. Car runs fine ...but NO TACH. (Tach. wired to MSD tach signal outlet). I have tried unhooking the tach. adapter (bought it because other things I had read said I needed it?) ... but no change.

I assume hooking the tach. lead back to the coil would NOT be good for the tach. internals?

MSD emails say I may need to put a capacitor in-line? ...or replace my tach??

Anyone else had this occur?

SEveral people have reported not being able to use early tachs from cars with Kettering ignition on conversions to CDI units, including MSD. The usual solution in the past was to replace the tach with a later model for CDI systems.

Another possible solution is to build a small circuit to give the 'impulse' signal that an early tach is expecting ... which, as you suspected, is not as high as the CDI output to the coil primary '+' terminal!

The following article suggests a small two-transistor circuit to drive early impulse tachs. The original article (first URL) has fig.13 on p.13 and discussion in the last paragraph.

http://www.not2fast.com/ignition/cap-dis-ignit-cdi.pdf

http://www.molla.org/DIY-CDI/SC-DIY-CDI-article-hires.pdf

http://forums.pelicanparts.com/uploa...1186348556.jpg

http://forums.pelicanparts.com/uploa...1186348977.gif

QUOTE: " Another possible solution is to build a small circuit to give the 'impulse' signal that an early tach is expecting "
_______________________

Warren,
BUILD? ME?? Looks like I'll have to enroll in M.I.T. next semister!

My ancient brain is not geared toward electronics. I guess I will be looking for a late model Tach. Is there a particular model/year/version I need ...to work with this MSD6 unit?

Thanks for all your help.

EDIT: Or can I just send my older tach. off to a place that rebuilds them (newer internals?)

"MSD emails say I may need to put a capacitor in-line? ...or replace my tach??"

You shouldn't need to replace your tach. I have the 6AL box and a stock 69T tach that works.

"Anyone else had this occur?"

Yes and it was a pita to get the MSD and tach to work.

Don't use the tach output signal from the MSD box. You have to change tachs or tach internals to use that signal.

Connect the tach adapter red wire to switched 12V.

Connect the white from the tach adapter to the trigger wire between the MSD box and points/point substitute with a diode to prevent current from flowing to the MSD box or points.

Connect the purple/black tach signal wire to the white wire from the tach adapter.

Good luck,
Jim

edited to correct diode location.

Jim,
Very interesting! This is NOT the way the wiring diagram from MSD shows it done. Nor did they (MSD) make such a suggestion to me.

Not disputing that it works for you and I will try it on my car. Just curious as to how you arrived at that configuration? Experimentation?

Thanks.

You could always install the MSD tach adapter

QUOTE: I currently have a MSD6 Box, MSD coil and 8920 Tach adapter installed.

I think I found the solution here at the Pelican BBS.

Note I edited my first response on the diode location. IIRC the MSD tach adapter kit includes the diode.

Regards,
Jim

Thanks Jim, will let you know how mine responds to that wiring change.

JIMMY BOY...You are da MAN!!!!!!

Now certainly MSD has encountered this before? Why couldn't they have written me three, concise sentences (as did Jim), instead of saying;
1. You may need a new tach.
2. You may need to install a capacitor.

That's why I love this BB ...actual field experience from enthusiasts.

Tach. working Jim! Thanks!!

It would be impractical for MSD to research every possible application so they could tell you what works and what doesn't.

Your '69 tach expects a 24V square wave driven from the points terminal. When the points open, there's a ton of ringing and noise and a voltage spike that causes the tach's one-shot to trigger.

The MSD sends a 460V signal to a CDI-specific coil so you NEVER drive the tach off that. However, the factory produced an "intermediate unit" that lowered the voltage coming off the CDI coil and sent it to the old tach to make it work. Then in 1971, they changed the tach and ran it off the points trigger for the CDI.

The MSD signal is a 12V square wave with a 20% duty cycle. That's why it won't trigger your69 tachometer. When you hook up the tach adapter, which is basically a big inductor, you are effectively adding a coil in parallel to the points circuit. When the points close, a magnetic field builds up in the inductor, and when the points open, the field breaks down, which creates a high-voltage pulse that triggers the tach. Inelegant, and the inductor heats up and sucks power and has to go somewhere.

You would be far better off to send your tach to North Hollywood Speedometer or Palo Alto Speedometer and tell them you want it converted to MSD. It's not that expensive, accurate and while the gauge is open you can have the face rescreened if it's faded or other repairs made.

Duh...... Ok ...Good info. John for anyone reading I guess, but didn't I previously post that Jim's directions solved my problem?

My tach. is working.

Hmm- MSD told me the 8910 was what I needed to have my tach operate in my 67. I've not installed anything yet; is the 8910 wrong for my application?