MSD Ignition 6A USE Adapter 8920, wich adapter so use 8910 or 8920 Options

[jc914]

I just Installed a MSD 6a WHAT A difference 100% (IMG:style_emoticons/default/aktion035.gif) (IMG:style_emoticons/default/driving.gif) (IMG:style_emoticons/default/biggrin.gif)
Now my tach will not work. I have searched the forum for what I need to do. I have found no real answer some use the MSD adapter 8910 others the 8920.
I have a 73 fuel injected 2.0 with pertronix. According to MSD the 914 (mine) the tach wire used to go to the Negative of the bosch coil and it is called a Voltage Trigger and says to use the 8910. I have read others using the 8920 with no problem. below are two diagrams I found. Which is the correct one to use



8920
(IMG:http://i176.photobucket.com/albums/w177/JCD404/tach.jpg)


8910
(IMG:http://i176.photobucket.com/albums/w177/JCD404/tach3.jpg)
THANKS

I found this on line about the 8920
http://www.msdignition.com/forum/archive/i...php?t-8649.html

[Aaron Cox]

I dont think you need an adapter....

The dizzy tells the BOX when to fire the coil right?
Hmm... the only wire coming out of the dizzy to the box is a signal wire.....

you should be able to tee into that wire and have a tach pulse (it used to be the wire going to the negative side of the coil before the box was installed)

works with my mallory box and dizzy this way

Yes.. the white wire in both mallory and MSD 6a boxes is a trigger wire. 1 pulse per plug (the box decides when to do multispark discharge... not the dizzy)


Tee into the white wire out of the MSD 6a box

[ArtechnikA]

The dizzy tells the BOX when to fire the coil right?

Yes, but...

the stock tach is current-sensing. When you remove the original coil, the switching current (points or electric points-replacement gizmo...) is now only a few milliamps. This might not be enough to trigger a tach reliably, especially an older one.

MSD's (and the Crane HI-6) provide a tach-trigger output just for this purpose. Try that first if using the original points trigger connection doesn't work for you.

It's time for my rant on the MSD 'tach adapter.' It's just a friggin' loading coil that presents an inductive load for the points just like the original coil did. My first time down this path was with an early CIS Rabbit that required a reliable tach signal to keep the fuel pump energized. I was shocked at what I saw in the box after parting with my money, especially when i had to do it TWICE because the first one (recommended by the wizard boys at Autotronic) didn't work and "Ooh, I guess you really need the high-current one after all - did we mention electrical parts are not returnable?"

I don't recall the exact loadings but the 'regular' adapter is something like 3-4Ω and the "high-current" is about 2.8Ω

If that sounds like the nominal resistance of an ignition coil - bingo... So if you've still got one in your parts stash, it's worth a try. (tape over Terminal 4...)

It's half-a-transformer, and I've always suspected that you could go to your local big-box hardware store and get a doorbell or thermostat transformer and have it work just fine - but I didn't have to go down that route so it is left as an exercise to the reader...

The 'tach adapter' can't be costing them more than a couple of bucks, copper content notwithstanding, and their "value pricing" designed to capitalize on the desperation of a car guy who needs 'just one more last bit' to get his project running rubs me the wrong way.

Or just buy a Monster tach that can run with the voltage-trigger levels that are compatible with CD ignitions... It'll fit right in the dash...