MSD 8910 tach adapter

[Rick Cabell]

A friend brought me a MDS system 6429 box and the 8910 tach adapter. Car runs fine but no tach. It's a 78 3.0 engine in a 73 body. I called tech support at MSD and they said the adapter needs to be the 8920. I know this is not correct because I saw this set up working in the car he got it out of. So, there is a white, and a red wire on the tach adapter, so where do they hook up to?

[suprxman]

I have an 8920 on my 80 sc and it works fine. I used the diagram I got online at MSD.

[304065]

Rick,

An 8910 is a doorbell transformer in a plastic box. Really. This is a very electrically dirty way to drive the tach, but it works- but usually if you have a pre-october 1970 tachometer.

The way it works is that the coil has one leg connected to +12v, and the other leg connected to the points. The /1 input to the tach is connected to the points also.

When the points close, the coil charges, just like an ignition coil. When the points open, a big whopper of an EMF fires up the tachometer black/purple wire and slams into the tach, where it triggers the tach's monostable multivibrator circuit-- basically a voltage pulse that triggers a transistor to shut off. Some guys use a diode in line to damp down the ringing of the coil, but that's an elegant solution.

This tends to burn up the points just like old-school kettering ignition but it works.

Anyway, I don't know what a 6429 is, can't find that part number, but if it's a 6AL, why not just use the tacho drive terminal, a 1/4" terminal on the side of the box, to drive the tach?

What tach is in the car? What numbers on the back? What date code?

[jtf914]

I have a brand new 8920 still sealed in the factory packaging and I need an 8910 if your friend wants to swap.

- Justin

[Bob Ashlock]

Admittedly self-promoting here, but if the MSD Adapters don't work out for you, please consider trying one of my TACH-ADAPT units. This unit has helped solve similar problems for many members on this forum. Home Page

[304065]

Originally Posted by Bob Ashlock

Admittedly self-promoting here, but if the MSD Adapters don't work out for you, please consider trying one of my TACH-ADAPT units. This unit has helped solve similar problems for many members on this forum. Home Page

Bob, what is the duty cycle of your unit's output? Is it an 11 volt square wave?

[Rick Cabell]

date stamp on the tach is 4/72.
The nay sayers all say to buy the tach adapter right out of the gate. That is why I didn't try the spade on the MSD . John, it was a typo on the MSD unit, it is a 6 AL 6420, not a 6429. Monday morning I am back it. Time to go home for the weekend.

[larrym]

i have run one for 15 yrs - no problems

there is another recent MSD thread you should search & read on this site

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