I've bought an OEM Tic Toc Tach and plan to install it into my 69 Cutlass. I've looked at the small amount of Rally Pac info in the factory service manual and although I found it helpful as far as the gauges R & R it doesn't shed any light on a retrofit into an HEI equipped car that doesn't have a Rally Pac harness. (I already have good oil pressure, temp, and voltage under dash gauges so I am only planning the tach installation for now).
I have a tach wire that comes out of my HEI and I'm interested to know if that lead can connect directly to the Guage tach wire or if I need another component in-between (Resister, Diode,??)? Should there be a fuse of some value between. Any experienced guidance here is appreciated. - Steven

 

I'm hoping the answer is "hook the tach wire to the tach wire with a 2amp fuse in between". I'm just not certain the signal output from the HEI is directly compatible with the tach input on an OEM tachometer.

 

i haven't personally done this, yet. it should be as simple as you stated above. things to consider are that the hei varies the dwell time according to rpm, and the primary voltage spike, when the module opens the ground line to the coil, will likely be of a higher voltage.

will the dwell time variation cause erroneous readings on a factory tach designed for a constant dwell time? i don't know.

will the input circuit be able to handle a larger voltage spike on it's input line? most likely yes, imo, they used to over-design things back in the good ol' days.


bill

 

I'm at this point hopeful that the HEI output was designed to be backwards compatible, that's just not always the case.

 

I think it will work fine. The only time there are issues to the best of my knowledge is with a the multi spark stuff. Then you need a signal adapter.

 

The TACH output on the HEI cap is exactly the same as the tach output on a points ignition system. Hook up the wire to the Rally Pac and don't look back. It works fine.

 

X2, I have connected a tic tock tach to a HEI and it worked fine. If you are using the factory style braided tach wire, it should already have a fuse.

 

My car has the factory Rally Pac, the tach works just fine with the HEI, and it works fine with a MSD 6AL ignition.

 

It will work fine, the tach out put even worked fine on my dinosaur 65 tach.

 

I think this is what this is......

 

Sammy,
The wire coming out to the fuse looks to be the same as what is being sold for 1969. I don't have that wire and fuse in mine. - Steven

 

Interesting, MSD told me I'll have to run a $65 8920 tach adaptor off the tach lead for the 6AL for my original rallye pac tac.

http://www.msdignition.com/forum/showthread.php?t=19322