current vs Voltage sensing tach

[Scarab73]

Hi Guys,

I've read a number of threads here related to getting the early 240 Z's factory tachs working with the newer MSD ignition boxes and HEI or MSD distributors. I get the idea that a tach-adapter of some type is needed, at times.

My question: At what point, did the change-over occur from "current sensing" to voltage sensing in the Z cars?

Thanks for any replies.

Edited June 21, 2012 by Scarab73

[z-ya]

Hi Guys,

I've read a number of threads here related to getting the early 240 Z's factory tachs working with the newer MSD ignition boxes and HEI or MSD distributors. I get the idea that a tach-adapter of some type is needed, at times.

My question: At what point, did the change-over occur from "current sensing" to voltage sensing in the Z cars?

Thanks for any replies.

 

I believe in 1973. So you can find a 1973 tach, or swap the guts from a 280Z tach into your current sensing 240Z tach.

[RTz]

If your tach. is working correctly, and it's connected to the positive side of the coil, it's current triggered. If it's connected to the negative side, it is voltage triggered.

[Scarab73]

If your tach. is working correctly, and it's connected to the positive side of the coil, it's current triggered. If it's connected to the negative side, it is voltage triggered.

 

 

Hi,

Thanks for your comment, but, the tach is "not" connected. The car is a v8 conversion and has a MSD dist, Blaster 2 coil and MSD 6AL ign. b0x.

I'm familiar with the early mustangs and how these upgrades can interrupt the Rally-Pac's functionality. The early cars were current sensing until the early 70s.

Edited June 21, 2012 by RTz

[Scarab73]

I believe in 1973. So you can find a 1973 tach, or swap the guts from a 280Z tach into your current sensing 240Z tach.

 

...and thanks that's an idea to consider. I have a wiring diagram, but, it's not very legible. They tach wires look like grn-wht and blk-wht? Not sure.

[Zmanco]

My 73 had a current triggered tach. I think the cutover to voltage triggered was when they changed to electronic ignition which I believe was 74 in the US with the introduction of the 260Z. Perhaps someone with a 260Z can confirm.

 

Z-ya is correct though, it's pretty simple to swap a voltage triggered tach into an earlier housing. I did that after giving up on finding a healthy current triggered tach.

 

FYI, the MSD adapter for current triggered tachs is a poor design that is affected by temperature. I'd avoid it, but if you have to use it, don't mount it in the engine compartment.

[Scarab73]

My 73 had a current triggered tach. I think the cutover to voltage triggered was when they changed to electronic ignition which I believe was 74 in the US with the introduction of the 260Z. Perhaps someone with a 260Z can confirm.

 

Z-ya is correct though, it's pretty simple to swap a voltage triggered tach into an earlier housing. I did that after giving up on finding a healthy current triggered tach.

 

FYI, the MSD adapter for current triggered tachs is a poor design that is affected by temperature. I'd avoid it, but if you have to use it, don't mount it in the engine compartment.

 

Thanks. That some good info, especially about the tach-adapter and heat.