Funky aftermarket tach

[Lee R]

My 74 620 has an aftermarket tach that looks like a Sun knockoff. It constantly shows idle at 1800, doesn;t go over 3200 when I shift in any gear, and when I do shift or slow down it bounces and waves back at me between 1000 and 5500 RPM. I checked all the connections and they are hooked up correctly and grounded. When I disconnect the green from the distributor it drops to zero. Should I just assume the tach is bad? :unsure:

 

 

[nismo dr]

most tach's have a switch for 4-6-8 cyl's, don't know if that would cause this, but its worth looking at

[Lee R]

most tach's have a switch for 4-6-8 cyl's, don't know if that would cause this, but its worth looking at

 

Set to 4. Also has an adjuster on the back to set the tach at 0. This was done as well.

[metalmonkey47]

I went through the same ordeal when i was setting up my gauges a while back.

 

 

Most aftermarket tachs only read a square wave signal, which these old Datsun ignitions don't read.

 

You can use THIS, or get an aftermarket IGN box with a tach output wire..

[banzai510(hainz)]

I never owned a Datsun with a tach. I do just fine and know when to shift

[Lee R]

I went through the same ordeal when i was setting up my gauges a while back.

 

 

Most aftermarket tachs only read a square wave signal, which these old Datsun ignitions don't read.

 

You can use THIS, or get an aftermarket IGN box with a tach output wire..

 

You now I have an MSD box and coil. Looked online and it looks like instead of the green wire coneecting into the distributor, it could go to the neg on the coil. Gotta check the box when I get home for a tach output. Got shocked once a long time ago and never bothered with wiring again. LOL!

 

Thanks metal!

[metalmonkey47]

YUP! lol if you're using the green wire, that's exactly why it's acting funky

 

 

Didn't even think to ask where you had it wired