induction tach on matchbox distributor won't read above 2700rpm?

[t bear]

got an older induction tach wired in (through the signal to the coil), spent the better part of a day dashboard wrestling, once it was in and showed idle speed i finished the loom all nice and buttoned it up. on the first freeway trip i realized that the tach won't go over 2700 r's, anyone have any idea what to look at? to much juice from the electronic ignition? no idea what to do, but i'll be bummed if i have to take the dash apart again. thanks for looking.

[Str8_69]

Try switching to the other coil terminal for your signal, and double check your ground, if that isn't it, prolly just a bad tach.

[banzai510(hainz)]

is there a jumper or switch in back for 4 6 or 8 cylinder aplications

[Str8_69]

is there a jumper or switch in back for 4 6 or 8 cylinder aplications

 

Good call Hainz, forgot about that option... might not be 4 cyl. compatible, lots of older tachs were 8 cyl only.

[t bear]

is there a jumper or switch in back for 4 6 or 8 cylinder aplications

 

it's a stock unit from an earlier 620, so that shouldn't be an issue unless there was a 1973 datsun skunkworks 620 V6. man, that would be rad. i'll try the other side of the coil, but i'm pretty sure it's supposed to go on the pos side. not today though... man it's nasty out. thanks for the ideas.

[banzai510(hainz)]

Positive side of the coil is like putting it straigh the the +side of the battery.

You want the minus side. as like in Point ignitions its a open then ground situation. the + side has always POWER just when the points close it completes the circut coil charges up. then points open. the magnetic field around the coil collapses thus shooting the main HIGH voltage out to the pug wire.

 

A tach usually senses the on/off.

 

 

Im not a Datsun Tach expert but heard if the parallel type(what I usually refer to,After market ones) the then the completes the circut type(In series). where you have a in and out(thru the tach then to the coil).

Im only guessin here on this. But if the Tack had a open circut in it then the coil would not get 12volts. Thuis could be rumer Or Im stoned. But I read this soemwhere before

 

 

However Inductive is usually when a probe holds the other wire and it senses it. LIKE WHEN YOU PUT THE HOLDER FROM A TIMMING LIGHT AROUND THE SPARKPLUG WIRE. This would be inductive to me.

[datsunaholic]

Actually the inductive tachs will work either on the positive or negative, but NOT if the positive wire is feeding anything else. Like the matchbox module. 510 ones ran on the positive side because the dual-points had separate wiring back to the coil. All the inductive pickup does is sense pulses of current. It's a much cleaner "break" on the Negative side (float open/hard ground). The Positive side has OK "break" at lower RPMs but once the pulses get too close together the inductive pickup can't read them. With only the coil connected, there's still a hard "break" in current when the coil discharges. But with an ignition module, there is always current passing through the wire.

[t bear]

well, i wired it every way i could think of and it still ran perfect... up to 2700r's. must be broke. went and bought a chincy sunpro tach for 40$ and rigged it in the cluster. it looks okay, but at least it works. thanks for the ideas.