Which resistor? How do I figure it out... NEW PHOTOs

[GrabberOrange69]

Gurus,

I bought this neat old vintage tach and mounting cup that looks neat on the dash. I hooked it up to dad's '66 289 just to test it out, only to discover it reads exactly twice the RPM it should. There is no switch on the back of the unit, or anything that looks like it may be adjustable. I have figured out I need a resistor to switch from what I presume to be 4 cyl compatibility, to the 8 cyl.

Am I on the right track?

:confu

 

[AzPete]

Re: Which resistor? How do I figure it out...

I was under the impression that they read the pulses........4/6/8.....some old tachs were only for one type engine.

Got any info on the tach...brand/model/numbers?

 

[B67FSTB]

Re: Which resistor? How do I figure it out...

You can try it by putting a resistor in serie with the tach .Try a 1K 1/2 watt resistor and see what happens.
But any info on the tach would be welcome.
If its an old tach it probably read the average voltage on the minus pole of the coil.
my 0.02 opinion

 

[Midlife]

Re: Which resistor? How do I figure it out...

I've seen tachs that have switch settings for the # of cylinders. Have you opened up the back of the case to see if a switch of some sort is there?

 

[66gt350]

Re: Which resistor? How do I figure it out...

I've seen tachs that have switch settings for the # of cylinders. Have you opened up the back of the case to see if a switch of some sort is there?

On some of the Autometer tachs, there's not a switch but you clip a small loop of wire.

 

[GrabberOrange69]

Re: Which resistor? How do I figure it out...

I'm taking it apart - will post pictures and see what I've got.

 

[GrabberOrange69]

Re: Which resistor? How do I figure it out...

Green wire is "+"
Black wire is "-"
Red wire powers the backlighting

No marks anywhere to switch between 4/6/8 cyl.

Ideas?

 

[AzPete]

That white screw head may be the one.....seems to have a marker tab on it.

6 would by in the top center position with 4 to the left (as it sits) and 8 to the right. They all should be within a 10 - 2 clock position.

 

[B67FSTB]

Isn't that a mechanical tach ???


Correct myself : NO !

 

[B67FSTB]

green is the trigger signal :confu

Should be mounted on the MIN pole of the coil.

 

[GrabberOrange69]

green is the trigger signal :confu

Should be mounted on the MIN pole of the coil.

Thought about that too...but those white plastic screws hold the instrument together...no switch.

A different tach but check this out...http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=360342973720&viewitem=&sspagename=STRK%3AMEWAX%3AIT...the last photo shows a conglomeration of resistors. i'm trying to figure out what resistor I'd need to make mine a "8 cyl", and how to route it.

Might be a bridge too far...

 

[AzPete]

I believe on that one....you cut one resistor for add more cyl...use all for 4, 1 cut for 6, and 2 cut for 8.

Is there anything insde the next level....under the black cover?

 

[Mach1Rider]

Re: Which resistor? How do I figure it out...

Green wire is "+"
Black wire is "-"
Red wire powers the backlighting

No marks anywhere to switch between 4/6/8 cyl.

Ideas?

:wth.... Green wire to neg coil side.
black wire to ground.
red wire to inst lamp wire. (this will allow dimming with dash lights.)