ELECMAN

I bought an after market tach guage for my 72 cutlass 455 ci engine .It fits in the clock spot the problem I'm having is where does the tach lead connect to.I have a pink,Brown,and Black wire come out of the distributor going to the top of the cap.The black wire fell out on the top of the cap dont know where that goes.but before it fell out I put the wire coming out of the tach to the tach slot on the cap got no reading what do I do from here Thanks.

ent72olds

Tach lead usually connects to the (if I remember correctly?) pos side of the ign coil....the black would be ground...a third wire may be backlight....

svnt442

Only if you want to blow the tach. It connects to the neg side of the coil.
Are you running an HEI? If so it clearly states "tach" on the cap.

If you got no reading then you probably have a bad tach.

Most aftermarket tachs are white, green, red and black for the wires.
Red to SWITCHED power, black to ground, white to the dash lights and green to the coil.

ent72olds

Thanks...I didn't remember correctly!

ELECMAN

what do you mean by HEI

svnt442

HEI = high energy ignition. The cap looks like this:



The stock points cap looks like this:

Lady72nRob71

The HEI uses an electronic pickup and a coil mounted inside the cap to give higher voltage to the plugs and require less maintanence.
Reference pics above to determine if an engine has HEI or not.

joe_padavano

The easy rule of thumb is that the tach connects to the same side of the coil as the distributor. The distributor SHOULD be connected to the negative side of the coil, but the coil may be hooked up backwards.

D Appeldorn

maybe i'm wrong, but don't they need like oil and wather temp meter the ignitionkey 12volt and a ground to work?
ground- 12volts - and sender?

if mounted corectly ad power source you can trie the + or - side of the coil. one should work.

ELECMAN

OK connected the wire from the tach to the tach port on the distrubator.the problem I'm having is at idle it reads about 2500 rpm at a stand still shift the car in drive still the same thing.started to drive the car and the tach came down to 1500 rpm and started to increase when I stepped on the gas.called the tech that supplied the tach and said it cant be the tach because the tach gives a polsating 12 volts.he said it could be my points on the cap does that sound right.

svnt442

That doesn't make any sense. First of all with an HEI there are no points. It's all done electronically so if the car is running then the module is good. Second there could be something wrong with the tach. Is that the tach you are using in that pic?

ELECMAN

yes

svnt442

Make sure the case is properly grounded. It usually grounds through one of the mounting screws. Just make sure they are all tight.
Other than that I would try and test it on another running V8 and see what it does.

OLD SKL 69

I agree with Randy, sounds like you need to check the ground first. If everything is hooked up and grounded properly, and it still does not work, then I would suspect that it's the tach.

joe_padavano

Note that the ground should be through a metal ground strap on the plastic dash insert. Be sure these are in place. Alternately, use a temporary ground wire.