[91cobra5.0]

now that my alternator is good i want to put my tach back in but it is giving me trouble.

when it is not hooked up at all the tach registers about 10,000 rpms when i hook it up and turn the car on it reads about 12,000 rpms constantl

green wire = negative on coil
black= negative battery post
red= posative battery post

[91cobra5.0]

my air/fuel guage doesnt get a reading either for some reason

[91cobra5.0]

help!!

[fogged306]

What wire are you using for the coil input, do you have stock ignition?

[91cobra5.0]

mostly, its got a msd ingnition coil, im putting it on the negative side, its the same wire that it was hooked up to when it worked but now it wont get a reading and my air-fuel guage is doing the same thing

[terribleonexx88]

check the positive for knicks

[91cobra5.0]

no kinks i just re ran bran spakin new wires

[91cobra5.0]

maybe the cable for the negative post is loose???

and what if i tie into the stock tach. say if i cut the wire for the stock tach and run it for the new tach?

[fogged306]

got a test light or a multimeter?

[91cobra5.0]

not at my house but i can get one

[fogged306]

get one and while the car is running check the lead that you tapped for the tach on the coil. I've never put a tach in a car before, but I'm going to assume it needs a "tach" signal to tell what RPM's the engine is at. I have to find tach on vehicles on a daily basis and I'm pretty sure its the same wire we're talking about here.

Which one will you have test light or multimeter?

and is there a switch on your tach for different cylinder settings?

[91cobra5.0]

theres not a switch, its 2 wires that you either cross or wtever depending on how many cylenders and im gona just say that i can get my hands on a test light

[91cobra5.0]

it should be the same wire going from the ignition coil to the stock guage tach right?

[fogged306]

with a test light the wire should flash rapidly when the car is running and the puisle speed should go up with RPM, sometimes its hard to tell that last part, depending on the test light. If its a nice one.

Can you find a link to some info on your specific tach model?.. mainly I'm looking for instructions cause I need to know what you do with those wires to determine how many cylinders, unless you know offhand and are 100% sure.

[fogged306]

Originally Posted by 91cobra5.0 it should be the same wire going from the ignition coil to the stock guage tach right?

Possibly, or at least it should be on the same circuit. On these cars it was dark green/yellow IIRC. but if your tach said neg coil wire, sending it a tach signal that hasn't been diode isolated could possibly fry your tach? Not sure about that one.. seems unlikely though.

[91cobra5.0]

i found it a while ago off the autometer website, and i even tryed experimenting with the wires crossed for different cylender types, didnt change anything. in the morning im going to try a different ground and then ill go get a light

[The Novice]

Dont you need a tach adaptor for those aftermarket guages? If not then the point of those things? But i thought ya did....

[91cobra5.0]

ummm i dont think there was one on it originally

[e3sean]

i would say maybe the tach is bad but u are having problems with your air/fuelr gauge also.... well if that is the case your air/doesnt run on the - coil wire find the common wire u have to both gauges... you dont have a good ground or u have weak voltage getting to them... check and make sure you have the grounding strap from the firewall to the block.. and check your fuses

[e3sean]

and there should be 4 wires for the tach the green one is accessory that goes to 12v key on there should be a white wire that goes to the - coil wire