Signal on a tach wire

[ryanerb]

I have 2003 chevrolet silverado truck. i know there is a specific tachometer wire for doing alarm installs for things such as remote start. I was wondering what kind of signal is on this wire. i want to take the wire and use it to display the RPM on 4 digits of 7 seg lcd. i did a search and saw other tach designs on this site, but a lot of them seemed to be tapping into some coil wire or something. i would rather just use this tach wire cause its nice and easy. square wave, sign wave? how much does it pulse? do i need an algorithim? if this is all covered in another post, just tell me to search. sorry.

 

[evandude]

the tach wire is generally the negative wire of the ignition coil, if i recall correctly. I think the signal is alternately pulled to 12v and grounded, but considering it's off the ignition coil it will be noisy and have large spikes I would imagine, so you'd want to clean it up, maybe a zener and a low-pass filter would do the job.

I do know that if you probe the wire with a DMM set to read AC volts, you'll get a reading of a couple volts that increases with RPM's. at least that's a way to find the right wire.

 

[gerty]

Look on your distrutor cap and you'll see a terminal marked "tach". It's where the wires plug in from the coil,, there'll be an empty hole for a wire (1/4" quick connect). Grounding this terminal will not allow the motor to run and may cause damage to the ignition system. Don't know how a remote start would use the tach wire?

 

[Nigel Goodwin]

I would imagine a remote start uses the tach signal to know when the engine has started, so it can switch the starter motor off.

 

[evandude]

I would imagine a remote start uses the tach signal to know when the engine has started, so it can switch the starter motor off.

you're correct, the reason I knew about checking it with an AC voltmeter is because I installed a remote start on a family member's car over the holidays, and that's exactly what it was used for. one of the first things you had to do before using the remote start system was start the car with the key, and push some sequence of buttons and then the remote start unit would "learn" the tach signal so it knew at what RPM range the car idled, and would be considered running.

 

[jrz126]

I would imagine a remote start uses the tach signal to know when the engine has started, so it can switch the starter motor off.

Not exactly, the alarms are typically programmed to turn the starter for a set time, (mine can be set anywhere between .6 and 3 seconds, mine is set at 0.8). Then the remote start will look at the the tach wire, and see if there is a pulse, if not, it will try to turn the motor over again. The remote starts can also use a voltage sense wire to check if it is running. (looking for the couple of volts increase due to the alternator running).

Look on your distrutor cap and you'll see a terminal marked "tach". It's where the wires plug in from the coil,, there'll be an empty hole for a wire (1/4" quick connect). Grounding this terminal will not allow the motor to run and may cause damage to the ignition system. Don't know how a remote start would use the tach wire?

Pretty much all vehicles no longer have distributors. With a distributor, you cant really adjust the timing, (unless you physically turn the distributor). The computer in the car is able to adjust the timing real time.


now to the OP...I believe its a square wave, (if the sensor is anything like the one in my 95 Monte Carlo). Do you have a haynes manual? They might tell you in there.