Tach and RPM?
Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=65092
Printed Date: May 19, 2024 at 6:28 AM
Topic: Tach and RPM?
Posted By: equate
Subject: Tach and RPM?
Date Posted: October 27, 2005 at 12:02 AM
I am installing a remote starter. the problem is that it doesn't have tach learning. It has one wire names "RPM INPUT". Is it the same with TACH input? But it can not detect the TACH signal. I called the factory. They said if RPM is reach 600RPM, the unit will think the enigne is started. I am wondering that when enigne is running, the frequncy on TACH wire is the same showing on the tachometer?
Replies:
Posted By: equate
Date Posted: October 27, 2005 at 11:05 AM
oK, I say it in a simply way. If there is a frequncy 60HZ on TACH wire, how much RPM frequncy on Tachometer? What's the relationship between them? How to convert frequncy on TACH wire to RPM on tachometer?
Posted By: equate
Date Posted: November 16, 2005 at 10:30 PM
Anybody can give me oscilloscope record of TACH signal? Any car is fine. Thanks a lot.
Posted By: dualsport
Date Posted: November 16, 2005 at 11:59 PM
Here's a good picture of the scope trace on the ignition primary, with description of the specific points-
Posted By: equate
Date Posted: November 17, 2005 at 12:08 AM
dualsport wrote:
Here's a good picture of the scope trace on the ignition primary, with description of the specific points-
Why is it becoming DC after 3ms? I don't think so. Because TACH signal is AC.
Posted By: dualsport
Date Posted: November 17, 2005 at 12:12 AM
You have to look at the timebase of the trace- were you expecting it to look like a sine wave?
Posted By: equate
Date Posted: November 17, 2005 at 12:19 AM
It should be this: https://www.picotech.com/auto/waveforms/rpm_sensor.html but not really sure it is measured on TACH wire or not.
Posted By: dualsport
Date Posted: November 17, 2005 at 12:45 AM
Don't think that's typical of the majority of cars; tach is usually driven from the negative side of the ignition coil or fuel injection drive. That rpm sensor signal may be for the ECU rather than the tach, but if it works, just as well.
Assume this is for a remote start function?
Posted By: equate
Date Posted: November 17, 2005 at 1:32 AM
dualsport wrote:
Don't think that's typical of the majority of cars; tach is usually driven from the negative side of the ignition coil or fuel injection drive. That rpm sensor signal may be for the ECU rather than the tach, but if it works, just as well.
Assume this is for a remote start function?
Yes. I think so. So can you can one waveform of TACH? I dont have oscilloscope.
Posted By: dualsport
Date Posted: November 17, 2005 at 1:42 AM
If you need to find the specific wire for your car, here's a useful link for a database of cars and the location where the tach signal is accessible.
Posted By: dualsport
Date Posted: November 17, 2005 at 1:48 AM
This is a comprehensive listIf it's not listed, I'm sure the guys on this site have seen it before and could give you the info on what you're looking for.
Posted By: equate
Date Posted: November 17, 2005 at 10:31 AM
I know this. I am helping my friend to get the tach wire on some special Asian version vehicles. For example, which is the tach wire of the following vehicles: 1. TOYOTA VIOS 1998 2. TOYOTA TERCEL 1997 3. BENZ E280 1996
Posted By: Preyhunter
Date Posted: November 17, 2005 at 10:46 AM
2. TOYOTA TERCEL 1997
Tachometer BLACK/ blue AC check connector
note: The check connector (diagnostic) is located in the left rear corner of the engine compartment. The tach can also be found at the ignitor in the same location.
Posted By: Preyhunter
Date Posted: November 17, 2005 at 10:50 AM
Maybe someone else can answer this question...is the tach wire the ONLY wire coming off the coil that would show AC? If so, you could just put your DMM on the coil wires to see which one shows AC. I wouldn't recommend going this route at this time, because there may be other wires showing AC. I simply do not know.
Posted By: cntrylvr79
Date Posted: November 17, 2005 at 10:55 AM
When in doubt you can always get a tach source at an injector. If you look at the injectors you'll see that one wire will be the same color at all of them. With your meter set to ac voltage, simply probe the other wire and rev the engine a bit. You should get an ac reading that goes up as you rev the engine. Good luck hope this helps.
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Cause I'm So white and nerdy...
First Class Certi-fried installer
Posted By: dualsport
Date Posted: November 17, 2005 at 11:02 AM
No listing for the Vios yet, but the other two are listed-
1997 Toyota Tercel Security Information
Tach BLACK/ Blue In Blue 12 Pin Connector in Passenger's Kick Panel
1996 Mercedes E-Class Security Information
Tach Black Coil
Frequency of the pulses would be dependent on the number of cylinders of the car, are you looking for specifics on that, or just want to find out where to tap into a suitable signal?
Posted By: KPierson
Date Posted: November 18, 2005 at 7:02 AM
That tach signal will vary widely from car to car. The wave shown in the 2nd link (slowly rising, quickly dropping off) appears to be a signal to drive a distributor. If you grab a signal off of a crank angle wire (or cam position) it will be a perfect square wave. Fuel injectors are typically quick spikes. And, I know people will disagree with me on this one (someone somewhere always does), but tach wires are NOT AC signals. They are pulsing DC. Pulsing DC can be read (not accurately though) by using the AC function on a meter. This is because the tach signal has a frequency associated with it, and the AC function looks for a frequency. The voltage that makes up the tach signal comes from the DC car battery. ------------- Kevin Pierson
Posted By: dualsport
Date Posted: November 18, 2005 at 9:04 AM
Yeah, pulsed DC is more like it than AC, since the polarity isn't really reversing, as it would with the AC in something like household line voltage. That trace was to an ignition coil input; the output from the coil is thousands of volts, and goes to the distributor (unless it's a distributorless setup). That side you don't want to touch; that'll curl yer hair.  Might be safer to stick with the ign coil input or injector signals rather than tap into the crank angle sensor, unless the connection can be made properly and weatherproofed, because the lower level signals are going to be more sensitive to anything that might degrade the signal.
Posted By: equate
Date Posted: November 18, 2005 at 11:44 AM
Thanks for all above. For TOYOTA TERCEL, I have a picture here. I don't really know what is compartment of the engine. 
Is it the black pen circel part that I can get the TACH wire?
Posted By: pss5075104
Date Posted: November 18, 2005 at 4:31 PM
i think it is always a hell of a lot easyer just to go to the injector.
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D
MECP First Class Certified Installer.
Nothing is impossable, it's just more expensive.
Posted By: equate
Date Posted: November 18, 2005 at 8:00 PM
I am thinking if use injector, it might lost some features of the alarm. For example, 2000RPM lock door, idel 2500 RPM engine stop, etc....
Posted By: equate
Date Posted: November 19, 2005 at 1:20 AM
I use test light to test the TACH signal. The red LED is staying on. The green LED is flashing. How come it does? Does it mean the positive voltage is alway on this wire, and the negative is a pulse signal? How can one wire has the negative and positive signal together???? It confuses me very much
Posted By: KarTuneMan
Date Posted: November 19, 2005 at 1:52 AM
all we need is year make and model.........OR should I say all I need is.............
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Posted By: ringojam
Date Posted: November 19, 2005 at 10:33 AM
 Dont use a test light to test for tac signal! use dvm only  ------------- Jamaica home of the worlds fastest man.
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