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the12volt`s installbay - Mobile Electronics Forums the12volt's install bay / Car Security and Convenience - Hot Topics

Subject Topic: Tach wire general comments (Topic Closed Topic Closed)

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ronco
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Joined: Juneá05, 2007
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Posted: Juneá05, 2007 at 12:45 PM - IP Logged  

There seems to be a lot of confusion regarding where/how to find the so called tach wire. Here are a few comments:

The name of the game here is to get a signal that typically is an alternating frequency that is varying at a frequency that is representative of how many RPMs the engine is turning so that the RS can determine if the engine is running or not. In my opinion, this is the only way to go especially in cold weather for more reliable starts

The "old" style ignition system is pretty straight forward. A cam shaft inside the distributor opens an closes a set of contacts(points) which when closed, allow current to flow in the coil primary. The frequency of these "pulses of current"  are directly proportional to how fast (RPMs) the engine is running. So merely running a wire to the "hot side" of the coil primary ( the other connection on the coil is typically tied to ground) and we have the "tach signal" for the RS system.Note that we need to run a wire through the firewall to tap this signal on the ignition coil.

The "new" style electronic ignition systems (aka "waste spark" systems )is whole different world. This system is computer controlled and basically uses three sensors ( 7 x Crankshaft position sensor, 24 x Crankshaft sensor and camshaft position sensor) to calculate engine rpm, when to fire the injector solenoids and how much to advance/retard spark among other things. So there really is no "tach wire" per se.

The three sensors typically are two "hall effect" and one "reluctance pickup" (the 7 X crank sensor which is the HEART of the system without which the engine will NOT start). So it probably is not a good idea to try to tap one of these signals to use as the tach signal. Probably the best way to "create" a tach wire is to pick off one of the fuel injector solenoid signals ( any one will do)right at the Power train control module ( aka "the computer"). This way there's no need to go through the firewall. However, you must be absolutely sure of the correct wire for as we know, a screwed up 'computer" replacement is huge bucks. Undoubtably, the best way to get it right is to have the actual service manual for the vehicle which will give the computer connector signals and pin outs. Basically, each injector solenoid has 2 wires. One wire is "common" ( the same color at each solenoid) and is tied to +12V through usually a 20 amp fuse labeled  INJ/COIL in the fuse panel. Each individual other wire on each solenoid goes through the harness and back to the computer which switches (pulses) that wire to ground to activate the injector at a frequency proportional to engine RPM . So all you need to do is tie on to one of these wires ( identified by the service manual) and bingo, there's your "tach signal".

One other key point about computer controlled systems. If you disconnect power to the computer, it "forgets" stored emissions control data and the car ( in most Startes) will NOT pass inspection because the emissions tests will read "incomplete" until the car has been driven about a hundred miles under various speeds , engine start/stop,  load conditions etc and the computer has a chance to compile a history of "good" ECM data. So to keep your customer happy, take a look at the inspection sticker. If the car is due for inspection ask your cusdtomer to get it inspected before you begin work. This prevents the situation where the guy comes back screaming at you that his car won't pass inspection since you worked on it.

Hope this long (too long) post helps someone.

Ronco

" The man who knows "how" will always have a job.....the man who knows "why" will always be his boss"

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KarTuneMan
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Posted: Juneá05, 2007 at 12:50 PM - IP Logged  

Wow..... ok.   For tach, I simply go where the tech info tells me to go.
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dswift
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Posted: Juneá05, 2007 at 4:04 PM - IP Logged  

Sweet deal. I wish i knew about this website a long time ago. Man,
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ronco
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Posted: Juneá05, 2007 at 5:28 PM - IP Logged  

I forgot to add that using one of the injector solenoid signals is not the only way to "create" a tach wire. On many cars , the 7 X Crankshaft position sensor is sent to the Ignition control module and is converted there to 2 signals (7 X reference High and 7 X reference Low) which are sent as inputs to the Power Train Control Module (computer). You can tap the 7 X reference high signal as the "Tach wire" signal right at the computer connector. Again, just make sure you've got the right wire!

Ronco
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KPierson
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Posted: Juneá05, 2007 at 9:39 PM - IP Logged  

Ronco,

What kind of cars do you typically work on?  You ever work on any imports?


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ronco
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Posted: Juneá06, 2007 at 7:21 AM - IP Logged  

Hi Kevin, to answer your question, I'll work on ANY car for which I have the correct manual (documentation, schematics, etc). I am an X Navy electronics technician and for years have enjoyed working on all types of vehicles especially their electrical systems. I'll send an email to you at KP Technologies so we can talk off line.
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KPierson
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Posted: Juneá06, 2007 at 8:12 AM - IP Logged  

I was just curious because of your generalizations.  From your ignition descriptions I would guess that you spend a lot of time working on domestic cars, and probably a lot of GMs.


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ronco
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Posted: Juneá08, 2007 at 11:39 AM - IP Logged  

Hi Knife,

I'm assuming your installing a tachometer guage?  No problem

Your motor has a conventional distributor with conventional coil. The coil has one high voltage lead going to the center of the distributor.

There are two smaller low voltage wires on the coil...one is labeled (-) and the other (+). If your system is a Negative ground system with the negative battery terminal connected to ground, then connect the "tach lead to the (+) on the coil......if it is a postive ground system, then the positive battery terminal will be connectected to ground and the tach lead goes to the (-) terminal on the coil.

in any case, don't worry... even if you connect to the wrong terminal it won't hurt anything.. if the tach doesn't work, just try the other terminal

Good luck !

Ronco

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knifegrinder
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Posted: Juneá08, 2007 at 6:23 PM - IP Logged  

Mine has a plug in with no +s or -s. I'll just have to try both of them. Ones orange and green and the other is black.
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howie ll
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Posted: Juneá10, 2007 at 7:38 AM - IP Logged  

Lovely comments but here in the real world, up to date service manuals  from dealers, don't make me laugh, then we have Peugeots where the injector port wiring is covered in epoxy (don't ask it's French). Actually I get the pin-out data books off snap -on etc . I use a snap-on tester with 2 LED's which will pick up the AC without damage, also with a neg ground vehicle go to the NEG side of coil not pos. (I've never had a reading I would call accurate with a DMM).
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