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Carfix4you, ASE Certified Technician
Category: Car
Satisfied Customers: 617
Experience:  ASE Re-certified Technician, 22 yrs. Dealer and Independent Repair World Class High Performance Expe
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Remote starter..12v..tachometer..I tap into, the alternator? Thanks

Customer Question

I have a remote starter device that needs a wire that changes state (0 to 12v or 12 to 0v) only when the motor is running. Many people recommend the tachometer wire. What else could I tap into, the alternator?

Submitted: 8 years ago.
Category: Car
Expert:  Carfix4you replied 8 years ago.
HiCustomer Thank you for asking your question on JustAnswer. The other Experts and I are working on your answer. By the way, it would help us to know:
-What year is your Car?
-What make and model is your Car?
-How many miles are on your Car?
-What type of engine is in your Car?
-What have you already tried?

Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.
Customer: replied 8 years ago.
Thanks. This is a general question. In other words the solution should work for most cars so we don't have a specific car to refer to. We're designing our own remote start system. We need to be able to tell when the vehicle is actually running (and NOT just trying to turn over) in order to stop triggering the engine start function.

We have tried the alternator but we want to know what's the most common method for this solution. People have suggested that we use the tach wire but our device responds to the presence (or absence) of 12 volts only not pulses of 6 volts like what we're seeing on the tach wire.

I thought that I would be live chatting with a technician. This leave-a-message process is a bit too slow for our situation. Is there a way that we can communicate more quickly?

Expert:  Carfix4you replied 8 years ago.
Sorry, chat has been disabled from the system. JA rules only allow me to communicate thru this site. From what you stated, the alternator would be a likely candidate thru the voltage regulator. The voltage regulator has a circuit that supplies the generator lite in the instrument cluster a ground signal(12v already supplied by instrument cluster) when the alternator is not charging. When it starts to charge when the engine is started, it changes that signal to 12+ volts, which turns the lite off.
Customer: replied 8 years ago.
Thanks. Does this occur in all cars? I ask this because I have a 2002 Lancer that has 3 wires to the alternator. I checked all. Only one changes from 0 to 12v but it happens BEFORE the engine turned over. Is this normal? I think it should only indicate 12v when the car is running.
Expert:  Carfix4you replied 8 years ago.
This is a normal operation for all alternators that have an internal voltage regulator. Here is the wires highlited on the 2002 that could go from ground signal to power when the alternator starts producing 12 volts.graphic
Customer: replied 8 years ago.
Thanks. I see that there a two possible places (wires) that I can tap into. What I want to confirm is the 'nature' of those wires. Will those wires only read 12v when the car is 'cranking' or when the car is actually running.

I need to have these wires hot ONLY when the car is running. It seems like when I did my tests earlier, I got a 'false positive' meaning that the wire in question was reading 12v BEFORE the car actually started.

Do you understand my reason for asking this question?
Expert:  Carfix4you replied 8 years ago.
One of them(I believe it is going to be the L-W wire) will only produce 12v when the engine is runnung. The other is the field wire from the ECU to coltrol how much the alternator charges. You mat have to isolate that circuit from the ECU .
Customer: replied 8 years ago.
>You may have to isolate that circuit...< Can you explain? Is there a way to know for sure the nature of this L-W wire.
Expert:  Carfix4you replied 8 years ago.
Isolate means cut the wire from every component other than the alternator. Then nature of that wire will be either 12v or ground signal when the alternator is charging.
Customer: replied 8 years ago.
Let me get this straight. I should cut (isolate) this wire from the circuitry of the car and connect it to my device? If this is the case, what will I affect by doing this? Does the system of the car not need this wire connected?

Expert:  Carfix4you replied 8 years ago.
This should be done momentarily for testing purposes only.
Customer: replied 8 years ago.
How does JA work? Will another tech chime in with a suggestion? I'd like to cross-check what you are suggesting. Also, does your resources describe the color of the wire and the location of the wire in question?
Expert:  Carfix4you replied 8 years ago.
I only get paid when you accept. If you could get one of your electronic engineers to ask these questions, I'm sure he or she will agree with me. If you would like another opinion, I can opt-out, and that means all this for nill.
Customer: replied 8 years ago.

Not that! I am going to talk to my guys here and accept when I get a confirmation. Did your source give you wire color/location information?

Expert:  Carfix4you replied 8 years ago.
I don't understand your question. My sources are 3 of the top automotive databases in the world. I have an associates degree in automotive electronics, and over 22 yrs of fixing automotive electronics.