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Q: Adding cruise control on a factory non equipped car.

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I have a 2010 mazda3 I sport. My car did not come with cruise control. I went the junkyard and got the controls for cruise and installed them. My brakelight switch has 4 wires going to it wich signifies it is 4 pronged. I read from oreilys that a 4 prong has cruise control abilities. I also performed a Guage self diagnostic and it showed a cruise control light. So, I have the light, the switches, and I think, the right brakelight switch. The dealer wants to charge me $110 to "see" if my ecm has cruise control capabilities and to turn it on if it does. I talked to a couple of different mazda dealer service desks, all of which told me they can't tell me before they look on the computer. Are you able to give me a probability of likeliness that the ecm will have a cruise control on it? So I don't end up spending more of my money on a useless endeavor.

My car has 87000 miles.
My car has a manual transmission.

A: The unfortunate truth is the dealer is corr...

The unfortunate truth is the dealer is correct. What you are doing is in theory feasible but not common at all. I haven't run across anyone who has done what you are doing and I have often wondered how such an endeavor might work. My point here is, I am not surprised the dealer isn't sure because it's likely they have never done this themselves. The dealer doesn't set up the PCM (Powertrain Control Module) in the cars they sell. The manufacturer does and guessing what happened when a car is being built is not common knowledge.

Anytime you modify any car, there are always details such as this that you will possibly have to address. When you choose to modify a car, you are now the engineer. The simple answer is that I can't tell you with certainty whether or not the PCM in your car has cruise control capability. The only way to find out is to allow the dealer to check in out. Independent shops might possibly be able to, but the equipment they have is aftermarket and not produced by Mazda. The electronic equipment the dealer has is. The dealer equipment will most likely be able to do what you need.

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