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Discussion Starter #1
Hi, I have a 2008 Mazda 6. On cold idle, the rpm is fine at 800rpm. When the car warms up it goes down to like 700rpm+ which is still fine. The problem I have is that if I turn on the headlights the engine sounds like it wants to shut off and the rpm goes up and down 600+ to sometimes 580rpm. If I press the power window switch it does the same rough idle. If I press the AC it goes back up to 700+rpm. I check the battery it is 12.5v I put in another battery and it's doing the same thing. The alternator shows 13.5v when it's not under load when I turn on the heat lights I've seen it drop to as low as 12.9 but then fluctuate down in the 13.0 to 13.5 range. I checked the alternator I connect the black lead to negative and positive lead to the alternator I get 0.05 I put the positive lead on all ground and I get 0.05. I check headlights 15a fuse I get 0.02 resistance. I connect to a scan tool it shows engine load value going up higher when headlights are on. I also see vacuum value go lower. What could be my problem?
 

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Discussion Starter #3
Check the charging system
What do you mean by check the charging system? What do I do? I use a multimeter and rev up the engine to 1.5rpm I connect the black lead to negative and positive lead to the alternator I get 0.05 I put the positive lead on all ground and I get 0.05. Positive lead on the positive terminal and negative lead on negative terminal I get 13.5v to 13.0v it goes up and down and the lowest I have seen is 12.9v. This is when the headlights are on. I ordered topdon battery tester will that check the charging system or how do I fully check the charging system?
 

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That's a good start.
Most folks take the car to an auto parts house and have it tested for free in the States. Alternatively we remove the alternator and have it bench tested, also for free.
My guess would be a failing alternator, (likely voltage regulator). The Mazda alternators are serviceable and you can replace the bearings, brushes, regulators, etc... However, given your questions it may be in your interest to plan for an alternator replacement. In the interim you should make sure all grounds are clean and the battery in in good condition (electrolyte levels and a load test).
Good luck

For reference my v6 shows around 14.0v-14.2v after morning start and will drop to around 12.9v-13.5v when warm and operating normally, (higher with seat heaters, defrost, and lights on)
 

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No, since its in the alternator and thats under the motor basically.
Since he is in The Bahamas (or maybe just his ISP) I am guessing it is the 4cyl so on the back yes?


What's the alternator charging range in a 2008 Mazda 6? Is it easy to swap the voltage regulator?
This may be easier for you.
 

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Discussion Starter #8
I bought the Mazda from Japan and it's a 2.3l 4cyl. I did some more testing for ground and positive leads all are reporting normal.
Before I turn on the headlights I get 13.35v to 13.70v but when I turn on the headlights the voltage drops to 12.70v+ and gradually goes back up to 13.5v and fluctuates 13.5v to 13.2v or less, but does not go to the 12v range. Does that confirm the voltage regulator is bad?
 

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Discussion Starter #10
I did have some powertrain codes before but I cleared them with a scan tool and they haven't come back as yet. It might have happened because of this problem I don't know but the main fuse connected to the alternator wire in the fusebox the 125a fuse blow I believe and the mechanic put an 80amp fuse on it. That's when I had a problem starting the car. These are the codes that were stored P0560, P1794 and U3003
 

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Discussion Starter #14
Thanks for the reply but question why would the fuse amp matters? I believe Mazda's alternator is rated maybe between 100 to 110amps would a lower rated fuse in my case 80amp stop the alternator from putting out 100amps or 110amps under heavy load headlights, etc is on?
 

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would a lower rated fuse in my case 80amp stop the alternator from putting out 100amps or 110amps under heavy load headlights, etc is on?
Exactly. Think of it like a wide straw and thick shake. Pinch it half way closed and you have to suck harder to get the shake out.
 

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Discussion Starter #16
Exactly. Think of it like a wide straw and thick shake. Pinch it half way closed and you have to suck harder to get the shake out.
I been to the store they only had a 100amp fuse. I'll have to buy the 120amp fuse online. I install the 100amp fuse and I am still losing power. I check the multimeter and when I turn on the headlights it goes down to 12.8v and gradually go up and down to 13.7v - 13.0v. When I get the fuse and that's not the problem does that mean the voltage regulator in the alternator is bad?
 

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I been to the store they only had a 100amp fuse. I'll have to buy the 120amp fuse online. I install the 100amp fuse and I am still losing power. I check the multimeter and when I turn on the headlights it goes down to 12.8v and gradually go up and down to 13.7v - 13.0v. When I get the fuse and that's not the problem does that mean the voltage regulator in the alternator is bad?
Not necessarily. It could be a bad wiring harness too. Where are you plugging your leads in? On the battery terminals?
 

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Discussion Starter #18
Not necessarily. It could be a bad wiring harness too. Where are you plugging your leads in? On the battery terminals?
Battery terminals, negative battery terminal to fuse box, positive battery terminal to engine ground, all reading the same volts. I'm not 100% sure but the car sounds better with the 100amp fuse. I'll have to do more testing when the 125amp fuse arrives.
 

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Discussion Starter #19
I also notice something, I have the emergency brakes on so the car doesn't move. When I put the car in drive and press the brakes I notice the rpm goes down and it sounds like it's losing power, but when I take my foot off the brakes the rpm goes up and it sounds normal. I am guessing the rpm goes down and the car losing power because of the brake lights?
 
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