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Old 01-20-2012, 10:28 PM   #1
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The short of it...
Battery light came on and replaced battery. Light went off a few weeks later. Came back on intermittently. Pulled into work after drive and car was squeeling and light came back on. Took it to shop and they said alternator bolt had come loose and belt was rubbing. Needed new belt and alternator. Got new alternator (aftermarket) and light still on. Replaced with another alternator. Still on. Replaced with Honda alternator. Still on. Mechanic asks if I had issue with temp gauge not working as they noticed it. No I had not.

Was told it might be MCU/Multiplex but I just wanted my car back as the alternator was confirmed working and battery is good.

Took care home. Temp gauge didn't work on way home. At end of drive on highway car would idle at 2k rpm and bounce a little to 2.2k. Turned off and back on and idle ok. Temp gauge came back on. Battery light still on though.

Today had same issue. In stop and go traffic had idle problem but it corrected itself about halfway on the drive. Also temp gauge didn't work but when idle problem corrected temp gauge came back. Additionally I can't get fan to blow heat to my feet, only to my face. Nothing comes out when moving dial to feet only. Quite odd. All other gauges fine.

Any ideas what it could be? Was told codes can't be read because battery light is on.

Before I drop $$$ on a MCU would love to know what anyone thinks could be the issue. Much appreciated.

Sorry. Car is a 2001 Civic EX, Auto

Last edited by toyomatt84; 01-21-2012 at 06:08 AM.
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Old 01-21-2012, 06:09 AM   #2
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Default Re: Battery light still on, new battery, new alternator plus new issues

It sounds like a few potential problems could be at hand. The multiplexer could be shot, you may have an idle air control valve that is sticking and needing to be cleaned, you could have a blown coolant temp sensor, or even your ECU could be malfunctioning and need replacement.
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Old 01-21-2012, 07:37 AM   #3
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Default Re: Battery light still on, new battery, new alternator plus new issues

Thanks Matt. I should preface by saying the fan issue, temp gauge and idle problem were not there before the alternator was replaced.
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Old 01-21-2012, 08:47 AM   #4
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Default Re: Battery light still on, new battery, new alternator plus new issues

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Originally Posted by diggs24 View Post
Thanks Matt. I should preface by saying the fan issue, temp gauge and idle problem were not there before the alternator was replaced.
That leads me to believe that there is an issue with either the multiplexer or ECU.
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Old 01-21-2012, 05:45 PM   #5
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Default Re: Battery light still on, new battery, new alternator plus new issues

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That leads me to believe that there is an issue with either the multiplexer or ECU.
As was my assumption. I'm going to try the multiplex first as it doesn't require a trip to the dealer as it would with a ECU reprogram (from what I've read). Thanks for all your help.
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Old 01-21-2012, 10:38 PM   #6
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Default Re: Battery light still on, new battery, new alternator plus new issues

I started having a similar problem myself (02 civic auto). I was driving home late at night, and all my accessories went dead. Heat, stereo, turn signals all dead. Head lights went super dim, throttle response was sluggish and acceleration was erratic. Once I got it home the car would not start again so I jumped it and after that it was like nothing ever happened. Starts every time, everything works again.

Fast forward a week, I'm driving home again, late night again and its starts to do the same thing. Only this time when I came to a stop the idle dropped and the car completely died. Got a jump, drove it home and voila. Starts up every time and everything works again.

I noticed that when this begins my abs light comes on. Followed by the check engine light, then the battery light. Gauges going dead when I push on the brake. This time my check engine light is staying on but I'm gonna check it again in the morning and see if its still on. I first thought my battery was on the fritz (original battery) but I'm not so sure anymore since it seems to hold a charge?

Whats weird to me is that I daily my car and I've been driving around all week and nothing went wrong. Now I ran into the same problem a week later, again on a saturday night at around the same time as last week.

Possible Multiplex, ecu issue for me as well? I kinda want to replace the battery to see what that does but I'm planning on getting the engine code pulled if my check engine light is still on.

I remember my mom's car which is an 05 accord had an issue where it would need a jump on occasion for no reason. Found out the battery was a pos so got that replaced and now everything is fine. She never had everything die on her while she was driving though. Thats kinda why I want to replace my battery.
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Old 01-22-2012, 09:55 AM   #7
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Default Re: Battery light still on, new battery, new alternator plus new issues

When my multiplex box failed, I kept on driving to get home, charge light did not come on, however the battery drained from the car being driven, charge light did not illuminate until the car dropped dead from not having enough electricity for fuel pump, ecm, ignition systems. These multiplex boxes are absolute garbage. Americans are lucky Honda of America does not rip off consumers on these parts, they are priced about a third of what Honda Canada Inc charges just for adding daytime running light function to the multiplex box.
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Old 01-13-2013, 09:59 AM   #8
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Default Re: Battery light still on, new battery, new alternator plus new issues

OP, have you solved this problem??

I am experiencing the SAME issues after my alternator gave up. I changed it, now everything is working but sometimes the coolant temp gage isn't working, I have to remove my key from car and start it again in order for it to work AND the batt. light is still on...

Anybody?
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Old 01-13-2013, 11:19 AM   #9
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Default Re: Battery light still on, new battery, new alternator plus new issues

Hondas only like Honda alternators from my experience. I doubt the eld causing these issues but it could be bad. Only a factory Honda scan tool will tell you though
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Old 01-13-2013, 09:08 PM   #10
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Default Re: Battery light still on, new battery, new alternator plus new issues

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Originally Posted by Racer02gtstang View Post
Hondas only like Honda alternators from my experience. I doubt the eld causing these issues but it could be bad. Only a factory Honda scan tool will tell you though
an aftermarket alternator could cause my idle to be weird, the coolant temp guage not to work proprely and the battery light to stay on?

voltage when motor is running is perfect... The alternator is 100% working...
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Old 01-14-2013, 05:57 PM   #11
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Default Re: Battery light still on, new battery, new alternator plus new issues

anyone?
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Old 01-14-2013, 06:56 PM   #12
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Default Re: Battery light still on, new battery, new alternator plus new issues

Low voltage can damage the MCU or ECM/PCM. Get a service manual and diagnose for a bad MCU or ECM/PCM.

@ eviee1973: Dam, the US one is expensive enough. Next time it goes out get the US version and wire the DRLs to the headlight circuit, then get in the habit of turning the lights on every time you drive. LEDs for the DRLs would be perfect for that application.
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Old 01-16-2013, 01:19 PM   #13
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Default Re: Battery light still on, new battery, new alternator plus new issues

UPDATE :

After running a bit (my temp gauge not working) if I shut the car off and restart it, the gauge goes to it's temperature... But it doesn't move while the car is running?

Wtf?

Last edited by thecca; 01-16-2013 at 05:05 PM.
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Old 01-23-2013, 09:37 PM   #14
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Yes, I did resolve it.

Bolt for alternator ground came loose, alternator arced and damaged my coolant sensor and pin B10 on the ECU. I bought a new coolant sensor and replaced it (like $18 off ebay and literally 5 minutes of my time) and that fixed the temp gauge/idle problem. It did take a few days for it to start working though...not sure why.

I bought a used ECU off ebay for $90. Dug behind the glovebox and pretty much had it ready to be pulled out. Called a dealer and asked if they'd marry a used ECU to my car. They said yes and $200 later I had my used ECU in and the light is gone. Beats the $1,200 the other dealer wanted to put in a new one.

Also, my alternator and battery were replaced and were deemed 100% working (heck I drove it for months like that...light just stayed on). Sounds like you need a new sensor and ECU. Pull yours out to get the right part number.
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Last edited by toyomatt84; 01-23-2013 at 09:54 PM.
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Old 01-28-2013, 02:42 PM   #15
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Default Re: Battery light still on, new battery, new alternator plus new issues

Thanks for the reply OP!

I think that it's my whole ECU. My alternator didn't came loose, the voltage regulator fkcued up and it was craning 18v everywhere so I guess it blew something in my ECU.

I'll call my honda dealer to ask if they can program them.

I can't drive the car with a new ECU right? Unless they program it? So I'll have to install it there?
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Old 02-05-2013, 07:15 PM   #16
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Default Re: Battery light still on, new battery, new alternator plus new issues

Quote:
Originally Posted by thecca View Post
Thanks for the reply OP!

I think that it's my whole ECU. My alternator didn't came loose, the voltage regulator fkcued up and it was craning 18v everywhere so I guess it blew something in my ECU.

I'll call my honda dealer to ask if they can program them.

I can't drive the car with a new ECU right? Unless they program it? So I'll have to install it there?
Correct. Dealer has to marry it to your car. I just had everything unbuttoned and the glovebox removed so all they had to do was remove 1 bolt to get it out.
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