alternator with low voltage after warmup

[jls_frog]

I replaced my alternator in December due to the voltage regulator going out in it, I bought the replacement from a local alternator rebuild place. The other day my radar detector shut off while i was sitting at a light, so i switched it over to display voltage. While driving my car was around 12.9 - 13.5 volts, but idle it would go all the way down to 11.9 volts. But as the car warmed up the voltage would slowly go down, even as i was driving. So yesterday i pulled the alternator and took it to get a replacement, the shop tested it and said it was perfectly fine, but just in case it had a problem after it was warming up they gave me another freshly rebuilt alternator that they tested also.

I installed the alternator last night and took the car around the block when i left my place it had 14.1 volts, but by the time i got home it was reading 11.5 volts. I have tested the voltage with a multimeter at the alternator terminal and it is reading the same low voltages maybe .2 volts higher then the battery. Today i checked the belt, and all the plugs on the alternator and everything seems to be fine. When i started the car this morning it had 14.3 volts again but after a few minutes of ideling it drops to 12 volts. The red wire in the alternator plug has 12v constantly to it. The charging relay is a new factory relay replaced in November. The are no dash lights on, scanned the ecu with a obd2 and no codes come up. If i turn on the lights, heater/ a/c and put a draw on the system it now drops to 10.5 volts.

Also once the voltage on the alternator drops the idle on the car goes up to 1500-1700 rpm.

any ideas?

 

[jls_frog]

I looked today online and could not find were do those normally ground at? When i used my volt meter with positive on the battery and the neg connected to those pins it did show 12.x volts.

 

[luv2rallye]

If you mean the voltmeter's positive lead on battery plus and voltmeter's negative lead on those pins, of course it would show 12v since those pins are grounded and you are then effectively measuring right across the battery.

No I meant unplugging ECU and measuring resistance from those pins in the harness to ground. They're probably ok since the ECU wouldn't work if they weren't but just to make sure you don't have any resistance there.

 

[turboglenn]

If you're not opposed to it a saturn alt (or chevy "CS" style) is much simpler and allows you to setup your own chargins system as well as pulling the signal it refferences to know how much volts toput put from a source that will to an extent allow you to control the alt's output.

In my thread "important improvements on the saturn alt swap" in teh custom fab section there's a lot of links to a site i hold dear in my electrical ventures on the DSM by using info they gathered about all the "good and bad" from the charging systems used by all the major detroit car makers and the pros and cons

here's a link to their site and there's more intfo direct to our situations on my thread (i've foudn the same issues as you on my 2g and was going through mitsu alts every 6 months when i decided to go saturn..since then i've had one fail from normal use and one fail from being covered in oil from a turbo oil feed line leak (i mean it was SOAKED, might as well have dipped it in oil and installed it )

MadElectrical.com - Electrical Tech

 

[jls_frog]

Pulled the alternator again and had it tested it put out 14.5 volts with no problem. Also when i ground the black with yellow stripe wire at the alternator the battery light, brake and a/t temp light all come on like they should. also tested the green wire with it up plugged from the ecu and there is no voltage on it nor is there any voltage on the ecu with that part of the harness un-plugged.

turboglenn, thanks for the advice but I'm trying to get the alternator i have working i have actually read your thread a few times trying to figure out what wire did what on the alternator.

 

[Sir Joke]

I had a similar issue, tried different alts, different ecu, then i relocated the alternator and put a saturn alt in and havent had a issue since. I couldnt run my halos and my gauges would dim when i put my lights on. Idle sucked and now i have no issues.

 

[juntjoo]

1) Is the alt relay you replaced the "Mitsubishi generator relay" next to the BCS?

2) Does the battery (charge) light come on when you initially turn the key from off to on (but not start)?

3) Does the alt white wire have near 0v or near +12v?

How do you check the battery charge light? Mine also does not go on with the key in.

 

[luv2rallye]

How do you check the battery charge light? Mine also does not go on with the key in.

The charge light is the light that looks like a battery icon. If it does not go on when first turning the key to On before starting engine, it's probably burnt out and you'll have to remove the instrument gauge cluster to get at it.

 

[luv2rallye]

How do you check the battery charge light? Mine also does not go on with the key in.

The charge light is the light that looks like a battery icon. If it does not go on when first turning the key to On before starting engine, it's probably burnt out and you'll have to remove the instrument gauge cluster to get at it.

 

[juntjoo]

Or what if my alternator isn't working?

 

[Mitsubishiown]

Ever find out the fix to this?

 

[EclipsedFLA]

Or what if my alternator isn't working?

If that is the case, you would definitely be experiencing troubles, My OEM 2g alt died twice, causing sparatic idling, and radio etc shutting off. I put a galant alt in and still having low voltage issues. My guess would be somewhere around the ECU. I would say, as suggested pulling the ECU and testing it.

 

[mattasi]

I really would not be surprized if the OP problem was heat soak from the DP. I have simular issues untill I made and installed a heat shield that goes in between the alt and the DP.

The alternator shop voided my warranty on the alternator because I was going threw regulators like nothing. They said something was wrong with my car. I told them that I am an electric tech and that nothing was wrong with the car electricly. I even offered to have them look at the car, but they didn't want to. Ended up that it was heat soak, killing the regulators in less than 500 miles.

 

[EclipsedFLA]

I really would not be surprized if the OP problem was heat soak from the DP. I have simular issues untill I made and installed a heat shield that goes in between the alt and the DP.

I wonder if the OEM shield will work over the o2, But will heat soak cause low amp on a brand new alternator?

 

[mattasi]

I wonder if the OEM shield will work over the o2, But will heat soak cause low amp on a brand new alternator?

Not sure what you are saying here, sorry man.

 

[EclipsedFLA]

my apologies, it was two seperate statements,first I was pondering if the stock o2housing heat shield will suffice for retaining heat. And secondly I understand heat soak will/can cause a new alternator to show fatigue? And cause the voltage drop? Is that correct?

 

[wmrcs]

I've been into car audio for the past few years and have had a digital voltage meter to watch it as I was playing the music, and I always learned that in colder weather the voltage was a few tenths higher and in warmer it was a few tenths lower. For example, I have a Jeep and when I start it in the morning it idles at 14.7v and then as it warms up (engine and weather) its idle voltage goes down to about 14.1 or 14.2. Is your voltage lower than that?

edit: Wow well I feel like an idiot. I didn't see your voltages in the original post. *facepalm*

 

[mattasi]

my apologies, it was two seperate statements,first I was pondering if the stock o2housing heat shield will suffice for retaining heat. And secondly I understand heat soak will/can cause a new alternator to show fatigue? And cause the voltage drop? Is that correct?

The stock one can always be improved. Maybe add another homemade shield inbetween the DP and alternator. If the heat soak is really bad like in my case, then your brand new alternator would have went out within 500 miles.

I've been into car audio for the past few years and have had a digital voltage meter to watch it as I was playing the music, and I always learned that in colder weather the voltage was a few tenths higher and in warmer it was a few tenths lower. For example, I have a Jeep and when I start it in the morning it idles at 14.7v and then as it warms up (engine and weather) its idle voltage goes down to about 14.1 or 14.2. Is your voltage lower than that?

edit: Wow well I feel like an idiot. I didn't see your voltages in the original post. *facepalm*

Yes this voltage drop is correct. And if it goes down much more than that at full temp, then look for ways to cool the alternator.

 

[EclipsedFLA]

The stock one can always be improved. Maybe add another homemade shield inbetween the DP and alternator. If the heat soak is really bad like in my case, then your brand new alternator would have went out within 500 miles.

Thank you. good knowledge