How Long Can You Drive on a "Bad" Alternator?

Status
Not open for further replies.
SwampSurvivor

SwampSurvivor

Joined
Apr 9, 2008
Messages
16,474
Location
Central NY
In the fashion of some high profile people here ... I am making a lot of topics.

But I'm curious about this.

I put a new alternator in the Jeep Saturday and I am noticing a few things with the new one. And this is stuff that has been going on since I have owned the vehicle starting 9/2010.

So I have noticed the following

  • New alternator doesn't drop voltage at idle - the old one would run at 13.5 - 14 after a startup, but would drop to 12 or 12.5 after 20 or 30 minutes. Lights, blower motor and electric fan on it would drop down li>
    • New Alternator doesn't "whistle" under load. Not a whistle of a bad bearing (though that was bad too), but it had an electrical whistle with a higher load on it. I have noticed this on pretty much every Cherokee of this era with the factory alternator
    • Alternator turns on immediately after startup. The old one would take anywhere from 5-10 seconds after startup to start charging. New one is immediate as soon as I let off the key and the engine is running
    • Jeep Cranks much quicker in the morning. Before - and since I have had it - it had always sounded like it had a dead battery in the morning. I think that may have been a result of the battery not getting a full charge.


  • I'm now interested to see if it has the bad parasitic draw that it has always had. The dropping voltage at idle makes me think that it had a bad diode that may have been draining the battery when it was sitting.

    Is it possible the old alternator was bad the entire time I have owned the vehicle?

    Next is going to be replacing the factory battery cables. I will be using premade ones.
 
Whistle under load is a symptom of poor rectification, or poor ground. The old one could have been bad or you could have a grounding problem.

I just went through that with a new Chinese alternator (100% new). Had a whistle coming into the radio but voltage looked okay. I really wanted to buy an oscilloscope to see the distortion, so I could point at the screen and say "Ah hah, one diode is bad!", but didn't. Finally pulled it and replaced the eight diodes in it and the whistling is gone.

Oh, and with only one bad diode you can drive forever. More common is two diodes blow and then you see your headlights getting brighter and darker at idle.
 
Originally Posted By: Brad_C
Sounds like the old alternator had an intelligent regulator and the new one has an old style "one voltage to rule them all" unit.


Externally regulated on these Jeeps.

Originally Posted By: spackard
Whistle under load is a symptom of poor rectification, or poor ground. The old one could have been bad or you could have a grounding problem.

I just went through that with a new Chinese alternator (100% new). Had a whistle coming into the radio but voltage looked okay. I really wanted to buy an oscilloscope to see the distortion, so I could point at the screen and say "Ah hah, one diode is bad!", but didn't. Finally pulled it and replaced the eight diodes in it and the whistling is gone.

Oh, and with only one bad diode you can drive forever. More common is two diodes blow and then you see your headlights getting brighter and darker at idle.


The headlights have been getting dark at idle for a while now. Not a lot, but definitely somewhat dark.

I did clean up the grounds after the Jeep took a swim and I cleaned the alternator. But didn't seem to make any difference. Cleaned up the grounds and the positive cable running from it.
 
Whistle could be a bad diode which could also play some [censored] with the rest of your electrical system. DC systems don't really like AC current getting tossed around.
 
I thought ma mopar had the computer regulate the alt. Surprised they act different.
 
So, sounds like it probably had a bad diode which was causing it to whine and the voltage to drop off and to parasitic draw the battery.

I have been daily driving it with the low price of gas, so I wont be letting it sit long enough for it to drain the battery ... for a month or so, at least.

Used to be 3-4 weeks and the battery would be drained to the point it wouldn't start on its own.

Now to put a winch on
smile.gif
 
How Long Can You Drive on a "Bad" Alternator?

It's usually about a half an hour before your car stalls and you get stuck no the side of the road. Yes, this is based upon personal experience. Yes, I have had a voltmeter installed in my cars ever since.
 
Chrysler charging systems are PCM regulated and most are temperature compensated.Far ahead of anybody else's systems.
 
Originally Posted By: zzyzzx
How Long Can You Drive on a "Bad" Alternator?

It's usually about a half an hour before your car stalls and you get stuck no the side of the road. Yes, this is based upon personal experience. Yes, I have had a voltmeter installed in my cars ever since.


Completely failed, yes. But I believe I have been driving on an ailing alternator for a while.

After the Jeep took a swim, I have tested twice how long it will run with no alternator. Seems 45 minutes to 1 hour about it before it dies.
 
You can drive on a 'bad' alternator until it either dies completely killing the battery, it sends out enough AC voltage to let the magic smoke out of half the electronics in the car (and sometimes certain wire harnesses with any corrosion or high resistance connectors), until it explodes the battery, or until it seizes and snaps the belt I'd guess.
 
Last edited:
I'm going to let it sit for a day or two and see if it still has the parasitic draw that I have been chasing for 5 years.
 
Originally Posted By: Miller88
I'm going to let it sit for a day or two and see if it still has the parasitic draw that I have been chasing for 5 years.

That's easy enough to check with a DVOM that has a current function, can get a cheap one at Harbor Freight... And yes they work fine, checked both of mine against high $$$ Simpson, were dead on...

Originally Posted By: NHGUY
Chrysler charging systems are PCM regulated and most are temperature compensated.Far ahead of anybody else's systems.


Maybe at one time but AFAIK about all mfgrs are using PCM regulated alts... My '07 Grand Marquis has one...
 
Status
Not open for further replies.

Similar threads

I
Battery or alternator?
2
Replies
37
Views
913
dumkid
G
Bad Alternator?
2
Replies
23
Views
1K
Donald
3
Wife's 2013 Hyundai seized up today 94k miles
6 7 8
Replies
158
Views
8K
350Rocket
B
What causes a running vehicle to shut down in extremely cold weather?
2 3 4
Replies
74
Views
5K
gfh77665
Kurtatron
The dangers of a long beard.
2 3
Replies
51
Views
1K
willbur
Back
Top