Voltage Drop When Rad Fan Kicks On

[Deerhunter4]

When at idle at approximately 700 rpms and the fan kicks on I get a large voltage drop and rpms dip down to 400 and then surge back to 1000 before leveling back off at 700. Voltage goes from 13v down to 11.5v and then returns to 13v. I replaced the fan hoping it was bad with no change. I replaced the alternator a week or so ago and everything is ok except for when the fan kicks on. The AC compressor cycles normally with voltage fluctuations. I can turn on all accessories and lights with no issues so I thought the new alternator was working as it should. I believe it’s a 135 amp. Any ideas on what to look for that might cause the hard start on the fan? Thanks.

Car is a 2008 GT manual. All stock. 135k miles.

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[1950StangJump$]

What you describe was typical on cars 30 years ago, especially if a small motor and low amp alternator.

But, never heard of it on an S197 Mustang. My first instinct is that your alternator is a cheapo. What brand is it?

Inevitably, someone will pipe in and tell you to either get a high amp PA Performance alternator or get the 2010 alternator from Department of Boost. They might work, but it might also be that your 135 amp would normally work but you got a bad one or a cheap one.

 

[Deerhunter4]

Thanks. I’ll check on the brand but i do know it’s a remanufactured job. I guess I could pull it off and see if the parts store will swap it out.

Seems that if I’m not running a big amp or aftermarket lights the 135 amp should be ok if it’s working like it should.


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[BEAST]

Clean/replace all of your grounds. Not just the terminal on the battery, all of the ends.

 

[Juice]

How old is the battery? At idle, alternator output is the weakest, and the battery is supposed to handle loads such as the cooling fan. Since your voltage drops well below battery voltage, I would load test the battery.

 

[Deerhunter4]

How old is the battery? At idle, alternator output is the weakest, and the battery is supposed to handle loads such as the cooling fan. Since your voltage drops well below battery voltage, I would load test the battery.

Date on the battery is 4/18 so it appears to only be one year old. It was ran down completely when my first alternator failed. I put it on a slow charger before I reinstalled it after replacing the alternator but I guess it could have been damaged by the faulty alternator?


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[Deerhunter4]

I cleaned both battery posts and three grounds in the engine bay. Seems to have helped a little. The surge is about 100 less rpms both ways. I’ve looked around and don’t see any other obvious grounds. Any particular place I should look for more?

I think I’ll also get the battery load tested later when I go to town. It’s just a duralast so maybe if it’s bad autozone will replace it.


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[Juice]

Yea, you can suck a lot of life out of an alternator charging up the battery. They arent designed to deliver sustained max output. Only to put back what the starter used and feed the car while in-use.

 

[JeremyH]

Check fan plug and wires at bec as well.

 

[Deerhunter4]

So I pulled the alternator and cleaned all the mounting points. I used my dremel to get all surfaces shiny including the connection for the wire connected to the alternator that goes to the battery. Everything seems ok. I still get a small dip in rpms when the fan kicks on but it’s not nearly as dramatic as before and it doesn’t rev up to 1000 rpms after the dip like it was before. Wish I had an amp meter so I could check the output from the alternator.




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[fdjizm]

I got that when I used a duralast alternator one time, even the fuel pressure would fluctuate at idle.
Upgraded to a 2010 mustang alternator haven’t seen it since.

 

[06monera96]

I can say, the huge drop would be huge AMP drain.
Could be the start of the BEC starting to lose contact or the connectors with corrosion causing huge AMP spikes during On
Or could just be a dying rad fan motor.

 

[DiMora]

OEM alternators are horrible; I killed two in under 50,000 miles. Buy a DOB unit.