Altenator question...
Tuned
Joined: Aug 2011
Posts: 376
Likes: 4
From: San Antonio, TX
Cool. I'm learning a lot. That's why I like the forum so much. So a car alternator is A/C power? I don't know squat about electricity. And the weather has no affect? I'm glad to know that because I don't want to move up north and have electrical concerns I've never had. 
And I didn't mean to hijack. HAHAHA
And I didn't mean to hijack. HAHAHA
Postmaster
Joined: Apr 2006
Posts: 3,524
Likes: 2
From: CA Bay Area
When the light are dimming, it doesn't have to mean the idle output is too low, but could also mean that the high rpm output is too high. I had alternator issues and have it going close to 15V at high idle.
Now my lights to bare eye stay the same bright with engine on or off.
Have good batteries that keep close to 13V.
Put real voltmeter on the truck, or we can speculate forever.
Now my lights to bare eye stay the same bright with engine on or off.
Have good batteries that keep close to 13V.
Put real voltmeter on the truck, or we can speculate forever.
Lead Driver
Joined: Sep 2007
Posts: 8,180
Likes: 669
From: Scio, OR
Modern day alternators produce A/C, which is converted internally to DC by a diode bridge rectifier. I believe a 6 diode rectifier is most common.
So they output DC. Unless one of the diodes is bad, which can cause them to output some really messy looking A/C like power.
So they output DC. Unless one of the diodes is bad, which can cause them to output some really messy looking A/C like power.
Tuned
Joined: Aug 2011
Posts: 376
Likes: 4
From: San Antonio, TX
Cool. So the Alternator pushes A/C (alternating) through the rectifier diodes removing the negative 120 degree offset waveforms which causes the positive waves to come to an almost complete peak making it D/C (Direct), and a bad diode will in turn cause your meter to show A/C current because the positive waveforms are disrupted by one pesky little negative offset. Crazy stuff.
Lead Driver
Joined: Sep 2007
Posts: 8,180
Likes: 669
From: Scio, OR
Right. A good quality DVOM set to A/C voltage will show A/C output on an alternator with that particular failure. There should be near 0 A/C current registering.
I think however (just my opinion), that some of the cheapo DVOM's might pick up the slightly pulsed DC output of a normally operating alternator and register is as A/C current. So you have to be aware of that when you do that test.
I think however (just my opinion), that some of the cheapo DVOM's might pick up the slightly pulsed DC output of a normally operating alternator and register is as A/C current. So you have to be aware of that when you do that test.
Tuned
Joined: Aug 2011
Posts: 376
Likes: 4
From: San Antonio, TX
dcham you're an alright dude! I know a lot of people that test their stuff with a 13.99 flee market hung sung meter from china that gets an entire 50 test cycles a second in there! And then they think "well I tested everything, I can't find a problem. I use a pretty decent fluke but have never actually run an alternator test to look far A/C voltage via a bad diode. I guess when one craps out I just get lucky at track it down to an alternator because D/C output is low and I'm done. I generally change my batteries every 3 years whether it needs them or not. I'll sell them to my empolyees for the core charge and they may get another 3 years out of them for 20 bucks a battery. I do the same with the wifes car. If I get in my truck when it's 115 outside the ******* better start or I'll be severely angry for something as stupid as a battery. Just not worth my trouble to get stranded for the price of 2 batteries every three years. Then I have to get a jump from someone and it will probalby be a Chevy or Dodge and then I hear about it.
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Tuned
Joined: Aug 2006
Posts: 405
Likes: 42
Man you guys have been hitting this thread HARD since the last time I saw it!! I LIKE IT!!! 
I finally got a chance to measure with a DVOM and amp meter. At high idle, at the batteries is 14.5 VDC. With the HVAC blower on high and the bright lights on I was able to measure 4 amps at the altenator. This measurement was taken at the ten minute mark after starting in about 30 degree temp. The only variance between high idle and normal idle was the voltage was at 14.4 instead of 14.5 nbd I would think??
Thoughts??
I finally got a chance to measure with a DVOM and amp meter. At high idle, at the batteries is 14.5 VDC. With the HVAC blower on high and the bright lights on I was able to measure 4 amps at the altenator. This measurement was taken at the ten minute mark after starting in about 30 degree temp. The only variance between high idle and normal idle was the voltage was at 14.4 instead of 14.5 nbd I would think??
Thoughts??
Lead Driver
Joined: Sep 2007
Posts: 8,180
Likes: 669
From: Scio, OR
14.5V is a touch high, but within spec. Especially if the batteries are requiring a charge. I'll see 14.1 after I start and after the glow plugs go out, for awhile, then it'll work it's way down to 13.2 to 13.5.
How are you measuring amps? Cause it's taking way more than 4 amps to even make that engine idle. The only way to measure the kind of amperage you should be seeing is with a clamp ammeter, or an ammeter using a resistor shunt. Clamps are pretty common but the shunt type are pretty rare.
How are you measuring amps? Cause it's taking way more than 4 amps to even make that engine idle. The only way to measure the kind of amperage you should be seeing is with a clamp ammeter, or an ammeter using a resistor shunt. Clamps are pretty common but the shunt type are pretty rare.
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Tuned
Joined: Aug 2006
Posts: 405
Likes: 42
Yes I was using a clamp style ammeter. I clamped it around the main cable coming off the back of the alternator. I couldn't believe the low reading myself, I had to read it several times just to believe my eyes!
Tuned
Joined: Aug 2011
Posts: 376
Likes: 4
From: San Antonio, TX
Yeah, I was sapping info from DCham. He would say something then I would google it to fully understand then reply back and so on and so forth and junk and stuff. I own a good DVOM and know the difference between a good one and a bad one. However, my electrical experience is limited to classes as an underground utilities troop. Basically it's just enough info to inform you of what dangers are present when you're digging things up and makes you dangerous so I stay away from it. A car is low voltage but I know it takes milliamps to kill you. Either way, I don't want to be horsing arond and blow up some expensive stuff on my truck. Your problem is interesting therefore I'm tuned in. Mainly to ask questions though and learn. I'm eager to see your solution if there is one.
Fleet Mechanic
Joined: Jul 2005
Posts: 1,800
Likes: 52
From: Southern Colorado
I just replaced my alternator. The reason was that with 2 good batts., the 'battery' light on the dash would flicker after starting. I measured the voltage at the battery and it was fluctuating right after startup (while the glow plugs were on) and also at hot idle. The voltage would cycle from battery to alt (14.x) in 1 second intervals - back and forth.
A local alternator shop declared the bearings and diodes dead and quoted $120 to rebuild. I bought a Tye 'all new' alternator from Rock Auto (supposedly a Ford supplier - the alternator is all new, but made in China). Paid $126, and didn't know it was Chinese when I ordered it. I hooked it up and it waits to start charging until the glow plugs turn off, and then goes full output voltage of 14.36v and stays steady. If I turn on the headlights and fan, it drops 0.2 volts or so. The battery light no longer flickers, and clearly the new alternator is doing the job.
Clearly, this 'cycling' of the old alt. was due to an internal problem. At least I have the old alt to use as a core - it was a Checker-auto type junkie thing dated 2007 for the reman.
A local alternator shop declared the bearings and diodes dead and quoted $120 to rebuild. I bought a Tye 'all new' alternator from Rock Auto (supposedly a Ford supplier - the alternator is all new, but made in China). Paid $126, and didn't know it was Chinese when I ordered it. I hooked it up and it waits to start charging until the glow plugs turn off, and then goes full output voltage of 14.36v and stays steady. If I turn on the headlights and fan, it drops 0.2 volts or so. The battery light no longer flickers, and clearly the new alternator is doing the job.
Clearly, this 'cycling' of the old alt. was due to an internal problem. At least I have the old alt to use as a core - it was a Checker-auto type junkie thing dated 2007 for the reman.
Postmaster
Joined: Apr 2006
Posts: 3,524
Likes: 2
From: CA Bay Area
That might answer the question why batteries on Fords need to be replaced so often. The one on our Mercedes lasted over 12 years.
I will have to check my new truck. The old one with new alternator was putting less than 14 v.
I will have to check my new truck. The old one with new alternator was putting less than 14 v.
Tuned
Joined: Aug 2011
Posts: 376
Likes: 4
From: San Antonio, TX
Ah man, Chinese? Hey, I'll give you credit though. At least there is one other person in this country that at least thinks about that besides me. I make my wife take s**t back from China. It takes her for ever to shop now. Did you know that Penn fishing rods and reels are now outsourced to China? I was heartbroken!
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