C5 4.2 Alternator problems
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AudiWorld Junior Member
Joined: Oct 2010
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C5 4.2 Alternator problems
I’ll try to explain this as clear as possible…
I changed the alternator on my ’01 4.2 last summer. Everything worked fine up until the first snow storm. The red battery light turned on while driving and voltage dropped below 12 on the gauge. Brought the car in a garage, checked all the electrical connections to the alternator and battery. Everything was snug and the battery was charging again, gauge between 12 and 14V.
The problem did come back a month later. The alternator does not start charging the battery unless i rev the engine over 3-4 rpm.
When there’s slushy snow or water, I do get the red battery light on the dash, and it sort of flickers away, until it completely goes off. It does not affect the charging, gauge does not move from 12-14 V range.
My questions are, how can i test the exciter wire and what else should i look at, that might be causing this problem?
The alternator is still in warranty and they offered to test it for free, IF i take it off the car. Hoping to check the other possibilities before doing that job again.
Thanks for your time!
I changed the alternator on my ’01 4.2 last summer. Everything worked fine up until the first snow storm. The red battery light turned on while driving and voltage dropped below 12 on the gauge. Brought the car in a garage, checked all the electrical connections to the alternator and battery. Everything was snug and the battery was charging again, gauge between 12 and 14V.
The problem did come back a month later. The alternator does not start charging the battery unless i rev the engine over 3-4 rpm.
When there’s slushy snow or water, I do get the red battery light on the dash, and it sort of flickers away, until it completely goes off. It does not affect the charging, gauge does not move from 12-14 V range.
My questions are, how can i test the exciter wire and what else should i look at, that might be causing this problem?
The alternator is still in warranty and they offered to test it for free, IF i take it off the car. Hoping to check the other possibilities before doing that job again.
Thanks for your time!
AudiWorld Senior Member
Joined: Oct 2009
Posts: 840
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Does your car have an "underbelly pan"? the plastic cover that prevents dirt, snow water entering the bottom of engine?
Regarding the alternator, is it OEM or aftermarket? Also, check codes to see anything meaningful comes out there.
From what you are saying, it looks like water is getting in there somehow and possibly creating a short. Take a look as to why that is happening. It won't be too long before your new alternator goes bad if this continues. Another thing to check is your battery. It sounds like it is good. This happened to me one time. The battery charges and so forth but loses the charge quickly. Not the same problem but, try a new battery if the current one is old.
Regarding the alternator, is it OEM or aftermarket? Also, check codes to see anything meaningful comes out there.
From what you are saying, it looks like water is getting in there somehow and possibly creating a short. Take a look as to why that is happening. It won't be too long before your new alternator goes bad if this continues. Another thing to check is your battery. It sounds like it is good. This happened to me one time. The battery charges and so forth but loses the charge quickly. Not the same problem but, try a new battery if the current one is old.
AudiWorld Senior Member
Joined: Mar 2004
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Was the "new" alternator rebuilt or really new? I experienced similar issues, where you had to punch it to get the charging started, with an elcheapo rebuilt years ago. Turns out the rebuild was only bearings and brushes. There were a couple of diodes burned out. Depending on the design, you could have 6 bridge diodes, and with one out, the alternator will charge, but sometimes needs a kick to get started.
You can test for diodes with a digital meter capable of reading AC volts. Connect the meter between the BAT terminal and ground and the AC volt ripple should be less than 0.5 volts. You can also test for diode leakage. A diode pack may be available, or an alternator shop can rebuild it. Or just buy a new alternator.
You can test for diodes with a digital meter capable of reading AC volts. Connect the meter between the BAT terminal and ground and the AC volt ripple should be less than 0.5 volts. You can also test for diode leakage. A diode pack may be available, or an alternator shop can rebuild it. Or just buy a new alternator.
AudiWorld Senior Member
Joined: Mar 2005
Posts: 809
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I would be concerned about the replacement alternator. I've seen many aftermarkets NOT really be rebuilt correctly and have bad internal electrical components. If the alternator gets wet and the alternator light comes on, its either a slipping belt or something inside the alternator is not fully insulated as it should be.
Thread Starter
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I forgot to mention:
I don't have the belly pan unfortunately, did not come with the car. I have not been able to locate one, except the dealer, which is asking close to 600$ with taxes i think...
I'm waiting to see if Shokan will get some any time soon. They have been out of stock of them for a while.
The battery is brand new as well and holds charge. It starts even at -20 celsius after sitting over night.
The alternator was rebuilt and I was told it's a full rebuild, using only the old casing and pulley. They also included 1 yr warranty. If it is the alternator, it will be the first thing i do once spring comes along.
What i was wondering is if i bring the car somewhere to test the alternator or even if i take off the alternator, when they do output test it will show it's giving out enough current to charge the battery.
The main problem is that it doesn't get the signal to start charging when i turn on the car. Is there any way for me to test the exciter wire to make sure it's telling the alternator to charge?
And when they do test the alternator, can they test to see if it "receives" this signal correctly?
Thanks for the quick replies guys!
I don't have the belly pan unfortunately, did not come with the car. I have not been able to locate one, except the dealer, which is asking close to 600$ with taxes i think...
I'm waiting to see if Shokan will get some any time soon. They have been out of stock of them for a while.
The battery is brand new as well and holds charge. It starts even at -20 celsius after sitting over night.
The alternator was rebuilt and I was told it's a full rebuild, using only the old casing and pulley. They also included 1 yr warranty. If it is the alternator, it will be the first thing i do once spring comes along.
What i was wondering is if i bring the car somewhere to test the alternator or even if i take off the alternator, when they do output test it will show it's giving out enough current to charge the battery.
The main problem is that it doesn't get the signal to start charging when i turn on the car. Is there any way for me to test the exciter wire to make sure it's telling the alternator to charge?
And when they do test the alternator, can they test to see if it "receives" this signal correctly?
Thanks for the quick replies guys!
Tech Guru
Joined: Oct 2002
Posts: 34,430
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Turn the key on without starting the car. Is the battery light lit? If yes, wiring is fine.
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Thread Starter
AudiWorld Junior Member
Joined: Oct 2010
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I will try that after work!
I also know the voltage reading by the car is good because when i start the car and DON'T rev the engine, the battery drops below 10V after a while and the yellow battery sign in the LCD pops up.
It's the red battery sign in the cluster that flickers in intensity when i pass a big puddle or something. Again probably due to no belly pan...
I also know the voltage reading by the car is good because when i start the car and DON'T rev the engine, the battery drops below 10V after a while and the yellow battery sign in the LCD pops up.
It's the red battery sign in the cluster that flickers in intensity when i pass a big puddle or something. Again probably due to no belly pan...
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Thread Starter
AudiWorld Junior Member
Joined: Oct 2010
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I will try to get it vag-com'd over the weekend.