Alternator output fluctuating

As the title says, ALT output fluctuates between about 12.5 to 15.5 volts (read more). (2001 Jetta, 410k miles, Bosch reman, 120amp ALT)

Background, the car was brought to me several days ago. The owner said it did not start good and that he had installed a new battery. The battery is from Walmart and completely fills the case. I never bothered to look for the amp rating. It does have a 10/18 sticker on it. Also, due to personal issues, I let the car sit for several days before starting it to do diagnostics. I noticed that the RPMs appeared to be going up and down a wee bit. But, by the time I got VCDS connected, the engine had smoothed out. The only DTC was for the CTS.

Anyway, the above was several days ago. So, last evening the engine barely started. I moved the car inside my garage and installed a new Coolant Temp Sensor. Then I left it with a trickle charge on until this morning. A few minutes after removing the trickle charge, I checked the voltage, 12.5. But, the engine did start okay in my 60f garage.

So, anyway, the fluctuation in voltage causes the engine RPMs to noticeably change and I noticed the Serp belt/Tensioner acted as if it was pulsating. I'm assuming the higher volt out-put is causing a slight drag on the engine so it compensates to keep the RPMs constant. (nothing on and park brake up)

Interestingly, after a few minutes (2 or 3) the ALT settles down and shows 14.3 volts and the Serp belt is as sooth as silk. I shut the engine down and restarted, again 14.2 volts and no pulsating. I waited a three or four minutes and restarted, again 14.2 volts and no pulsating.

I did install a new ALT pulley (INA) last spring on this car.

I'm letting it sit outside the garage and cool down to ambient (19f here at the moment). I plan to start it later today to see results.

So, any thoughts about what might be going on?

Yep. If the brushes in the voltage regulator are sticky, then it could be they're only making occasional contact with the slip rings until it gets warm. Add in short trips and you're looking at an undervoltage situation with eventual battery failure, because the alternator just isn't doing its job when it is most needed on startup.


I pulled the alternator a couple of months ago from my wife's Jetta and one of the brushes (those carbon sticks) was about 1 cm longer than the other. A new voltage regulator is a no-brainer requirement except for the fact that one of the slip rings is well grooved. sigh...


Cheers,


PH

Slip rings are available here. https://www.aspwholesale.com/slip-ring-bo-p2384.html

What year is this car?

Antother potential problem with the brushes is that they could be stuck in the holder.


Had that happen once. Coolant had gotten into the alternator and mixed with the carbon dust....

Sorry, it's 2001 Jetta with a Bosch reman 120amp ALT (410k miles, second ALT)

Yeah, I was afraid this is a brush issue ... And, it's not going to get any better!

But, I've never seen anything like this. Even with my bad hearing, I noticed the engine hunting when I fired it up the very first time. Which lead me to believe there was something amiss with the IP. But, then when I scanned with VCDS, observed various functions, etc., everything seemed okay.

Well, its likely the H7 Maxx 800 CCA Walmart battery has been compromised by the lack of adequate charging. The 1/2 amp trickle charge should have bumped the volts up to about 12.75 or higher. It was about 5 minutes after I disconnected the trickle charge when I thought to check the volts... not good at 12.50 volts after about 12 hours charging.

Thanks!

I've had the brushes stuck in the holder too, cleaned them and all was well again.

Slightly off topic, but I was about to post a question regarding alternator output. When I first start our Golf the red charge light stays lit for a while. If I rev it the light goes out but it goes out after a while even if I don't.
Charge voltage when the light has gone out is 14.2V.

There are two small wires on the alternator connector, one is the igniter but what's the other?
If the igniter is broken somewhere, would the alternator still charge normally when it gets going?
No fault codes stored.

Nokivasara, I'll let someone else answer your question. (I think one wire goes to the light in the cluster and the other to the ECU.)

Okay here's the latest.

Apparently my trickle charger was not trickling.

This afternoon, with the car sitting outside since 8:30 am (19f then, and 37f high today), it started okay. The ALT continued to do as it did previously. Then, with VCDS connected, multi-meter connected and a single read digital, it was showing 14.44 volts once it smoothed out. It showed 14.29 volts (small up & down) with head lights on dim and heater fan on position 2. When I revved the engine up to 2400 RPMs charging stayed basically the same. I did several off and restarts with the ALT commencing the fluctuation in charging each time and then settling on 14.44 volts. So, I let it idle for about 30 minutes. When I cut the engine off the battery was showing 12.85 volts. After about three or four minutes it was down to 12.79 then it continued to drop and settled on 12.67 volts after more than 30 minutes.

So, I think the battery is okay.

Further, I looked up in my PC where I had worked on this car last March. I installed a used Bosch ALT with new Bosch brush/voltage regulator set and a new INA pulley. So, I suspect there may be a diode(s) going kaput or that the Bosch (Made in Hungary) regulator is given up the ghost.

But, again, I've not never seen an ALT act this way.

Sounds like it’s a Bosch regulator. I’d replace it to see if it resolves your issue. Otherwise it’s the entire alternator, but really sounds like just the regulator. When you have the alternator out of the vehicle be sure to check the play in the pulley and replace it if necessary.


Sent from my iPhone using Tapatalk

If you replace the regulator, please post the pn and where you bought it. I've been looking to pick one up as a spare for when I need it (better to have one on hand ahead of time) and it seems there are a lot of pn's that are interchangeable. (my original one ends in -350). I see BOSCH branded regulators for $30 shipped. https://www.ebay.com/itm/Fits-2002-...egulator-Bosch-84123WT-2004-2003/292748761644

This is the one that comes up for my 06 Jetta TDi (MKV) - BRM Engine.

https://www.ecstuning.com/b-genuine-volkswagen-audi-parts/voltage-regulator/06f903803b/


Sent from my iPhone using Tapatalk

Here's a link to Autohausaz for a Bosch Voltage Regulator... best deal I've found.

https://www.autohausaz.com/pn/F00M144136

Edit: Here is another source for a Bosch Regulator:
https://www.partsgeek.com/catalog/2002/volkswagen/jetta/engine_electrical/voltage_regulator.html

The guy that owns this 2001 Jetta is ordering this Alternator today. I suspect it is made in China. He said he knows a guy that purchased a DB Electric Starter and it has been a good one. So, we'll see.. His dime, not mine!

https://www.ebay.com/itm/NEW-ALTERNATOR-for-1-9-1-9L-Diesel-BEETLE-99-00-01-02-03-04-05-06-1999-2006/132219988984?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

Andy, remember the hidden ground underneath the air filter box on the BRM? When that ground was loose on my car, I would get fluctuating charging voltage, sometime a click but no crank from the starter and a glow plug light.

I know it's a different car than what you are diagnosing, but I would start by checking grounds. After that, pull the regulator, check the brushes and slip rings.
As a reference point I replaced my regulator at 185K miles before I had any issues. The brushes were pretty short but not arcing or sticking yet however the slip rings were grooved.

I was able to clean the slip rings up by taking the alternator apart and spinning the alternator with an electric drill and my Clutched pulley tool while using sandpaper on the slip rings. Almost forgot, the bearing were just fine.

Bought one of these to keep on hand when I need it.
https://www.ebay.com/itm/VOLTAGE-RE...ASSAT-AUDI-FITS-BOSCH-ALTERNATOR/181518835649

03TDICommuter said:

Bought one of these to keep on hand when I need it.
https://www.ebay.com/itm/VOLTAGE-RE...ASSAT-AUDI-FITS-BOSCH-ALTERNATOR/181518835649

Click to expand...

If you have a lot of miles, I would not wait until the brushes completely wear out. There could be arcing that would destroy the slip rings in the alternator.

The regulator you purchased is not an actual Bosch regulator and may not work correctly because it appears to be an inexpensive Chinese part. Hopefully, it won't damage your alternator.

Ol'Rattler said:

If you have a lot of miles, I would not wait until the brushes completely wear out. There could be arcing that would destroy the slip rings in the alternator.
The regulator you purchased is not an actual Bosch regulator and may not work correctly because it appears to be an inexpensive Chinese part. Hopefully, it won't damage your alternator.

Click to expand...

Thanks for the advice. I did as much googling as possible to see if Transpo parts were any good and so far have not found any bad info. That vendor has sold quite a few of them and no bad reviews.

Transpo was a good company, US-based. then they sold out to China. Not sure what has happened to their quality since.