| Author |
Message |
bobhill8
Samba Member
Joined: June 09, 2017
Posts: 777
Location: MA
|
 Posted: Tue May 06, 2025 11:00 am Post subject: Why did this alternator fail a bench test? Help me understand |
 |
|
Hey all, So I had an old alternator in a parts bin from a previous van purchase. I brought it to Advance Auto Parts and had them bench test it. They said it failed and I said "Why?" and they showed me this readout:
The guy couldn't explain except to say that it failed according to the machine. I don't know enough about alternators to have a clue, but wonder if anyone could enlighten me what the two failed categories mean, and if the alternator could be rebuilt. It's just a spare so it's not critical, but would be good to know. I COULD install it in my van and try it, but that is a PITA, as we know. Thanks.
_________________
1986 Westy
1971 Squareback |
|
| Back to top |
|
 |
ALIKA T3
Samba Member
Joined: July 30, 2009
Posts: 7105
Location: Honolulu,Hawaii and France
|
|
| Back to top |
|
|
DanHoug
Samba Member
Joined: December 05, 2016
Posts: 5594
Location: Bemidji, MN
|
| Posted: Tue May 06, 2025 1:47 pm Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
i'm calling this alternator good. i don't know what the 'lamp voltage' tests are testing exactly but a quick perusal of the interweb shows multiple people with that specific failure on good alternators at the Autozone test stations. i'd say, shooting from hip, that it has something to do with the high rpm required on Bosch alternators in vintage cars to engage charging. gotta blip the throttle to get 'em to put the light out and get 'em to charge.
https://www.bimmerforums.com/forum/showthread.php?2469099-Battery-Light-Flickering&p=30994893
_________________
-dan
60% of what you find on the internet is wrong, including this post.
'87 Westy & '89 Westy both 2.1 4spd
Past projects can be found at--
www.thefixitworkshop.com |
|
| Back to top |
|
|
airschooled
Air-Schooled
Joined: April 04, 2012
Posts: 13407
Location: West Coast, USA
|
| Posted: Tue May 06, 2025 2:07 pm Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
There are a few types of alternators, regarding how they trigger the idiot light and what the voltage regulator does and how. What kind of alternator is this? I bet it's fine…
_________________
One-on-one tech help for your vintage Volkswagen:
www.airschooled.com
https://www.patreon.com/airschooled |
|
| Back to top |
|
|
Hammy1
Samba Member
Joined: May 08, 2012
Posts: 220
Location: OTTAWA, ON
|
| Posted: Tue May 06, 2025 4:55 pm Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
Not to high jack the thread but are the lights required for the alternator to function properly? Or can you just run 12v+ straight to the exciter wire?
Regards,
Jon h. |
|
| Back to top |
|
|
?Waldo?
Samba Member
Joined: February 22, 2006
Posts: 9927
Location: Where?
|
| Posted: Tue May 06, 2025 5:45 pm Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
| Probably the main reason the machine said it failed was because they sell more alternators that way. |
|
| Back to top |
|
|
bobhill8
Samba Member
Joined: June 09, 2017
Posts: 777
Location: MA
|
| Posted: Tue May 06, 2025 6:19 pm Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
Thanks crew. I agree that there is certainly the possibility that the alternator is functioning and luckily I don't need it currently (Pun!). It is an older Bosch, but that might be better than a newer "Bosch". Guess I have to put it in to be sure, but I still say it's kind of a PITA to swap it out just to test. I guess it's all relative. Not planning on driving around with the thing in my van, so I guess if my present alternator stops working I can try it then. Thanks Dan and others for hypothesizing.
_________________
1986 Westy
1971 Squareback |
|
| Back to top |
|
|
CarstenR
Samba Member
Joined: August 07, 2023
Posts: 69
Location: Michigan
|
| Posted: Fri May 16, 2025 5:58 am Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
I may be way off-base here, but when I originally got the Opa Bus (1981 Westy) I purchased a cigarette-lighter-plug-USB-and-voltage-readout because I was suspicious of the battery. When I finally got the bus to start I discovered the alternator wasn't charging, according to my dohickey. After getting the alternator rebuilt and still not seeing the proper voltage when the bus was running I did more research and learned about the 'blue wire' and the fact that the engine had to hit a certain rev count before the alternator would kick in if that blue wire wasn't properly connected. I wonder if the test of the alternator wasn't done to account for this idiosyncrasy(?)...
_________________
Carsten Ramcke
Allen Park, MI
1981 Aircooled Westfalia: the 'Opa Bus' |
|
| Back to top |
|
|
tjet
Samba Member
Joined: June 10, 2014
Posts: 3716
Location: Az
|
| Posted: Fri May 16, 2025 7:33 am Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
| CarstenR wrote: |
| I may be way off-base here, but when I originally got the Opa Bus (1981 Westy) I purchased a cigarette-lighter-plug-USB-and-voltage-readout because I was suspicious of the battery. When I finally got the bus to start I discovered the alternator wasn't charging, according to my dohickey. After getting the alternator rebuilt and still not seeing the proper voltage when the bus was running I did more research and learned about the 'blue wire' and the fact that the engine had to hit a certain rev count before the alternator would kick in if that blue wire wasn't properly connected. I wonder if the test of the alternator wasn't done to account for this idiosyncrasy(?)... |
The alternator in my van needs the same throttle blip to turn it on. After that, it stays on, regardless of RPM.
Not sure if that's how is supposed to be, but that's how it been since I've owned it. |
|
| Back to top |
|
|
jlrftype7
Samba Member
Joined: July 24, 2018
Posts: 4435
Location: Chicago
|
| Posted: Fri May 16, 2025 10:25 am Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
| tjet wrote: |
| CarstenR wrote: |
| I may be way off-base here, but when I originally got the Opa Bus (1981 Westy) I purchased a cigarette-lighter-plug-USB-and-voltage-readout because I was suspicious of the battery. When I finally got the bus to start I discovered the alternator wasn't charging, according to my dohickey. After getting the alternator rebuilt and still not seeing the proper voltage when the bus was running I did more research and learned about the 'blue wire' and the fact that the engine had to hit a certain rev count before the alternator would kick in if that blue wire wasn't properly connected. I wonder if the test of the alternator wasn't done to account for this idiosyncrasy(?)... |
The alternator in my van needs the same throttle blip to turn it on. After that, it stays on, regardless of RPM.
Not sure if that's how is supposed to be, but that's how it been since I've owned it. |
Ours was the same until I installed the 'other' special GoWesty harness that they make for this issue. Once installed, no more throttle reving needed to wake up the Charging Circuit. 
_________________
'68 Westy- my first VW and vehicle/Bus- long gone.- sold it to a traveling Swiss couple....
'67 Type 3 Fastback, my 2nd car- gone
'69 Semi-Auto Stick Shift Beetle-gone
2017 MINI Coopers, our current DDs
‘84 Tin Top - Hilga....Auto |
|
| Back to top |
|
|
harvgwen
Samba Member
Joined: February 11, 2021
Posts: 274
Location: Okanagan Valley, British Columbia
|
| Posted: Sat May 17, 2025 6:07 pm Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
Tjet I know there are several different ways of exciting the alternators in our vans, and I think VW and Subie are different. (Not VW or Subie) when my 01 Discovery did this, the exciter wire was broken.
_________________
1986 Westfalia Vanagon, 2.2 Subaru
Okanagan Valley, British Columbia |
|
| Back to top |
|
|
jlrftype7
Samba Member
Joined: July 24, 2018
Posts: 4435
Location: Chicago
|
| Posted: Sun May 18, 2025 5:18 am Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
| harvgwen wrote: |
| Tjet I know there are several different ways of exciting the alternators in our vans, and I think VW and Subie are different. (Not VW or Subie) when my 01 Discovery did this, the exciter wire was broken. |
Yes, the wire came in a little short at the harness, and the copper work hardened as well. It didn’t take much for the wire to break on either the Discovery 2s or the Range Rover .
At least the Alternator and the harness were on top and readily accessible instead of buried on the engine lower down… 
_________________
'68 Westy- my first VW and vehicle/Bus- long gone.- sold it to a traveling Swiss couple....
'67 Type 3 Fastback, my 2nd car- gone
'69 Semi-Auto Stick Shift Beetle-gone
2017 MINI Coopers, our current DDs
‘84 Tin Top - Hilga....Auto |
|
| Back to top |
|
|
Wildthings
Samba Member
Joined: March 13, 2005
Posts: 52085
|
| Posted: Sun May 18, 2025 7:29 am Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
| Your "lamp voltage at stop" is wrong, which means there is a problem with the small set of diodes in the alternator. |
|
| Back to top |
|
|
RawUmber
Samba Member
Joined: June 23, 2019
Posts: 297
|
| Posted: Sun May 18, 2025 12:36 pm Post subject: Re: Why did this alternator fail a bench test? Help me understand |
|
|
From the looks of the readout (my WAG), the Minimum and Max columns are expected numbers for comparison, whereas the Value column contains the actual readings of your alternator. Apologies if I'm stating the obvious.
Therefore:
the "Lamp ON at Stop Condition" didn't happen as it should, specifically because:
the "Lamp Voltage at Stop" was 9.1V where it was supposed to be a maximum of 5V.
There's a good chance that swapping out the voltage regulator/brush holder module with a known good one will resolve, given that the test was appropriate to begin with.
Below is a schematic of a Bosch EL type regulator. The lamp is the circle with an X inside, to the right of "15".
|
|
| Back to top |
|
|
|
