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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| now what?
noticed a squeaky fan belt for a split second upon starting in the morning
so, one of the things that are turned by the belt puts up resistance
now at day 3 I get an undervoltage message and the battery is falling below 12V
I am thinking : alternator
what else should I be thinking/checking?
have no access to any diagnostic tools
Edited by 4x4abc 10/13/2014 12:10 AM
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amg463
Veteran
Date registered: Apr 2013
Location: Los Angeles, CA
Vehicle(s): 1994 G300
Posts: 217
| Re: no what?
idler pulley replaced already? |
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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| Re: now what?
both have been replaced recently
stethoscope says they are healthy
Edited by 4x4abc 10/13/2014 12:14 AM
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AlanMcR
Expert
Date registered: Apr 2006
Location: US, CA, Los Altos
Vehicle(s): G300DT E300DT 230SL
Posts: 3500
| RE: now what?
Spin the alternator by hand. It should be pretty quiet. If that test passes, then pull it and replace the regulator. Usually the problem is that the brushes have worn out. If you've been wading in muddy water then that greatly accelerates the wear. If you don't have a spare and the brushes are indeed worn out, you can usually make brushes that fit from a larger motor (washing machine, etc).
Edited by AlanMcR 10/13/2014 12:30 AM
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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| RE: now what?
I'll get the sucker out tomorrow (hated it last time!)
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J.R.
Elite Veteran
Date registered: Apr 2006
Location: Southern Maryland
Vehicle(s): 00 G500, 09 911 4S, 11 Cayenne S, 86 280GE (sold)
Posts: 828
| RE: now what?
Here is an image from my pile of odds and ends. I kept a spare set of brushes for what used to be numerous 1980s and early 90's cars in my stable using Bosch alternators. Not sure if the newer Bosch regulators are using these brushes but here's the image for what it's worth.
I recall some discussion years ago about whether or not the voltage regulator could be removed on the G500 without removing the alternator. I just find the posting.
Good luck.
Edited by J.R. 10/13/2014 10:51 AM
 (Bosch Alternator Brushes.JPG)
Attachments
----------------
 Bosch Alternator Brushes.JPG (27KB - 0 downloads)
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olasdeplata
Extreme Veteran
Date registered: Sep 2006
Location: At the Beach in Hatteras
Vehicle(s): 280GE convertible 84, 300E 89
Posts: 396
| Re: now what?
A bad battery in my case burned two sets of brushes before I realized the problem was upstream... And it will start. Not sure if someone has a better trick to place back the brushes with the springs compressed. I taped them, pull as much as the tape as I can, and then when the car is started it burns any remains of the tape. |
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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| RE: now what?
so, sprayed a liberal amount of electrical contact cleaner on the alternator
seems to work fine now
I'll keep you posted if this will last |
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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| RE: now what?
didn't last
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bram_r
Expert
Date registered: Apr 2007
Location: the Netherlands
Vehicle(s): 1984 MB 280GE, 1982 MB 300GD, 1986 Subaru XT 4WD
Posts: 1659
| Re: now what?
It worked for 22 minutes 
Sorry, can't help you with this issue.
Edited by bram_r 10/13/2014 12:45 PM
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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| Re: now what?
Mariano,
can you elaborate on the bad battery and how it caused problems downstream?
should I, could I, disconnect the battery with engine running and test the alternator? |
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Warren T
Extreme Veteran
Date registered: May 2006
Location: Montreal
Vehicle(s): 230GE,300GD,300D
Posts: 519
| Re: now what?
NEVER run engine (alternator) with battery disconected.
Warren |
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J.R.
Elite Veteran
Date registered: Apr 2006
Location: Southern Maryland
Vehicle(s): 00 G500, 09 911 4S, 11 Cayenne S, 86 280GE (sold)
Posts: 828
| RE: now what?
4x4abc - 10/13/2014 11:50 AM
didn't last
Harald,
On 3-4 occasions I have had a Bosch voltage regulator fail (all failed due to a brush wearing out) and there was never a direct cockpit indication. On two occasions I detected an impending engine shutdown because the water temp gauge started to go down...I had about 2 minutes left in the battery's reserve. On one occasion the regulator was dieing intermittently. Sometimes the failing brush was making good contact, other times it would not. Your use of the cleaner may be a false lead.
I've not had one fail in my G500...yet.
My guess is the voltage regulator has an issue, probably one of the brushes has worn out. It was a rare vehicle in my 1980s and early 1990s to see a voltage regulator go past 125,000 - 150,000 miles. I think, no I hope, that Bosch alternators and their VRs have gotten better.
It might not be wise to start the engine off the battery and then disconnect the running truck from the battery. My reasoning is that you have confirmed that the alternator is not producing. If you then disconnect the battery, not only will the engine die, but every electrical component on the truck, every computer that was looking for power will shut down, an abnormal shutdown at best with no damage. However, our G's are power hungry devices and I would not want to find out whether such a procedure would damage those components if I had a choice. You are turning the negative cable connection into a switch, there will be a spark and it might not be pretty.
The alternator is designed to handle all of the loads and fast charge the battery. The battery is there as a starter battery and a supplier during brief peak demands (every electrical item on high in the truck).
If your battery is holding voltage after a charge and then provides a normal discharge using its reserve, it would be hard to argue that the battery contributed to the early demise of the alternator.
Good luck!
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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| RE: now what?
did more tests
charge battery externally (about 13.8V)
start car
measure voltage (charger still connected)
>14V meaning the alternator is working (my charger will not produce that much)
disconnect external power source
still >14V
after about 20 minutes voltage drops below 12.6V
and keeps dropping with engine running
turn off engine
restart
>14V for some time
then slow drop in voltage again = alternator not producing
so, the alternator works only for a while then fails
also noticed: aux fans are not starting (and it's hot here)
do the aux fans have a fuse?
where? |
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Titus Pullo
Elite Veteran
Date registered: May 2007
Location: Ottawa, ON, Canada
Vehicle(s): '90 Wolf 240GD, '06 G270 CDI.
Posts: 1010
| Re: now what?
about the fans: I am not sure but its possible that the computer prevents them from running when a low battery is noticed in order to prevent the fast battery discharge. What I am trying to say is fix the charge/battery issue first and worry about the fans after. Also, I will suggest to replace the battery with a known good one just to localize the problem (if battery available of course).
Edited by Titus Pullo 10/13/2014 9:21 PM
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Floobydust
Veteran
Date registered: Mar 2014
Location: Massachusetts, USA
Vehicle(s): 1995 E320 Cabrio, 2003 G500, 2000 SL500, 1980 TR8
Posts: 284
| RE: now what?
J.R. - 10/13/2014 7:52 PM
The alternator is designed to handle all of the loads and fast charge the battery. The battery is there as a starter battery and a supplier during brief peak demands (every electrical item on high in the truck).
The battery provides two more small, but very important functions as well. First, the battery acts as a "filter capacitor" to eliminate the 3-phase voltage ripple from the raw alternator output. On a car with a very simple electrical system, the raw 3 phase ripple could probably be tolerated, but cars with modern electronics would be much less tolerate ripple. The second function is that "filter capacitor" effect reduces the feedback loop cut-off frequency for the voltage regulator circuit. All feedback systems have a minimum and/or maximum frequency beyond which they become unstable. Without a battery in the circuit, the VR could become unstable, resulting in large voltage fluctuations at the output. Some VRs are designed to shut down under these circumstances and some alternators are built with zener diodes as rectifiers so as to limit the maximum voltage obtainable (once Vz is exceeded, the rectifier stack effectively begins to short itself out). I'm not sure about the Bosch VR, but I wouldn't risk it.
Harold - As for your alternator, it seems to me that you have a simple case of an intermittent VR or VR brushes. The "squeak" at start up occurs because the battery voltage is lower than the set voltage on the VR, so the VR transistor is turned on to apply full field current which in turn causes a very heavy starting torque on the alternator shaft. This can become a vicious circle because the high field currents necessary to charge the battery can in turn cause the brushes and/or regulator output transistor to overheat and become intermittent. This in turn lets the battery discharge. Thus you get the "it runs for a while" symptom. New brushes and VR usually solved the problem.
I hope this helps a little bit. Good luck!
- FD
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AlanMcR
Expert
Date registered: Apr 2006
Location: US, CA, Los Altos
Vehicle(s): G300DT E300DT 230SL
Posts: 3500
| RE: now what?
I'll second that. Verify that the battery is good by either using a known good replacement, or charging away from the vehicle and watching to see that it holds a charge. Still, the alternator bushing could be just barely touching. Once things heat up they could shift around to a point where they no longer touch = no charge.
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4x4abc
Date registered: Apr 2006
Location: La Paz, Baja California, Mexico
Vehicle(s): 02 G500
| RE: now what?
I will check it tomorrow
but with no access to parts I don't really have to hurry |
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AlanMcR
Expert
Date registered: Apr 2006
Location: US, CA, Los Altos
Vehicle(s): G300DT E300DT 230SL
Posts: 3500
| RE: now what?
I'd bet there are quite a few ruined major appliances laying about. Any one of them will have a big motor with brushes that could be shaved down and put to work. Re-purposing motor brushes is one of the classic shade-tree mechanic fixes, and a good one to know in order to get oneself out of the back country. For a short term fix you can stretch out the spring that pushes the brushes out and/or jamb something behind the spring.
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olasdeplata
Extreme Veteran
Date registered: Sep 2006
Location: At the Beach in Hatteras
Vehicle(s): 280GE convertible 84, 300E 89
Posts: 396
| Re: now what?
Harald
I had a one year old Odissey large battery, that of course I did NOT expect it to fail. Well, the battery will start losing juice and it will go from 13.6 with engine running to 12, and then 11.6... I change the brushes assuming that the voltage regulator was the problem, with intermittent charging, but the issue persisted. Charged battery externally, and it will show 12 volts, but will drop by the next day. Burned the new brushes, change them again and decided to have the battery tested. It was bad. Got a new one under warranty.
A bad voltage regulator can ruin a battery and a bad battery can ruin a voltage regulator.
The easiest way to test this to swap batteries with another car and see if the problem reoccurs. If it does, then it is the alternator, and in 90% of the cases, the voltage regulator inside it. |
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