[W126 Coupe] Strange gauge lights error, any ideas?

[Josh Allen]

Ok, I think I've found the issue.

I followed the suggestions of the ETM and a few others here, tested the 
resting voltage (12.3v), idle voltage (13.7v), and voltage with various 
electrical loads on (13.4v).  I then ran up the RPM with the loads 
(13.9v max), and without (14.2v max).

I then hooked up my oscilloscope and checked the B+ waveform, as the ETM 
suggests, to look at the condition of the rectifier bridge inside the 
alternator.  It came up with an obvious missing phase in the waveform, 
so it appears that one of the diodes has failed.

I've attached a pic of the waveform on the scope, for those who might 
find it of interest.

Thanks for all the suggestions, now I get to do battle with the parts 
house!

-Josh Allen



On 2/24/2010 10:31 PM, Josh Allen wrote:

> Markus,

> After testing today, it's sitting at 13.7-13.9 at idle, and 14.2 or so 

> at ~ 2k rpm.

> I was reading the same voltage indicated at the terminal block on the 

> fender, the battery positive terminal, and the B+ terminal on the 

> alternator.

>

> Looking in the charging system section of the ETM, I'm going to hook 

> my oscilloscope to the B+ terminal of the alternator and see if it has 

> an abnormal ripple, and failing that, I may just wander down to the 

> Napa and see what they can tell with their analyzer.  It's still under 

> warranty for most of another two years.

>

> The fuse on the top of the OVP was still good, I double checked, as 

> well as having the OVP apart to clean up it's pcb which had some nasty 

> flux residue or corrosion on the solder side of the board.

>

> (FYI: The alternator that is currently installed is PN:  RAY 2138381 

> http://www.napaonline.com/Search/Detail.aspx?R=RSE2138381_0218228923  

> with a 72 month warranty.)

>

> -Josh

>

>>

>>

>> On 2/24/2010 2:45 AM, Markus Meyer wrote:

>> Josh,

>> I'd start by checking battery voltages, resting (car not on) and with 

>> car running (this shows your alternator voltage), and go from there. 

>> Any idea what alternator was put in, where it came from, was it new, 

>> rebuilt, new regulator used, etc?

>> Once you rule out a charging problem, you can move on to other things 

>> like the OVP.

>> Markus

>>> Markus,

>>>

>>> The new alternator was whatever I could get from the Napa warehouse 

>>> store, since that was the only alternator available on short notice 

>>> at the time.

>>> It had a new regulator with it, and at the time was putting out 

>>> 14.5-14.7v when running, but I don't know currently.

>>>

>>> I'll check up on the charging system today, one way or another, I 

>>> just couldn't when I found it yesterday evening.

>>>

>>> Thanks,

>>> Josh

>

>

>

>

> Dan,

> The dim light is while the car is running, but not revved above idle.  

> Once I rev it up at all, the lights all glow brighter, in sync with 

> each other, but with the battery light the brightest at all times.

>

>

>

> On 2/24/2010 7:55 AM, Dan Landiss wrote:

>> Josh, could you clarify for me me whether the dim lights are BEFORE 

>> you start the car, or immediately AFTER? I recall some really crude 

>> method of distributing the grounds behind the instrument panel.

>>

>> on 2/24/2010 9:44 AM Josh Allen wrote:

>>>>

>>>>

>>>>     When I start the car, the charging system indicator glows

>>>>     dimly, as well as the brake wear light and the empty fuel

>>>>     system light, but both of the latter lights are around half the

>>>>     intensity of the charging system light.

>>>>

>> -- 

>>    

>

>

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