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

[Ernie Stephens]

This may all be a coincidence but since I had the car serviced yesterday everything seems to be working as it should.  One of the most annoying things that was happening is that I would have to put my transmission in neutral to start the car.  This problem started months ago on an intermittent basis where most of the time in would start in Park but it progressively got worse where recently I had to put it in neutral every time to start it.  I was planning on replacing the lockout switch myself as soon as the weather warmed up but now I'm not sure what to think since it starts readily in Park as well as the dash lights being off. Is this strange or what?
Ernie
  ----- Original Message ----- 
  From: Markus Meyer<mailto:markusfmeyer at gmail.com> 
  To: Mercedes Coupes Mailing Lists<mailto:mbcoupes at mbcoupes.com> 
  Sent: Thursday, February 25, 2010 2:38 PM
  Subject: Re: [W126 Coupe] Strange gauge lights error, any ideas?


  Suggestion, as I just went through a similar test.  I have several 126's, so I just stole the alternator from my SEC to keep my wife's SEL going, since the SEC is hibernating right now anyway.

  I about to get a Bosch AL129X alternator from the yards, found on late 80's, early 90's SAABs.  Same housing, 115 amps vs. 80 amps.

  I've been told most rebuilds are junk, so just be aware.  Dealer wants close to $350-400 for one!

  Markus


  On Thu, Feb 25, 2010 at 4:35 PM, Josh Allen <heyallen at null-ptr.net<mailto:heyallen at null-ptr.net>> wrote:

    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<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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