R129 300SL 24v - Starts then immediately dies

Discussion in 'Engine' started by DanMorgan, Jun 26, 2014.

  1. DanMorgan

    DanMorgan Active Member

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    Not having much luck at the moment, got a W124 300 thats not starting and now my R129 is starting to play up :wallbash:

    It takes a while to start, with my foot hard to the floor and once it eventually starts, it dies straight away.

    I had a rare occassion where it stayed on and I went on a 15 mile drive without problems, but that was fluke.

    Any ideas?
     
  2. WG M-B

    WG M-B Auhorised Forum Sponsor Authorised Forum Sponsor

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    The dizzy cap is real common on these
     
  3. OP
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    DanMorgan

    DanMorgan Active Member

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    Replaced that and the Rotar arm a couple of months ago. There is strong spark and when I do drive it, there is no misfiring or stalling like there was when I bought it (hence buying new ones)
     
  4. WG M-B

    WG M-B Auhorised Forum Sponsor Authorised Forum Sponsor

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    Fuel pressure? Have you checked that? The mas relay can fail
     
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    DanMorgan

    DanMorgan Active Member

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    Yup, there was fuel pressure and I have two MAS relays - tried them both and still no difference
     
  6. grober

    grober MB Master

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    Possibly the cold start valve or associated circuitry if its only when cold. Can be checked by removing valve and directing it into a suitable receptacle bottle while blocking off the hole in the manifold. OR the fuel pump relay is on the way out
     
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    DanMorgan

    DanMorgan Active Member

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    I think it may be the cold start valve myself. I forgot to mention, a Clifford alarm was removed - would this cause any of this? I have driven it a couple of times after this, so I doubt it
     
  8. WG M-B

    WG M-B Auhorised Forum Sponsor Authorised Forum Sponsor

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    If you think it's the cold start valve why not disconnect is and try it?
     
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    DanMorgan

    DanMorgan Active Member

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    I will give it ago - Currently raining :(

    Any other possible reasons why?
     
  10. WG M-B

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    The fpr is built into the mas relay I'm sure. I don't have a wiring diagram to hand so cannot tell you which wires to bridge in order to bypass it. I seem to remember they are to one side.
    I could get you the PIN numbers tomorrow
     
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  11. demetrios

    demetrios Active Member

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    My r129 500 started on Saturday and midway going roofless it switched off. My heart skipped a couple beats and then realised it had no fuel:eek:
     
  12. OP
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    DanMorgan

    DanMorgan Active Member

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    Just took the colds start valve off. It started straight away without cranking like it usually does, but still it died straight away
     
  13. macker64

    macker64 New Member

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    The OVP relay, if original, can also cause erratic problems on these cars.

    It lives beside the engine ECU, behind the main battery. The original has one fuse on top and the replacement OEM has 2.

    Very good idea to replace it. These cars are very sensitive to variations in voltage.

    When you do get it started, measure the voltage across the battery with a DVM, should be a stable 14 volts approx.

    If not, replace the REG pack on the alternator. Very easy to replace, two screws and your done.

    Best of luck!! :thumb:
     
  14. Vlad

    Vlad MB Enthusiast

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    I would also second having a look at the OVP relay.....these relays suffer from dry joints internally as they get older ,causing all sorts of issues.
     
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    DanMorgan

    DanMorgan Active Member

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    I had also replaced the OVP around 2 months ago, surely it wouldnt have packed up already?
     
  16. SL300-24

    SL300-24 MB Enthusiast

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    I would test the switch in the throttle body.

    THROTTLE HOUSING - W124, R129, SL300 24V MERCEDES-BENZ A0021402253 0021402253 A0021402253

    The one with the three pins.Engine off, Meter on ohms ideally on buzzer.
    Black probe on centre pin. Red probe on rhs pin should buzz. Open throttle should not buzz. That's the idle side.
    Left hand pin is for full throttle. Red probe on this should not buzz, open throttle wide and it should buzz. I suspect it is the idle side that has gone down.
    This means that the KE-Jetronic ECU is not receiving the correct signal and running open loop and is only working on the mechanical side without electronic control.
     
    Last edited: Jun 27, 2014
  17. Vlad

    Vlad MB Enthusiast

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    Did you replace with a new OVP relay or a second hand one?...new ones have a slight re-design.
     
  18. OP
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    DanMorgan

    DanMorgan Active Member

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    I replaced the Camshaft sensor and it worked and went on a 30 miles cruise trouble free. However the Idle speed is at 500, which is concerning
     

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