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Strange rev counter behaviour a clue to cold idle not working?

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by d215yq, Dec 7, 2015.

  1. d215yq

    d215yq Senior Member

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    Your Mercedes:
    1987 W124 300D 250k
    Have been having a rough cold start from my car ever since the temperatures have got lower than 10 degrees. Although the glowplug light never lights (i assume bulb broken), if i wait 15 seconds for the glowplug cycle to work, and press the accelerator a bit the car starts perfectly. However now the temperature is near zero, releasing the accelerator after 5 seconds will see it stall, it needs about 10-15 seconds at 1500rpm before release to idle.

    So I am thinking it isn't a glowplug issue but is a cold start idle issue. I've already played withthe relays behind the battery and switch cleanered them/changed all fuses in fusebox. One thing I notice is that when the ignition is on and I am waiting for the glowplugs, the rev counter acts like a temperature gauage! (6k rpm is when it is near zero, 4k when it is about 5 degrees, 2k when it is 10 degrees, above 10 degrees rev counter is on zero). There is also a clicking from the relays behind battery while car is cold. Once the car starts the rev counter works as normal (although soemtimes mid journey it works intermittently). So does this suggest that somehow the rev counter is shorting out/interfering with the feed to not allow the relay to do its job of making the car put more fuel in when cold?

    If so is there a particular wire or relay or circuit that needs further investigation?
     
  2. television

    television Always remembered RIP

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    The rev counter cannot interfere with the running of the car. when they do not always work it can be a sticky stop pin.

    The glow plug lamp should be on when the key is turned to ignition.
     
  3. OP
    d215yq

    d215yq Senior Member

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    Thanks as ever for quick reply. Perhaps I phrased the question wrong, is the fact the rev counter is somehow getting a feed from something that measures cold temperature a clue as to why the actuator or whatever it is that increases the fuel when cold is not getting this signal/not working?

    The glowplug lamp hasn't worked for a long time, but does the fact the car starts first time with the accelerator down a bit show they do work and it's actually a not enough fuel at idle problem rather than a glowplug one? Also on start up there is nothing more than a puff of white smoke the instant it starts, nothing more after.
     
    Last edited: Dec 7, 2015
  4. television

    television Always remembered RIP

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    Your Mercedes:
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    You have an air temp sensor and water temp sensor that set up the fueling mixtures.

    The rev counter was an extra on most diesels and it takes the reading from the engine CPS, often from the front one on a diesel, so this cannot be the problem.

    Just pulling the connector off either sensor would put the car in the max cold mode
     
  5. OP
    d215yq

    d215yq Senior Member

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    Thanks,

    Could you tell me where these two sensors are and I will try disconnecting them.
     

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