W201 (190E) alternator output queries.

Discussion in 'Engine' started by quenching, Oct 16, 2011.

  1. quenching

    quenching Active Member

    Feb 16, 2010
    '84 W201 2.0E, '91 W124 230TE estate
    I've already posted this query over on the 190 forum but lots of different people here who may be able to help.....

    Some advice needed from those who know more than I do about rough engine idle and a possible link to alternator output, that will be most people as I know very little :eek:

    I have a 1984 190E 2l petrol auto with 37k miles, used once a week or so from Feb to Nov and more regularly in winter, serviced according to MB schedule. Its always had an issue with what sounds and feels like the idle speed dropping a little too low and causes a "shudder" in the engine. Its more noticeable when the electrical load on the engine is increased by switching on headlights or rear window demister etc. A gentle lean on the accelerator to raise the idle speed by 100rpm or so and the shudder is gone. I've had idle control valve removed, cleaned overnight and refitted but that didn't solve the problem, however it never bothered me enough to investigate further. Time has come that it now bothers me so I want it resolved, only taken 2 years!

    So, as the increase in electrical load seems to cause the roughness/shudder/low idle speed, or whatever I should call it, I figured that the alternator was a good place to start, its charging the battery and the car never fails to start immediately, but the voltage seems a little low, never more that 13.6V when engine revved to 2000rpm, and about 13.1V on idle when gearbox in P (move it to D and it drops to 13.0V. I replaced the regulator on the alternator with a new one but that hasn't made any difference.

    Today I put a test meter across the battery terminals and here's a summary of what I've noted.

    Gearbox in P
    Idle speed: 700rpm, Voltage: 13.1V
    Generally runs smoothly but can very occasionally be a bit "lumpy".

    Gearbox in D
    Idle speed: 700rpm, Voltage: 13.0V
    Idle speed initially drops by 100rpm or so when shifting to D but recovers quickly, but engine seems rougher.

    Effect of increasing electrical load - the same effects in either P or D
    Switch on rear demister: Voltage 12.7V
    Plus switching on headlights: Voltage 12.3V (idle speed drops to 600rpm)
    Plus turn steering wheel to engage power steering pump: Voltage 11.5V
    Engine now very rough but recovers as electrical load is reduced.

    I've also noticed that the noise from the fuel pump changes tone as electrical load in increased, from a steady hum to a lower pitched pulsating hum.

    Should the alternator output be higher that the figures above? I believe the one in the car is still the original 1984 unit, a 55amp Bosch with part number 0120489300.

    The car has just been serviced and the mechanic (who is very experienced and knows his MB inside out) thinks the alternator is a likely cause, or possibly the the timing chain tensioner is not operating correctly and not slack in the chain is throwing the valve timing out, but he's suggested changing the alternator first as its the easier job. I don't doubt him for a second but I'd prefer to be more positive than just start swapping parts ( I can live with £10 for the regulator :) ). It also had new HT leads and distributor cap about 3k miles ago as they were in poor shape to due to age. The problem was there before these were changed.

    Any and all advice gratefully received, if more info would be helpful let me know and I'll do my best.

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