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Thread: 300E MAS Relay question

  1. #1
    E500E Guru JC220's Avatar
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    300E MAS Relay question

    OK so here goes. I have owned a 300E-24 Saloon since I was 18. Some 14 odd years later the same problem has re-surfaced for the 3rd time. (I found 2x old MAS relays tonight from this car) It kills the fuel pumps or rather they become very intermittent to the point where it won’t run. Out of the blue recently it did it again and wouldn’t start. I couldn’t hear the fuel pumps so I knew the age old issue had cropped up again.


    These little relays are expensive from MB (If they are still available I haven’t checked) and expensive used too. I am reluctant to dive into another used unit as it may have the same fault waiting. Sooooo.... I opened the current one up tonight and the circuit board inside was super clean with no issues visible to my eye. I jumpered the fuel pump pins and the car fires up fine only that the engine speed tachometer does not function.

    Now – if I wind the jumper wire around the fuel pump pins on the MAS relay the car starts up just fine again and the Engine tachometer does work again as normal.

    See pic below for what I mean.

    20180508_202158.jpg


    What I am thinking now is wiring in a new fuel pump relay which will function independent of the MAS relay to bypass that fault inside the unit. This would be a replaceable fuse topped relay with it’s own grounds and ignition feed. Do any of the CIS experts see a reason why this might be a very bad idea?

    I know – buy a new one you cheap $$@# I will later in the year maybe but my fleet is draining me currently!
    1987 w124 200 - 3.6 AMG build | 1990 w124 300E-24 | 1992 w124 500E

    1994 320CE Sportline | 1997 w140 S280 | 2007 w211 E220CDI

  2. #2
    Member northNH's Avatar
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    Re: 300E MAS Relay question

    When the MAS relay failed (2° to a shorted wire) on my 300CE M104, I had little problem finding a good used one on eBay for $50.
    A MAS relay for the M103, different #'s but physically identical to the M104 one, worked just fine until I found the M104 one, are way more plentiful and even cheaper. The M103 one is now in my trunk.

    Only difference I found betwixt the two was perhaps less aggressive acceleration at the very top end with the 103 one; possibly my imagination.

    I would drive to my grave with a used 103 one before I would buy a new 104 one, and you need to check your shorts...
    '92 300CE Black/burgundy Sportlined
    '94 320TE Silver/gray
    '86 BMW 635CSi Black/black 5spd
    '67 Volvo Amazon wagon Green/green 75Kmi time capsule
    '62 Volvo Amazon Black/red 4Dr 65Kmi time capsule

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    JC220 (05-08-2018)

  4. #3
    E500E Guru JC220's Avatar
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    Re: 300E MAS Relay question

    Quote Originally Posted by northNH View Post
    When the MAS relay failed (2° to a shorted wire) on my 300CE M104, I had little problem finding a good used one on eBay for $50.
    A MAS relay for the M103, different #'s but physically identical to the M104 one, worked just fine until I found the M104 one, are way more plentiful and even cheaper. The M103 one is now in my trunk.

    Only difference I found betwixt the two was perhaps less aggressive acceleration at the very top end with the 103 one; possibly my imagination.

    I would drive to my grave with a used 103 one before I would buy a new 104 one, and you need to check your shorts...
    Thanks for that. I was wondering would other MAS relay models fit. EPC shows only 2x numbers as compatible for my car and all of them are over $100 USD used on eBay. I am certain the same issue will develop again at some point in the future.

    My car has no A/C or check engine light. It seems bypassing the fuel relay function in the MAS has no I'll effects.......

    Aren't the fuel pumps just on or off? AKA no control over that circuit? I know the feature of a short prime when ignition on will be gone but that doesn't matter much since 99% of the time you just crank to start up in one motion
    1987 w124 200 - 3.6 AMG build | 1990 w124 300E-24 | 1992 w124 500E

    1994 320CE Sportline | 1997 w140 S280 | 2007 w211 E220CDI

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    E500E Guru JC220's Avatar
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    Re: 300E MAS Relay question

    In this pic you can see there is a regular relay in the circuit board (off the shelf relay shown for reference alongside)

    20180508_220205.jpg
    1987 w124 200 - 3.6 AMG build | 1990 w124 300E-24 | 1992 w124 500E

    1994 320CE Sportline | 1997 w140 S280 | 2007 w211 E220CDI

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    Member northNH's Avatar
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    Re: 300E MAS Relay question

    I think my EPC research back then found all 24V CIS-E M104s (300SL, 300CE and in ROW the more pedestrian body styles like yours) basically interchangeably used the same M104 MAS, EPC #s/info notwithstanding.
    True, the one I bought for $50 was amongst an eBay flock of $100-200 ones, but as my coupe was running well with the 103 relay (which cost me naught from a local 124 graveyard character) I had time on my side.

    Why are you certain it will happen again, or rather, why do you think you can't get to the bottom of it?

    The short that killed my (original at 150Kmiles) MAS relay developed slowly from unbeknownst-to-me water that had leaked into the vacum pump/fuel pump connector area below the right rear seat.
    First I had intermittant ASR/ABS limp-home, then occuring constantly but still occasionally running well.
    Shortly thereafter, no start/run without putting juice directly to fuel pump.
    Found the water, squared away stuff down there, still would not start.

    No good advice found, even from here!

    Ultimately jumpered stuff in the MAS, learned a bit, finally suspected MAS was bad.
    Swapped in a 103 one since I couldn't find a 104 one, held my breath, and she fired right up, without a glimmer of ABS/ASR lights.
    Ran fine till I swapped in the 104 one, as it still does today, without ANY CIS ASR/ABS warning light BS.

    In the Name of Science, faced with overpriced MAS'es your plan makes sense to me, but
    I bet somewhere your fuel pump wiring is shortlng out your MAS'es
    '92 300CE Black/burgundy Sportlined
    '94 320TE Silver/gray
    '86 BMW 635CSi Black/black 5spd
    '67 Volvo Amazon wagon Green/green 75Kmi time capsule
    '62 Volvo Amazon Black/red 4Dr 65Kmi time capsule

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    JC220 (05-09-2018)

  8. #6
    E500E Guru JC220's Avatar
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    Re: 300E MAS Relay question

    Quote Originally Posted by northNH View Post
    I think my EPC research back then found all 24V CIS-E M104s (300SL, 300CE and in ROW the more pedestrian body styles like yours) basically interchangeably used the same M104 MAS, EPC #s/info notwithstanding.
    True, the one I bought for $50 was amongst an eBay flock of $100-200 ones, but as my coupe was running well with the 103 relay (which cost me naught from a local 124 graveyard character) I had time on my side.

    Why are you certain it will happen again, or rather, why do you think you can't get to the bottom of it?

    The short that killed my (original at 150Kmiles) MAS relay developed slowly from unbeknownst-to-me water that had leaked into the vacum pump/fuel pump connector area below the right rear seat.
    First I had intermittant ASR/ABS limp-home, then occuring constantly but still occasionally running well.
    Shortly thereafter, no start/run without putting juice directly to fuel pump.
    Found the water, squared away stuff down there, still would not start.

    No good advice found, even from here!

    Ultimately jumpered stuff in the MAS, learned a bit, finally suspected MAS was bad.
    Swapped in a 103 one since I couldn't find a 104 one, held my breath, and she fired right up, without a glimmer of ABS/ASR lights.
    Ran fine till I swapped in the 104 one, as it still does today, without ANY CIS ASR/ABS warning light BS.

    In the Name of Science, faced with overpriced MAS'es your plan makes sense to me, but
    I bet somewhere your fuel pump wiring is shortlng out your MAS'es
    Thanks for that – all great info. I will also check my wiring back to the pumps as you suggest.


    The reason why I think another used MAS will fail again soon is because this seems to be their failure mode and very common too from google searches I did. No-one such as BBA reman can offer a repair for them so it’s onto another used one or stump up the new MB price.


    I tested again last night and again – there were no ill effects of running the jumper cable along with the MAS. So I will proceed to wire in a quality 4 pin fused relay to take over the fuel pump switching. The wiring will be done as such that the original car’s harness is unaffected and the relay can be readily unplugged / removed in the future.


    The car’s first annual test in 6 years is next week so I’m keen to get it running again and get some other prep work done before the test.
    1987 w124 200 - 3.6 AMG build | 1990 w124 300E-24 | 1992 w124 500E

    1994 320CE Sportline | 1997 w140 S280 | 2007 w211 E220CDI

  9. #7
    E500E Guru JC220's Avatar
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    Re: 300E MAS Relay question

    An update for anyone who finds this thread in the future – I did fit a separate relay to control the fuel pump switching and it’s working A1. I took the case off the failed MAS and soldered my connections directly off the pins + circuit board inside the unit. Then drilled the MAS case and let the wires out to the new fused relay. This way the car's wiring harness is untouched. I didn’t have time for many pics but here is a poor one to illustrate what I did!

    20180519_090947.jpg

    This is not a permanent solution, it got it running again for it’s MOT tonight and will serve as a spare in the trunk. I bought another used MAS from a 500E board member which will be here in a few days and I’ll swap that in instead. FWIW my dealer quoted around $600USD for a new one
    1987 w124 200 - 3.6 AMG build | 1990 w124 300E-24 | 1992 w124 500E

    1994 320CE Sportline | 1997 w140 S280 | 2007 w211 E220CDI

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