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Thread: WTB: Heater valve and regular W124 fuel sending unit

  1. #1
    Member mercedesbenz911's Avatar
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    WTB: Heater valve and regular W124 fuel sending unit

    Wanted for my 1994 E420:

    Heater valves assembly (mine goes full hot randomly when trying to 'pulse' the valve during summer) - should be same as 036?

    70L Fuel sending unit (mine is flickering) - should be the same for all non-036 W124 sedans if I am not mistaken.

    I don't have EPC to verify the above, going off memory.


    Shipped to 77479 please. Thanks!
    1982 280SL
    1994 E420
    1998 540i/6

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    .036 Hoonigan™
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    Re: WTB: Heater valve and regular W124 fuel sending unit

    The heater valve (monovalve) changed design in mid-1994 USA model year, IIRC. You'll need to either post a photo of your valve, or the last 6 digits of your VIN, to verify which type you need. Also, the valve is controlled by the pushbutton unit inside the car, so first make sure the monovalve is actually faulty before you bother replacing it.

    The fuel level gauge acting up may be the sending unit (which can be cleaned), or it can be the gauge. You are correct that the sending unit is same for all non-036 sedans, but new ones are not that expensive, you might consider that route before messing with a used one. There have been many thread discussing the gauge issue, here's one of 'em:
    https://www.500eboard.com/forums/showthread.php?t=247

    Dave M.
    1997 E420 (Bugeyes)
    1994 E420 (Blondie)
    1994 E500 (Q-ship)
    1992 500E (Mach 5)
    1987 300D (Sportline Stage 2)
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    Member mercedesbenz911's Avatar
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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Thanks Dave! The VIN is 109696. I'm fairly confident it's the valve itself due to the way it goes full hot instead of just pulsing the valve to bring the temps up slightly once the car cools down with A/C on. Then the system reacts again by going full "cold" and blasting the fan at high speed, only the heat is on full as well out the side and center vents. Restarting the car sometimes fixes it.

    I tried cleaning the sending unit years ago in my old W123 but it didn't work too well. I'll try again on this one before I give it up entirely.

    I also have a line on a 90L tank from an E300D which apparently is the same P/N as the 036...not sure I want to mess with cleaning out all the inevitable diesel gunk, but it would be nice to have that larger tank in the 420!
    1982 280SL
    1994 E420
    1998 540i/6

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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Uh-oh. The EPC indicates your VIN is after the cutoff, and should use the "late" valve, p/n 001-830-25-84, as of chassis C096887. Bad news is, that valve was only used on the 034/036 chassis, and only late production, so it's pretty tough to locate a used one. New is $470 list, or $285 at Husker. First double-check the type on your car though, the early valve is cheap at $130 list / <$80 Husker. I found the EPC VIN break to be incorrect but I can't remember which way the error went (earlier cars used the late valve, or vice-versa).

    Remember the valve simply acts on the signal from the pushbutton unit, if it's not getting the "pulse" signal - or in summer, the full-time 12v to keep it energized to shut off the hot water - a new valve won't help. As a test you can unplug it and temporarily apply +12v to the terminals, you should hear an audible click when voltage is applied. You can leave the +12v wired up and test drive for a few days to see if the heated air remains permanently off. If so, the valve is probably fine.

    The 90L tank would be a neat upgrade! Don't forget that the trunk carpet is different, you'd need to swap that also to keep stock appearance. There may not be much, if any, diesel gunk. Inspect internally with a flashlight through the sending unit hole.


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    Re: WTB: Heater valve and regular W124 fuel sending unit

    I've been here before- The EPC cutoff is wrong for the heater valve. One has 3 ports and one has 4 ports. Mine was had the later style even though the vin was :C086231

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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Ah, so the break is earlier... thanks for confirming, Michael. That just means the OP's car pretty definitely has the late style valve.

    I'm still not convinced it's faulty based on the description though. Sounds like a control issue to me.

    EDIT: Based on a couple different vehicle confirmations, the actual VIN break point is between C060xxx and C086231, earlier than the EPC states. This was Nov/Dec 1993 production.


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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Most radiator shops will clean the tank.

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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Thanks guys. It's been a while now, but I think I did test it when it was acting up and throwing heat on a 95* day. I applied voltage as Dave said but it was still throwing heat. Given the new price, and the fact that this old E420 is worth next to nothing, the search for used continues

    Funny, been down that path with the junky 90L diesel tank before. Had a 95 E300D at one time that would die on less than 1/2 tank of fuel and kept fouling the filters. I removed the tank and brought it to a radiator shop to have them clean it out. They looked inside and saw plastic, so they wouldn't touch it with chemicals. They ended up cutting a hole in the side, cleaning it out, and welding it back together!
    1982 280SL
    1994 E420
    1998 540i/6

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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Quote Originally Posted by mercedesbenz911 View Post
    Thanks guys. It's been a while now, but I think I did test it when it was acting up and throwing heat on a 95* day. I applied voltage as Dave said but it was still throwing heat.
    Are you certain you had the polarity correct...? Can you hear a positive click with voltage applied, and also when voltage is removed?

    I've only encountered one failure on this valve and it wasn't intermittent.


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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Alright Dave, you made me doubt myself so today I tested the valve and it certainly seems faulty. With the recent cold front and temps in the 50s, the car started out blowing warm air as expected. After 15 or so mins it turned HOT. The cabin temp sensor picked up the marked increase in temp and switched to cooling mode out the center vents. Fan speed picked up, cool fresh air came out the center vents while hot air came out the side vents. Lowering the temp only made the fan blow more ferociously. I got home, left the car on, and tested the plug at the monovalve. Full battery voltage as expected, yet ferocious heat out the side vents.

    I tried to take pics but it's pretty inaccessible as it is, made worse by operating with one arm since my left is broken, so apologies about the quality.

    If anyone has one of these later style valves please let met know!


    Also Ricardo just posted his DIY on cleaning the tank sending unit so I may try that when I am able to before replacing. After 200k miles I'm sure it's needed.
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    1982 280SL
    1994 E420
    1998 540i/6

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    Re: WTB: Heater valve and regular W124 fuel sending unit

    Getting cool air out the center, and heated out the side, is strange... but I think it's possible if the diverter flap is opened. (?)

    If you have full voltage at the monovalve solenoid, and full heat out any vents, that does seem to indicate the valve / solenoid has failed. Have you tried disconnecting the wires and applying 12v directly, as mentioned in the previous post? If there is no loud click, the valve / solenoid is defective.


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    Re: WTB: Heater valve and regular W124 fuel sending unit

    I'm having the same issue. Did I read somewhere that the monovalve can become clogged with old coolant? Wondering if mine just needs a good cleaning.

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    Re: WTB: Heater valve and regular W124 fuel sending unit

    My w126 was recently replaced but the gunk inside was pretty good. I changed it as preventative measure.
    1994 E500
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