Author Topic: Side effect: Fuel pump runs if brake fluid is empty and one door is open  (Read 6219 times)

WRe

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Hi,
I've been working on rear brake callipers and brake fluid was empty (control lamp was on; same for open door). I mentioned that my fuel pump started running when I opened the door (ignition turned off; no key in ignition switch). Anybody any ideas about this phenomenon?
...Wolfgang

Garry

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When you open the door on a 113-model car, the red brake warning light comes on. This is to signify that the electrical brake warning system is working OK. So why is the red brake light associated with opening the door? Legislation demanded brake fluid low and door ajar warnings. There was only one light position left on the kombi instrument, so they decided to use the same light for both functions.
First, a description of this W113 feature:
All Mercedes cars are designed so that when the key is switched to "on", the wiring section No 54 in electrical system is in use. Section No 30 is straight from 12V+ on battery. A primary rule is that section No 30 and No 54 are never to be connected, as this bypasses the ignition key and empties the battery.
The 113 is designed so that:
The courtesy light below the dash is connected to No 30. Light is (may be) lit as door is opened. Brake fluid warning is connected to 54 (Light is on when car is running and level is low). Now, the M-B designers wanted to find a way to check that the warning light bulb was OK. And, they did that by using the courtesy light door switch. This also resulted in a "door ajar" warning function. This design carries with it an unfortunate effect: If the brake fluid level is low, and the door is opened, 30 and 54 will be connected. Means ignition will go on, and the fuel feed pump will start up. Fuel can be squirted around if one is performing work on the fuel system in the car. A wise thing to do is to disconnect the battery before starting any maintenance work on the fuel system.
Now, a remedy:
Firstly check that the float does not have a pin hole in it and has sunk to the bottom of the brake fluid and thus giving the low fluid warning and subsequent connections when opening the door or.
If you are going to work on the car then, y simply fitting a diode in the circuit between the fluid level sender and this red light bulb in the combi instrument. A Si diode, 1A with the (+) side facing the bulb will do fine, and it may be easily hidden in the wiring harness of the car.
Garry Marks
Melbourne/ Kyneton, Australia
1965 MB 230SL Auto RHD Lt Blue 334G, Top 350H, 213 Leather, Tourist Delivery
1964 VW Kombi Split Screen (Muffin Top) Single Cab Utyility Blue 334G lower, top 350H. Restored
2005 MB A200
2008 MB SLK 55 AMG
2016 VW T6 4x4 pop top Camper

WRe

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Hi Garry,
thanks a lot for your excellent description!
...Wolfgang

zalsaigh

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I have never seen anything like this in my 40 years around different models of Mercedes. When the door opens, usually a courtesy light comes on. There is no connection between the courtesy light and the brake fluid warning light and the fuel pump. There is a mix up in electronics and I would advise to look at the wiring diagrams to sort this out. Good Luck.
Alchemist
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66andBlue

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I have never seen anything like this in my 40 years around different models of Mercedes. ...
That does not mean that Garry's description of the problem is wrong!
This is a well described phenomenon on US version 280SL cars with dual braking fluid circuit - it does not happen on a 230SL.

.. Anybody any ideas about this phenomenon?
Wolfgang,
this was also discussed here: http://www.pagodentreff.de/diskussionsforum/t7508-motor-geht-mehr.html
Alfred
1964 230SL manual 4-speed 568H signal red
1966 230SL automatic 334G light blue (sold)
1968 280SL automatic (now 904G midnight blue)

Garry

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It is not just the US versions where it is wired that way, it occurred on my ’69 UK delivered 280SL back in 2006.  It worked exactly as designed by Mercedes Benz, it just took me several months to work out the problem of why, when opening the door the fuel pump was going on. It made it very difficult to work on the car with out disconnecting the battery, or keeping the door closed :o

 I had a pin hole in the float, and a new one kindly supplied by Benz Dr that remedied the problem.

Garry
« Last Edit: April 29, 2014, 08:47:34 by Garry »
Garry Marks
Melbourne/ Kyneton, Australia
1965 MB 230SL Auto RHD Lt Blue 334G, Top 350H, 213 Leather, Tourist Delivery
1964 VW Kombi Split Screen (Muffin Top) Single Cab Utyility Blue 334G lower, top 350H. Restored
2005 MB A200
2008 MB SLK 55 AMG
2016 VW T6 4x4 pop top Camper

WRe

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Hi Alfred,
thanks for the link to Pagodentreff.de, I remember it now, but I think this design is not limited to US models because mine is one of the last German models.
...Wolfgang

66andBlue

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Thank you Wolfgang, learned something new again.
I wonder whether the TÜV also required a warning light for the brake fluid level and not only the DOT here.
Alfred
1964 230SL manual 4-speed 568H signal red
1966 230SL automatic 334G light blue (sold)
1968 280SL automatic (now 904G midnight blue)

ejboyd5

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For my own use, and to fully understand the circuits involved, I took Garry's excellent analysis and slightly amended it.  When I was finished, I thought someone else might appreciate a slightly different version of the same analysis.  Here it is:

DOOR AJAR LIGHT, BRAKE FLUID LIGHT & FUEL PUMP ACTIVATION   
     When you open the door on a W113 model car, the red brake warning light comes on.  This light was originally installed to indicate a low fluid fault in the brake system.  So why is this red brake light associated with opening the door?  It is because continuing safety legislation demanded both low brake fluid and door ajar warnings.  As there was only one light position left on the kombi instrument, it was decided to use the same light for both functions.
     All Mercedes cars are designed so that wiring section No. 30 runs straight from 12V+ on battery and is always energized.  On the other hand, it is only when the ignition key is switched to "on", that wiring section No. 54 in the electrical system is energized.  A primary rule is that sections Nos. 30 and No 54 are never to be connected, since this bypasses the ignition key and empties the battery.
     The W113 is designed so that the courtesy light below the dash is connected to No. 30 and is lit as door is opened. The brake fluid warning is connected to No. 54 (light is on when car is running and fluid level is low).  The M-B designers wanted a way to check that the brake fluid warning light lamp was OK and they did this by connecting it to the courtesy light door switch that also allowed it to serve a "door ajar" warning function.  This design carries with it an unfortunate side effect: If the brake fluid level is low, and the door is opened, 30 and 54 will be connected through the common lamp and 34 will be activated the same as if the ignition key is turned to “on,” and the electric fuel feed pump will start up.  Fuel could be squirted around if one is performing work on the fuel system in the car.
     Now, some remedies:  a) A wise thing to do is to disconnect the battery before starting any maintenance work on the fuel system.  b) Check that the float does not have a pin hole in it and has sunk to the bottom of the brake fluid canister thus giving a false low fluid warning that might result in unintended electrical connections if the door is opened.  c) If you wish to modify the electrical systems, fit a diode in the circuit between the fluid level sender and the red light lamp in the kombi instrument. A Si diode, 1A, with the (+) side facing the lamp will do fine, and it may be easily hidden in the wiring harness of the car.

Peter van Es

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1970 280SL. Please do not mail or PM me questions on Pagoda's... I'm not likely to know the answer.  Please post on the forum instead!

mbzse

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Quote from: WRe
Hi Garry,
thanks a lot for your excellent description!
Well, yes... perhaps some comment is due here. Of course Garry did not write this text at all (and he did not claim to have done so).
Text resides, since some time back, in our Technical Manual (Wiki), as Peter states in reply No 9.

The analysis and write-up was done more than ten years ago by Ola Kristoferson (in Swedish). I added to it and translated it, and I placed this text initially on the MercedesVeterans Forum website (run by Alex G way back) for the benefit of Pagoda owners. Subsequently as it came into our Wiki, it was slightly modified, and now "ejboyd5" further modified it.
/Hans in Sweden
« Last Edit: April 30, 2014, 19:46:49 by mbzse »
/Hans S

Garry

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Hans is absolutely right, I like Hans took Ola’s original text and added my bit that I learnt with my experience with a pin hole in the float and like Hans and now like EJ we have end up with the excellent overall description of the problem and possible solutions that Peter will put in the Tech Manual for all to find when they do a search of the manual for their fixes.

I would have responded earlier but have been back in hospital, again.

Garry
Garry Marks
Melbourne/ Kyneton, Australia
1965 MB 230SL Auto RHD Lt Blue 334G, Top 350H, 213 Leather, Tourist Delivery
1964 VW Kombi Split Screen (Muffin Top) Single Cab Utyility Blue 334G lower, top 350H. Restored
2005 MB A200
2008 MB SLK 55 AMG
2016 VW T6 4x4 pop top Camper

Peter van Es

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And that is why ALL full members can freely add to, improve and modify ANY description in the Technical Manual. After all, it is our collaborative, shared knowledge that make this site so wonderful.

If you want to see who added/modified what, and when in the Technical Manual, click here: http://www.sl113.org/wiki/Site/AllRecentChanges

As you can see, Alfred (66andblue), and I make many changes, but there are contributions by Garry, pj and others. But please feel free to help out...

Peter
1970 280SL. Please do not mail or PM me questions on Pagoda's... I'm not likely to know the answer.  Please post on the forum instead!

glenn

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Engine keeps running in the 'Off' position, too??