Door sensor / fuel pump grounding issue

[GB]

1971 pagoda - new to me (as is this group).  Car will only shut off when doors are closed. If either door is open and key is switched to off position, engine continues to run. I hear this is not so uncommon - any tips are appreciated. Thanks!

[Vander]

Check your brake fluid level, or check the float in the master cylinder reservoir to see if it sunk. Yes, I know it sounds goofy.

[GB]

Interesting - I am also having brake issues “dead pedal”. Just received a replacement brake booster. Could be related??

[lpeterssen]

Dear friend:

Yes the previous suggestion is correct.  Mercedes did a mistake on the design of this particular circuit. If the brake level sensor are active because of low fluid level, it will accidentally back feed and energize T15 circuits if at the same time doors are open (that includes the fuel pump, the coil and many others).  That is possible since both circuits share the same warning light at the central instrument cluster.   If that happens much current will travel trough a very thin cable, so it is imperative to disconnect the brake fluid sensors in a case where they are defective, stuck, etc.

These was because Mercedes used a positive signal to power up the low brake fluid alert light at the instrument cluster, which  also serve as alert light. When doors are opened. This later circuit is always hot. So it is possible to pass current from this circuit to the T15 circuits if the low brake fluid sensors are in the circuit close position.  See current flow on diagram attached.

On later Mercedes as W108 this situation was corrected and the brake fluid level reservoir light was activated by a negative (ground) signal instead and also the alert bulb is to shared with any other service.

One possibility to solve this mistake is to install a DIODE on the feed cable to the brake level switch so that electricity can only travel from circuit 5 to the sensor and not backward. (Cable black/red/blue)

See diagram attached.
Best regards
L.peterssen

[GB]

Thank you - will give it a try and report back

[GB]

So I removed the sensor connector - and that eliminated the problem. Do you recommend replacing the reservoir and sensors? Any issue driving the car with the sensor disconnected? ( other than making sure to check brake fluid level) . See pic

[GB]

So I removed the sensor connector - and that eliminated the problem. Do you recommend replacing the reservoir and sensors? Any issue driving the car with the sensor disconnected? ( other than making sure to check brake fluid level) . See pic

[Pawel66]

Perhaps check why is this sensor sending signal. Take out the cap/float and look at it if there is anything obvious to fix or to replace. Make sure no dust gets into the brake fluid reservoir as you remove the cap.

[lpeterssen]

Dear Gb

If by removing the brake fluid connector circled in RED the problem stopped, then that particular float is stuck or the sensor is internally damaged so that it gives a close circuit signal.

Nevertheless the main one is circled in GREEN.  Through that one pass All the signals that go to the warning light at the instrument cluster.

You can drive with that disconnected checking fluid level each time you drive the car.

Of course the better solution is to replace or repair the damaged sensor. It is the same sensor as used on Mercedes W108, W114 etc, so is no so expensive.

See picture attached.

Best regards
L.peterssen

[Garry]

When I had the exact same problem with mine, it was a pin hole in the float.
 Very hard to see unless you pull the cap and then take out the float to realise it was partially filled with brake fluid. Replaced the float and problem gone.


Garry

[GB]

Thanks for the great advice . I ordered a new complete reservoir with sensors just to be safe.

[Pawel66]

Well, you have a nice looking original reservoir, maybe you can just replace the float or the cap... I would check if I were you...

[Delvis]

I had the same problem. Ordered a new brake fluid reservoir complete with floats & sensors from MB classic centre.

please note that the new brake fluid reservoir tank connection points omg the bottom are not inline and off-centre. the new ones will mount at a slight angle when looking from the top. so slightly different than the original. I simply pulled the float/sensor/connector assembly from the new reservoir and installed into old unit. works - problem gone and keeps the original look.

all the best & stay well!

Devlin