Humming in engine compartment

[auret]

There is a soft, high pitched humming/whine in the engine compartment when I open either of the doors, audible when the engine is not running. It appears to be coming from the injector pump or nearby. What is it and is it normal? Thanks.

[waltklatt]

Auret,
Try pushing in the door light pin and see if the noise disappears.
If the noise goes away then you have a crossed wire somewhere.
The low brake fluid warning and the open door warning both go to the same light switch.
If you research on the forum messages, you will come across others who have come across the same problem and the engine runs on after turning the key off and leaving the door open.
Hope this helps,
Walter
1967 220SL-diesel

[auret]

Hi Walt, thanks for the reply. The noise does disappear when you push in the pin or close the door. I'm wondering what is creating the noise in proximity to the injector pump. Sounds almost like an electrical fuel pump. If I can establish what it is then I can back track to find the fault.

[waqas]

Can you narrow down the source of the sound?  Have you checked around the brake reservoir caps?

[Garry]

Auret,

I have had the same problem in the past.  It is the petrol pump working due to a closed circuit caused by the float in the brake reservoir being full of brake fluid and not closing the circuit when it is in the 'brake warning light' mode.  As Walter pointed, out there is an interconnection between the door opening/brake fluid warning light and when the float that sets off that light fails it also opens the circuit to have the petrol pump operate with the door open.
The fix is to open your brake fluid reservoir and take out the floats.  one will have fluid in it. To test it at least try to get the fluid out and reassemble and see if that then stops the pump coming on. If so then replace the float as it will only fill again in time. Dr Benz (Dan) had one for me.

Garry

[Benz Dr.]

I know next to nothing about electronics but I've been told that you can put a diode in line somewhere in that wiring system and this will prevent the problem. I guess it acts like a one way valve only allowing current to pass in one direction.

[Peter van Es]

That Diode trick is described here: http://www.sl113.org/forums/index.php?topic=9331.msg61730#msg61730

Peter

[auret]

What a wonderfull support group! Garry was dead on the money. I opened the brake fluid reservoir and one of the floats was filled with brake fluid causing the switch to remain closed. I removed the brass pole - looks a bit like a nail that is attached to the float - so the switch will remain open for now. The humming noise is gone. I should add 2 things.
1. This noise has been there for at least one year to a lesser extent.
2.  I recently damaged the contact switch in the drivers side door panel and removed this while awaiting a replacement. Since then the red light on the instrument cluster has been flashing on and off at times but unpredictably.

I suggest that anybody who has this humming sound when they open a door check the switches in the brake fluid reservoir. The dash light has gone off and the noise has gone since opening this switch.

[waltklatt]

2.  I recently damaged the contact switch in the drivers side door panel and removed this while awaiting a replacement. Since then the red light on the instrument cluster has been flashing on and off at times but unpredictably.

Auret,
Glad you nailed it with Garry's help.  
If you need another contact switch, I have one to send to you.
Walter
1967 220SL-diesel

[auret]

Walter, thanks a lot. I did order anonther switch - should arrive on Monday. Will get back to you if any problems.

[Garry]

Just be aware Auret, that while you have that brass pole out that you do not have any warning on your fluid level in the brake resevoir if you leave it like that for any time.  Worth keeping an eye on the fluid level.

Garry

[auret]

I checked both floats in the brake fluid reservoir. They were both about half filled with brake fluid. One had come loose and was drifting around in the reservoir not attached to the brass terminal of the switch. This was clearly the one at fault and the noise stopped when the brass terminal was removed. The other funtioned normally with the brake fluid in it. I examined both the floats and they were both the same. There is a tiny pin hole in the tunnel in the float about half way up. This allows brake fluid to fill half the float and then this appears to act as ballast. The little rubber stopper in the float which anchors the brass pole seems to be the problem. In the faulty one it had been completely consumed by the brake fluid. I made two stoppers out of plastic and drilled a tiny hole down the middle for the brass pole. I removed the brake fluid from the floats with a syringe and a thin needle. Both switches function normally now. I expect the floats to fill up half way with brake fluid as a result of the the heating and cooling of the air in the floats. They will hopefully continue to operate normally. I will report back if there are any problems in the future.