Sometimes, when the key is in "on" position, regardless of whether or not it's running, I hear randomly timed clicks coming from a relay which I think is behind the glove box. But I can hear it also from under the hood. I thought it was the boost cutoff valve but it's not...then in the past couple of days, it sporadically starts to correlate with my pressing of the brake pedal. That lasts a few minutes, then it stops clicking. Any ideas? Thanks!

update



It started happening again! I could NOT figure out where it was coming from. This time, it was correlated with the brake pedal. When I depress it, I hear a click; when I release it, another click. There is a clicking relay somewhere behind the glove box (or below slightly), and a click outside in the engine compartment on the firewall, near the center, but down low, near the bottom...I can't see anything that is clicking, but I was out and couldn't get down underneath. Based on the clicks, I believe there are actually two different relays clicking, nearly (but not precisely) at the identical time. It happens when the ignition switch is "on", whether or not the engine is running. No switch inside the cabin that I could find affects it...I tried turning most things on and off and that had no impact on the clicking.

The weird thing: When I start moving faster than 2 or 3 mph, it stops happening. When I stop, it happens. Weird, eh?

And best of all: I captured it on video. Please watch that. You'll be able to hear the clicking clearly.

Clicking sound in 1980 Mercedes 300SD (w116) - YouTube

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1980 300SD (daily driver), 1981 300D (Inca Red)

Check your vacuum levels. There's a main climate control vacuum switch behind the glovebox. It allows the climate control system to operate above a certain vacuum level. It's not a relay and I don't know if it makes much sound but if it's effected by using the brake (booster) and goes away with higher rpm's it could be easily correlated with vacuum levels.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Do you have a CC relay that site above your PS pump and the Valve cover. If so check that. Once I got my Tack working that box clicks like clockwork. I have an 83.

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Mad_Matt's Current Stable:
Oil Burners:
09 Jetta TDI The highway queen @ 105k
83 300D. Sold
Gassers:
16 A4 sport line
10 Passat
83 Recaro Trans AM WS6 Bone Stock Survivor
11 Harley Ultra

Interesting responses. Kerry, I have good vacuum, and the CC works. It holds vacuum overnight, even a few days. Also, when I mentioned pressing the brake pedal, it is just slight pressure (the electric brake switch is all that has to be activated) that makes the click. I don't have to press any harder than that. It's very fast -- I can do it much faster than I show in the video if I want.

Matt -- interestingly, the clicking does upset the tachometer. Sometimes it bounces, but when I hear the clicks, often the tach jumps. I went out and tried to find the click but it was none of the relays or valves that are at the top edge of the firewall.

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1980 300SD (daily driver), 1981 300D (Inca Red)

'Scary, how much that sounds like a Turn Signal Relay...(But, it's in wrong Local)

WAG is some sort of "Grounding" Problem in the Wiring.
Pressing the Brake Pedal enough to activate the Switch AND having something
else other than the Brake Lights OR the Cruise Control respond is usually a BAD
Ground somewhere.

Does this Chassis have an Aftermarket Towing Wiring Harness (Hack Job) installed? [You may have to remove the Trunk's Liners to SEE it.]

Does this Chassis have an Aftermarket: Stereo/Alarm/Wireless entry system?

Attached Thumbnails

 

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'84 300SD sold
124.128

it sure is strange, eh? Tonight, driving home, I was able to prove to myself that the clicking only happens below 3 or 4 mph. In fact, if I coast to a stop (keeping my foot off the brake, in N), I'll hear a click as my speed slows nearly to a stop. Then, at this point, the clicking will happen if I touch the brake pedal, and click again when I release it. Now, when I accelerate, I'll hear another click as I pass 2 to 4 mph, and after that, no clicks will happen when I touch the brake. WEIRD!

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1980 300SD (daily driver), 1981 300D (Inca Red)

I think I know what it is. There is a switch (a pendulum switch by all accounts) on a 116 which causes the transmission to downshift to first when the car comes to a stop. If you don't come to a stop, the transmission will stay in 2nd and not downshift to first. No one has ever posted a picture of that switch on the forum to my knowledge but it's on the passenger side of the car, inside the car somewhere. I'll bet it's something associated with that switch causing the clicking. I believe the switch is activated via the brakes and downshifts the transmissin to 1st once the car comes to a stop and the pendulum swings backwards. There are a couple of references to that switch in a few threads.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

There is the kick-down relay on the passenger side floorboard by where your right foot would be. That makes an audible click as you step on and release the brake (if it's working).

If I understood you right it only happens when you step on and release the brake.

Just an idea.

Duke1

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1978 300SD

sorry Kerry....didn't read yours until after I just posted mine. I agree with you though.

Duke1

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1978 300SD

The brake pedal making the clicking stop makes me think of the cruise control actuator. Mine used to click and would stop when I pressed the brake.

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"