I have a '90 w124 2.5L turbo but my comment is universal.

I have a medical condition called multiple chemical sensitivity/environmental illness. It's a mouthful. It's now much better but 10 years ago it crippled me. The smallest amount of dust, solvent or combustion product and my body would go into shock.

I'm obviously grateful to be doing much better. However, diesel fumes still bother me. I have done a very simple mod to keep the fresh-air inlet at a constant 80% recirc setting. It is a big improvement.

I would now like to make a modification to my original mod and that is to only have the vehicle in the recirc mode while stopped or in stop-and-go traffic. If I can find an electrical signal that will indicate that the car is stopped I will use that to drive a circuit which will then energize the recirc pod.

Ideally this will let the car draw fresh air while driving on the highway but close it while in congestion. The brake signal wont work and I have thought of somehow taking the signal from the tach and using that. But I am a chemical/material science guy and my knowledge of electronics is limited.

What would be ideal would be a signal that would shift to one state while stationary or traveling less than 10 to 20 mph and then shift to another state at higher speeds.

Thanks.

124 has ABS, no? Then the hub sensor should do it. Older ABS uses photoreceptor sensors, whereas newer ABS uses magentic sensors. Either one provides a similar digital signal that should give you the data you need to program your IC.

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

One word of caution, tampering with - including tapping into - the ABS sensors could render the system inoperable. DANGER DANGER.

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

if you were concerned about tapping into the ABS system you could use a separate ABS sensor on one of the wheels. you would need to make a mount for it, should not be awfully difficult. then you will need some circuitry that will determine if the sensor is picking up movement in the wheel or not. then send that signal to your switching device for the vents. there is also a hall sender in the speedo cluster that may be of use for the same purpose of sensing road speed.

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punkinfair

Please tell me more about the Hall sensor. Where's it located? What's it do for the MB?

Car does have ABS. I'll have to think how easily I can use that option. Using a signal from the tach whenever the RPM fall below 1,000 or so seems easier.

I'll gladly take any suggestions you'll can come up with.

Thanks.

The Hall sensor is in the instrument cluster, near the speedometer cable. It is a plastic piece with a green wire. You will see that the green wire goes to a distribution block forward of the instrument cluster. One or more green wires come off of that block and there are unused terminals. In the W124 cars, the Hall-effect speed sensor is used by the outside temperature display (stops updating when the car is stopped) and in the climate control (slows the blower a little when the car is moving at highway speeds to compensate for the "ram air" effect). It should not be difficult (he said) to find a way to use this signal. I would recommend having the climate control switch to 100% recirculated air when the car is stopped.

In the meantime, just push the "recirculate" switch when you stop the car. That starts a timer and puts the system in 100% recirculated for 30 minutes.

A friend has a similar problem and finds that biodiesel is less harmful to her than petroleum diesel.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Simple switch on the acccelerator linkage? At idle the air supply system switches over. You can even trim it to keep recirculating at whatever point you desire.

That's why I posted the question!

What an elegant and simple solution!

In essence a microswitch that clicks when the linkage is at rest (or whatever point one chooses).

Thanks!

Greetings,

Next time I pull the dash I'll take a look at it. Some of my early grad work involved measuring the Hall effect so it has a charm for me.

I think the recirc switch only puts the car in 80% recirc and it only enters 100% recirc when the CCU is shutoff.

The recirc button performs differently then other MBs I've had. On all others when I engage the recirc there is a noticeable increase in blower speed. That does not occur in this car. Instead I hear a quiet relay engage underneath the back seat and the 6-port vacuum realy behind the glove-box opens the port to the recirc pod but no change in blower speed.

I currently run at continuous 80% recirc by having the source vac line tee into the recirc pod (80%). I then altered the action of the recirc switch such-that engage it will close the recirc from 80% to 100%.

I've now decided that I would like to a 3-way choice: 0%, 80% or 100% and have the option of manual and/or automatic operation.

Interesting what you said about your friend being less perturbed by bio-diesel. 10 to 12 years ago the affliction crippled me and I could not work for almost 3 years. It was a challenging process to recover. Next time I'm over by Berserkley or up in your area I'll get a tank of bio-diesel and see if the exhaust affects me differently.

I think that when you hear other cars' blowers "speed up", you're just hearing the increased airflow noise and often increased airflow from the recirc flap opening (usually at the passenger's feet). I've noticed this in cars for decades, even though I've not yet had a car that changes blower speed when recirc is pressed.

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Gone to the dark side

- Jeff

Thanks Jeff I appreciate your input.

Like everyone who buys a 20+ year old used car I'm trying to improve it. Knowing how it is suppose to behave helps.

Personally I prefer the method of using the hall-sensor like the outside-temp sensor does. If you use the pedal, it'll be switching as you touch the gas to move forward in a toll-booth line, traffic jam, etc. The hall-sensor can be tied to a timing circuit where you can set the MPH threshold so that it only opens when you're moving at a decent speed.

Cool idea though.

__________________

Gone to the dark side

- Jeff

So here are two relays that monitor throttle position. What do both of them do?


Thanks

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My 1989 "Service Manual/Automatic Climate Control/Model 124" says that there are only two choices for recirculation: 80% or 100%. Your 1990 might be different, I suppose. Unfortunately, in this part of this manual the English translation is ambiguous. One thing it does say is that the system tends to use more recycled air if it is very hot or very cold outside. That makes sense - less work for the system. The manual is very clear that when you press S24 (the recirculated air switch in the row above the PBU) you get 100% recirculated air.

As an unhelpful suggestion, I will note that our 1996 E300D has a carbon filter as part of its climate control system and it works well. Caveat: the aftermarket replacements are not very good; the factory carbon filter, expensive as it may be, is the one to get. BTW, I don't think it is possible to retrofit one to a 124.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The 124 came with cabin air filters in '94 & '95, unfortunately it is a different air box, ... the big piece under the dash, so not a practical retrofit.

__________________

Gone to the dark side

- Jeff