Looks like you folks have a good forum here. I hope to learn a lot from you.

I just recently got my A/C to work on my '82 240D. Are these engines supposed to idle up when the compressor kicks on or does the RPM's just drop?

Thanks in advance-

Joe

The fuel being injected is mechanicly set via a "dumb" screw on the injection pump. With the throttle linkage at "idle" the engine put's in the same fuel at 750rpm as it would at 3000rpm no matter the load put on the engine. So, yes, the RPMs just drop.

Welcome to the forum. It's good to see more new people show up.

Might want to turn the idle up if it drops too much. The idle may be too low to begin with.... I'm not sure but you may have an idle adjust knob inside the car, can't remember which 240's have them.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k

Related question: Are the 300D's the same? I thought I had read the injection pump was a self governing type of some sort that would make some attempt to hold the idle steady at varying load. Mine sure seems to, putting it in gear and/or turning the steering wheel does little to lower the engine RPM. I havent payed attention to the AC at idle actually...

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

Mine will drop around 100-250rpm (depending on the engine/trans temp at the time) when I put it into D and drop by roughly 50 rpm when I make large wheel turns when not moving.

I read something about this and the IP either here or in the FSM, I'll have to dig around and see..

Mine might drop that much, but no more than 250rpm. I don't think even that really.

How accurate are the dash tachs known to be?

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

I thought these injection pumps had some sort of governor, too. That's why I was wondering about the drop in RPM's.

After driving around and getting the engine up to operating temp, it seems to idle a little too high- I don't have a tach so I don't know the exact RPM. When the compressor kicks on, it drops dramatically. It starts to feel rough at that point. You guys think I might need to juice it up a bit throught the aneroid adjustment or are there other things you guys might suggest I look at first?

Thanks for all the help!

Joe

Adjusting the ALDA won't affect idle. You should try running some Purge or doing an italian tune-up to clear up the gunk. You might try to find a handheld optical/magnetinc tachometer device to verify what your rpms are. You could also get a tach from a later W123 turbo engine and swap your big clock with a tach (Do a search, it's been done before.)

The A/C must take 20+hp to turn. It may just be a sad fact that it's a power robbing device and you just have to learn to live with it.

Good luck.

Granted the compressor is probibly larger on the MB, but the RPM drop on a VW diesel isnt that bad with the AC on...


(brag) my 81 non-turbo will run 70mph steady with the AC on. Barring any hills.. go 50hp! (brag)

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

Yeah, I realize that- I was thinking about it in terms of increasing the amount of fuel being introduced into the cylinders. The reason I bring that up also is because this car shows 168,000+ and doesn't smoke on acceleration. Maybe it's running a little lean so therefore can't handle any additional load at idle?

I'm new to the automotive diesel so I'm just brainstorming.

Thanks for all the replies- you guys are great!

Joe

If this is any comfort, my 240D also goes from too high to too low when the compressor kicks in. It doesn't seem possible to adjust it to a comfortable speed via the cable and have it stay there. I've learned to just turn the air conditioner off when sitting at a light. That's the thing about a 240D - it requires a lot of driver involvement.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

I've heard some people with the theory that these injectoin pumps go lean across the board as they wear and time passes.
Hence the need for us to adjust the ALDA so often. It kind of makes sence.

Dunno what to do about it, short of spend a bunch of money..


If this is a big deal and common on the 240D, it can't be THAT hard to rig up an electric solonoid to bump the idle speed up when the AC's on. Many old carb equipped cars used this.

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One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K