This is for the 82 300D.

Houston's been HOT these past few weeks. I can bear it when I driving down the freeway cause I'll be around 2500-3000 rpms most of the time. BUT, if I'm in slow traffic or on surface streets with lots of stops at red lights, it'll start getting hot in the car QUICK! The compressor clutch will not engage until around 1500-1800 rpms. When I turn the compressor on at idle, the rpms will not drop, but once I get it to the 1500-1800 range, I hear the click of the clutch kicking in and the rpms drop a little bit. Once I get back to idle, it'll disengage and sweating resumes. From what I've read, the compressor clutch is activated by a magnet of some sort. Maybe it's something simple and the magnet needs to be cleaned, or it's something electrical, I don't know. Swapping out CCUs hasn't solved the problem so it's not that. Where should I start troubleshooting?

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

When was the last time you checked your refrigerant levels/pressures? The low pressure cutout could be preventing the compressor from engaging. You can jump the low pressure cutout for a short duration test.

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Bob
'82 300D Petrol B-G Metallic

That's pretty bizarre. Maybe the alternator output is inadequate at lower engine speeds. Will the clutch engage with the engine off?

Does the a/c work okay when the clutch is engaged?

I had a vehicle that would do a similar thing. At idle the compressor clutch would not kick in, and a little above idle it would throw. It steadily got worse (higher RPM required) over time. In my case it was the compressor clutch going bad. The winding insulation can break down over time and heat and start shorting out.

Your case may be something different, but the clutch going bad is a possibility. A relay controls the clutch, and the relay is thrown by the CCU in combination of several sensor inputs - one being the low pressure switch.

If you sit with the engine off but key on, when you switch the ACC from Economy to Full Climate Control (i.e. A/C) you should hear the relay click.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

The A/C works fine when the clutch engages. It won't engage with the engine off.

I had it recharged by the indy about 3-4 weeks ago, so the levels are fine.

I went out and tried that. I heard clicking, but I don't know if it was the relay or the vacuum components under the dash moving around. Is the A/C relay one of the "ice cube" ones located on top of the driver's side wheelwell or is it located elsewhere?

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

You might try swapping the compressor relay with the aux fan relay (under the black cover on the left wheel wheel.) Also ensure that the connector at the compressor is clean and tight. And replace fuse #8.

Yes.

Tried all three things, no change in the situation. It's possibly a fault with the clutch itself or something in the electrical system. I'll let my indy diagnose it tommorow.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

Can you verify that the clutch is getting power? If it is, and it's still not engaging, it pretty much has to be the clutch. Maybe the additional alternator output at higher rpm's is enough to take it over the edge.