I had the same problem with my '92, and a new radiator ultimately solved the problem. The temp was fine as long as the car was moving, but in traffic in Arizona heat the temp needle started crowding the red zone. I have a manual trans so the load from an automatic would only have made things worse.

However, when I pulled the old rad out, for kicks I hosed off the grid from back to front, and I'd say a quart or more of accumulated gunk (flies, feathers, grit, you name it) was dislodged. I have to wonder if that alone would have solved the problem. It's easy enough to do, so I recommend that before proceeding to a replacement. And no, you really can't do an effective job of hosing the grid with the rad still in the car.

FWIW, the replacement rad is an OEM style with plastic tanks. It's been in the car for close to 10 years now (again, in AZ heat) and it's worked just fine.

Also - the pusher fan in front of the rad is not OEM, but someone's add-on. That didn't appear on the 240s until they moved to the new AC system c. 1990-91. The OEM fan is controlled by a relay which is switched by the pressure switch in the refrigerant line on the passenger side, at the front of the frame rail. Since the AC system needs all tghe help it can get, I bypassed that switch so the fan comes on whenever the AC is on.

How hot is it? Do you have a brass/copper radiator or aluminum?

When I lived where it got over 100, my car would overheat at idle (with the aux fan working) with a recently professionally cleaned radiator. Once I switched to an aluminum radiator (2-row), this stopped happening.
Like you, I had a new water pump, thermostat, and coolant, yet still got to overheat temps.

Where are you, how hot is it, and what kind of radiator are you running? What pressure is your coolant reservoir cap?

The aux fan turns on in 2 situations - when the thermostat in the top of the radiator closes (I think this is at 190F or something), and when the AC is set to max, (there may be a third scenario - if the AC is on and the AC hits a certain pressure, but I'm not sure about this).

The aux fan depends on a relay identical to the RSR. If it or the wiring (or the wiring's insulation) to it fails, the fan will never come on.

I believe an 88 240 has the type of AC that should cycle on and off. You may have a bad sensor somewhere. Do you have the watch-glass port on your dried/receiver?

Good Luck!

EDIT: You mention the temp comp board and gauge. Did you remove/bypass the temp comp board? Have you verified that you're actually running too hot with an infrared thermometer on the head? Maybe this is just what the temp comp board was meant to hide.

HAve you ever taken a garden hose and hose out the fins of the radiator?

Take the Fan off the waterpump and get the shroud out of the way and then blast water from the engine compartment outward to get all the dust and years of gack out and off teh fins.
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Fix the fan first.

There was a decent editorial in one of my car magazines that made the point that car manufacturers never put stuff in their cars that is not needed!

Both our '88 745 GLE and our '90 744 TI had a plastic fan in front or their radiators.

I do not know about the '88. but my '90 had a bad fan - after I found the car's thermostat missing.

The new fan would run if I let the car idle for 5 minutes - even in winter - like when I scraped the windows.

Is there a switch in the top of your radiator? When you short it, the fan should run.

Keep in mind that a dead fan blocks some of your radiator.

When the fan went on my '88, I was able to find an after market, generic fan that I could attach to the original shroud.

It was too much trouble to attach it like the original, but it was a loose fit and I was able to tie it in with 10 loops of repair wire.

If you do use a generic fan, make sure it blows in the right direction

The black fan I think works only with the ac on. Unplug it from the system and connect it directly to the battery to see if it works. If it works see if there is a relay in line somewhere between the fan and the ac compressor. If the compressor doesn't turn on the fan won't turn on.
You said temp goes up, how high? You may want to have the radiator boiled to really clean it internally. Also test you fan clutch, should have some resistance.
Do a vacuum test on the ac to see if it holds.If it doesn't hold vacuum then you have a leak somewhere. What are you filling it with, r12 or 134a?

Your post says that this condition only occurs with the AC engaged? Is that correct? The flimsy plastic fan is in front of the AC condenser which is in front of the radiator. It is possible that the cooling fins of both condenser and radiator are gunked up and simply need cleaning. Radiators will become internally restricted so that coolant only flows through a reduced section of the radiator. Aftermarket flushing with chemical products doesn't work to clean them out. You could take it to a radiator shop for a backflush and flow test.

Best Regards

pat hayes