Thanks for that input GSXR. Yesterday, I removed the radiator, a replacement radiator from about 4 years ago, and blew 100 psi compressed air through the cooling fins of the radiator and while there was no oily gunked on crap restricting air flow, I did see a small amount of sand blow out. Also, I blew air through the AC condensor, also not much came out.

As mentioned, I can hold the aluminum fan still at temperatures at or around 110c according to my dash gauge. I dare not go any higher!

So as far as I can see, this fan clutch does little to nothing since the aux fan will come on at 105c, and it does, and when it does it returns the temp back down to around 85c to 90c. So now I'm wondering, is the fan clutch for the purpose of kicking in only if the aux fan cannot retain, or contain extreme temps. In other words, an auxiliary for the auxiliary, (isn't that an oxymoron?)a back-up for the auxiliary electric fan?

This is where I have difficulty.

If the fan is freewheeling at freeway speeds, how can it do anything? And, we absolutely know it is freewheeling at freeway speeds because we can't get the damn thing to couple at zero speed and no airflow.

So, it can't possibly be coupled at freeway speeds. The air temperature hitting the clutch is nowhere near 100°C.

I don't know Brian? If the engine temperature (internal) while say driving 65mph, goes above 105c, (hot sunny day on a desert highway) we know that the aux fan comes on, right! 105c coolant temps passing through the radiator will have cooler air blast into the 105c radiator lowering internal coolant temps, and with air coming out of the radiator at 105c, and continually dissipating, heat transfer, eventually lowering overall internal temps.

That 105c out going air will hit the fan clutch.

But when the production of heat generated within the engine exceeds the ability of heat transfer, (overwhelming) via radiator, heated air coming out of the radiator will climb higher, 110c, 115c, unless provisions for pulling more volumes of air through the radiator is increased. If 110c, 115c, air is leaving the radiator it will hit the fan clutch, thus actuating it. In this case, aux fan and primary fan with one regulated by internal temps, the aux with thermo switch, the later via ambient flow through the radiator.

If the internal temps cannot be controlled via the aux fan, ambient temps become available to over-ride and actuate the fan clutch, as a reserve, or as a means to work together to fight the resistance.

Maybe these fan clutches need condition described above in order to actuate, or engage, heated air blasting through the radiator, on that hot day on a desert highway

It seems that MB have gone out of their way to run the 603 hot. I suspect that the design engineers had no idea of the cyl. head cracking issue when they came up with the viscous fan design. And in fact, the head may have no problem at all with temps over 100 C due to the fan not running. It would be interesting if someone would actually conduct experiments to see if the head will crack at high temps. It would be necessary to use a number of heads to get usable results $$$. But that's another issue. Anyway, for us folks living in high heat areas, It makes sense to run the fan before its design temps. My fan would stop within aprox. 2 turns (but still not couple tight when hot) and it would spin free when cold. It would never make any discernable noise over the clatter. So i put in the viscous fluid from Toyota in hopes that it would basically couple all the time. Will soon find out after I get it all back together.

__________________
Dan
1981 240D
1987 300D Turbo
1973 220
Das Beste Oder Nichts

Firstly, if the head temperature is 105°C., the temperature at the top of the radiator is already slightly below this level.

Then, when the coolant gets down to the middle of the radiator, approximately equal to the height of the clutch, the temperature of the coolant is significantly below 105°C. Maybe already down to 90°C.

Finally, when air at a temperature of 45°C. passes through a radiator that is at 90°C, the air will not heat up to 90°C. The air will take up some of the heat from the radiator and exit at a temp. of something like 75°C. or less.

So, effectively, it's not possible for the air temperature near the clutch to ever reach 105°C. with a moving vehicle.

I understand all that Brian, and don't disagree with you, but higher radiator temps will raise the air temp leaving the radiator. It's these temps that are supposed to actuate that clutch fan. Whether they actually are ever high enough is I guess the question here. The point made from the other poster here that the early 603's #14 heads were not anticipated by MB to crack, do not actually ever get high enough to actuate that fan clutch presumably before the head cracks, or at least, too close for anyone to dare test!

I'm gaining more suspicion that there maybe nothing wrong with two brand new BEHR $171.00 fan clutches I have, that temps are not getting high enough in the radiator for the fan clutches to engage, unless that is that the engine and radiator are near, or in the red zone, or boil-over point, in which case our 603's with #14 heads are ready to crack! I'm not going to test that! I wish now that I had bought instead, another new auxiliary fan motor!

It's actually the opposite. The main fan does most of the engine cooling. The clutch kicks it up a notch when things get hot. The electric fan mostly aids airflow when the car is moving slowly or not at all, and engine speeds are low or at idle (basically when stopped in traffic). The electric fan is more of a help to improve AC performance at idle, than cooling the engine (although it does kick on high speed at 105C if your sending unit is still working - every original one I've come across has required replacement.)

__________________
Check out my website photos, documents, and movies!

Not exactly true. Although it's not often coupled at freeway speeds (mostly because engine temps aren't often above 100°C on the freeway), it can and will couple when the clutch "sees" high enough temps to couple.

Remember that the temp hitting the clutch does NOT have to reach 100°C!! Everyone please go read the section in the FSM about how the fan clutch operates and pay special attention to the numbers given. (Just remember my assertion that the new 606 clutch engages at higher temps.)

More random thoughts:

1) On a hot day (ambients about 100°F) with the AC on max, with engine temps at an indicated 100-105°C, my clutch is frequently coupled and I can see this by watching the fan as I shut down the engine. That's with the car parked and hood up - no freeway airflow blast present.

2) Wouldn't hurt to see if the dash gauge is accurate - might be you're chasing a problem that doesn't exist.

3) Your 4 year old radiator is highly suspect. Another guy with a 603 chased down a cooling problem (110-115°C under load climbing grades) and replaced everything, sometimes twice, while ignoring the radiator because it was only 1.5 years old. Guess what? A brand-new Behr radiator cured his problem instantly. You might want to start considering that option.

4) If your electric auxiliary fan is not working it should absolutely be replaced, especially if you have working A/C. You can get a used one fairly cheap (I know they're spendy to buy new.) Also make sure the 3-prong switch that's supposed to trigger it at 105°C is working (hard to test on the car unless you can get engine temps above 105°C.)

5) The 603 head is not some fragile thing that will self-destruct if it exceeds 110°C. It should be fine to 115-120°C as long as the cooling system is pressurized properly and has the correct mix of anti-freeze, so you don't get pockets of boiling coolant. It's simply an indication that the cooling system is not working properly and should be fixed. Of course temps above 120°C are NOT good and I would turn the heater on max, then shut the car down, if mine ever touched 119°C... but I wouldn't do that at 115°C.

6) In case I didn't make it clear already, the electric auxiliary fan does two things - improves AC peformance, and assists the main clutch fan when temps exceed 105C. Both cases are typically when the car is idling or moving very slowly. It is NOT intended to be a major factor in cooling the engine.

7) I still think the radiator should be replaced, or tested with an IR thermometer both in the center (in front of the clutch) as well as looking at temp deltas for input/output.

Pardon my ignorance, but what is FSM, and where can I access it to read?

Is this only for the W124?

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

That's basically true for almost all Mercedes ever built, at least up through the late 1990's. (I don't know enough about the newer ones to say for sure.) They all use the mechanical fan for the bulk of cooling duties, with the electric fan kicking on only when needed. My theory is that it's mostly due to noise - electric fans that move enough air to actually DO something are really loud, and people spending $50k on a car don't want it to sound like a Toyota. The mechanical fans are much, much quieter. Newer electric fans are better, which is why I'm not sure about the newer cars.

I prefer e-fans actually but you have to have a decent charging system to have ones that can move some serious airflow.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

1) FSM = Factory Service Manual. Commonly available in CD-ROM form
MB Tech Manuals from MBUSA cost $19.99 plus S&H
MB Tech Manuals from MBUSA
I would not attempt to do any serious work on a Mercedes without using the factory manuals.

2) Electric fans need a larger alternator. If you have an OM603, you're in luck, you can install a factory 143 or 150 amp unit for about $100 that performs amazingly well. Full details are in this thread. I have the 150A upgrade in both my W124's.