I think the same thing. One way to test a fan clutch is to tuck in behind a tractor trailer on a grade. If your temperatures start rising it's an indication that the fan is not sucking enough air. Behind the truck your speed and load are still the same but the air flow is significantly reduced so you need the fan clutch.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Just for Fun....

Grab an optical tachometer, mark (read tab) the fan at the fan blade / hub joint, find ambient temps > 90*C, get the motor temp to operating temp and you are ready.

Now as you raise the RPMS a good clutch will keep the fan speed increasing relative to increased RPM #s (remember they will not be the same due to natural pulley vs crank speed variants). This should continue up through approx. 3,000 - 3,500 rpms (highway) so long as the ambient temps remain high enough to keep the clutch engaged. Many listen for relative increases in fan noise or better known as the “Roar Test” J.

On a worn but not dead clutch you will see fan rpms somewhat raise with motor rpms into the lower 1,500 to maybe 2,500 rpms (city). However the "% lock" or ratio of crank revolutions to fan revolutions will continue to diminish with increased rpms. As you raise the motor rpms from there the fan speed doesn’t increase proportionally. Result being less air being pulled across the radiator because the clutch is worn or low on fluid while engine temps are rising due to increased combustion. This is one of the reasons we are often fooled by decent in town temperature gauge readings.

Now a completely shot fan clutch may give high in town temps and possibly lower highway temps provided ambient are relatively cool and rpm demands / fuel load is low simply because more air is pushed through the radiator at 55 mph than 25 mph

Re body styles and air flow: For our purposes here there is not a whole lot of difference in a 123 vs 126 while on the highway. The Bernoulli Effect of the body shapes will want to pull the volume of air up / below and around the car at speed due to increased body surface area creating lower air pressure (think air foil). This is why the fan is very important as it actually creates a lower air pressure area at the grill. Otherwise air flow will want to pass over/under the car.

Here’s and example of a really shot clutch test from another board:

{ Car #1 - Good Clutch:
With the engine at ~650rpm idle in park, pulley speed was ~810rpm, actual fan blade speed was ~790rpm (almost 98% lockup).
I didn't think to test it at higher engine RPM's, I will do that in the future for grins.

Car #2 - Bad Clutch:
With the engine at ~650rpm idle in park, pulley speed was ~810rpm, actual fan blade speed was ~275rpm (only ~35% lockup).
With the engine at 1500rpm idle in park, pulley speed was ~1900rpm, actual fan blade speed was ~500rpm (only ~25% lockup).
With the engine at 2500rpm idle in park, pulley speed was ~3200rpm, actual fan blade speed was ~500rpm (only ~15% lockup).

Conclusion: With a bad fan clutch, the fan is spinning slower at freeway speeds than it does with a good clutch at IDLE. }

The important thing to remember is what is referred here to as % lockup which is also relative to %lockup to combustion heat generated.

Hope this helps

P.S all is moot without a good radiator.
V.S. Tech...you were correct.

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1986 300SDL 440,xxx

At ambient temps of greater than 90*C, would not the motor already be at operating temperature?

The question was not whether a bad fan clutch results in slower fan speeds.

Could not the case be made for the Bernoulli Effect actually improving air flow through the radiator due to decreased static pressure in the engine compartment? You could grab an altimeter and plumb a static line into the engine compartment and see what happens at highway speed.

swapped out the clutch fan for another one, it looked promising for a while but the 2nd trip on the highway was back to same ol... 245 on the gauge.
I did take a IR thermometer and looked at 3 points, engine block between the front 2 plugss on the drivers side, the "intake manifold" just above point 1 and the water pump, front passenger side, gauge sais 175, 1 = 240, 2 = 170, 3 = 175, gauge reads about 220, 1 = 250, 2 = 185, 3 = 185, gauge reads 245, 1 = 280, 2 = 220, 3 = 220

Open letter to qwerty.....

I have found your attitude in this thread and others to be unnecessarily unfriendly. You seem to have good knowledge but a tendency get into putting down others.

On this forum, particularly the diesel section we have a lot of very experienced and helpful folks. When a person posts with a problem everybody normally tries to help with an occasional humorous remark.

We all win when the fellow finds the solution to his problem and gets it fixed.

Some problems like this one involve so many variables that the solution going in may involve a lot of searching to find a definitive answer.

It does not help when someone gets into negativism.

I am confident you can concentrate on helping solve problems and being more forum friendly.

Thank you for being a member and helping to solve our problems.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

The Dale Carnegie Course was not wasted on you, and if you never took it, you don't need it anyway.

Good thing you didn't see the first draft.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.