That is absolutely NOT true for many vehicles. I have PERSONALLY had a vehicle where a bad fan clutch would first show up at highway speed, and as it got worse progress to idling in traffic. Some vehicles, for whatever reason, don't get enough ram-air through the radiator to cool the engine.
Its especially true for a big boxy vehicle like a van which requires a lot of engine power just to maintain highway speeds. More power-> more cooling requirement.
I have also had this problem. If the fan doesn't turn fast enough at highway speed then it BLOCKS the flow of air through the radiator - leading to overheating.
Frank
one more update.. I just noticed after getting off the highway today that there is a tiny spot on the radiator itself near the top that is now leaking a minute amount of fluid but only when driving at highway speeds. Should I just go ahead and buy a new radiator at this point? I'm thinking there may be a large portion of corroded/rusted walls in this radiator with potential for pressure & fluid loss. Prior to this leak developing on the radiator the mechanic still insisted the cause of my overheating on the highway must be the fan clutch as it doesn't seem to go any faster when gas is applied. I'm planning on buying a new radiator from autozone this week @ $180 and also installing a new fan clutch myself from autozone. I will have to run myself over repeatedly (somehow) if the engine still runs hot after this...
Thanks
Here is a test for the fan clutch. With the engine fully heated up have someone shut it off and watch the fan. A good clutch will stop the fan almost instantly when hot.
Second test is to have the engine off and grab the fan blades and wiggle them. There should be very little play in it.
The leak up top is very common after a lazy person changes a thermostat. Rather than unhook the top hose, the lazy person bends it up out of the way to install the t-stat. This can and does crack the top hose nipple's solder seal or it pulls the plastic tank off the gasket if yours is plastic.
A bad fan clutch will vibrate at 55 mph....
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT'sMotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.