How To Test The Fan Clutch Case Study

This case study will help you to test a thermal fan clutch on any GM vehicle equipped with one.

Believe it or not, there's a specific and logical diagnostic strategy to diagnose a BAD fan clutch (that's causing your vehicle to overheat).

Now, the way I'm presenting the info, I'll jump into the story of an over-heating 2003 Express Van, what tests I did and what I finally replaced (which in this case and the purpose of this case study was the fan clutch).

Symptoms Of A BAD Thermal Fan Clutch

A BAD thermal fan clutch will cause your vehicle to overheat. To explain why, I'll go into a very brief and non-technical description of how the thermal fan clutch works (after all, we don't plan on reverse engineering the damn thing):

In a nutshell: There are two major components that make up a thermal fan clutch: A reservoir filled with Silicon fluid, that's inside the fan clutch itself, and a bimetallic coil on the outside.

This bimetallic coil is connected to a shaft that runs inside the fan clutch assembly. The bimetallic coil is designed in such a way that when exposed to the engine's increasing heat, it begins to expand. This expansion causes the shaft, it's attached to, to rotate counter-clockwise.

When the engine starts to cool down, so does the bimetallic coil. This decrease in temperature causes the coil to contract and so the shaft now rotates clockwise (and thus back to its original position).

This counter-clockwise and clockwise rotation of the shaft, as the bimetallic coil expands and contracts, causes the fan clutch to go into one of two modes of operation:

  1. Engaged Mode
    1. This mode causes the silicone fluid inside the fan clutch to move from one chamber to another, thus causing the fan clutch to lock up and turn close to engine RPM speed.
    2. This mode is the one that provides the maximum cooling effect by pulling the most air across the radiator and A/C Condenser.
    3. This mode has a direct effect on engine power and fuel economy which decreases both.
  2. Disengaged Mode.
    1. In this mode the movement of the silicone fluid is restricted, thus causing the fan clutch to free-wheel (to a certain degree in relation to the water pump's Pulley) and so the fan clutch does not turn at engine speed.
    2. This mode is the one that provides the least cooling effect by pulling the least air across the radiator and A/C Condenser.
    3. This mode has a minimum effect on engine power and fuel economy.

When a fan clutch goes BAD and causes your vehicle to overheat, it's usually because the fan clutch is unable to get into its ‘Engaged’ mode. Thankfully this can be tested (indirectly) by manually locking the fan clutch into its Engaged Mode and I'll show you how in this case study.

What Vehicles Can I Apply This Info To?

Although this is a case study of a 2003 Chevy Express Van (with a 6.0L) that belongs to a buddy of mine (Oscar), you can apply the testing tips and info to any thermal fan clutch equipped vehicle.

So, whether you're driving a GM 4.3L, 4.8L, 5.0L, 5.3L, 5.7L, or any Ford, Chrysler vehicle, etc., this info will help you.

OK, without further ado... let's jump right into the story:

The Complaint

My bud Oscar drives a 2003 Chevy Express Van (with a 6.0L), which is his work vehicle, and like most of the folks I know... The state of the economy has driven him to do most of the repair work on his van himself.

Every now and then, he'll have something that'll stump him and he'll call me, over the phone, to ask for advice. Most of the time, I'm able to help him over the phone... other times I have to intervene directly.

When he called me this time around, it was to tell me that his Express van was overheating and that he had already replaced a bunch of stuff. But that none of those things had solved the overheating problem. He was thinking he had a blown head gasket.

He had already replaced: water pump, thermostat, and had removed the radiator to have it rodded through (thinking the radiator was clogged up inside). Since he felt that there was nothing else to replace or blame, he needed me to take a look at his van and make sure it didn't have a blown head gasket.