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Testing Electric Fan clutch?
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Looks like one strategy is to use a friction surface for 1:1 drive and an eddy current when the friction surface is disengaged to underdrive.
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The fan clutch on the coaches is elecrically held into low and high speed by means of rare earth magnets. When the fan clutch is not energized in any way the the magnets has enough attraction to drag the fan around but you can grab it and hold it with no problem.
When low speed is asked for one set of magnets are energized and when high speed is activated another circuit with larger magnets is energized and the fan is “locked in” on high speed.
The program to control this fan clutch is created by Prevost and Detroit to meet the cooling requirements of the engine and the emmissions controls for that year’s engines.
Detoit supplies two wires from the engine ECM for fan control: 555 (low speed) and 563 (for high speed). They are both gound circuits and they both become active once the engine is started and no cooling fan is required. Prevost in turn uses two relays for fan control, one for low speed and one for high speed. The ground wires for the fan clutch run through each relay. They are wired through the relays on the normally closed contacts. Thus when they are not energized there are grounds supplied to both speeds through the relays.
When the engine starts and no cooling request is sent from the engine both wire 555 and 563 are grounds and the two relays are energized and both fan clutch grounds are now open.
When low or high speed are asked for the appropriate wire from the engine ECM loses its ground, its relay de-energizes and the ground wire from the fan clutch goes to ground and you get that speed.
All of this circuitry was done to keep the fan clutch circuit from being actively attached to the engine ECM.
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Excellent explanation, thanks Prevost…
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