[Dick]

Prevost used the two speed fan clutches with the introduction of the Series 60 engines. The first Detroit engines used were DDEC II configuration and had no fan control avaiable in the software. To make it function a thermal switch was used. It was mounted in the radiator cross pipe over the engine. It would turn on low or high speed as the temperature reached around 190 degrees F for low and around 200 degrees F for high speed. See diagram. : This control method was with the power side of the fan clutch. In other words when the temperature reached low set point, power was fed through the switch and down wire 291 to give us low speed. As the temperature continued to rise, the same would happen for the high speed and power was fed through the switch to wire 292 for the high speed. The grounds for the fan clutch were constant to chassis ground.

With the introduction of DDEC3 the present style of fan control was supplied by Detroit Diesel through their ECM.

This style electrical diagram: This is the setup with the two relays: one for low speed and one for high speed. Plus the fan clutch is fed 24 volts when the engine is started and we control the speed chosen by controling the ground part of the circuit. As noted in this forum trail there are different relay numbers for the XL and the H.

Looking at the above diagram we can see that with no power on the circuit both relays supply a path to ground through each relay. The relays themselves are fed 24 volts when the engine is startedand the grounds to conrol the relays come from the Detroit ECM wires 555 (low speed) and 563 (high speed). Thus the control comes from the engine computer based on a program agreed to by Prevost and Detroit Diesel.

When the engine starts and the engine is cold the ECM supplies a ground on both wires and the relays are engaged (picked up) and the grounds to the fan clutch are removed. The fan spins free as the engine rpms go up and down. The fan may appear to be engaged but is simply being pulled along by friction on the bearings.

With the newer control system you can lock in a speed electrically by pulling the relay and removing the 85 tab and reinserting the relay in its base. Now the relay cannot be engaged by the engine ECM as it is constantly fed a ground by the relay. And if you are going to do this temporary fix to get off the road you would choose the relay for high speed to modify. That way the fan is locked into 1 to 1 drive and is spinning at engine rpm just like the mechanical system on the earlier model coaches. This usually will get you to a repair place and safely off the road.

Also as noted there are tabs and bolts to mechanically lock in the fan but they are definitely there to get you off the road and to a repair shop as soon as possible. The tabs are not that thick or strong and the torque applied to the fan blade when locked in is enough to eventually brake off the tabs and then you have no means to control the fan any longer.

A side note: when you go to a Detroit dealer he will insert your engine number into the big computer and tell you there is an update that he can do to the engine computer on your vehicle. You must be sure tha they are giving you a BUS program that is specified for Prevost vehicles. Many of the updates are for truck engines and do not have 1. any fan control 2. fan control for a single speed fan. Plus the actual parameters of the bus programs were worked out with Prevost engineers and Detroit engineers to meet both manufacturing requirements and EPA certifications. If they cannot guarntee this bus program, you would be better to wait to have the update done. Some overheating issues were generated unknowingly by a simple ECM update.

[Archived]

Excellent post Prevost, thank you…