[Archived]

I would say it would be more of a solution if you get in a bind rather than a test just to see if it works.

R78 (low speed relay) & R79 (high speed relay) in the rear junction box control the fan clutch. Breaking the 85 tab off R78 will keep the fan locked in low anytime the ignition is on or until the temp calls for the high speed. Breaking the 85 tab off R79 will keep the fan locked in high. Your MPG will reflect it right away.

To make sure the fan is unlocking, it should spin free with the key off & when it is just sitting there at an idle; it will spin some but wont be locked in (try to resist sticking you hand in there). If your engine runs over temp, its not locking in! Not sure the specifics but the low speed should kick in at the low 200 mark and the high speed maybe 205 or so?

Somebody on here knows.

[AL]

Patrick has the relay numbers and the procedure correct. An H model has different relay numbers but the concept is the same.

The clutch is controlled by DDEC and generally speaking the low speed fan comes on at 204 F and the high speed at 208 F. If you have clutch issues and have to lock the clutch electrically the engine temp will be maintained by the engine thermostat but at lower temps.

The clutches can also be locked mechanically using the tabs and screws provided. The screws are usually in place at the end of the shaft for use to lock the fan, a temporary measure at best.

Since the dash gauges are not precise the temperatures mentioned are about what you see on the gauge, but all that is important is that you see the gauge cycling up and down a little as the coach is being driven. The cooling system is healthy if on a long steep hill on an excessively hot day you see the temos reach around 208 / 210 and then cycle back down as the high speed fan kicks in and moves a lot of air.

If you have any doubt about if the fan clutch is working break the 85 tab from a relay and with the coach on fast idle use the modified relay in both the low and high speed fan positions. You will definitely see the fan engage and the high speed moves enough air to blow the face off a dog.

[Archived]

Art,

FYI – If you have dash air and not Prevost OTR, when you turn on the A/C it will turn the fan on.

[Rodney]

When I go up a large hill and it’s hot out I noticed temp gauge starts rising and then it fall’s back down. To test if you had an old relay and cut tangs with engine off and key on would fan not turn. And what does the DDEC the 2 relays? and at idle engine cold fan does not turn very fast?

[Joe]

Art, don’t over think this………if your temps are cycling up and down as you ascend a steep hill on a hot day everything is working as it should.

Your DDEC is signalling the fan clutch to do what it is supposed to and if you absolutely have to prove it is working do as described above with a modified relay.

[Art]

So the low speed and hi speed on low speed is the clutch slipping all the time and Hi speed it’s totally locked up? and what goes wrong in the fan? Does it have clutch disk that fail?

[Tim]

I’ve never been around when one has failed on a bus but I have had a fan clutch go out on my truck. It will make LOTS of noise and you can wiggle it back and fourth b/c of a bearing failure. It gets real oily and messy.

[Bill]

So in the fan it’s self it has a oil clutch like a car and a electric clutch all at the fan?

[joel]

As long as it seems the design of the fan clutch is important has anyone considered it engages via a set of planetary gears to get the two speed operation?

Or is it possible there is some type of centrifugal effect at work to vary the speeds like in some transmissions and go carts?

If the low speed has a slipping clutch preventing it from reaching the full locked up second speed does it have a cooling system to reject the heat of friction?

I’m going to wait for folks that need to know how the fan clutch operates to visit me so I can watch them disassemble and old fan clutch I have replaced because the bearings were starting to make a little noise.

For those as lazy as me we can find out how one works just by looking up patents for fan clutches to see not only illustrations, but the text describing how it works in detail.

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1999 XLV X-6872,  liberty #440

[Archived]

Haven’t a clue how it looks (works) inside, but as long as it freewheels at cold and the 1st and 2nd speeds function AND IT DOESN”T MAKE NOISE, you’re golden. Obviously, there has to be a bearing inside to allow the fan to remain mostly stationary when cold and the pulley is turning. When my clutch failed, I could hear a grinding noise over the sound of the diesel when idling cold.

If it or the gearbox fail again during my life, I’ll probably dump it all and go to electric fans.

[Archived]

Thanks’ guys I just like to know how things work

[Archived]

Joel, the new Prevost coaches have electric fans now, replacing the gear box and belt driven fan.

[needtocamp]

Fan Clutch

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.

MORE FAN INFO

The fan clutch ot the Prevost is a two speed device. It is either off, low speed ( approximately 1/2 of engine rpm ) and high speed ( full engine rpm). It is controlled by the ECM of the engine.

The fan clutch is powered with 24 volts when you run the engine. The ECM controls the ground circuits for the two speeds through the two relays , R78 and R79. The circuit is a little confusing as the two relays provide gounds to the fan clutch when they are not engaged. Thus when the engine starts and the coolant temperature is low both relays receive ground signals to the relays to pick them up. This in turn removes the ground path for the fan clutch,both high and low speeds.Thus no ground signal from the ECM results in that fan speed to be active. That is why we have you break off the 85 tab on the high or low relay to have that speed on all the time to get you off the road.

Pull the high and low speed relays. Start the engine and see if the fan is still locked in on one of the speeds. If it is then there must be a short to ground on the wire from the relay to the fan clutch itself. If it just free wheels, then there may be an open between the ECM and the relays on either wire 555 (low speed) and 565 (high speed ).

If the engine thermostats are stuck open it may be hard to achieve normal engine temperatures, especially when the external temperature is low. You could try to run the engine with the relays pulled and see if the coolant temperature will go higher than the 180 -195 degree range. That will show that they are generally working. Don’t let the engine overheat and shut down

Fortunately with this problem you won’t overheat and shut down on the road

[Dick]

Good post Don. Jon – We May want this in repair nuggets and closed.

[Craig]

Excellent post Don. Two questions remain:

1. Where did you gleen that great information?

2. What mechanism does Prevost/Detroit use to effectuate the two speeds? Does it shift gears? If the change in speed is from 1:1 to 1:2 (or 2:1), that is pretty dramatic. How does that occur without there being a significant strain on the mechanism?

Thanks!