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Engine Temp Too Cool?
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My guess is you can rule out the thermostats otherwise with a fan on 100% of the time the engine will take forever to warm up it it ever does.
I do not know where the fan clutch gets its signal to engage, whether it is a sensor or if a sensor sends a temperature value to DDEC and DDEC is what controls fan engagement.
The problem does need to be fixed and the repair cost will come back to you in fuel savings. It takes engine power to spin the fan and I have heard that it can use as much as 30 HP. Additionally the engine is more efficient at higher temps so just averaging 200 degrees instead of 185 for example is more efficient use of fuel.
I will reach out to Bill Jensen and see if he can help.
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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.
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Thanks much.
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 ).
I pulled relays 78 and 79 and had my wife crank the bus. The fan was immediately engaged. If I understand correctly, I now need to be looking for one of the wires going between relay 78 or 79 and the fan clutch that may be shorted to ground? Or is the wire I am looking for going to the fan clutch from another source?
Am I looking for a broken wire? A wire with the copper exposed and touching metal? Something else entirely? Or any of the above?
Davy
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I am not a Prevost owner yet but I remember others say the A/C will cause a fan to engage.
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Davy,
If I were to begin looking, based on Bill’s advice and information I would start with the wire harness / terminal at the fan clutch and follow it looking for pinched or chafed wiring that could create the problem.
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Will do, Jon. Thank you. I could be all wet, but I wonder if my high idle gremlin could be caused from a related injury to a wire some where?
Thanks for the suggestion, Jim. I see the connection you were making. I was hoping (as always) that it was something that easy. I have read on PC that the radiator fan automatically switches to high on some coaches with dash air. I have bus air from the factory and mine fan does not go to high with the bus air on.
Davy
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If the fan engages with the relays removed then it most likely is a short to ground from one of the two ground wires from the fan clutch assembly. It also could be that the wire is shorted internally in te fan clutch,
You are correct in that there is no activation of the fan clutch on OTR A/C equipped shells.
Look at the relay bases for the two relays and see if one of the wires may have come loose from the base and has dropped down and is shorted there.
Is the fan in low or high speed when you start it without the relays? Depending on the speed you can look at the relay base and ground wire back to the fan clutch assembly for that speed. See what you can find there.
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If the fan engages with the relays removed then it most likely is a short to ground from one of the two ground wires from the fan clutch assembly. It also could be that the wire is shorted internally in the fan clutch,
You are correct in that there is no activation of the fan clutch on OTR A/C equipped shells.
Look at the relay bases for the two relays and see if one of the wires may have come loose from the base and has dropped down and is shorted there.
Is the fan in low or high speed when you start it without the relays? Depending on the speed you can look at the relay base and ground wire back to the fan clutch assembly for that speed. See what you can find there.
Thank you.
I believe it is the low speed fan that is on. I could easily be wrong, since it seems to be a habit with me, but I think it is low speed. Is relay 78 low speed? How are the wires attached to the relay base?
With the wires bundled and running along beneath the relays, it is difficult to see much there. I will try to check the relay wires first then move to the fan and work my way back. If I get to the compartment with the relays and have not found the problem, I may have to un-bundle some of the wires.
I never thought of the short being internally in the fan clutch until my Dad mentioned that as a possibility. Thank you for confirming that possibility.
I drove 400 miles Saturday evening and Sunday morning. All remains the same.
Thanks again for your help.
Davy
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R78 is the low speed relay. The wires are held into the relay base by the socket electrical fittings on the ends of the wires. They are pushed in from the bottom and lock into place.
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Thanks to all that helped me with this including Prevost.
The fan clutch was stuck in low all of the time, not even going to high when high was called for by R79.
Jeff Rowe determined it was the fan clutch and replaced it last week. Here is hoping I get 23 years out of this one.
Thanks again.
Davy
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