Multiple Items, will be a long read... Alarm at 3;30AM, 2 Stage Fan Clutch, and other

[Gil_J]

https://techpub.prevostcar.com/en/

Look at the menu options in the banner on the top.

BTW, the speed sensor is on the transmission.

[KenEhresman]

d060874p09r021.pdf on prevost site...

Couldn't see where to attach... in any event, I see where the relays supply the ground and looks like all the control voltages are 24V, so 24V relays... looks like ECM turns on speed 1, then there is a hardwired temp sensor that must turn on speed 2 as it says "DDEC III Engine Harness" whereas Speed 1 says "DDEC III ECM " which kink of makes some sense I guess, the fan speed 2 comes on when it it in danger zone.... while speed 1 is set from DDEC computer... also see where the AC can turn it on ( knew that, I run with AC on in the summer and it stays at 180 all day long, but in the winter, the compressor rarely kicks on so no fan action from that portion of the circuit... so far making sense... it also appears when you use the retarder, it trips it on, i would assume since it is blocking exhaust it can build heat, hence the forced fan ( didn't know that)

So I'll make a couple pre checks on CB130, but logic would say it is good since the 2nd of the 2 speed relays is working but check anyway, once each relay has voltage on the coil, I'll ground out the other side of the relay coil while not connected to the socket ( I have a test jig for this) and see if I get fan... this will test the relays and circuits from the box to the fan.... assuming all works, the only other potential issue will be from he DDEC computer harness to the coil negative.... I'll assume the DDEC sends that line to a logic low when it want fan action.... If it gets that complicated, I'll need a way to monitor that signal while driving...unless it also reports that up front somewhere? chasing that line through the DDEC diagram now... thanks for pointing me to the files, much easier to troubleshoot when you have schematics lol...

[Gil_J]

Ken, the DD engineers were pretty smart. The fan is energized when the relay is resting or deenergized. That doesn't still mean it can't be a relay. If it's the relay it will be due to contacts.

There's also a relay, R99 as I recall, that provides 24V to the fan clutch that's controlled by the key.

BTW, only use 24V relays with protection diodes on DDEC controlled relays. They are not that common but are available from the major electronics suppliers.

[truk4u]

Ken,

We also have a fan control problem with a member on PC and Prevost is helping, thought this could help you.

2/1/2018 3:40:00 PM PrevostService
	Fan Clutch Control	Top of Page
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.

[KenEhresman]

HI Tom, thanks for the insight.... Curious for more details.. Any idea at what temp the ECU lets R78 rest to kick the fan on low speed?
In my wiring diagram, it shows R79 being fed from the Wiring harness, not the ECU... I have not yet went back to the schematics, but assumed it wasn't being controlled by the ECU, but rather a sensor somewhere on the engine? I'll dig back in the schematics and chase that line.... My symptom is R78 not functioning, but R79 appears to function fine.... hence my question on what temp.... according to my dash gauge, and I can't say it's accurate, the R79 rest and kicks the fan on about 210 degrees.... assuming the retard or AC isn't on....

[KenEhresman]

Is listed, but according to my serial, I don't see any C313, it list it as PIN A on that connector, and the index reads as it is a DDEC INPUT connector... if anyone sees C313, Send the link my way.... I still suspect it is coming from a temp sensor on the engine itself feeding both the DDEC as well as the Fan #2 relay

[Gil_J]

Ken,

Has relay 78 been changed with a known good relay and has the low speed operation of the clutch been confirmed? The low speed operation is worked hard for those that don't leave their dash air on.

The engine sensor that provides engine temperature to the DDEC can't also supply a switch function to R79 because the DDEC is looking for a variable resistance to represent temperature and not a simple contact state change. I'll see if I can't pull the info and read it on my phone.

[Gil_J]

For Series 60 engines with motor serial numbers up to 06R0194000 inclusively
• 204F (96C) First speed
• 208F (98C) Second speed
For Series 50 & 60 engines with motor serial numbers from 06R0194000
• 196F (91C) First speed
• 203F (95C) Second speed

[Gil_J]

Ken,

I've always thought the fan control for both low and high speed are DDEC controlled, but I do see only one DDEC speed control output. I also only see one engine mounted coolant sensor. I would have expected 2 engine mounted sensors. Still digging.

Screenshot_20180207-071216.jpg

[Gil_J]

Speed 2 does come from the DDEC. It's not illustrated because it's a OEM programmable output.