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

[KenEhresman]

ok, crap second time typing this... so here we go again... awoken at 3:30 by alarm... sounds like Polizei coming after me on the autobaun.... noticed a very faint version of this at 10 when we went to bed but could never locate so thought it was a coach or rv near us...nope, after 10 minutes of searching and another 10 of dismantling dash I found it. was just near the door hinge and thank gosh had a plug. fwiw, i did unplug the little black box and also the keyless entry brains thinking one or the other was turning it on, but neither had an effect... it was kinda cutting in and out a little cracking so I'm pretty sure something is failing but not sure what circuit or purpose this serves... not related to air pressure, tag or other chassis stuff, but did look like it was a prevost chassis plug and labeling... anyone know what this little gem is and what purpose it alerts us to?

next, With bad weather I got a weird Pro Driver Error, VSS Sensor issue... which with some research was kicking out the correct codes for Vehicle Speed Sensor.... was intermittent and seemed to come on after driving through snow and ice... I'm suspecting a connector somewhere... the dash speedo continued to work, but all other speed related functions such as cruise stopped working...it would clear upon turning coach on and off then was ok until bad weather again. Any wiring diagrams or schematics for the pro-driver unit, or this VSS sensor location and plugs would be helpful.

finally, I"m pretty sure my low speed fan relay isn't working... it's cold, and not florida cold lol... so this will have to wait until at least a 40 degree day, but any schematics of for fan clutch system would be great. It runs pretty consistent at 210 ish, and when it get to 220ish the fan kicks on and brings it back down to 200 ish... i removed the 190 thermostats and replaced with 175's which help but not enough.... also I can work around this by turning on the dash ac compressor switch, which cycles the rear compressor every once in a while which keeps it a little lower about 190 ish. On a hot summer day, when i run the dash AC it runs right at 175-180 so I know the ac compressor circuit it locked in much more for dash air.... any help here appreciated.

[KenEhresman]

One more item, While running coach the coolant circulates from engine through aqua hot to heat water.. at faucet it appears to get hot.... the front and rear zones for the aqua hot heater though never really blow the heat like I would think they should.... it is pretty much just cold. When my webasto is firing right ( it is not again" it will heat it and work correctly, with plenty of heat coming from those small heat exchangers and the circulation pumps all work correct. wondering how these aqua hot heat exchangers are built? they don't seem to efficiently take the heat from the running coach which is 175 on the laser to the input side of the aqua hot and transfer it to the closed loop system of antifreeze in the fired heat exchanger and registers in the coach... anyone experience similar issues? dash heat will blast you out, but the rest of coach is forced to be running on ac rooftop heat strips to stay warm....

[BoaterAl]

Would it be possible to get some info on your coach ? Converter, year, model posted would help. Likely another owner might have experienced this issue.
Just trying to help,
Thanks,
AL

[KenEhresman]

Sure, I forgot that part... its and XL chassis, 1999 last of the rivets.... and a featherlite conversion... from what I have been told, this particular featherlite was build up in Oklahoma, and seems to be a bit of a one off coach...

thx

[Gil_J]

The alarm may be the engine bay fire alarm. If they get wet they give false alarms. The sensor is directly over the head of the engine.

Got to run, so I'll answer the other questions later, if no one else does.

[KenEhresman]

I had accidentally left the driver rear bay open that houses the aquahot... maybe there is a detector there that got wet? it did rain that day.... drizzle anyway... the coach had been sitting for 3 days idle.... got me thinking there is a sensor in that bay?

[Gil_J]

Your fan clutch is always supplied 12V. It is activated by supplying a negative return. You may be able to provide a direct return at the fan clutch connector. If so, turn the key on, but don't start the engine. If you hand spin the fan and apply the return jumper you should see the fan slow down.

If you have a known good relay, break the coil tabs off. Pull the low speed fan relay. Again, ignition on, engine off. Inserting the broken tabbed relay should result in the fan slowing down. If this works, the problem is the relay. If it doesn't the return feed is not making it to the clutch or the clutch is bad.

The low speed relay is worked hard, so its failure after 15 or so years isn't a surprise. At less than $5 a relay, just replace all 3 fan relays.

[KenEhresman]

12v and not 24? i'll guess these relays are in the back engine compartment connection box?

[Gil_J]

Okay, I'm going from memory. Don't hold me to the 12V. Yes, the relays are in the compartment above the engine. Hopefully, you still have the decal on the engine bay door with the relay identifiers and location.

Did you know you can get the wiring diagrams and air drawings off the Prevost site?

[KenEhresman]

I didn't but i'll try to search for them... thx

[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.


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2/1/2018 3:40:00 PM
PrevostService

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Fan Clutch Control
Top of Page (https://prevostcommunity.com/forum/threaddetails.asp?Category=General+Discussions&ThreadID=8660#myTop)


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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[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.

14803

[Gil_J]

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

[KenEhresman]

SO I guess now we just need to know if any has the code for the DDEC to see what temps set these outputs.... and I thought I had you stumped lol....I should have known better :-)

[Gil_J]

Ken, see previous post for temp set points. My Detroit shop told me these were hard coded, which seems strange. Pittsburgh Power can probably change them, if anyone can. Once you fix the low speed problem, all should be fine.

[Gil_J]

For those following this thread, a fan that won't turn on will seldom be a problem associated with the DDEC or its wiring. A failure of the DDEC control or its wiring will result in the fan operating.

[KenEhresman]

relays replaced.... no noticeable change I can see... maybe runs 5 degrees cooler? Will take old units ad break off leg and take for a ride on low speed one to see what the temps run with constant fan on low and report back in a week or 2...