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Author Topic: Looking for "Blower Delay Circuit" on MC9  (Read 388 times)
TheHeavenlyChillbillies
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« on: November 04, 2017, 02:20:35 PM »

Hi there.
I found mention of a blower delay circuit in an old thread but have found no reference to it in the MC9 manual.  I am trying to make an inter-tie between the house and start batteries that activates/deactivates a relay based on when the bus is running.  Any help would be appreciated. 

Thanks.

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Geoff
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« Reply #1 on: November 04, 2017, 05:44:48 PM »

I think I know what you mean.  An oil pressure switch will work.
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Geoff
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« Reply #2 on: November 04, 2017, 09:03:10 PM »

Look at the electrical schematics, specifically the Heating and Air Conditioning schematic, and Charging and Starting schematic. You'll find the Discharge and Blower Cut-in Relay in the rear junction box. This will activate at about 60 psi. You could use it to activate your crossover relay.

BTW, I highly recommend you use a simple switch to activate the crossover manually. There are times you really don't want it charging your house batteries until you are actually underway and have higher RPMs.
« Last Edit: November 04, 2017, 09:05:21 PM by gumpy » Logged

Craig Shepard
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TheHeavenlyChillbillies
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« Reply #3 on: November 05, 2017, 06:34:39 AM »

Hi there.  That seems easy enough.  What are the concerns about running an inverter/charging house batteries at low RPMs?  It seems that the engine kicks up automatically when a load is put on the inverter.  Is it a strain on the generator(alternator)?

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buswarrior
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« Reply #4 on: November 05, 2017, 08:24:53 AM »

The inverter/charger isn't doing anything, if the engine alternator is doing the charging?

The alternator will just make as much amps as it can for the RPM being operated.

Just drive the coach.

Also, have a way to dis-engage the inter-tie. Some folks use circuits that require shut-down, and you may not want that?

happy coaching!
buswarrior
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bevans6
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« Reply #5 on: November 05, 2017, 08:59:06 AM »

It would help to know what alternator you have, on which engine.  If you have the popular 8V-71N and Delco 50DN alternator, then the alternator is putting out around 150 amps at idle.  If you are running an inverter, that translates to around a 25 amp 120 volt AC load, so one air conditioner is OK, two is a stretch at idle.  The engine governor will adjust the idle speed to be the same even with the power load to run the alternator, so if your inverter kicks in you may hear the engine note change as it picks up power, but stays at the same rpm.  The 50DN was specifically designed as a transit bus alternator that can keep heavy loads operating at momentary pauses to idle, stop and go traffic, that sort of thing, and most MCI's came with a high idle setting designed to let the engine continue to support high loads for long periods of time parking - running the stock air conditioners or heaters all day waiting in a parking lot, that sort of thing.

The blower cut in relay is controlled by the relay output of the alternator and the low air pressure switch.  That means the engine must be running so the alternator is charging AND the air pressure must be up before the cabin heat and air conditioning blowers are allowed to turn on.  The air pressure switch creates a delay only if the bus is started with no or low air pressure.  If the air pressure is up, no delay.  Controlling the big cabin blowers with the alternator relay output is so the blowers can't run if the engine stalls for any reason, and run the batteries down.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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