MCI - how does low air switch control alternator delayed charging?

[bevans6]

My MCI MC-5C, and I presume some other buses, has an air pressure delayed charging system.  The alternator doesn't turn on until air pressure has built up to the point that the low air pressure switch closes.  The reason for this is to delay the charging load until the stock (long gone, to be honest) air cylinder belt tensioner has applied adequate tension to the alternator belts (MC-5C, four belts drive the alternator, unlike other systems where there is a gear driven alternator).  This has to be done by interfering in some way with the field coil energizing circuit, either by turning positive off to the voltage regulator or by interrupting the F1 signal from the regulator to the alternator.  I just don't have the schematic for this in my manual, and I can't find it on-line.  Can anyone post the alternator control schematic that shows how this air pressure charging delay is achieved, or even just describe it for me?

Aside:  I do have a schematic for an MC-9 with a gear driven alternator that shows the blower discharge relay and how it controls the air pressure delay for heavy loads like the AC and heater fans.  It shows that the delay is via the Nason air pressure switch and the output from the Relay terminal of the alternator.  This is a separate circuit, I think, from the main alternator delay.  It simply doesn't allow the heavy loads to turn on unless the alternator is charging, turning the loads off if the alternator fails to charge for any reason.

Thanks, Brian

[Dreadnought]

On your 5c where is your alternator located? On my 5 mine is at the rear (near the cabin, nearer the front of the coach) of the engine. It's accessed via one of the side hatches and twin belts drive it. Other pictures I've seen of other buses often show them driven at the front of the engine

[bevans6]

Mine is mounted to the rear of the engine, on a cradle that is part of the engine mounting cradle, driven from a four sheave pulley on the cam accessory drive.  It is oil cooled, 250 amps rated.  My understanding is that earlier MC-5's had similar systems with a lower output air cooled 50DN alternator.  Edit:  there is only one belt on in this picture, basically to keep the alternator from flopping over when I was installing it.  It's driven by four matched 1/2" vee belts.  You can see it here:

[TomC]

Unless you still have the over the road A/C, why don't you switch to a normal alternator. I know at Freightliner we were selling 160amp alternators for less than $200 new. Good Luck, TomC

[Dreadnought]

Unless you still have the over the road A/C, why don't you switch to a normal alternator. I know at Freightliner we were selling 160amp alternators for less than $200 new. Good Luck, TomC

Were they also liquid cooled or air cooled?

I have a weedy 65 amp one, and even on a 24volt system, I don't know if Ill be able to power the refridgerator and one of the overhead AC units while driving with it.

[Tony LEE]

The question remains

Can anyone post the alternator control schematic that shows how this air pressure charging delay is achieved, or even just describe it for me?

[bevans6]

My alternator works perfectly, I just don't have the schematic to show how the delay works.  The delay works fine, but several times a year someone posts a question on MCI alternators and I would like to be able to answer the questions.  Someone on another forum is having a charging issue with a MC5, and that prompted my question.

Tom, I run a Magnum 4000 watt inverter while driving to run the rooftop AC unit, so I do use and need the big alternator.  I am one of the few that is a big fan of the Delco 50DN.  I think it's a great alternator, particularly because it came with the bus for free... 

Dreadnaught, a 65 amp alternator will not do more than run the lights and charge the start batteries.  It won't run any of the original heating or cooling system accessories, and won't come close to running a rooftop AC unit.

[gumpy]

The Penn Air switch grounds the coil of the field relay for the alternator.  

Should be a schematic called Charging and Starting in the electrical section of your manual.  If you don't have it, check the MC8 manual. Probably the same. Penn air switch closes at 65 psi and grounds the field relay coil which connects positive to regulator POS terminal.

Note that the telltale lights are a separate circuit connected to the discharge and blower cut-in relay, which is controlled by the alternator R1 terminal.

[Oonrahnjay]

...Tom, I run a Magnum 4000 watt inverter while driving to run the rooftop AC unit, so I do use and need the big alternator.  I am one of the few that is a big fan of the Delco 50DN.  I think it's a great alternator, particularly because it came with the bus for free...   

     Brian, what do you think should be the "safety factor" for an alternator.  I don't mind using their rating "all the way" for brief periods, but making full rating is a big thermal load and mechanical load.  Some people say "don't use an alternator continuously for more than 80% of its rated output" others 75%, etc.  Got any thoughts?

[bevans6]

If you are using the stock 50DN, they are pretty much able to produce continuously at their rated output, as I understand it.  Particularly the oil cooled versions, it's why they are oil cooled in the first place.  If you are using an automotive air-cooled lower capacity replacement alternator, I have always used 60% as the maximum continuous load.  That said, my understanding is that some aftermarket alternators designed for supporting big audio systems in cars are designed for high maximum load and higher output at idle speeds, at least the marketing says so.  Our engines run so slowly that you need to look at pulley sizes and ratios to really understand what your replacement alternator might be doing.  If you have a typical 3" pulley on your alternator and you are running it from a 6" cam pulley, you'll have virtually zero output at idle and about 70% rated output at highway cruise rpm.  On a Detroit two stroke, all the available pulleys are at crank speed, mostly, and an alternator's rated output is at 6000 rpm (there is a standard test rating for alternators).

[luvrbus]

I too have often wondered why the older MCI buses use that setup,it never made any sense to me no other bus uses it with a 50D

[gumpy]

I too have often wondered why the older MCI buses use that setup,it never made any sense to me no other bus uses it with a 50D

The reason for the air switch disable is because of the air tensioner on the belt driven alternators. They did not activate the regulator until the air pressure was sufficient to tension the belts properly.  Later models with the direct drive alternators don't delay the power, The telltale lights still indicate it's not charging until air pressure comes up, but it really is.

[luvrbus]

Still makes no sense why you even need a air tension on a belt driven alternator the Eagle was a belt driven 50D they never used a air tension   

[daddysgirl]

The Penn Air switch grounds the coil of the field relay for the alternator.  

Should be a schematic called Charging and Starting in the electrical section of your manual.  If you don't have it, check the MC8 manual. Probably the same. Penn air switch closes at 65 psi and grounds the field relay coil which connects positive to regulator POS terminal.

Note that the telltale lights are a separate circuit connected to the discharge and blower cut-in relay, which is controlled by the alternator R1 terminal.

Yep.
And thanks to Craig...I know he's right.
I've got the schematics, but there are several under starting and charging (motor control might be helpful) based on the unit number for the MC8.

[bevans6]

If it's like the air tensioner on the other belts, it's so a driver can change the belts with no tools.  Blow off the air pressure, belt is as loose as it can be, and you can change them and the bus automatically applies the correct tension with no fiddling.  If that's not the reason why, it's at the least a happy byproduct.