MCI 9 6V92 generator issue

[RickB]

Hi All,

Have a good friend with a 6V92 call me last night with the following issue. He said is gen light was on and wondered if he could drive it home 4-5 miles I told him I thought that it would be fine. Then he calls me back and says the light went out again. Long story short here's the symptoms:

When he starts the motor his gen light comes on and stays illuminated for 4-5 minutes, in that time none of his peripherals will work. fans and blowers etc.

He said last night that he couldn't disengage the parking brake until the gen light went out. I don't get that at all.It's not like the generator has an air actuated clutch that needs to build up air.

He also said that long after his low air light goes out the lo air buzzer keeps buzzing. I told him to watch and see if the buzzing was stopping at 90 lbs.

He has the old style voltage regulator so I told him to make sure he's getting just under 28 volts to the batteries and to make sure that they are full of water and that they didn't freeze and to check all the grounds.

Any other thoughts, idea's

Thanks,

Rick

[RickB]

I forgot to mention that he turned off the motor last night when the parking brake wouldn't release and after sitting 10-15 minutes it started and worked fine. Does that sound like a circuit breaker overheating and then resetting itself after it cools?

Thanks,

Rick

[buswarrior]

This is a fascinating situation!

What is the background on this coach? Some unorthodox protection circuitry inhibiting the brakes by design?

Depending on it's configuration, the "no gen" light works in conjunction with the air pressure, the relay controls both, in order to not load a belt driven alternator until sufficient air pressure is built up, but that shouldn't prevent the parking brake to release, unless the air pressure was too low for a release at the time.

What was the air pressure reading on the dash gauge while this was going on?

Maybe want to spray the low air relay attached to the rear junction box with some lube spray?

keep the details coming!

happy coaching!
buswarrior

[luvrbus]

Rick, not being a MCI person don't know if this will help but on a Eagle it has a 6 volt relay that energizers the alt and regulator to the charge mode and his problems are about the same as when that relay starts to go bad on a Eagle .
I would think both would be the same with 50D alternator with the MCI using a 12V relay for the 24V system. But the brake deal ?

good luck

[bevans6]

Your friend's bus operation sounds perfectly normal to me.  The generator is controlled by the air pressure in the Aux system, along with the high current loads (I believe the relays are called the 24V Discharge Relay and the Blower Cut-in Relay, and they are in the rear distribution box, and the air switch that controls them is in the little box on the rear bulkhead, it is the exact same switch that controls the low air warning light and buzzer, so it is to be expected that the two are connected in their operation.  The brake deal is also connected to the air system, you can't release the parking brake until there is 100 psi or greater in the  air system.  So overall, unless there is more to it, the operation of everything seems normal and he's trying to do things before he has full air pressure?

One thing - the low air buzzer is also the buzzer for the hot water, and low oil pressure alarms, and they are all back-feed protected by diodes, so one of those could be causing the buzzer to stay on after the tell-tale has gone out.  If the engine is idling, I would think of the 3 psi low oil alarm sensor personally.

Brian

[RickB]

Brian,
That sounds reasonable to me but it is either an option or an older/newer bus because my bus doesn't operate this way. My gen light goes off as soon as my motor starts and my buzzer goes off at the same time as the buzzer.
Thanks for the help
Rick

[luvrbus]

You must be able to wire the MCI in different ways I have a friend with a 9 with the gear driven alt with a 6V92 and it starts charging when the engine starts then another friend has a 8 with belts and his won't charge till it has 90 lbs of air.  
Glad I have a Eagle   lol  


good luck

[bevans6]

It's totally possible and not very hard to bypass the air switch.  It's for belt driven alternators, as Luvrbus points out, and it's because of the air cylinder belt tensioner.  Your friends bus is acting exactly the way mine does every day.  Nothing much works until there is pressure.  On the buzzer, all I wanted to point  out is that it's not just the low air buzzer, it buzzes for at least two other alarms as well.  The low air switch turns on the tell-tale and activates the buzzer with a single switch, so if the light is out and the buzzer is still on, it's not buzzing for low air.  On my bus, I changed it so that it is  indeed just the low air buzzer, but that is not stock.

One thing, perhaps even trivia - the air pressure gauge reads pressure in the dry tank, while the low air switch (that activates the alarm and switches the various relays we have been talking about) reads pressure in the auxiliary tank, at least on my 1980 MC-5C.  I have heard that some earlier MCI's had the gauge read pressure from the auxiliary tank.

Brian

[luvrbus]

I don't think the air belt tension cylinder has anything to do with his belt driven alt on his 8 not charging till he has air pressure he doesn't have one.




good luck

[JackConrad]

Our 73' version MC-8 has the gear driven alternator, but still had the switch to prevent the alternator from charging until there is air pressure. This switch also prevents the AC from working until you have air pressure (AC belt tension controlled by an air cylinder).  Jack

[bevans6]

BTW, my comments are not to say that something isn't wrong with the bus, just that mine happens to be very similar and acts the same way as what has been described, is all.  If it just started doing this, maybe something has gone wrong.

Brian

[Jerry32]

Same here , mine keeps the not gen light on till 90 psi and at that time the buzzer stops too. Brakes can then be released and go. jerry

[JackConrad]

  On my bus, I changed it so that it is  indeed just the low air buzzer, but that is not stock.

I have heard that some earlier MCI's had the gauge read pressure from the auxiliary tank.

  We did the same thing, our low air is still the buzzer, but our low oil pressure pressue is now beeper and our hot engine is the same as the low oil except it is a constant tone.
   On our MC-8 the air pressure gauge reads the pressure in the aux. tank and the sender is in the line that feeds the belt tensioner, engine stop & fast idle solenoids (these feed of the accessory tank).  We removed the OEM air controlled switch on the side of the rear panel (installed the buzzer sender in place of the Penn switch), since the we do not have the OEM AC or a air tensioned alternator. This allows the alternator to start charging as soon as the engine starts.  Jack

[bevans6]

jack, I have a two questions for you on how your pressure gauge reads.  Apologies to everyone for the thread drift...

Your pressure gauge reads from the auxiliary tank, which is downstream of the pressure protection valve.  When your bus airs from from completely drained, the pressure protection valve will not let air into the auxiliary tank until the pressure reaches about 65 psi.  Does your gauge read zero and then jump up to 65 - 70 PSI quite suddenly?

My other question comes from the thread on electric compressors and pressure protection valves.  Reading the Bendix literature it's clear that they all prevent backfeeding from the aux. tank side back into the dry tank side to a greater or lesser degree, depending on which type is installed,

"If system air pressure decreases below the specified closing
pressure, the regulating spring will close the inlet valve. The
remaining air pressure at either the supply or delivery side,
(depending on where the pressure drop occurs) will be
retained."

So, from that it's possible for there to be a total loss of pressure in the dry tank, and as soon as the pressure in the dry tank falls below the closing pressure of the valve (10 to 20 psi below the opening pressure) the aux. tank will retain it's 45 - 55 psi.  Will your gauge then read 45 psi, not the zero that your brakes actually have?  Will your low air warning come on?  The parking brake/emergency brake tank is protected with a check valve, so it should retain full pressure if that should occur.

Just wondering if I have worked it out right.  It's just striking me that the low air sensor might be better off on the dry tank side of the PPV, not the aux tank side.

Brian

[Hartley]

Guys, Wow lets take a simple problem of not enough AIR pressure and make a grand
and confusing GUESSING GAME out of it.

The guy apparently doesn't have a clue that you need AIR pressure to make the Light go off, Brakes Release and Buzzers to stop making noise.

It has absolutely nothing to do with the alternator, regulator or anything electrical.

AIR Leaks, Poor Compressor Performance, Too Cold to build air pressure is possible
until stuff warms up.

The simplest answer is usually the correct one.... AIR PRESSURE !!!!!

Too Many "Engineers" not enough paying attention (again!)...