What Gage cable Battery to Starter

[bevans6]

On my MCI MC-5C, the master battery switch inside the luggage compartment interrupts the 24 volt  cable from the batteries to everything else.  Note to self - never throw that switch when the alternator is charging...   On race cars with alternators we use a battery master switch that has a subsidiary terminal with a resistor to ground the field terminal on the alternator, to force it to stop generating as soon as the switch is thrown.  I'd have to really think about how the field terminal on the 50DN is controlled to see if the same function exists, making the issue of throwing the switch while the alternator is charging moot, but easier to not throw the switch.

On my bus, 12 volts is connected to the center terminal of the battery pair, along with the Vanner.  Since the switch interrupts the 24 volt side, 12 volts continues to be present in the bus regardless.  Aside from disconnecting the battery or removing the fuse, there is no way to turn it off.

Brian

[Jerry32]

I have two busses here ( GM 4905 & MCI 102A3 ) and both original have the batteries going through a cutoff switch of which I use when parked to avoid loading batteries and discharging them. Both start as soon as you hit the start swich so I don't see the problem as this also makes it so you can kill all power to the bus to work out wiring without dangwer of shorts . Jerry

[JackConrad]

On our MC-8, the OEM battery switch was located in the battery compartment (in front of front bay on passenger side) and, when disconnected, shut off everything. The switch had one cable from battery on one (input) terminal and 3 cables/wires on the other (output) terminal When I relocated the bus batteries to the engine compartment, we maintained this same wiring by relocating the switch with the batteries and connecting the 3 output cables/wires in the OEM battery compartment together with a 1/2" brass bolt and nut.  I try to  always shut off the bus batteries when we arrive at our destinations, although I sometime forget. So far, we have never had any starting problems.  Jack

[Len Silva]

I don't have the schematics in front of me but I believe the Eagle 15 used three group 31 batteries and had a disconnect switch for each of them, wired directly to the battery.  I particularly like this idea as you could easily isolate a bad battery and keep going.  IIRC they also had a switch selectable voltmeter right there in the battery compartment which could look at each battery individually (with the other two disconnected).  Were I to ever build another bus, that's how I would do it.

[roadrunnertex]

According to the GMC wiring diagrams for intercity coaches (P8M4905A) and (P8M4108A).
It shows the battery master disconnect system is in-line with the complete electrical system.
When you open the master switch the complete electrical system is "NOT" powered.
And yes I understand that if you were to open the switch while the engine is running the alternator will have a voltage spike and it will do damage to the components in the system.
jlv

[Sean]

...  I'd have to really think about how the field terminal on the 50DN is controlled to see if the same function exists, making the issue of throwing the switch while the alternator is charging moot,

The field on the 50DN is controlled by an external regulator.  Some are P-type and some are N-type.  The regulator governs the field excitation based on input from a "battery sense" lead, which is intended to be connected directly to battery positive.  This will automatically compensate for any loss in the alternator to battery cabling, including, if present, a battery isolator.

This is the issue I was talking about earlier, when I said that you should be sure that the battery sense lead is on the same side of the disconnect as the alternator output.

On my bus, 12 volts is connected to the center terminal of the battery pair, along with the Vanner.  Since the switch interrupts the 24 volt side, 12 volts continues to be present in the bus regardless.

I have another word of caution regarding disconnect switches and Vanner equalizers.

If you have a Vanner it should be connected on the battery side of any disconnect switches, not on the opposite side.  And, yes, this does mean that the Vanner will be running all the time and that it's idle current will be depleting the batteries.

The problem with doing it otherwise, though, is that the Vanner needs to have the ground connection be the last one made and the first one broken.  Changing the connection or disconnection sequence can fry the equalizer.

Also, I recommend that you add a disconnect on the 12-volt tap if you have loads there.

Folks who have any 12-volt equipment on a 24-volt coach must exercise caution in the battery connection and disconnection sequence, whether or not an equalizer is present.  That's because a disconnected ground with connected 12- and 24-volt loads can apply a reverse 12-volt polarity to the 12-volt loads, possibly damaging any polarity-sensitive equipment such as stereos, GPS receivers, video screens, etc.

The complete disconnection sequence would be:
1. Open the 12-volt load disconnect.
2. Open the 24-volt load disconnect.
3. Remove the battery ground.
4. Remove the 24-volt positive connection(s)
5. Remove the 12-volt positive connection(s)

Connection is the reverse of the above steps.  This sequence assures that there is no possibility of reverse polarity, and also provides for the required ground-before-hot sequence for the Vanner (if any).

For the inquisitive, we used Intellitec latching disconnect solenoids on our chassis system, and connected them to a single toggle switch, so the 12- and 24-volt loads are disconnected or connected simultaneously with a simple flip of a single switch on the dashboard.  Our Vanner remains connected at all times, as does the alternator (and the starter, where this discussion began).

Our disconnects are only rated for 100 amps, but the loads and associated wiring are lower than that.

-Sean
http://OurOdyssey.BlogSpot.com

[cody]

Let me see if I understand this correctly, ALL the other companies from GM thru Prevost are wired up wrong?  Somehow I find that not only unlikely but bordering on impossible to believe.

[bevans6]

"If you have a Vanner it should be connected on the battery side of any disconnect switches, not on the opposite side.  And, yes, this does mean that the Vanner will be running all the time and that it's idle current will be depleting the batteries."

I really wondered about that, and considered changing it, since the ground is constant  and the 24 volt to the Vanner is definitely switched.  But it's the stock install by MCI lo those many years ago, and it seems to have survived just fine.  I decided if it's not broke, don't fix it, also known as the "let sleeping dogs lie" method...    The warnings by Vanner may be overly cautious, although another type of equalizer I have has the same warning.  Maybe the warning really is against switching the 24 volt off when drawing significant power from the 12 volt.  I can see that irritating it.

I did install a switchable breaker for the 12 volt feed to replace the in-line fuse, but I tend to leave it on most of the time so that the radio doesn't lose it's mind.  It's not a perfect scenario, but there only a couple of things powered by that 12 volt supply.


Brian

[BG6]

What I can tell you, though, is that generally speaking, main starter cables are wired directly, and with good reason

My 1990 96A3 has a master disconnect in the battery bay, as shown in the factory schematics.  The cable from the hot terminal on the switch to 24VDC hot is the only connection to that end of the battery bank.

You can run a LOT of current through a switch for brief periods -- the rating on a switch is the OPERATING current, meaning how much it will make or break.  This is important, because switch arcing can weld and burn contacts.  That's why the starter is operated through a solenoid, with is a really-heavy-duty switch.

I agree with the 4/0 cable, even if the battery is only 2 feet from the starter.  Starting the engine is the most important job the battery can do, and the more power you send to the starter, the better. 

[Sean]

I'd like to clarify some remarks I made earlier, because it occurs to me they could be misconstrued.

I am not necessarily advocating to anyone here who has a working configuration, whether that is factory-installed or otherwise, that they should go back and rewire their disconnect switches.  (Although I will suggest to those people that they at least check to see that their sense lead is connected in the right place.)

We started here with a question from someone who has no electric starter at all, and so is faced with the prospect of running new wire and making new connections for it, and my original advice was specifically targeted to that question.

And my numbered recommendations several posts ago were intended for anyone faced with adding or changing wiring, as so many of us have had to do in the course of our conversions.  If you are going to add or replace a disconnect switch, add a battery equalizer, or replace or change alternator wiring (as you would, for example, to implement a battery isolator system), then I strongly advise you to follow those recommendations.

If, OTOH, you have a working system wired differently that has not caused you any trouble, you might consider changing to these recommendations when and if you are able.  And, importantly, you should check any disconnect switches for corrosion, pitting, or loose contacts on a regular basis, and exercise caution when operating the switch so that an alternator is never separated from a battery while running.

Lastly, to clarify that when I said I advised against the practice of installing disconnects at the batteries, what I meant was not that disconnects should not be added.  Rather, that, if you are going to install a disconnect where none existed previously, it should not require any extra effort or materials to put it in the correct place: between the charging system and the loads.

I hope that clears up any confusion.

Let me see if I understand this correctly, ALL the other companies from GM thru Prevost are wired up wrong?  Somehow I find that not only unlikely but bordering on impossible to believe.

See above.

Also, don't forget that manufacturers have many competing objectives when they design and build vehicles.  A classic example is reusable fasteners vs. single-use fasteners: fastening a panel such as a dashboard with screws and washers will facilitate servicing anything behind the dash later, but having a single-piece molded dash with snap-fit plastic fasteners molded in will be much cheaper to build.  You will notice that almost all cars use this latter type of interior panel.  For those of us who work on our own vehicles and care about serviceability and access, it would be tempting to say "they did it wrong" but, in reality, they are trying to meet a price point.

Likewise heavy-duty manufacturers have the same trade-offs to make.  I can't speak to why any of the manufacturers you mentioned chose to put the disconnects ahead of the alternator connection, but what I can tell you is that it is contrary to the recommendation of the alternator manufacturers and it is not the optimal choice.

As I said above, the reason why I point it out here is because folks here are generally modifying these electrical systems.  Those modifications in many cases could not have been anticipated by the OEM and the OEM designs certainly do not account for them

For example, OEMs generally do not assume that there will be an entire second battery bank and that such a second bank might be interconnected to the chassis bank with a solenoid.  Or that there might be an isolator between the alternator and the chassis batteries.

So let me give you a scenario where a modification might cause a problem.  Let's say you have a disconnect between the chassis batteries and the alternator.  Let's further say that you've installed a solenoid between the chassis system and house system -- not an uncommon addition -- for the purpose of "emergency start" when there is a problem with the chassis batteries.

It is now possible to start the coach with the chassis disconnect switch open.  Then as soon as the "emergency start" button is released, the alternator will suddenly be working completely unloaded.  While the chances are small, this could, in fact, damage the alternator.

Again, I am not suggesting anyone run out and change a working OEM system.  But if you are making modifications to your chassis electrical system, even if that is as simple as cross-connecting it to the house system, it's best if you are aware of the potential dangers ahead of time.  When you are planning these sorts of additions and modifications, it is usually a simple and inexpensive matter to make sure the disconnects are in the proper place, which may not be where the OEM put them.

-Sean
http://OurOdyssey.BlogSpot.com

[cody]

I guess if a person felt the urge they could also wire a welder into the kitchen circuit, what I'm asking is, are the companies that manufacture buses all wiring them wrong by putting the disconnect switch where the factory put it, clearly somebody here is wrong, incidentally, my eagle also has the disconnect and it's factory installed.

[Sean]

You can run a LOT of current through a switch for brief periods -- the rating on a switch is the OPERATING current, meaning how much it will make or break.

Actually, heavy-duty battery switches should carry both kinds of ratings, which is why I said that you'd need a switch with a "non-breaking rating" sufficient to carry the starter current.

You are correct that a switch which could make or break that much current would be much different.

Again, if your coach builder put the starter across a switch from the batteries, that switch is probably properly rated for starting current, or at least we would hope that the coach builder used the right switch.  But that does not mean that the switch used in the OP's air-start coach is so rated.  Also, as Bob wrote earlier, older coaches can have all manner of "amateur" wiring modifications, and disconnects that have been installed by other than the coach builder might not be properly rated.  I have personally seen cases where switches have been replaced by sometimes well-meaning but usually rushed technicians with items that did not meet the required specifications.

-Sean
http://OurOdyssey.BlogSpot.com

[BG6]

If you have a Vanner it should be connected on the battery side of any disconnect switches, not on the opposite side.  And, yes, this does mean that the Vanner will be running all the time and that it's idle current will be depleting the batteries.

The problem with doing it otherwise, though, is that the Vanner needs to have the ground connection be the last one made and the first one broken.  Changing the connection or disconnection sequence can fry the equalizer.

Sounds like it would be a good idea to put a switch into the Vanner ground line, labeled "Disconnect FIRST, Connect LAST"

Looking at the MCI schematics, I have to sit corrected -- all Vanner connections are on the battery side of the main disconnect.  That means for long-term parking, it will need to be disconnected.  I'm thinking of a SPST for the ground and a DPST to cut the other two, so it would be:

1)  Cut Vanner ground
2)  Cut Vanner hots
3)  Main disconnect

Then the other way around when it's time to roll.

[Gary '79 5C]

Good Points brought up for discussion,

I like the master sw killing everything for two reasons, One the batt's are then protected from any drains etc.etc.

Second, God forbid any of us get a engine fire, I would try to shut off the master on my way to the back.

Have a great day.

[bobofthenorth]

It has been a very interesting discussion and I have to agree with Gary, I like the idea of everything being clearly and absolutely cut off when I throw the disco.  Pretty well everything we do is a compromise and it is good to know what the trade offs are.  I personally wouldn't have thought of this one.  Sean raised a good point and I understand the risk but I still think the risk of a battery drain is several orders of magnitude higher than the risk of destroying an alternator.  That, in my mind anyway, justifies the "risk" that I am taking.  I don't know much about risk theory but it is the product of cost times probability that matters.  Looking at cost or probability alone is not adequate.  We dealt with a lot of older equipment over the years and I know for a fact that battery drains were a huge cost that could easily be controlled by the addition of cut off switches.  It took us a while to figure that out but once we had it figured the first thing we did whenever we added some antique to our fleet was to add a battery shut off.