Electrical question...

rampeyboy · May 07, 2011, 02:19:18 PM

Guys I feel dumb for even asking, but here goes.

I had previously determined that my Scenicruiser is positive ground by following where the cables go. Or, so I thought.

All along my non drivable bus has shown "no charge" and I was told it wasn't charging. Today while fooling around running the engine, I noticed the no charge light was out. I go to the battery with my DVOM and at idle it is showing 14.66 volts. I had the positive lead of the DVOM on the positive battery cable. Reversing the DVOM leads (pos on DVOM to negative post of battery) shows -14.66 volts. Turning the engine off the battery(2 group 31's) shows 13.01 volts at rest. This shouldn't be this hard, but I been in the sun all day and my head hurts. haha can't wait for the race to start!

So does this mean I have been wrong all along and it is negative ground?

Boyce

zubzub · May 07, 2011, 02:35:50 PM

My understanding of + or - ground is the only thing affected are some gages and the alarm system rectifier stuff. IOW you can install the batts either way and run the bus, but various gages won't work and if you have an alt it is polarity specific (do they even have + ground Alt?) but a generator will charge either way once it's fields have been flashed/polarized. The fact that the charge light went out even though the gage read no charge could mean many things including that the charge light could have burnt out.
However sounds like you are charging, which is good. I had trouble understanding what you were doing with the volt meter/batteries other than you had 14.6 V engine on and 13 off which means charge. It is no big deal to switch most buses to negative ground, it is not a strong point of mine others will chime in.

rampeyboy · May 07, 2011, 02:42:43 PM

I have not switched the battery cables from pos to negative, only the leads on the volt meter. If I put the red lead of the volt meter on the positive post, it reads 14.66 volts. If I swap the volt meter leads so that the red volt meter lead is on the negative battery post, the volt meter reads -14.66 volts.

This is consistent with a normal negative ground system, right?

MY confusion is that after following the leads some months ago, I have been convinced it was positive ground!

Now I am questioning my sanity!
Boyce

bobofthenorth · May 07, 2011, 02:48:17 PM

You've been out in the sun too long.

And I mean that in the kindest possible way.

The VOM will read + voltages when connected across the battery "normally" and - voltages when connected the other way. Put your red lead from the VOM on the positive battery post and put your black lead on a clean section of the bus frame. If you get ~12 volts then you are negative ground. If you get nothing then put the black lead on the negative battery post and put the red lead on the frame. If you get ~12 volts that way then you are positive ground.

rampeyboy · May 07, 2011, 02:50:22 PM

thanks for understanding! haha, been inside now an hour, let me go rethink this thing!

JohnEd · May 07, 2011, 03:59:59 PM

No need to think at this point......What BOTN said so well.

John

pvcces · May 07, 2011, 04:27:38 PM

BOTN and has it right. The batteries would not work very well if their polarity was reversed. That's what you were checking. The question is: how are the cables connected to the bus?

For what it's worth.

Tom Caffrey

rampeyboy · May 07, 2011, 06:54:26 PM

BOTN has it. Black volt meter lead to the negative post, and red lead to the bus body gives me 12.xx volts(engine off). So my original thought of it being positive grounded is correct. I was just confused by seeing 14.66 volts (running) with the VM leads red to positive post and black to negative post. I would have expected -14.66 since it is positive ground...

Oh well. Now that my gear driven alternator (or generator) is charging, I need to rethink my plan of eliminating it!

Boyce

buswarrior · May 07, 2011, 08:52:05 PM

positive and negative are still + and -.

The only difference is which one is connected to the coach body.

Keep the smoke in!

happy coaching!
buswarrior

artvonne · May 07, 2011, 08:59:53 PM

Take your VOM and measure voltage from the negative battery terminal to the Bus chassis. Cargo bay floor, whatever. If your VOM shows any indication of voltage you have positive ground. If it reads zero you have negative ground. If your reading zero, try the positive post just to be sure your getting a good ground on the other lead.

rampeyboy · May 08, 2011, 04:32:59 AM

I'm certain it is positive ground now. Verified by the volt meter and by following the cables. I did some searching online last night on how to change the diodes in the gear driven 50DN. No luck so far. Anyone know of a tutorial on this? Can it be done working through the floor, or does the thing have to come off the engine and be torn down?

Boyce

babell2 · May 08, 2011, 06:35:46 AM

Quote from: rampeyboy on May 08, 2011, 04:32:59 AM
I'm certain it is positive ground now. Verified by the volt meter and by following the cables. I did some searching online last night on how to change the diodes in the gear driven 50DN. No luck so far. Anyone know of a tutorial on this? Can it be done working through the floor, or does the thing have to come off the engine and be torn down?

Boyce

Do you have an overwhelming desire to re invent the wheel? Just placard the battery box "POSITIVE GROUND" and let the systems run. If you want to standardize the coach with the rest of the world you would have to swap every motor winding wires, Alternator, and ,battery connections. Seems like a lot to do just to do something.

Brice

JohnEd · May 08, 2011, 09:49:21 AM

I can get those diodes for you or the Part number...if you wish.

Here is the alarm bell that went off for me when I read your post: gear driven alternator. Clifford, Yoda of Busville, has said that he wouldn't keep a "gear driven" alternator on ant DD engine he owned. Or words to that affect. The problem is that if you seize a bearing in that alt you will lunch the gear and that gear train also does the cam drive circuit. Loose the cam and you destroy the engine completely. That's over $12 grand for most of us.

I wouldn't follow Clifford s advice cause I think that gear driven alt is more efficient than doing it with belts. i wouldn't trust the thing and would get it a careful listen with a stethoscope and check end play and get the armature turned.serviced if there were any doubt. $100 worth of insurance but then there are so many hundred buck issues that are also important.

HTH,

John

luvrbus · May 08, 2011, 10:24:05 AM

RB, the diodes can be changed with the alternator on the engine remove the screws in the end plate you will see the diodes check first by testing with a OM take a reading then reverse the leads a good diode will give you 2 different readings high and low a 300 on both reading that one is bad

good luck

rampeyboy · May 08, 2011, 11:15:57 AM

Brice if it were anywhere near drivable, I wouldn't worry so much about it. But, it is a real restoration project, with more not working than working as far as I can tell. The only things on the bus that will care if it's pos or neg ground are the charging system (which until yesterday was DOA) the gauges (again DOA until yesterday) and maybe the overheat shut down (which probably doesn't work and if it did should I trust it to when/if I need it? I have to replace some/all floors, and when I do will be a great time to address those so hard to get to diodes!
Boyce

Electrical question...

artvonne