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Author Topic: 24 volt start on 12 volt system  (Read 4616 times)
bobofthenorth
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« on: July 27, 2006, 09:28:21 AM »

The original converter on my bus changed it from 24 volt start to 12 volt but didn't relocate the batteries.  We've been applying bandaids to that ever since we bought the bus.  I've pretty well got it licked but I'm pretty hard on starters and I think I've got another failure coming on.  This one has lasted 18 months so that is an improvement over the 3 that I put on in the first year.  I don't want to even think about switching the whole bus back to 24 volt but John Deere used a 24 volt starter on a 12 volt system on some of their farm tractors in the 60's.  Has anyone ever done this on a bus?  The JD system had a solenoid that put the batteries in series for starting and switched them back to parallel for running. 
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R.J.(Bob) Evans
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larryh
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« Reply #1 on: July 27, 2006, 11:12:43 AM »

There were a lot of trucks and some still do use a series parellel switch to start on 24 volt thats all then goes back to twelve volt for all other systems if you buy a series parellel swotch you should get wiring diagram or any good motor book on trucks should give you what you need. reccoment 4 series 31 12V batts

Larry H
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« Reply #2 on: July 27, 2006, 04:42:19 PM »

I have a '72 flex with 8V71 & auto. when the starter went out 6 years ago I was in Tall. Fl. when I went to
a starter rebuild shop the man told me that someone had put a 24V stater in a system that had always been
12V. The alt. & wirering & starter & lights on that bus was biult 12V. The feild coils on the starter is what makes
differance he told me. I haven't had a problem with the right starter in 6 years. And now I plan to put that system
in my '77 MC8 because all that motorhome suff seems to be cheaper in 12V. So maby all you need is the right
starter. just a thought.
Ray with the littlehouse [south of seattle]
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ChuckMC8
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« Reply #3 on: July 27, 2006, 07:04:12 PM »

Ray, (with the littlehouse) you should consider leaving the MCI 24 volt system as it is. Take a opportunnity to hear an Detroit engine start with 12v and compare same type with 24 volts. The 24 volt starter just "Jumps" on it while the 12 is much slower. The 12 volt accessories are easily handled with a 24 volt  house battery bank, mine (I also have 77 MC8) I use 8 six golf cart batteries with a Trace SW4024 inverter. Since the house bank and coach bank are both 24 volt, the bus alternator charges both systems, This system is capable of running 2 roof airs while underway...without the Onan.
  All 12 volt loads, including the bus headlights are  handled with a Vanner 24/12 equalizer.

 this setup (+/-)  is common on a lot of MCI conversions...because it works.....but its certainly not the only way
« Last Edit: July 27, 2006, 07:06:23 PM by ChuckMC8 » Logged

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TomC
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« Reply #4 on: July 27, 2006, 08:31:03 PM »

Two ways to go come to mind.  First is to use the Delco MT42 24v starter that is a hugely heavy starter with a series/parallel switch.  So you can start with 24v but run the bus on 12v.  On trucks, this system is mostly not used anymore since superior 12v starters are available.
The new Delco MT39 is a 12v starter that is gear driven, have the size and weight and supposed to last twice as long as the MT42.  It is sort of bazzar when you start one of the new trucks with the MT39 and it sounds like a Toyota starter.  But they work very well especially in low battery situation.  Personally, I would use the new MT39.  Good Luck, TomC 
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #5 on: July 28, 2006, 04:54:20 AM »

I drove a '56 Autocar(among others...showing my age) with a parallel switch and it was a pain. I know technology has progressed but IMHO if you can stay away from that type of system you will be better off. The old KISS principle. Conversely, I drove a numbers of later model ('60s & '70s) trucks with Detriots & Cummings that started on 12V & had very little problems as long as I kept up the maintainance. Good Luck!
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« Reply #6 on: July 28, 2006, 06:07:08 AM »

Would it not be a lot easier, and in my opinion better and less expensive, to just increase the size of the battery cables. From your statement I have to assume that is the problem.
my Eagle was 12 volt starting and I never replaced a starter in the 15 years i owned her, so I think you need to fix the basic problem. Richard


The original converter on my bus changed it from 24 volt start to 12 volt but didn't relocate the batteries.  We've been applying bandaids to that ever since we bought the bus.  I've pretty well got it licked but I'm pretty hard on starters and I think I've got another failure coming on.  This one has lasted 18 months so that is an improvement over the 3 that I put on in the first year.  I don't want to even think about switching the whole bus back to 24 volt but John Deere used a 24 volt starter on a 12 volt system on some of their farm tractors in the 60's.  Has anyone ever done this on a bus?  The JD system had a solenoid that put the batteries in series for starting and switched them back to parallel for running. 
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« Reply #7 on: July 28, 2006, 07:59:12 AM »

Another point along the lines of battery cables.  My transit bus has the batteries on the left side almost in the middle of the bus (front to back).  Each battery cable is a 000 cable that goes uncut directly to the starter-so two big cables on the starter solenoid.  The ground is also two big cables from the batteries directly to the chassis, then two big cables from the chassis to the starter.  Plenty of starting power, the lights dim very little when starting inticating that there is little resistance to the starter.  If your starter was 24volt originally, then changed to a 12v, the starting amps are double and if the cables are original, won't be big enough.  Highly recommend using 000 cable times 2.  Good Luck, TomC
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Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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« Reply #8 on: July 28, 2006, 10:36:06 AM »

We've already doubled the cable - 2 runs of 4-0 on the hot & ground.  I agree with the KISS comments - I don't like the basic concept of 24/12 systems but I am at my wits end over this starter.  Someday it is going to let me down on the road.  I'm going to look into the heavier starter - I'm close to a solution now, just a little better starter would likely solve the problem permanently.
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R.J.(Bob) Evans
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« Reply #9 on: July 28, 2006, 11:58:27 AM »

Another point along the lines of battery cables.  My transit bus has the batteries on the left side almost in the middle of the bus (front to back).  Each battery cable is a 000 cable that goes uncut directly to the starter-so two big cables on the starter solenoid.  The ground is also two big cables

Is 000 cable the same as 3/0?

My inverter recommends 4/0 for up to 15 feet and 350 MCM beyond that.  I may be slightly over 15 feet.  Will I be okay with 4/0?  Can I run two 4/0 cables in parallel since I can get 4/0 for 25% of the cost of 350 MCM?

Brian Elfert
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Brian Diehl
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« Reply #10 on: July 28, 2006, 12:05:11 PM »

Yes, 000 is the same as 3/0.  So 0000 is the same as 4/0.
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Len Silva
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« Reply #11 on: July 28, 2006, 12:10:41 PM »

3/0 is 168 MCM and 4/0 is 212 MCM
So 2 runs of 3/0 would be 336 MCM and 2 runs of 4/0 would be 424 MCM.
(MCM is thousand circular mils)

Len
« Last Edit: July 28, 2006, 12:12:13 PM by Len Silva » Logged


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« Reply #12 on: July 28, 2006, 05:35:45 PM »

Bob....be different,go with an airstarter,and let the batteries power something else..........*smiling*....Good luck with your situation..........Frank
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« Reply #13 on: July 28, 2006, 07:57:58 PM »

Bob....be different,go with an airstarter,and let the batteries power something else..........*smiling*....Good luck with your situation..........Frank


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« Last Edit: July 31, 2006, 02:24:47 PM by DrivingMissLazy » Logged
bobofthenorth
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« Reply #14 on: July 31, 2006, 01:12:40 PM »

I found a place nearby that has the 39MT Delco starter.  I just need to pull the old one out and take it with me to match up to the new one.  Of course I won't know for a year or so whether this has helped (unless I know in a month or so that it hasn't).

Stay tuned.

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R.J.(Bob) Evans
1981 Prevost 8-92, 10 spd
My website
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Simply growing older is not the same as living.
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