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Author Topic: Trying to use a DrawTite ModuLite to tow a trailer....need help getting 12V!!!!!  (Read 3904 times)
ilyafish
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« on: March 11, 2009, 10:20:30 PM »

Okay, so last time i took my coach out i had this ghetto rigged, so i figured i want to get it right this time.

Basically, the trailer that i have, the brake lights and turn signals share the same bulb.  On the MC9, that is not the case obviously. Now not sure if any of you are familiar with the device, but DrawTite makes a thing called a ModuLite which basically takes these seperate signals, and combines them into one for situations such as this.  However, it is a 12v device and all the leads are 24v.  I'm not too educated when it comes to electrical stuff, but i figured i'd go to radio shack and buy a bunch of resistors and wire them up and get 12v out of the 24v.  I managed to get 12v after putting on a few .5a resistors starting at 100k ohms, then, going to 47k ohms and then finally 10k ohms.  This brought it down to 12v.  I then connected all the wires to the wires coming out of the modulite for the stop, left turn, right turn, and tail.  As soon as i connected the wires, i was reading only about 1v, if that.  Once i disconnected the wires from the modulite....back to 12v.  After messing around all night and checking everything over 20 times, i realized that the resistors had taken a dump.  So now im a bit stumped.  I still have no idea why i was reading 12v up until i connected to the modulite, and then less than a volt.

I would like to avoid putting a set of auxiliary lights on my trailer for the brake or turn signals (whichever one i decide) pretty much at all costs.  So i am just trying to figure out what my problem is.  Why did the resistors blow...not enough amps?  I'm pretty stumped!  Any other alternative to hooking up the wires for a trailer that shares the brake and turn signals with the same bulb?
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« Reply #1 on: March 11, 2009, 10:34:08 PM »

All you need is four 24-volt automotive "cube" relays, about $2 each.  Get the ones with diode suppressors in them.

Jim Shepherd just did an article on this in BCM, even listing suppliers and part numbers.

Here's a wiring diagram:



Pick up the +12v source from the center-tap of your batteries.

-Sean
http://OurOdyssey.BlogSpot.com
« Last Edit: March 11, 2009, 10:36:10 PM by Sean » Logged

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« Reply #2 on: March 11, 2009, 11:18:53 PM »

do you have any information on who would carry this stuff, or does this have to be purchased online?

thank you so much!!
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« Reply #3 on: March 12, 2009, 12:43:03 AM »

24-volt relays are going to be hard to find locally in a brick-and-mortar store.

You can order them from Digi-Key (www.digikey.com).  You want:
Tyco, part number PB684-ND (24V with diode)

You might be able to find them at a heavy-duty truck part store as well, especially if they cater to the local bus company.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #4 on: March 12, 2009, 02:36:10 AM »

check with ronthebusnut.com
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« Reply #5 on: March 12, 2009, 06:32:27 AM »

As Sean said, the March issue will have an article on using standard ISO Bosch type relays to make a very robust trailer wiring converter.  In addition to the Sean's wiring diagram, I reference Craig Shepard's wiring schematic using MCI relays.  Craig sells the complete conversion system if you don't want to wire up your own.  His website is:  http://www.gumpydog.com/ (it seems to be down right now).

Because of the rapid cycling of the relays (during flashing), I strongly recommend a relay with a built in diode, like the one Sean recommended, or you could wire in your own. 

Dealing with DigiKey is very easy and you will find great prices.  The relays are less than $5.  In the article I also list a source of a prewired socket, but again, you will need to order that online.

If you plan to use your toad taillights, you will need to install special diodes designed to isolate the signal from the bus from getting mixed into the toad electronics.

If you don't subscribe to BCM, I will be posting the article on my website in the next few days.  However, I hope you are a subscriber.

Jim
« Last Edit: March 12, 2009, 06:35:29 AM by rv_safetyman » Logged

Jim Shepherd
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« Reply #6 on: March 12, 2009, 06:57:04 AM »

instead of using 24v relays is it possible to hook 2 12v relays up together?
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« Reply #7 on: March 12, 2009, 07:00:25 AM »

 FWIW
    Sean's copyrighted diagram is pretty straight forward the only thing I would suggest is to add another
 cube on the break side for a trailer brake controller. I did that just for future ease of diagnostics.

    In some states the trailer brake laws apply to some pretty light situations. BSTS

  Skip
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« Reply #8 on: March 12, 2009, 07:03:40 AM »

I am kinda lost here.  You have been given good advice and want to reinvent the wheel.  

I tried that when I did my system and quickly learned that the masters like Sean, Craig, and Jack Conrad, had done all of the ground work and had the same basic solution (different relays, but same approach)

The coil in the relay is designed to be triggered by a specific voltage.  You can't put them in series.

Not sure why you have an aversion to spending less than 5 minutes to order the proper parts from Digikey and get on with doing the job properly

Jim
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Jim Shepherd
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« Reply #9 on: March 12, 2009, 07:13:01 AM »

because i have to leave in 12 hours with the bus and trailer
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Own: 1981 MCI MC9 w/
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« Reply #10 on: March 12, 2009, 07:23:04 AM »

Oops, forgot to give us part of the puzzle Wink

Craig's schematic uses MCI relays.  Do you have any spares?  Any local source?  If you can find them or a local source for the Bosch type 24 V relay, you will be in business.  Then all you need is the 12V supply, but that should not be an issue.

Jim
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Jim Shepherd
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« Reply #11 on: March 12, 2009, 07:27:05 AM »

ok awesome....time to start looking!  now this eliminates the need for the modulite system correct?
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« Reply #12 on: March 12, 2009, 07:38:49 AM »

You will not need the module.  You will need 4 of the MCI relays (5 if you need to do the brakes). 

If Craig's site is down, drop me a note (jim at rvasfetysystems dot com) or give me a call (three zero three 478 -3501) and I will email you a PDF of his diagram.

Jim
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Jim Shepherd
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« Reply #13 on: March 12, 2009, 08:01:04 AM »

Give Luke at US Coach a call.  He's about an hour south of you, in Vineland.  He might have some in stock. (856) 794-3104

IMO, you will be better off making however many phone calls you need to track down a local supply of 24-volt relays than to try to MacGyver the Draw-Tite module.  You can't simply use resistors, as you've discovered, because the amount of current the module draws varies depending on which lamps are operating.

In an emergency, you can try substituting cheap 12-volt relays for the 24-volt items, but leave out the marker light circuit (so no night driving).  The 12-volt coils in the relays will work on 24 volts for a while, but then they will burn out.  Since neither the turn signals nor the brake lights are on full-time, you might get away with this for a few days.  Buy a couple of spares, and check the assembly frequently to make sure the relays have not melted or are otherwise presenting a fire hazard (the 12-volt coils will overheat on 24 volts).

When you find the correct 24-volt relays, simply replace the 12-volt ones -- you'll likely use female spades to connect to the relay terminals, so it's plug-and-play.

-Sean
http://OurOdyssey.BlogSpot.com
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« Reply #14 on: March 12, 2009, 08:08:11 AM »

You can use 12 volt relays if you place a resistor in series with one of the relay coil wires.

Then the only thing you will need is a long wire to get 12 volts tapped from your battery box back to the relay unit.
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« Reply #15 on: March 12, 2009, 08:28:08 AM »

I use the Modulite, but my lighting is all 12 volt.  My battery compartment is directly in front of the engine so I was able to pick up a 12 volt feed in there even though my bus is primarily 24 volt.  I did have a problem with the Modulite not working last year and ran out to get another one, but it did the same thing.  Someone had reversed the connections to one of the llights in the bus.  We fixed that and now I have an extra Modulite.

Now, on an MCI you don't have 12 volt lighting and probably no easy way to get 12 volt power to a Modulite so I recommedn you build your own converter as recommended.
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Sean
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« Reply #16 on: March 12, 2009, 08:57:12 AM »

You can use 12 volt relays if you place a resistor in series with one of the relay coil wires.


Dave is right, but I suspect the resistor you need will be just as hard to find as the correct relay.

You would need a 105-Ohm resistor in series with each relay's coil if you use a typical automotive 12v relay such as a Tyco or Bosch.  However, it needs to be a 2-Watt rated resistor -- the little jobbies you can buy at Radio Shack are only 1/4-Watt and will burn out, as you discovered.

-Sean
http://OurOdyssey.BlogSpot.com

(Edited to correct values -- I was looking at the wrong spec sheet)
« Last Edit: March 12, 2009, 09:01:51 AM by Sean » Logged

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« Reply #17 on: March 12, 2009, 11:36:17 AM »

Check your Messages. Shocked
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« Reply #18 on: March 12, 2009, 02:12:16 PM »

Dave is right, but I suspect the resistor you need will be just as hard to find as the correct relay.

You would need a 105-Ohm resistor in series with each relay's coil if you use a typical automotive 12v relay such as a Tyco or Bosch.  However, it needs to be a 2-Watt rated resistor -- the little jobbies you can buy at Radio Shack are only 1/4-Watt and will burn out, as you discovered.

Based on some recent experience which I have had shopping for resistors at Rat Shack  Grin  I can state that they do in fact usually have a decent selection of high wattage ceramic resistors in addition to the fractional wattage ones. 
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« Reply #19 on: March 12, 2009, 04:23:52 PM »

Or you can run 2 12v relay in series for 24v and use the one of 2 relay contact. No problem what so ever. Resistor is more problem that 2 relays. However if you can find & get 24v version now than it less parts.

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« Reply #20 on: March 13, 2009, 03:23:55 AM »

Finding stuff in stock at Radio Shack is always a crapshoot...but you can increase wattage ratings by running resisters in parallel. Of course you have to select higher resistances compensate for the loss of resistance.

http://www.radioshack.com/product/index.jsp?productId=2062320 = 470 ohm 1/2W 5% Carbon Film Resistor pk/5 for $0.99

4 of those in parallel would be 470/4 = 117 ohms at 0.5 x 4 = 2 watts. That's close enough to Sean's number to work, assuming his assumptions are correct.

Of course, if you find 24V relays, that's still the better solution.
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« Reply #21 on: March 13, 2009, 02:54:52 PM »

I've got one of Gumpy's systems installed in my MCI. It is a simple thing to install and everything always works the way it should. WHen you return from this trip I'd order one from Craig.  FWIW
FRed
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« Reply #22 on: March 14, 2009, 07:49:09 AM »

You maybe down the road by now, However if the OTR A/C has been removed, you have (2 -3) 24V Heavy Duty relays for the former evap and blower motors available for salvage.

On my 5C they were located in the forward bay, driver side mounted top of bay. Saved me $200 for two 200 amp used relays.

Good Luck, I looked around for a week before having my light to go on !!

Gary
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« Reply #23 on: March 14, 2009, 03:47:41 PM »

go to a toyota dealer or any trailer supply store and buy a toyota land cruiser tail light converter. all you need to do then is put 24v bulbs in the two trailer sockets .
early cruisers were all 24 v.
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