Building the Tail Light Converter Assembly

Click on the pictures to view larger images.

The MC8 we purchased came with a towing receiver hitch installed, but the tail light wiring consisted of a 4 prong flat connector spliced somehow into the 24 volt bus light wires. After considerable investigation into the various ways to approach towing a vehicle, I decided the best way to set up our 1991 Explorer was to tow it four wheels down, and wire permanent diodes into it's tail light wiring circuits with a 6 prong connector mounted in the front bumper (see separate writeup at The Toad Modifications). This meant that I needed to wire the bus to provide for 12 volt lights, also.

The first problem faced is that the bus is 24 volts, and the Explorer is 12 volts. The second problem, is the bus has separate brake and turn signals, and the Explorer uses combined brake and turn signals. So a converter needed to be installed to convert from the bus's 24 volt 4 circuit configuration (tail, brake, left-turn, right-turn) to the Explorer's 12 volt 3 circuit configuration (tail, brake/left-turn, brake/right-turn). I had a schematic available to build my own converter using 4 relays, and began by pricing this option. It seemed logical to me that the best way to approach it would be to use the same 24 volt DPDT relays that are common throughout the MCI coach. This way, I would only need to carry one spare 24 volt relay, and the converter relays could also be used as spares in the bus, if necessary. The relays cost about $18 from MCI parts service, although compatible relays are a bit cheaper from other sources. The sockets cost about $9. Four relays and sockets are necessary to build the converter assembly, plus wiring and connectors, for a total cost of approximately $120. While I had seen cheaper 24 volt relays, they are not compatible with the bus relays, so I would lose the convenience of having common relays throughout the coach. The total cost would be reduced by only a few dollars, anyway, so I opted to use the standard MCI type relays.

Then, I found what appeared to be a better solution. The bus garage where I have my maintenance work done had designed a relay converter box to do exactly what I needed. It would convert the 24 volt bus circuits to 12 volt toad circuits, combining the brake and turn signals as required. It would mount easily into the coach, would connect into the existing circuits with minimal effort, and did not cost much more than I had estimated to build the system myself. I purchased one, and installed it in the MC8. Very simple and straight forward. Mounted the box, attached a few wires to the rear junction box. Attached a few more to the trailer connector. Voila! Tail lights on the Toad. Only one problem - it didn't work properly!

When the brakes were applied, and the right turn signal was activated simultaneously, the left turn signal would blink also, making it look as if the emergency flashers were on (this explains why the woman passed me on the right side while I was waiting at a stop light to make a wide right hand turn). I removed the unit and returned it. The shop manager was very understanding, and exchanged it for a new unit. This one I installed in the MC9.

Again, it mounted easily. I used the same harness I had made for the MC8, and in a matter of a couple hours, I had it wired in place. Unfortunately, it worked the same way as the previous one. Obviously, there was a flaw in the design, and it was never thoroughly tested by the manufacturer. I took that one back, also.

I have to say that the fellow I purchased the units from was very good about it all. He was quite surprised that I was having these problems, and was certain the devices worked properly. He said his shop had sold and installed several of the units in commercial buses, and no problems had ever been found. He even told me that he bench tested one with marker lights after I called about the second unit being bad, but when I arrived at the shop that afternoon to return it, he had already dismantled his test assembly. After discussing it with him for awhile, he mentioned that a charter company client had indeed had the same problem I was having. When I asked what their solution was, he indicated they had finally given up on it, and had hung marker lights off the trailer they were pulling. So, he credited my account for the returns, and asked me to help him test the unit when he gets the problem figured out, which I will be more than happy to do. He even gave me a schematic for building the converter myself using relays. Basically, his schematic is the same as the one I already had, which confirmed that it should work just fine.

Since I already had 4 relays, I purchased sockets for them from the bus garage. I bought diodes for the relay coils at Radio Shack, and ring terminals at NAPA. After determining the layout I wanted, I built custom wires to connect the relay terminals as needed, following the wiring schematics.

000724.00 - Relay bases wired together 000724.01 - Test fit relay bases to mounting plate

Then I painted a small piece of steel, and riveted the relay sockets to it. The whole assembly was then screwed to the bulkhead in the engine compartment, next to the air cleaner. The rear junction box harness was attached, followed by the harness to the trailer connecter.

000725.00 - Converter installed in bus

Everything worked perfectly the first try! Fantastic! Let there be taillights!

When I built the converter, we were scheduled to leave on a family vacation in a couple days, and I needed lights on the toad, so I didn't have time to enclose the unit to protect it from moisture. I also couldn't find any fused terminal blocks, so the circuits were not fused. I corrected this when I rebuilt the unit in the rear cap.

This assembly is very easy to construct and install, and makes towing a vehicle or trailer behind the coach a simple matter. Just plug in and go!

Here's the basic schematic I used to wire the relays. I've added 3 outputs which can be used to drive separate brake and turn signals, if your toad is so equipped and you want to go through the hassle of connecting them separately. Personally, I wouldn't bother. These outputs can also be used to drive 12 volt lights which you might wish to add to the coach. Note that I have not actually tested these outputs, so if you choose to implement them, you should verify they are correct.

I built three converter units for some friends of mine. I mounted them on a piece of poly cutting board. This worked well for mounting. It drills well, and takes screws well. How it holds up over time will remain to be seen.

Here are photos of the process of building one of the units.

071001 - Here's a photo of the current layout of these units.

I have been making these units for other busnuts who don't have the time or desire to wire one up for themselves. They need to be installed in an area that is protected from dirt, moisture and road spray. They should be mounted inside the rear cap area, or in a weather tight electrical box mounted in the engine compartment. In addition, a fuse protected 10 ga wire should be run from the batteries or from the A/C junction box in the front bay to the converter to carry the 12 volt feed. The original 12 volt option in the MC8 and MC9 only included a 16 gauge wire, which is a bit small when you start connecting several incandescent tail light bulbs. The production units include four 10 amp rated octal relays and three 10 amp automatic resetting circuit breakers on the output signals. I have put together a detailed instruction sheet for installing the unit on an MC8 or MC9. Installation on other buses will require further information to determine proper connection to the bus electrical system.

"So how do I get one of these awesome units?" you ask? Well, I'll lay it out for you in plain english.

The price for the units I build, as of May, 2008, is $200. This includes USPS priority mail shipping in the United States. I try to keep all the parts on hand, so it typically requires only a day's notice to build one (assuming I'm home and not out traveling in the bus somewhere fun). If you pay with bona-fide certified funds (money order, cashier's check), I can usually ship it within one day. I will accept a personal check and ship the unit after the check clears the bank (typically 10 days). If you want to send me a bad check, counterfeit money order or cashiers check, you should first read the short story "The Most Dangerous Game" by Richard Connell. If you prefer to use Paypal, the cost is $207, which covers the fee Paypal charges me to receive money. If you need it yesterday, you're probably out of luck, but if by chance I have one built and ready to ship, I can ship it via overnight FedEx, UPS, or DHL. The cost will be $200 plus whatever the delivery company charges (the last one I shipped overnight cost $85 just for the shipping!!). It's up to you. I'll do whatever I can (within reason) to accomodate your needs, but remember, poor planning on your part does not constitute an emergency on my part.

If you would like to discuss purchasing one , or email me at .

Tips and Tricks

Omron Part # MK2P-S-DC24
DigiKey Part # Z799-ND
Omron Part # PF083A-E
Digikey Part # Z805-ND
Jameco Part # 141145