From: Vadim Kogan (vadim@XCF.Berkeley.EDU)
Date: Thu Feb 17 2005 - 09:21:40 PST
On Thu, Feb 17, 2005 at 08:23:49AM -0800, Vadim Kogan wrote:
> FYI, it's not terribly hard to get a civilian trailer behind a proper 24v
> vehicle. If somebody is interested, I could provide some details on how I've
> done it (12ton civ. trailer 12v all around, electric brakes + setup for a
> HMMWV and a 5ton to tow it).
Hmm... 2 people have already asked :-) Ok then, let's see...
I'll try to describe both setups at the same time. They are only slightly
different.
In the HMMWV, I already had 12 volt subsystem via a 24->12 DC-DC converter,
which is capable of doing 60 amps continuous with peaks up to 66 amps. More
importantly, it will reset itself in case of overload (i.e. there is no
fuse to replace).
In the 5ton there was no 12 volts to use, so it was done by means of putting
additional batteries and tapping off those. This is OK for a short trip, but
not your best solution for a permanent setup, unless you have something to
change them off the engine with.
Both vehicles had an operational standard military trailer connector at the
back, which provides 24v for all the lights. That connector has nothing to
control the brakes.
You will need a brake controller, which is a device that will apply electric
brakes of the trailer proportional to your braking. There are multiple types
of brake controllers out there:
- those that measure inertia and have to be mounted strictly horizontal. This
is the cheapest kind out there. The crappiest kind too.
- those that measure inertia with a solid state device and don't have to be
mounted absolutely horizontally. These is the sligtly more expensive ones
and slightly less crappy.
- those that measure brake pedal displacement by means of a linkage to the
brake pedal (usually cable-controlled). Again, a bit more expensive and a
bit better.
- those that measure the pressure one of the brake lines (or air pressure if
you have real air brakes). These are the best kind and the most expensive
ones.
After talking to people I trust more than myself about the above choices, I
chose to use the 4th kind (BrakeSmart), which was relatively expensive. The
good part was that I can disconnect the brake controller itself from the
wiring harness that goes to it and bring it to another vehicle. So, in this
case, there were 2 pressure sensors and wiring harnesses in two vehicles,
but only one brake controller that moved between them as needed.
Whichever way you go, the brake controller will need most likely at least 4
inputs:
- ground
- brakes-on (12v)
- 12v (see later)
- something to sense the brakes, whether internal or external.
Brakes-on is effectively the output of your brake switch. This would be
activating the controller in the same sense that ignition activates things.
You will have a happy brake switch somewhere, except that it'll put out 24v.
So, what you need to do is "splice" into that 24v output. I say "splice"
because I would actually suggest making a little Y cable with standard
military connectors, which will plug in between the output of the brake
switch and whatever it was going to, that is you don't cut any wires and can
easily remove the whole setup. On the output of the brake switch is split
off to go to the brake controller, you'll need to put a little 24v relay.
Since it needs to handle the "on/off" signal, it can be very small. I used
one that I think can handle up to 2 amps (and it was DPDT, but who cares).
It appeared to be a little solid state thing. You'll need to give it ground,
12 volts from your 12 volt subsystem and the 24v output of the brake switch.
The output of the relay will go directly to the brake controller. The
smallish relay I've used was so small that it fits a (large) shrink tube, so
the end result was a blob of silicone inside a shrink tube on one of the
wires, not very pretty, but rather safe and secure.
The 12v input needs to be more powerful. You need to figure out how much it
needs to handle, based on your trailer. Your brake controller documentation
would mention how much power it'll deliver max, given the number of brakes
on the trailer (more wheels -> more brakes).
The output of the brake controller will be a 12v line that should go to the
trailer brakes.
At this point you should have all the inputs to the brake controller
handled. You'll need to run 2 wires back to the trailer connector. The two
additional wires will be the output of the brake controller (12v, quite a
few amps potentially, don't use wimpy wire) and 12 volts out of your 12 volt
subsystem. Put a usable connector on the back of that. I just got a 4-wire
connector at Kragen and used 2 for 12v and 2 for brakes. It was a nice
connector too, since it had a cover attached to it (to be used when the
connector is not in use). Try to run the wires nicely: use an appropriate
conduit and secure it along the way. Try to follow a path of an existing
harness.
We chose to have the rest of the electronics on the trailer itself, thus
reducing the number of things that need to be done on another MV to tow it.
If you think that you'll tow it with only one vehicle ever you may choose to
adjust the stuff below to put it on the vehicle itself. If you are thinking
that you might have to tow various civilian trailers, then you probably want
to do the rest on the vehicle, so that you can attach to a random civ
trailer.
What you need is 3 good 24v relays, a project box, a commercial trailer
recepticle (the 7 pin kind, matches the other non-military end of a regular
military trailer cable, available at NAPA) and the matching connector to
whatever you used to bring 12v and brake controller output to the back of
the truck.
On the trailer you should have 4 interesting inputs: left, right,
parking/perimter lights, brakes. Brake lights are really left and right on
at the same time.
Brake input is coming from the brake controller, but the other three come
from the military connector. I don't have the papers here, so I don't
remember which pins were used. But it doesn't matter, you need to check it
yourself. The fat pin should be ground (check against some other ground).
The rest should behave, based on the turn signal position, brakes applied,
etc. That is, you should map your own trailer connector to make sure
everything is woring as it should. Yes, some pins will be unused, but I
didn't want to cut into wires and use them for my needs, I preferred to have
a separate connector. IMPORTANT: check your military trailer cable and make
sure that the pins you're using are properly wired.
At the end, attach the trailer cable to the vehicle and map the other end of
the trailer cable. You should find everything you need there (left, right,
perimeter). You should see that left and right flash and that they both go
on when you hit the brakes.
Now, you need to mount 3 relays in the project box. Each relay will have 12v
input (that you're bringing from your 12v subsystem), ground, 24v input from
the trailer connector (coming from your vehicle: left, right, perimeter) and
the output will go to the matching (left, right, perimeter) wire on the
trailer.
The brake controller output goes directly into the trailer's brake wire.
You should be done! Make sure you read and understand all of the brake
controller documentation. Make sure you set it up to match your trailer and
that you adjust it based on your vehicle and your load (primarily do a few
test runs with the expected load in a safe environment to make sure that the
gain adjustment on the brake controller is good. You should have both
trailer and the vehicle brake about the same and not have too much strain on
the pintle).
OTOH, if you have something largish like a 5ton, you are likely to be able
to get away without any trailer brakes at all! If you're heavy and have a
lot of traction in the rear (where the tongue of the trailer will provide
extra weight), you will find that it stops OK with the trailer. For a
variety of reasons, we had to tow about 10 tons (trailer + halftrack) behind
a 5ton without trailer brakes working, and tests showed that it was handling
OK, and it indeed was handling fine all the way to the destination. However,
it is still important to have the lights or at least the turn signals.
All of the parts were purchased at a local electronics shop, except for the
connectors, which were purchased at NAPA and Kragen.
Hope that helps. Ask more if you need, some of the questions I can answer
now, some I'll have to look around for the old papers to answer.
Vadim.
This archive was generated by hypermail 2.1.4 : Sat May 07 2005 - 20:39:52 PDT