[Note: cross-posted to both HML and MIL-VEH]
A suggestion for all M998 HMMWV owners: Check the brake light switch
on the brake pedal, and make sure it's working properly!
The short story is that I believe there's a design flaw in the HMMWV's
brake light switch that can make fail rapidly, especially in a truck
with very little brake pedal free-play. Basically, AM General used a
rotary switch in an application which I believe requires a snap-action
or plunger switch. I'm referring to early M998 HMMWVs like my 1986
model; I do not know if newer HMMWVs or civilian Hummers use the same
type of brake light switch.
Read on if you want the longer version...
The brake light switch on the HMMWV is a sealed rotary switch,
actuated by a lever which is connected to the brake pedal by a
linkage, and spring-loaded into the "on" position. The brake pedal
return spring holds it "off" against the pressure of the smaller
spring inside the switch.
Several months ago, a co-worker who happened to pull in to the parking
lot behind me warned me that my brake lights weren't coming on. I
re-adjusted the brake light switch so that the brake lights would come
on with light pressure. I later discovered that the brake lights would
not turn back off reliably. Examination of the switch revealed that
there was a fairly wide high-resistance region between "on" and "off".
It was wide enough that the switch could not be adjusted to turn all
the way on with light brake pressure (such as when slowing the vehicle
down just a bit more rapidly than engine braking would allow), but
still turn all the way off when the pedal is released. My truck has
very little free play in the brake pedal, and it takes very little
brake pedal pressure to stop the truck. Even when I'm slowing down
primarily by engine braking, I like to apply the brake pedal lightly,
just to light up the brake lights and warn the impatient guy
tailgating my slow truck that I'm going to be rolling even more slowly
soon. :-)
I hacked together a home-made bracket out of parts from Home Depot,
and used it to mount a snap-action lever switch as a temporary
replacement for my brake light switch. It works very well: very light
brake pedal pressure positively turns the brake lights all the way on,
and they positively turn all the way off when pressure is removed. On
the other hand, it's not water-proof, and it's a bit flimsy.
A couple months or so ago, I replaced the brake pedal switch with a
brand new one from a sealed package. I tested it before installing it,
and it was fine. I installed it and adjusted it to work properly.
Tonight, while sitting in my truck and doing nothing (as an
alternative to sitting in front of the Evil Brain-Wasting TV and doing
nothing), I noticed that the new switch appears to have failed just
like the original, after less than 500 miles.
I've thought about it, and I think that this is a design flaw. A
rotary switch just doesn't belong in an application where it's
carrying several amps, and where it will normally be feathered right
at the edge of the "on" region, rather then positively turned all the
way on every time. That'll cause arcing and local heating at the edges
of the contacts where they first make contact, and pit them there,
making that high-resistance region between "on" and "off". I can
actually hear the arcing if the engine is off, and I press the pedal
with my hand while kneeling next to the truck. This sort of
application demands a snap-action switch (like the ones commonly used
as limit switches), or at least a plunger-type switch that has a large
contact area even when held right at the "on" boundary.
I will probably re-design my replacement switch and bracket to be
waterproof and more sturdy sometime. In the mean time, here are some
pictures of my cheesy temporary replacement. The first two show the
home-made bracket (the silver part), the lever switch (the black
part), and the original brake light switch (the goldish part), bolted
together, but with the truck removed:
http://www.qsl.net/ke6myk/tmp/mvc-522f.jpg
http://www.qsl.net/ke6myk/tmp/mvc-523f.jpg
In those pictures, the rotary switch's lever is in the "on" position.
I use the normally-closed contacts on the snap-action switch, and the
lever from the original switch actuates the snap-action lever when the
brake pedal is not depressed. The original switch remains in the truck
to form part of my new switch mechanism, but is disconnected from the
wiring.
Here are a couple of pictures of it installed in the truck:
http://www.qsl.net/ke6myk/tmp/mvc-525f.jpg
http://www.qsl.net/ke6myk/tmp/mvc-526f.jpg
In those pictures, I've added wiring and connectors, and insulated the
lugs on the snap-action switch with some rubber-lined mastic tape
(basically, it's fancy, thick, gooey electrical tape).
If and when I get around to making a better switch, I'll update my web
page with a description and pictures.
Any comments or questions about this are welcome! In particular, does
anybody know if this problem has been addressed before? If not, I have
half a mind to write in to PS Magazine... :-)
-- Mark J. Blair, KE6MYK <mblair1@home.net> PGP 2.6.2 public key available from http://www.keyserver.net/ Web page: http://www.qsl.net/ke6myk/ DO NOT SEND ANY UNSOLICITED COMMERCIAL EMAIL TO THIS SITE PLEASE SEND PLAIN ASCII TEXT ONLY -- NO HTML OR QUOTED-PRINTABLE
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