I haven't been following the discussions on this subject, but I need to
point out that this problem is age and use related, not a design problem.
If you can feel heat in a switch or push-on contact, indeed in ANY
electrical part other than a resistor, it is in trouble and headed for
failure. The sooner you find and cure the problem, the better. (Note that
this comment does not apply to the dashlight dimmer rheostat, ignition
ballast or the blower speed control resistors - they are supposed to run
hot, and they are designed to take it!).
The problem we face is that the spring tension on the various sliding
contacts, and the surface smoothness and flatness of those contacts has
deteriorated over thousands of operations to the point where there is
serious loss of conductivity at the contact. This shows up as increased
voltage drop across the contact in question. If you put the + lead of your
voltmeter on the "B" terminal of your headlight switch, and the - lead on
the wire which goes from the switch to the headlight dimmer switch, note
that with the switch in the "ON position and the lights on "Bright", you can
read this voltage drop directly. If it is anything more than about 1/10
volt, your switch is going to be getting hot.
Since the heat disippated in each contact is the product of the current
through the contact (in the case of headlight switches this can be 15 AMPs!)
times the voltage drop across the connection, (say for instance 1/2 volt),
the heat generated at just one contact is 7.5 watts! Since the contact is
only able to get rid of that heat through radiation and the thermal
conductivity of its mounting provision, plus the wires it is connected to,
it can get very hot indeed. This leads to a snowballing effect, as the
overheated phenolic board in which the contact is staked shrinks or even
chars from the heat, the contact gets looser, thus getting even hotter,
until finally the whole switch fails. Once the situation has gone that
far, the only cure is to replace the ruined parts.
I have rebuilt quite a few switches, using pretty obvious techniques. I
polish the mating surfaces of sliding contacts, tighten any spring forced
contact pressure by replacing or restaking contacts, and in extreme cases
replace the phenolic or other circuit board material with new substrate
purchased from Radio Shack. This ain't rocket science, folks - anyone can
do this.
I wrote up a couple of these detailed procedures for the IML site in the
last couple of years - an archive search should turn them up, but seriously
folks, you don't need directions - just common sense and some skill with
small hand tools!
Dick Benjamin
----- Original Message -----
From: W Bell
To: mailing-list@xxxxxxxxxxxxxxxx
Sent: Saturday, March 06, 2004 10:18 PM
Subject: IML: Daniel Stern Lighting
I know that it's always better to use a relay to control sizable amperage
circuits, BUT why are the headlight switches cooking if there's a circuit
breaker in the circuit too? In the case of the '72 era Chrysler C-body
cars, the circuit breaker was external to the switch. Other cars have the
circuit breaker in the switch. As it isn't a fused circuit (in a
safety-related item like headlights), there will surely be a circuit breaker
in the system somewhere.
Seems like I recall a discussion about using a relay to run the headlights
so they'd get full system voltage and resultantly be brighter. I also
recall some people trying to put higher wattage bulbs in their plastic
headlight housings and the housings melting from the heat.
Could it be that the headlight switch (with the supposd 30amp breaker) could
be situated in a place on the instrument panel that gets little air
circulation or is a poor heat sink? Or there was an inferior grade of
lubricant in the switch from the factory that let internal corrosion happen?
All the headlight switch is doing is transferring current with little or no
resistance, but touch the connector on the back of the headlight with them
working and that's where you'll find some heat... If, perhaps, the contacts
are too small for the current being carried, come resistance and heat can
result over a period of time, especially if the lube goes away and internal
corrosion happens, but still if the circuit breaker is working, it should
kick out before significant heat happens, I would think.
Considering the financial situation at Chrysler in the latter-middle 1970s
(when the 1980s cars were being finalized), it seems that many of their
product-related problems were due to decisions related to cost instead of
just poor designs (as many desired to believe). LCD displays take very
little power to run, just as LEDs take basically zilch power to run. The
less heat produced from an electrical device, typically the less power it
takes to run it.
Just some thoughts,
W Bell
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