Re: [Amc-list] F('ing)ailing parts 2

["Swygert, Francis G MSgt 436 CES/CECM" <Francis.Swygert3@xxxxxxxxxxxx>]

>From Tom J (edited)
It seems to me now that this is an engine-oil sensor, that some
bean-counter smart-ass "cross checked" in a catalog, noted
matching mechanical/electrical specs and OK'd the substitution
and subsequent inventory shrinkage. And overlooked that brakes
generate 1000PSI where an engine can't generate 100PSI.

The immediate failure was a pitted contact -- there's a phenolic
block with two brass contacts poking through, radially opposite.

* There was brake fluid (DOT5 silicone) inside. The rubber/crimp
seal is poor by design. There was ooky black on the electrical
parts; maybe the rubber isn't silicone compatible? Seems unlikely,
rubber otherwise OK. Might have simply been carbon from arcing,
in solution.
----------------

I mentioned the pressure in a previous post. Now that you mention the
immediate failure of the contacts, and that this is really and oil
pressure switch, brings to light something else! Brakes are used a lot
-- lots of on/off action. An oil pressure switch closes the contacts as
soon as the engine is started and the contacts stay closed until the
engine is shut off. It's just a ground circuit for one small light bulb
-- not carrying the load for 2 or more (third brake light, trailer?)
bulbs that are also higher current than the warning light. So the
contacts aren't made to carry the load NOR is the diaphragm rated for
the pressure! It seems to me that even though the contacts failed first,
the seal was evidently starting to fail and would have, though it would
have lasted a bit longer with heavier electric contacts. Your whole
findings report needs to go to NAPA, especially the tear-down. That
along with the pressure and electrical differences might convince them
to change the part number. They should be able to research the Ron
Francis part source and get the correct part. Even if they don't, I'd
rather see them list the switch as "no longer available" than sell a
component that will fail in 3-6 months of regular use. The only reason
they get by with it is that most cars old enough to be designed with a
pressure switch aren't driven regularly. If you drive it only on sunny
days the switch will last several years.

I overcame the problem by making a bracket for a 1973 or so AMC pedal
mounted switch. No more pressure switch to fail! I think this is a
common retrofit for regularly driven vehicles due to the failure rate of
the standard switches. Retrofit to a pedal switch or spend the $20-$25
for a Ron Francis switch, which was designed for the load (pressure and
electric). 
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