I can see where the technique you described for resetting the differential pressure switch
could work on SOME units. A very sharp jab of the brakes might cause the pressure switch
piston to bounce back and forth far enough to allow the spring-loaded electrical switch
plunger to drop back into the piston's centering groove.
For those of you who haven't had the pleasure of dealing with these switches, here's a bit
more on how a typical differential pressure switch works. To my knowledge, all of these
switches have a piston which is shoved to one side or the other by a difference in
pressure between one brake circuit and the other. This usually only happens when a brake
circuit fails, although overly agressive bleeding of a system can cause it as well. On
MOST units, there is a spring-loaded plunger built into the electrical contact part of the
pressure switch that presses down into a tapered groove in the piston. This helps keep the
piston centered. When one brake circuit fails, the high pressure differential forces the
piston to one end or the other (depending on which circuit fails) and the spring-loaded
plunger is forced to ride up the side of the tapered centering groove and onto a flat part
of the piston, closing (or opening) an electrical contact. Because the plunger is now
sitting on a flat part of the piston instead of in the tapered groove, there is no longer
any centering action and the switch tends to remain in this position. To recenter the
piston, pressure must be applied from the other side to push the piston in the opposite
direction and allow the spring-loaded switch plunger to slide back down into the tapered
centering groove. Whew...
Here's one way to recenter the piston for THIS type of switch, but it does NOT work on all
units:
After repairing any problem in the failed circuit, turn the electrical system on, loosen a
bleeder screw in the circuit that had failed, then press gently on the brakes until the
failure light goes out on the dashboard, retighten the bleeder screw, and release the
brake pedal. Done.
Or, as Pat mentioned, you might try a sharp jab or two of the brakes to try to bounce the
piston back and forth far enough to allow the spring loaded plunger to drop back down into
the centering groove. Be sure not to kick the brakes hard enough to break anything.
HOWEVER, there is another kind of pressure differential switch in which the piston is
centered via a spring at each end of the piston. When one circuit fails, the plunger from
the electrical contact part of the switch falls down into a VERTICAL-sided groove and it
will NOT allow the piston to recenter until you actually unscrew the electrical contact
part of the switch, which raises the plunger out of the groove and allows the springs to
recenter the piston. If you have this kind of differential pressure switch, you HAVE to
unscrew the electrical switch portion or you will not be able to center it. In fact, if
you try other methods on this type of switch, you might end up bending or otherwise
damaging the plunger. On the plus side, it is pretty easy to simply unscrew this part of
the switch, then screw it back in. It may require a new O-ring, depending on the switch.
Of course, there could be (and probably are) other switch types that I'm not familiar
with.
Therefore, my overall recommendation is to follow the manufacturer's published technique
for your particular unit so you don't break anything.
By the way, Pat, I don't remember the name of the company that did the faulty "rebuild" of
the dual master cylinder that I described earlier. I should have been a bit smarter and
written it down so that I can avoid them in the future!
My advice to everyone is to check parts carefully in the store before you take them home
(I know, you can't always do this, especially with mail order).
It used to be that if I saw a problem with a gadget that I just bought, I'd often fix the
stupid thing myself, but in recent years, I've gotten much more active about complaining.
My suggestion is that if you see a problem, DON'T EVEN TRY TO USE THE PART OR FIX
IT...SEND IT BACK AND COMPLAIN...LOUDLY. It's a tragedy that some companies let
potentially dangerous products leave their factories. Unfortunately, it seems to be
getting more common, which is pretty spooky when you think about it. As long as I'm on my
soapbox, I'd encourage everyone to be good consumer advocates and push hard for better
workmanship and quality control. Make a big stink when something is poorly made. In a
textbook economy, if EVERYONE quit buying junk (myself included), no one would make junk
any more. Only outfits that made quality merchandise would survive. I also think that the
increased competition would make the price for quality tend to go down. We'll never get
rid of all the junk being made, but we might be able to reduce it.
Thanks,
Alan
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