The EVTM schematic for my '98 Explorer shows the +12V feed to the brake
pressure switch as 'HOT AT ALL TIMES' (through a 20Amp fuse). Same for my
'97 Grand Marquis.
I believe that Ford's recall just adds an in-line fuse. To prevent a
defective switch from overheating, it should be rated very low, possibly
0.25A or less. This would lead to the assumption that the brake pressure
switch is not for servo power, but just a signal line. Can anyone confirm?
Heck, I may just stick my multimeter in there and take it for a spin, to
watch the current draw...
Thats what I did on my 02. Whoa. Not exactly. I didn't measure
the current. I only verified the switch harness connector was hot only
with the key on.
I looked back multiple years on my service DVD. On Explorers, it
appears they changed from "hot at all times" to "hot in start or run"
for the 2002's. Thats the same year they went to having the speed
control smarts buried in the PCM instead of a separate servo.
Unplugged the connector, hooked an ampmeter instead of the brake pressure
switch, and took if for a ride. Results: with speed control engaged, the
current was between 0.45A and 0.60A. It started at the high value and went
down after a while, apparently as things were warming up. I expected the
current to increase momentarily when pushing 'accelerate', because of the
increased load on the motor as it pulls the cable, but could not see any
change. The reading goes practically to zero (less than 1mA) with CC
So looks like the fuse that I am thinking of splicing into the harness
should be rated no less than 1A. Is this low enough to protect a defective
switch from catching fire? Hopefully. But in any case, it's much better than
the 20A that the fuse in the 'always hot' feed will allow.
Disclaimer: I am providing this information for entertainment value only.
This was just an experiment that I performed on my own vehicle and I don't
know if the results apply to anything else. If you are not experienced with
automotive electrical measurements, please don't repeat my experiment - this
is an 'always hot' circuit, and you may damage something or start a fire,
even with the ignition switch off.
I don't have any knowledge about the cruise control servo or the brake
pressure switch and their failure modes. I am not advising anyone to tamper
with the harness or install anything on their own... If you do that, I take
no responsibility whatsoever for the consequences.
Interesting results. I wonder what else is on that 20A fuse? Is it
possible to just change the fuse to a lower value? If all you get is .6
A, it would seem a 20A fuse would allow an overcurrent more likely to
occur than lets say a 5 amp fuse.
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