Panic stop--why?

Yesterday I'm cruising along at 35 mph, the light ahead changes, and
I gradually brake for a stop. Suddenly, when I'm about 15 feet from
the car ahead, it's like the brakes fade. I press the brake pedal
as hard as I can and the Jeep finally stops and I feel a clunk like
the transmission as just dropped out of first gear. (This is NOT a
case of my foot accidentally hitting both the accelerator pedal and
the brake pedal.) The rest of the trip was uneventful.
What's going on? I do not want to experience a repeat performance.

'98 Wrangler, 6 cylinder, automatic transmission, 30" offroad tires.
Used once a week or so. Typical drive profile is: 30 minutes of
freeway followed by 45 minutes of very rough trail, then back two
days later.

Other squawks (probably unrelated):
-On very cold days, which are rare around here, the transmission
will not upshift from second to third gear for the first couple
of miles of driving.

-The air conditioner works for 20 minutes or so, then does not work
for the rest of a trip. (System pressures are normal but the
compressor clutch does not engage.)

--
Jack Myers / Westminster, California, USA

Just think of how stupid the average person is, and then realize half of
them are even stupider! --George Carlin

A vacuum leak or bad check valve in the brake booster will do that to
you. You used up all the vac in the booster with the long braking,
nothing left for the final stop. Pressing real hard gives you manual
brakes and you stopped correctly. I'd look for a mushy or cracked vac
hose to the booster. Consider replacing the booster, about $70 for the
part IIRC

On 12/31/2010 11:06 AM, Jack Myers wrote:
> Yesterday I'm cruising along at 35 mph, the light ahead changes, and
> I gradually brake for a stop. Suddenly, when I'm about 15 feet from
> the car ahead, it's like the brakes fade. I press the brake pedal
> as hard as I can and the Jeep finally stops and I feel a clunk like
> the transmission as just dropped out of first gear. (This is NOT a
> case of my foot accidentally hitting both the accelerator pedal and
> the brake pedal.) The rest of the trip was uneventful.
> What's going on? I do not want to experience a repeat performance.
>
> '98 Wrangler, 6 cylinder, automatic transmission, 30" offroad tires.
> Used once a week or so. Typical drive profile is: 30 minutes of
> freeway followed by 45 minutes of very rough trail, then back two
> days later.
>
> Other squawks (probably unrelated):
> -On very cold days, which are rare around here, the transmission
> will not upshift from second to third gear for the first couple
> of miles of driving.
>
> -The air conditioner works for 20 minutes or so, then does not work
> for the rest of a trip. (System pressures are normal but the
> compressor clutch does not engage.)
>

Jack Myers wrote:

> Other squawks (probably unrelated):
> -On very cold days, which are rare around here, the transmission
> will not upshift from second to third gear for the first couple
> of miles of driving.
>
> -The air conditioner works for 20 minutes or so, then does not work
> for the rest of a trip. (System pressures are normal but the
> compressor clutch does not engage.)

Roy pretty much nailed your brake problem. I'd lean toward the leak in the
booster thought - an external leak doesn't quite fit the symptoms.

For the up-shift, how long since the transmission fluid was changed? That
cold up-shift issue is usually my first clue that it's time to service the
tranny (unless cold = what we have this morning: -8 F ).

The a/c question probably means that the cold temps above don't apply to you
;-) I'd be looking for a freeze up in the expansion valve. In South Texas,
that was a common problem and the real cure was usually to evacuate the a/c
system, let it sit with a really high vacuum for several hours to dry it out
then re-charge. You may need a new accumulator - they have a dryer element
in them to remove moisture and that may be saturated. Relatively cheap part
if you already have the system evacuated.

--
Will Honea

Will Honea wrote:
> Jack Myers wrote:
>
>> Other squawks (probably unrelated):
>> -On very cold days, which are rare around here, the transmission
>> will not upshift from second to third gear for the first couple
>> of miles of driving.
>>
>> -The air conditioner works for 20 minutes or so, then does not work
>> for the rest of a trip. (System pressures are normal but the
>> compressor clutch does not engage.)
>
> Roy pretty much nailed your brake problem. I'd lean toward the leak
> in the booster thought - an external leak doesn't quite fit the
> symptoms.

Agreed. Take a can of carb cleaner and spray around the vac hoses
with the engine running. if you find a leak the motor will stammer.
Pay close attention to the booster hose where it flexes between the
motor and the booster. It tends to crack. Also follow all the "hard
lines" and check the boots as they crack as well. I replaced all my
vac lines when the Jeep was about ten years old. Lots of cracks, splits,
and it wasn't that difficult.

> For the up-shift, how long since the transmission fluid was changed?
> That cold up-shift issue is usually my first clue that it's time to
> service the tranny (unless cold = what we have this morning: -8 F ).

This Jeep is new enough that the tranny will store codes. If you have
a DRB-II/III scantool (or access to a shop that will pull codes for you)
that might help in isolating the issue. But do check fluid level.
Cold (real cold) trans fluid will cause problems. It does in my ZJ.

> The a/c question probably means that the cold temps above don't apply
> to you ;-) I'd be looking for a freeze up in the expansion valve. In
> South Texas, that was a common problem and the real cure was usually
> to evacuate the a/c system, let it sit with a really high vacuum for
> several hours to dry it out then re-charge. You may need a new
> accumulator - they have a dryer element in them to remove moisture
> and that may be saturated. Relatively cheap part if you already have
> the system evacuated.

Power for the clutch runs through two pressures senders. A low side
located on the accumulator (big can) and high side located on the
compressor. The low side sensor could be failing or you might be low.
Alternatively if someone stuffed too much refrigerant in there you might
be too high. Hard to tell from behind a keyboard.

Next time it cuts out, disconnect the plug from the accumulator sensor
and short the connector's pins out. If the AC clutch kicks back on
then it's probably the sensor. That part can be replaced without having
to mess with the refrigerant. You just screw it out (fairly quickly)
as there is a tiny valve (like a tire valve) under it. Then install
the new unit. use a little soapy water to check for leaks and don't
overtighten it or you can strip out the aluminum mount.

--
DougW
a high side.

Jack Myers wrote:
> Yesterday I'm cruising along at 35 mph, the light ahead changes, and
> I gradually brake for a stop. Suddenly, when I'm about 15 feet from
> the car ahead, it's like the brakes fade. I press the brake pedal
> as hard as I can and the Jeep finally stops and I feel a clunk like
> the transmission as just dropped out of first gear. (This is NOT a
> case of my foot accidentally hitting both the accelerator pedal and
> the brake pedal.) The rest of the trip was uneventful.
> What's going on? I do not want to experience a repeat performance.

Oh.. how was the weather? wet/rainy/icy? Sometimes drum brakes can
build up ice inside and cause problems like that. A real bad case
I saw once was ice buildup on the caliper! Also check the fluid
level and examine the lines to see if one is blowing a bubble.

--
DougW