cruise control stops working randomly

I have a 2002 Chrysler Concorde Lxi 3.5L V6. Sometimes, the cruise
control will shut off and I won't be able to turn it back on until I
restart the car.

It happens if I have it on and running and then disable it by hitting
the brake or if I've just turned it on and hit the "set" button.
Sometimes, I won't have to do anything, I'll just be driving along at
70mph and the cruise control will just shut off.
In each case, the green "cruise control" light on the dash shuts off.

If anybody has any suggestions as to what may be going on, I'd
appreciate hearing them.

Thanks,
PaulH

 

A scan tool will be needed to find out what the inputs and outputs are doing
and will also show what the last cutout was

Glenn Beasley
Chrysler Tech

 

This won't be an exact answer, but we've seen an indication recently on
the 300M Club forums (for this discussion, 300M = Concorde), that the
brake switch needs replacing when this happens.

Nothing definite has been determined (by repeated success), but the
switch does have three contacts - two of which are inputs to the PCM for
purposes of controlling the cruise control. One of those contacts makes
and breaks a bit of current (for the cruise control servo unit solenoid
valves) and has a tendency to burn and get intermittent. When the PCM
does not see the two cruise control-related contacts (the other one is
the low-current brake sense signal to deactivate the cruise control when
the brakes are applied) work pretty much in tandem (due to the one not
consistently making electrical contact) as they normally would, for
safety liability concerns, it disables the cruise control
(simultaneously, the light on the dash turns off and power to the servos
is cut off). Just a theory so far, it's the only explanation that
makes sense, and, like I said, appears to be supported by success in
replacing the brake switch for people who have this problem, which there
have been a few of lately.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

<shnip>

Might the same diagnosis (brake switch) apply to a '94 LHS?

My cruise comes on for a few seconds, then shuts off.

 

I can't say for sure. I have two 2nd gen. Concordes. The 1st gen. LH's
are probably very similar, but I'd have to see their schematics.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

When the set switch is pressed the cruise control lite should turn on
reminding you that the cruise control is on. When the brake is depressed ,
it shuts the cruise servo off. BUT the lite on the dash should still alert
you that the cruise control is still on. So what you are saying is the brake
switch is replaced because the cruise (dash lite turns off) or the fact that
the cruise control just kicks out???

Glenn

 

Glenn, You are correct that when things are working normally and you hit
the brakes, the dash light stays on, and the cruise "kicks out" (with
dash light still on). However, when this switch contact gets flaky, the
PCM apparently senses that something is amiss (because it sees the two
contacts not operating in tandem like they normally would), so the PCM
turns the cruise completely off - yes - the dash light goes out. I
figure this is an anti-liability feature they built into the PCM
firmware that takes such action if it notices *any* anomalies whatsoever
to avoid any possibility of anything resembling disasterous "sudden
aceleration" situations and creating a lot of bad publicity, accidents,
lawsuits, etc. (think "Audi 5000").

Again, this is speculation, but appears to be borne out by the problem
being corrected when the switch is replaced. A guy on the 300M forums
turned us on to this by observing that that particular contact appeared
burnt when he was having this problem and he examined the brake switch,
plus, the problem went away when he replaced the switch.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

Also, Glenn - I was just looking thru the PCM codes list, and there are
several codes specific to the cruise control. They're worded so
ambiguously as to be hard to know if they relate to the brake switch
failure. I notice that they do not set the CEL. So your advice to have
the codes read may be right on.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

You know, I get a lot of advice to have the OBD II port read. I'll hit
AutoZone and have them do it. But, is there a good reader you
recommend?

Thanks,
PaulH

 

Not only to read the codes but also check to see what the last cut out was.
Its a feature that our scan tool has, it will show if the brake switch,
power switch,speed or ignition caused it to cut out. The same goes with
vehicle theft alarm.it will show what triggered the alarm. But reading the
inputs and outputs in the cruise control monitor should determine what the
cause was. The next time I work around a brake switch with cruise I will
disconnect the switch with the cruise on to see if the lite on the dash goes
off. Im just not sure

Glenn

 

Unless you snip a wire to separately disconnect one of the two switches
related to cruise control, I don't think that will do anything as far as
confirming or refuting the theory. If you simply unplug the switch
connector, both switches will appear to be open - the PCM will simply
say to itself "Oh - the brake is on" (with both switches simultaneously
agreeing to that) - it will keep the cruise power and dash light on. My
theory is that it looks for **discrepancies (differences)** between what
the two switcnes are telling it (i.e., one switch appearing to be open
and the other appearing to be closed at the same time).

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

looking at the wire diagram, snipping any wire at the switch doesn't give me
a reason to think the dash lite will go out

 

Then you missed what I was saying earlier. Snipping a switch wire will
not cause the light to go out directly. BUT if the PCM firmware is
looking at the two contact closures and openings to not be agreeing with
each other as a reason to shut the cruise control off as a
safety/liability precaution, then - yes - it would shut it down and the
light off.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

The brake lamp switch has three internal switches

Brake lamp actuation
Speed control deactivation, and
Brake sense for ABS and brake transmission shift interlock


The PCM just wants to see (brake sense circuit) , cut that wire, the lite on
the dash will still be lit and you wont have cruise control. with a possible
fault code stored

 

If the fault code gets stored, the PCM won't cut the cruise control
(and dash light) off?

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

If the fault code is stored with an active (hard fault) the cruise will not
work anyhow, the lite on the dash will still be on. Customer complaint is
cruise will not work. If the switch or circuit from the switch has a problem
the dash lite will not come on.

 

maxpower wrote:

OK - makes sense. So we're left wondering what would cause the light to
turn off (along with the cruise control) after it is working for a
while. Any ideas?

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')

 

Bill I never said it couldn't happen, I have seen switches, relays and
sensors do some crazy things that just don't seem possible. But if I were to
get such a complaint, I would have to start with using the scan tool to
check inputs and outputs paying attention to the power, ground side of the
switch. Clockspring and Circuit. And or connecting Co-pilot to the vehicle

 

I removed and cleaned the contacts on the brake switch and I haven't
seen the problem since.

Thanks, all!