300M Check Engine Light

Over the years the Check Engine Light on our '02 300M has activated, and
the "key dance" has indicated that the fault code is P0440 or P0441.
I've deactivated the light by disconnecting the battery or pulling the
appropriate fuses, and the CEL has usually stayed dark for a long time.
It's never re-illuminated in time for a regular service so they can
check it out.

Not long ago the CEL illuminated again, and I hadn't got around to
either doing the key dance or pulling the fuses. But then when I drove
the car on Friday I noticed that the CEL was not on. I mentioned it to
my wife, who said, "Oh yes. All the instrument panel lights flashed off
and on, and then when they came on again the CEL was off."

What could have happened here? Spontaneous reboot of the computer?

Perce

 

Perce -

My best guess is that something sucked the system voltage below the PCM
reset level momentarily. One likely cause is that the battery has an
intermittent internal short (that's often how they act towards end of
life). Other possibilities are a momentary main bus short (to ground)
elsewhere, or a poor (intermittent) connection either on the power
system or ground.

For poor connections, check: (1) Pos. and neg. right at battery (tends
to be a lot of corrosion unless checked and cleaned periodically on
these cars due to hidden battery location), (2) Positive jump post
connections (*not* the battery post - the place where you would jump the
car off - nect to the air cleaner) - *all* power (with the exception of
the connection between the battery and alternator) go thru that junction
and rely on good clamping, and (3) Negative jump post - located on the
passenger side strut tower - that is the main vehicle ground point.

How old is the battery?

On the two codes you're getting - you do know that one of the more
frequent causes of that are gas cap not being screwed on tight enough to
seal, or defective gas cap. The fact that those codes are few and far
between for you give hope to that. If that's not it, there is a valve
and a couple of hoses that are the other common causes of that in the
2nd gen. LH cars.

 

New battery last November.

And removing power by disconnecting the battery or pulling the fuses for
just a few seconds doesn't usually reset the computer, does it? I've
read that one should wait 10 minutes or so.

I'd read that, and I replaced the gas cap. CEL still illuminates from
time to time.

It did switch off on its own at least once before -- without anyone
noticing flashing instrument panel lights.

Perce

 

You are correct - *however*, that is without shorting the bus to ground
- IOW, capacitors in the front ends of the various modules have to
discharge by leakage currents for the bus voltage to drop to the PCM's
reset levels - that's the reason for the 10 to 15 minute wait period.

Experts know that *IF* you were to disconnect the battery and short the
bus to ground (it would be better to use a resistor to limit current),
the reset of the PCM occurs right away (only as long as it takes to
discharge the total capacitance hanging on the bus) - which would also
happen if the battery were to internally short, bringing the power bus
momentarily to ground.

Then you likely have a problem in that valve or a break/crack in one of
the purge system hoses (IIRC, one of the hoses is just under the air
filter box) - details are available on some of the LH car forums
(www.dodgeintrepid.net and www.lhforums.net). I've never had this
problem on either of my Concordes, so I tend not to remember the exact
details.

Yes - some of these codes/CEL's automatically disappear after some fixed
number (50 I think?) warm-up/cool-down cycles without the fault recurring.

 

The CEL came on again, and I had pulled the fuses a few days ago. The
CEL was still off.

Earlier this evening -- still daylight, so the lights were not on -- I
had to wait for a train, so I switched off the motor. After a minute or
so I heard a rapid clicking noise from the left-hand end of the dash,
and when I turned the key the instrument panel lights were dimmer than
they had been, but I was able to readjust them back to normal brightness.

I think I must have a bad (intermittent) relay -- or a bad connection to
one of the relays. Finding which one could be a problem.

Perce

 

What year is your M? You need an FSM so you can study the schematics to
figure which relays, etc., relate to your symptoms. I can email you a
link to access an FSM for some years (and certain years are so similar
that you're probably covered by an adjacent year).

 

It's an '02, and I do have the service manual -- which I have not yet
consulted. But there is a layout guide on the inside of the cover on the
end of the dash, but no relays seem to have anything to do with the
instrument panel -- fuses, yes, but no relays. I pulled and reinstalled
the fuses, but I don't know whether that will accomplish anything.

Perce

 

Hi!

I hear that and immediately think "bad ground". Or a flakey ground
connection somewhere. Bad or flakey grounds will cause all manner of screwy
behavior.

It's worth a look...

William