300M Check Engine Light

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

and 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