Last week I was driving my '86 240DL sedan that has only 295,000 miles on it, when I came to a stop light and happened to look down at the instrument cluster and noticed that all the idiot lights were weakly pulsating. I had time to pop the trans into neutral and rev the engine a little and the lights all went out until I let it come back down to idle. I took it to AutoZone and had them test the charging system......all checked out OK. The alternator belt is less than 1 year old and nice and snug. The battery and alternator are 5 years old and have about 90K miles on them. The problem went away that day, but happened again today. I had the a/c and headlights on as usual. Any suggestions at this point? The lights that pulse when the car is at idle are:
BATTERY/OIL/PARKING BRAKE/BRAKE FAILURE/BULB OUT/ lights.
The car starts perfectly with or without the a/c already on, but usually I try to make sure the compressor is off before starting the car.

Thanks for your thoughts and suggestions in advance.

I once had a relatively new chevy truck that started giving me problems (flickering lights, poor starting) simply because of a loose battery cable. Make sure your connections are all clean and tight. Also, five years is getting pretty old for a battery. You could have a bad cell in it, etc.

The 1986 240s are susceptible to wiring harness rot. Is yours OK?

There are but three wires connected to the alternator: The fat red one which connects to the hot side of the starter. Then thin black one that connects the alternator frame to the block. The thin red one that connects to the brushes and then to the gray connector on the firewall.

All three wires must be OK for the system to work.

The lights that pulse on your car are the ones that should be on when the engine is stopped while the key is in Position II. Those 5 and the oil light.

To turn the lights on, the ground side must be grounded. (The oil light is grounded inside the oil pressure sensor. Pressure = no ground.)

The other 5 are on the same grounding wire - the red thin wire on the alternator. If it is grounded, the lights come on. It can be grounded in the proper way - through the brushes when the alternator is not spinning, OR through a failure of the harness usually in the under-the-engine-front harness.

Your pulsating lights, do they pulse with engine movement at idle speed? If yes, it's likely the harness. If not, then you probably have worn the brushes down to being too short. Easy fix - new brush/regulator set. Can be done without pulling the alternator.

Good Luck,

Bob

:>)

PS The a/c system has a 10-sec. delay that begins when the alternator light goes out. That's to avoid having the compressor coming on when starting.

B.C.
Thanks for responding so quickly. My the dash lights pulsate with the engine idling. Since the alternator has a lifetime warranty from autozone, wouldn't it be simpler to replace the alternator instead of just the brushes? I'll look at it this weekend. Thanks.......Xraybob.