I have a '90 Grand Marquis 5.0 EFI with 125,000 miles on it. The only
major problem I have had so far was replacing the transmission.
Starting today, when I have the A/C running or the Defroster on
(compressor running) all is fine and silent but if I have my lights
on, headlights or running lights, I get a loud humming sound coming
from the compressor. As soon as I turn the lights out the noise goes
Are you sure the noise is not coming from the alternator? It sounds like when
you put on additional accessories, you are putting additional load on the
alternator, which can make noise. In that case it might be time to change the
If it is definately coming from the compressor, the only thing I can think of
is that either the extra electrical strain put on the alternator is causing
it's pulley to spin less freely, causing possible belt slippage.
Or, the alternator is defective, not putting out the right voltage under heavy
load, which means the compressor is not getting the full voltage it's suppose
to be operating at.
I would start with the basics first. See if the belts are tight. If that's ok,
hook up a voltmeter to the battery and see if the alternator is charging right.
It should be at around 14 volts with no accessories on. Then turn on the A/C
and the lights and check the voltage. If there is a big drop, it could be the
well you might check the drop in voltage that is going to the
compressor's clutch coil... with the lights on the voltage might drop and
then instead of getting full voltage(12volts) you might just be getting
8-9 volts and this might not be holding the clutch tight engough and that
might be the vibration noise you hear???? just a guess but somewhere to
or it might be the compressor clutch coil going bad????
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.