strange occurance

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 away.
Any ideas?
Thank you!
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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 alternator. 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 alternator.
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On 29 May 2004 14:40:54 GMT, snipped-for-privacy@aol.com (SizzleMP) wrote:

You are most probably right about the alternator. I just did a cursory look when listening for the sound. I will check the things you mentioned today and post back.
Thank you.
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What happens if the compressor is off, and you turn on the headlights? I suspect the problem is the alterntor or electical system and not the compressor.

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wrote:

I am sure you are right about it being an electrical system problem. Checking it today.
Thank you.

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I'll bet the noise is coming from the alternator not the compressor.
mike hunt
forked wrote:

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Alternator bearings.
--
Steve
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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 look..... or it might be the compressor clutch coil going bad????
forked wrote:

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