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Old 06-18-2003, 04:07 PM
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90 300se a/c try this one tech masters!

Greetings all!!!
I will explain what I have done first. My compressor started to leak. So I decided to get a new compressor and while the desire to stay with r-12, I changed all the hoses, condenser, R+D, pressure and temp switches all from Mercedes Benz. I have replaced the high and low fan relays too! The only thing I did not touch was the expansion valve and evap core.

My problem is two fold! When I am in the car and turn the "a/c" on everything seems very nice....cold etc etc. I just never see the compressor kick off. Now I have done the adjustments to the clutch so that when I disconnect the pressure switch I see the clutch disengage. So I rule out the over tighten clutch. Could this be a problem of the MAS unit not letting it go electronically?
The second problem and I have seen this from several posts is that the aux fan never comes on until the temp gauge is around 110 degrees. I agree that they will come on at around 110 degrees when the a/c is not on for the sake of bringing the temperature down for the engine.
But they must come on for the a/c system at an early point. For example in stop and go traffic you need air to pass through the condenser so that you get ample of cool air for the passenger.
My concern is that while traveling and forcing air to move though the condenser I great artic cold air. Now if I put 12v direct and make the aux fan come on I get great cold air at standstill. As an experiment and safety in mind, I want to wiring up a direct 12v to the aux fan (switched) while in downtown or traffic for manual testing So again is this all controlled by the MAS control unit? I read Steve Brotherton's article and was just wondering.
Thank you all for this wonderful forum and the people who join to make it happen!

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Old 06-18-2003, 04:13 PM
Gilly's Avatar
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Join Date: Mar 2001
Location: Evansville WI
Posts: 9,616
Try bridging the high pressure switch and see if the fans come on low speed.

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Old 06-18-2003, 04:30 PM
1991300SEL's Avatar
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Join Date: May 2003
Posts: 545
You should get low speed fans when you turn the A/C on. The high speed fans come on around 102-105(ball park). The low speed fans may cyle off when the clutch disengages.
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Old 06-19-2003, 07:47 AM
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You did not indicate your location in your signature and did not indicate ambient temperature while your compressor is not kicking out. This is important to know. Under high ambient conditions, the compressor may very well not kick out.

Good luck,
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Old 06-19-2003, 10:36 AM
Registered User
Join Date: May 2003
Location: Long Island, NY
Posts: 83
Go to the following website for modifications which will make your car run cooler and make your AC more efficient:

This is an excellent website which contains much useful info.
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Old 06-19-2003, 11:26 AM
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Please don't use a sealant. There is much recovered refrigerant being contaminated with sealants. We have a finite amount of R12 remaining, what's on the shelf and what we can successfully recover.

There is no guarantee that sealant will work, but if it is recovered, it is guaranteed to contaminate whatever refrigerant is in the recovery cylinder, plus it can destroy the recovery/recycle machine that belongs to that tech that is doing his best to earn a living.

My $0.02,
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Old 06-19-2003, 12:17 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Don't worry about the fans. Use this technique. Get a digetal thermometer and place it in the center vent. Place the temp at "Min" and the blower at low blow.

Drive atleast 50mph for ten minutes and watch the temperature. The thing should cycle the compressor of at about 40 degrees. If you don't cycle the temp should continue to below 30 degrees where icing would occur. This is the only reason we wish the compressor to cycle.

If either cycling or icing doesn't occur then try it again in the cool of the morning. If it still won't cycle at low blow then you have an inefficient A/C. I'm not sure but possible causes could include a recirc door inhaling too much outside air.

Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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