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Old 12-02-2001, 05:27 PM
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Question A/C Compressor Clutch Won't Engage '88 420SEL

1988 420SEL 103k miles. See at

A/C Compressor clutch will not engage. I checked the single blue wire that, when hot, will engage the clutch, it is not hot. I can apply a 12v jumper wire where the blue wire plugs into the compressor and the clutch WILL engage. I switched out the "N6 A/C Compressor Control Unit" with one from my '89 560SEL (exact same part number), but to no avail.

Where should I go from here? It's for sale in Texas ($7888) and will not sell w/o A/C! Thank you.

Last edited by mscotta; 12-02-2001 at 05:40 PM.
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Old 12-02-2001, 09:51 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Check the low pressure switch at the drier and bridge the two wires with the system on. If it comes on, you have a faulty switch or low freon level.
Donnie Drummonds
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Old 12-03-2001, 10:27 PM
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Cool R-12 Freon? Freeze 12?

Donnie: Your new shop is looking GREAT! Too bad I live in Austin, Texas. Anyway, you were absolutely correct. I jumped the two wires at the low pressure switch and on came the compressor, however, no cool air. The window on the drier shows nothing, but I'm almost sure it needs R-12 freon. I installed R-12 many times in the early 80's and I have access to a good R-12 gauge set, but where am I to find the freon???

What is your opinion about the newer replacement freons for R-12 such as "Freeze 12"? Will they work in a 1988 Mercedes? What must I do before I install Freeze 12? Can I still buy R-12?

Again, Thanks! mscotta
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Old 12-04-2001, 09:24 PM
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R-12 still available in Mexico. Not been for sale in states for over 2 years. Find and repair your leak in system, go to K-Mart and buy conversion kit for R-134 refrigerant. It comes comple with installation hose and new valves for your system. Costs about 49 bucks and comes with complete instructions.

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Old 12-05-2001, 10:05 AM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,512
R-12 is most assuredly still for sale in the 'States. I priced it this summer when doing some a/c work. Just about every McParts store carries the small blow off cans, if not the larger 30lb containers. Last I checked the price was running $25-$30/can. You must have a 'section 609' mobile a/c certification to purchase R-12. I've had mine for almost 10 years. The certification can be gotten online - I think the URL is

R-12 has not been manufactured in the U.S. for several years, but there is still plenty available for sale.

I don't like the conversion kits for a number of reasons. To properly convert from R-12 to R-134 the system should be flushed to remove the old mineral oil used to lubricate the compressor. Mineral oil is not miscible in R-134a, hence just collects in low spots in the system. Too much oil also lowers performance - a bad thing considering that R-134a doesn't usually cool as well as R-12. Lastly, different compressors require different viscosity oil. Some like ISO 42, others ISO 68, and yet others ISO 100. Just dropping in the POE oil charge which comes in the death kits may or may not be sufficient to keep the compressor alive.

I think your best bet is to scare up some R-12 and just recharge. The certification can be had for ~$20 or so, then go look on ebay for R-12; it's likely to be getting cheap this time of year.
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