W126 420SEL #5 A/C fuse blowing - PeachParts Mercedes-Benz Forum

RSRobbins · #1 10-02-2003, 05:39 PM

Hi Everyone....

I was traveling at about 65 mph yesterday afternoon with the A/C running fine when it quit. #5 fuse was blown and I replaced it with a new 8 ceramic. It blew again.

Replaced it again and found that fuse blows only when A/C is on. It's fine if just the blower motor is running.

Anyone familiar with this symptom? The A/C blows cold for about 10 minutes so I don't think the compressor is frozen. But in any event I'm wondering if a compressor malfunction could have anything to do with it. I just don't want the shop telling me "you need a new compressor" if it doesn't even seem like a possible cause....know what I mean?

Thanks for your help!

Ralph

mbdoc · #2 10-03-2003, 08:08 AM

Might test the A/C clutch for excessive current draw, normal draw is 4A +-1.

RSRobbins · #3 10-03-2003, 02:00 PM

Thanks for your reply M.B. Doc...really appreciate your time. Car is in the shop...tech sez it "might" be the compressor. Its not seized, that I know as it will blow cold for about 6 mins before the fuse blows.

What would cause "excessive" current draw at the clutch or anywhere else in around the compressor and what might be the cure? Any ideas would be appreciated. I'm just wondering what electrical about the compressor could malfunction. I'll buy a new one if I have to, but sure hate to throw away the big bucks.

Thanks again!

Ralph

SqueakyBrake · #4 07-12-2004, 01:38 PM

Advice on how to test the current draw of the A/C clutch safely.

I imagine that I would have to use a higher amp fuse for testing purposes, where are the testing 'points' is what I don't know. Never had this problem before and all of a sudden, A/C is a no go and it's hot! My fuse does not last for any amount of time, as others have posted, as soon as the a/c clutch engages the fuse blows.

JohninFla · #5 07-26-2004, 08:29 AM

How did you end up with this issue? I am experiencing the same thing with my 1989 560SEL, just recently, and would like to find the solution. It is hot here, too!!

SqueakyBrake · #6 08-01-2004, 04:15 PM

Still hot, still blowing fuses

I can't find it on the service CD's, where are the testing points for the a/c clutch.

stevebfl · #7 08-01-2004, 04:30 PM

As Doc says the current of the clutch coil is about 4amps. An easier test might be to measure the resistance. If E=IR and E is 12v and I is 4 amps how much should the resistance be.

(Hint: in between 2 and 4 ohms). The resistance can easily be accessed by removing the Klima relay and measuring pin 87 to ground. Don't even have to bend over.

The answer if the resistance is one or less as it probably would be E=IR: 12v= 12a x 1ohm 12v= 24a x .5ohm It blewa 8 amp fuse. I'm guessing less than an ohm. The real answer in these cases is a rebuilt clutch from MB. If you are sneeky enough one doesn't even have to discharge the system. The rebuilt clutch should cost about 150. It is a rare enough of a problem that used can work well also.

JohninFla · #8 08-24-2004, 10:24 AM

Replaced clutch with a used part. Coil in back of clutch was bad. A/C is cold again!!!

MrCjames · #9 08-24-2004, 12:55 PM

You may want to also test the "air injection pump clutch," it can be a contributing factor to fuse #5 blowing intermittently. I believe there was a bulletin regarding this issue, group 83 or group 14 not certain at the moment.

Mike Murrell · #10 09-04-2004, 07:56 PM

The amperage spike that takes out the 8 amp fuse can be intermittent. Am dealing with this now as well.

As Doc said, you expect to see 4A +/- 1. I see just a hair over 5A at initial start-up. Once it warms a bit, the amperage reading drops to about 4.6A-4.8A with A/C on.

Another way to test is to pull the #5 fuse and connect your VOM with the 10A scale in place. One lead goes to the bottom fuse connection point; the other to the top. Start the car; turn on the A/C. Again, be sure your VOM is using the 10A scale.

Take a reading with the A/C on, then one with it off and subt. the difference. That should give you an idea what the draw is.