Hi all, I've just removed my AC Compressor from my 98 XK8 Convertible to replace the clutch coil and reseal the whole unit. Apart from blowing the clutch fuse it was exhibiting dye on the front and all body seals but as it was actually functioning as a Compressor a rebuild was my choice.

Question: My Compressor looks original but has a small light brown block wired into the single supply wire. I'm assuming this is a ballast resistor? It shows no resistance but the coil on its own shows 3.8 ohms which is smack on the recommendation.
Does anyone know the value of this resistor and its purpose? Most 10PA17C compressors don't seem to have one. Will my Compressor work OK without it when a new coil is fitted?

I did note that the live wire had been chafed by the pulley and may have been the cause of the blown fuse when it got close to the body of the Compressor.
Thanks in anticipation.
Phil

 

After some off line advice, I've been told that the block is in fact a Blocking Diode that ensures the voltage spike caused by the clutch disengagement doesn't travel back the ECU and damage it.
My question about readings still stands. What resistance should I find in either direction?
Thanks guys
Phil

 

Well if its a diode then infinity in one direction and a few ohms in the other. The point is that there will be a 0,7v drop when it supplies current to the clutch. When the current is cut off to the clutch there will be a large voltage spike ..300v ish as the magnetic field in the clutch collapses. The diode prevents that being applied to the system.

You may find a diode test position on your multimeter which will show infinity in one direction and 0.7v in the other

 

Thanks Pistnbroke. I'll get checking tomorrow.

 

If your meter lacks a diode test function, it's possible to get fooled. The device under test may appear to be very high or infinite resistance in both directions. (Some meters will fool you this way; some won't.) If you get that result, worth rechecking with a meter equipped for diode test.

BTW, there's a similar voltage spike suppressor in the convertible top pump, but that one is two diodes back-to-back. These thingies sometimes fail to a short-circuit condition in both directions. When that happens, the pump blows its 40 amp fuse and is then often assumed to be DOA. But it's a cheap, easy fix to replace that voltage suppressor.

 

Thanks Dennis07. I have measured the diode now and I have infinite in one direction and 580 ohms in the other. I can't find any data on what the anode reading should be. Am I right to take it as being serviceable?

The failure I get is a constantly blowing fuse but I do believe that this has been caused by a chafed power lead right near where it exits the coil just behind the pulley. It doesn't look like it's quite touching the compressor body but the voltage spike that's been mentioned is probably enough to jump the gap, I would say.

Thanks to all again.
Phil

 

Phil,

The diode is very likely OK.

The chafed power lead seems like a prime suspect to me too. Let us know if fixing that does the trick.

 

If you were using the diode test function on a multimeter the 580 indicates 0.58v as the forward voltage drop ..580ohms would stop it working

 

Ah ha! Thanks for the explanation. I'm still learning how diodes work and that has certainly cleared up my confusion.

 

diodes is easy ..only lets current flow in one direction. The diode is rated for its max forward current say 10A go over this and it burns out usually open circuit. Its also rated for voltage say 600v which is the max voltage it will withstand when opposing current flow over that it goes bang open circuit again.
Usually there is a white band on one end of the diode .this indicates the end that current will come out when passing forward current.
Often these are fitted across the supply in say a car radio. If someone connects the polarity wrong the the diode passes current and blows an internal fuse .Fuse and diode need replacing.
Next time MOSFETS !!
PS lived in Noosa for 10 years and have elderly relatives in Sydney.

 

Love Noosa! Probably a bit warm for me but a beautiful part of the world.

What a great collection of knowledgeable people willing to share. Thanks again for your help.

I now have the compressor rebuilt with new seals and a coil. Next job is fitting it. Hopefully it won't take me as long to get it back in. Over 4 hours under a hoist at my age certainly takes its toll!
Phil

 

I wish you the best of luck with those damn buried pipe unions. New compressor and new O-rings and I still managed to 'engineer' a new leak

 

The diode is probably OK. We can't think of a diode as having a fixed resistance value like 580 ohms.

In the "reverse" direction a diode acts like an open circuit (unless we apply enough voltage to fry it). Simple enough. In the "forward" direction, not so simple. Starting with a very low voltage applied, it continues to act like an open circuit. Then as we near what's called the "offset" voltage (a few tenths of a volt) the diode begins to conduct, and with increasing voltage it exhibits less and less resistance. So ... It does NOT exhibit constant resistance like a resistor does. Test using a very low voltage, it shows high resistance; test using a higher voltage, it shows less resistance.

If a test meter is applying, say, 1 volt to test a diode ... something not too far above the diode's offset voltage ... a healthy diode can show up as hundreds of ohms. [edit] It's also possible the component you're testing includes a diode and a resistor.

 

Hi Dennis07. Just letting you know that everything is back together after another few hours under the hoist and success! I now have working and surprisingly efficient air conditioning. By resealing the unit and changing the clutch coil myself, I not only have saved myself a heap of money but have been able to verify the condition of most components. The compressor was in amazing condition for 25 years old. It was the original 97 build unit. It also helps having a relative in the trade who vacuumed and regassed for me. Another satisfactory ending!

 

Phil,
That's great! Thanks for letting us know, Always nice to hear when one of these ends well.