A/C Clutch Relay Questons

Bernie Benz b.benz at charter.net
Tue Mar 30 09:22:54 EST 2004

You're doing good Tom!

The diode is necessary to provide a suppression path for the inductive
energy stored in the compressor clutch coil when the relay contacts open.
If it fails shorted the compressor runs all the time, your case.  If it
fails open, this stored inductive energy will cause a voltage spike on the
electrical system that may be K volts and will blow electronics like your
ECU and will be heard on the radio.  Don't run the AC without this diode!
Use the 3A diode, it must be capable of handling the full clutch current at
the instant that the contacts open.  The cathode must be to the + supply
relay contact such that it will not conduct normal current to the clutch.
If you get it in backwards it will act just like your shorted diode.

Good luck,  Bernie 

> From: TooManyAudis at aol.com
> I evac'd and recharged the a/c system on my 200q yesterday.  All works well,
> with the climate control doing everything right (airflow goes to the proper
> vents and everything!), except that the compressor runs on econ and off.  In
> searching the archives, the likely cause was a shorted diode that connnects
> between #30 and #87 on the relay.  Sure enough, there was continuity between
> those 
> two pins, and when I cut one of the diode leads, the relay acted "normally"
> being open in the normal condition and continuity between the two pins when I
> manually closed the relay.
> So far, so good.  One of the writeups mentioned replacing the defective diode
> with a Radio Shack 50v 2Amp diode.  That would be fine, except my radio shack
> only had 1 amp and 3 amp diodes from which to choose.  So, I got one of each.
> The question is, which one to use?  I am thinking the 3 amp, but I will defer
> to those more knowledgeable on the topic.  Also, the diode is directional.
> What is the proper orientation?
> This will be the first time doing a hack job on a circuit board, although I
> have done the basic soldering for universal oxygen sensors many, many times.
> Any tips for soldering/unsoldering?
> Thanks,
> -- Tom
> 91 200q
> 88 80q
> 92 VW Cabriolet
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