What is this diode???

[kewljay223]

so im trying to get the ac up and running and i accidentally cut the original wires going to the compressor... but i think i figured it out. this is for my 95 truck using the compressor from the TBI motor.

i found the lt blue wire coming from the relay that goes to the HPS (located on back of R4 pump correct?) which i cut. i need to hook it to one side of the connector then the other side, dk grn, will jump over the top of pump to green wire for clutch right? then just ground the blk wire... there was a splice on the green that went to the pcm but i guess the new pcm needs data instead of 12v... so i just leave it out i guess. does this sound right so far???

but what the heck is the diode for? i drew a big red arrow if you didnt notice.... i tried searching and it seems like its built into the factory harness... well thats long gone.... so do i need this thing? if so, what are the specs on it cuz ill need to go to radio shack and pick one up and wire it in...

i just cant see what it does and why its there. anybody know what the deal is?

[GMCwantsLS1]

When you apply a voltage to a coil is creates a magnetic field. When you remove the voltage the magnetic field collapses and creates a reverse polarity voltage and can be many times the value of the original applied voltage (X4 in my o-scope experiments). This creates a transient voltage pulse that can damage other components in the circuit that are not rated for this polarity or the higher voltage created, things like semiconductors and caps have a maximum voltage limit and breakdown if exceeded. Having a reversed biased diode across the coil allows the diode to conduct for reverse polarity voltages and creates a 'short circuit' across the coil that allows the pulse to be dissipated in the resistance of the coil wiring

[kewljay223]

ok that makes sense. but if thats the case, then why arent there any diodes on most of the other coils in the vehicle? wouldnt the same thing happen to them also? or is it because its such a large coil and its directly connected to the pcm which is supplying a ground, this would prevent a backfed voltage to the pcm?

ok well it sounds like i should pick up a diode for it. do you happen to know what size i need? or how i could figure it out? measure the current draw accros the coil when actuated or something similar?

[GMCwantsLS1]

If you control the coil with a relay and no other items attached to the contacts, you might not need the diode. It is a fairly large coil on the compressor clutch and it does have the PCM attached with it. Almost any regular diode will do. It doesn't really move current just a spike of voltage for a split second.

[kewljay223]

cool man. makes total sense. im going to try running the ac without the pcm hooked up for now and see how it works. if i need the pcm to get the signal, ill grab a diode and throw it in. appreciate the explanation.