Diode on AC compressor 86 corvette

[Jim Hannon]

I have been trouble shooting AC problems on my 1986 L98 and noticed a diode on the compressor plug was broken in half. What is this for? Where can I get a new one? Ran all test in FSM and getting 12V to green wire and black wire located at the plug on the compressor. The system is full of freon. Compressor works when I did the jump test. Thanks in advance

 

[vetteoz]

noticed a diode on the compressor plug was broken in half.
What is this for?

Protects the clutch coil from back flow current when the clutch dis engages
Power can only go one way ( to ground ) through it

Possibly get a replacement at the likes of Radio Shack.

 

[boomdriver]

I have been trouble shooting AC problems on my 1986 L98 and noticed a diode on the compressor plug was broken in half. What is this for? Where can I get a new one? Ran all test in FSM and getting 12V to green wire and black wire located at the plug on the compressor. The system is full of freon. Compressor works when I did the jump test. Thanks in advance

I had similar issues last yr. After looking at everything else, we cut the diode and its been working since. This works to protect the clutch coil as Oz states, but the need is only under certain conditions that only a bad diode can cause.....Many yr models don;t have this.

 

[LLC5]

The diode is actually there to prevent the compressor clutch surge voltage damaging the vehicle electronics when the compressor clutch disengages.

 

[boomdriver]

The diode is actually there to prevent the compressor clutch surge voltage damaging the vehicle electronics when the compressor clutch disengages.

Ok, I "get" the theory in what you;re saying, but my problem in understanding that thing..... is that this particular circuit remains 'hot' rain or shine, a/c or heat,. basically anytime the key is turned to "run". The Opt-68 BCM manages the path of the power in this loop where the same wire jumps from switch to switch and finally back to its source.

Its not actually a circuit with a positive & negetive, its only a 'field' coil that creates a small but powerful magnetic field by way of 2 miles of ultra fine wire wrapped in a circle. If one side (blk for ex) was cut and grounded, nothing would work.The fuse would blow for the a/c.
The coil is energized by the completion of the loop that the wire makes back at the BCM. I've had more than one tech say the same thing...that its leftover technology from older design control systems thats not necessary especially with the BCM as the source of power/grnd for the whole system.
The manual air might be better suited for a diode there, but I don;t see the need with opt-68. All the diode does is short out the field if it fails. Mine was'nt even blown and caused a short.

Oh well..not a big deal. Mines been running fine for many years without a diode. It was last yr when I tried to replace the plug with a new one that had the diode when the trouble started...soon traced to the diode and cut out and forgotten.

 

[LLC5]

Ok, I "get" the theory in what you;re saying, but my problem in understanding that thing..... is that this particular circuit remains 'hot' rain or shine, a/c or heat,. basically anytime the key is turned to "run". The Opt-68 BCM manages the path of the power in this loop where the same wire jumps from switch to switch and finally back to its source.

Its not actually a circuit with a positive & negetive, its only a 'field' coil that creates a small but powerful magnetic field by way of 2 miles of ultra fine wire wrapped in a circle. If one side (blk for ex) was cut and grounded, nothing would work.The fuse would blow for the a/c.
The coil is energized by the completion of the loop that the wire makes back at the BCM. I've had more than one tech say the same thing...that its leftover technology from older design control systems thats not necessary especially with the BCM as the source of power/grnd for the whole system.
The manual air might be better suited for a diode there, but I don;t see the need with opt-68. All the diode does is short out the field if it fails. Mine was'nt even blown and caused a short.

Oh well..not a big deal. Mines been running fine for many years without a diode. It was last yr when I tried to replace the plug with a new one that had the diode when the trouble started...soon traced to the diode and cut out and forgotten.

When the a/c clutch coil is grounded and the compressor engages, there is a magnetic field in the coil which pulls the hub in and turns the compressor. When the compressor cycles off and the coil loses it's ground path there is a spike voltage from the clutch coil (just like an ignition coil) that can damage the electronics.

 

[boomdriver]

When the a/c clutch coil is grounded and the compressor engages, there is a magnetic field in the coil which pulls the hub in and turns the compressor. When the compressor cycles off and the coil loses it's ground path there is a spike voltage from the clutch coil (just like an ignition coil) that can damage the electronics.

OK....

I do understand, just never had the misfortune of experiencing that. My only experience was trouble with the diode itself.
I've lost 2 BCMs over the yrs, but thats understandable as hot as those get with resistance from the fan motor and the rest of the a/c electronics...

Fair 'nuff.

 

[BigJimZ16]

OK....

I do understand, just never had the misfortune of experiencing that. My only experience was trouble with the diode itself.
I've lost 2 BCMs over the yrs, but thats understandable as hot as those get with resistance from the fan motor and the rest of the a/c electronics...

Fair 'nuff.

We have similar problems in the Industrial Controls arena. The voltage spike induced by the collapsing field of brake coils, clutch coils, realy coils, etc., is usually many magnitudes above the supply voltage. This spike is enough to either damage the device driver at the control or to induce noise into data paths that may run parallel to the conductors. With the diode across the coil at the remote location, the voltage spike is "shorted out" by the now forward biased diode since the induced spike is of the reverse polarity. Thats why the cathode of the diode on the AC clutch coil is connected to the positive connection.

 

[boomdriver]

We have similar problems in the Industrial Controls arena. The voltage spike induced by the collapsing field of brake coils, clutch coils, realy coils, etc., is usually many magnitudes above the supply voltage. This spike is enough to either damage the device driver at the control or to induce noise into data paths that may run parallel to the conductors. With the diode across the coil at the remote location, the voltage spike is "shorted out" by the now forward biased diode since the induced spike is of the reverse polarity. Thats why the cathode of the diode on the AC clutch coil is connected to the positive connection.

Interesting...

I'm knew of a slight surge as the field collapses thats like a backlash when the resistance suddenly vanishes, but I was unaware that it caused enough of a spike to do damage..The way this thing operates I assumed that the BCM easily handled any surges since its absorbes the resistance from the blower motor.

Strange...when I had the problem, I looked at the diode and turned it around, tested it (tested ok), and moved on. It was later that someone else took the step of "snipping" it and everything went back to functional. It was a direct short somehow. At the time I never cared to look further...It was summer and the a/c was working again. I did'nt want to know..!

Thats for the detailed explaination.

 

[geekinavette]

Interesting...

I'm knew of a slight surge as the field collapses thats like a backlash when the resistance suddenly vanishes, but I was unaware that it caused enough of a spike to do damage..The way this thing operates I assumed that the BCM easily handled any surges since its absorbes the resistance from the blower motor.

Strange...when I had the problem, I looked at the diode and turned it around, tested it (tested ok), and moved on. It was later that someone else took the step of "snipping" it and everything went back to functional. It was a direct short somehow. At the time I never cared to look further...It was summer and the a/c was working again. I did'nt want to know..!

Thats for the detailed explaination.

Turned it around? If you turned it around and re-connected it then you just created a short circuit that will definitely blow the AC compressor clutch fuse and most likely the diode itself.

Those back-EMF spikes can be huge...most definitely enough to cause damage.

 

[vetteboy86]

Depending on the conditions, the "inductive kick" could be in the magnitude of 100 plus volts.

 

[Curious George]

Most relays in a vehicle also have a built-in diode or resistor to reduce back EMF as well. To illustrate how strong the EMF can be, get a standard "Bosch" type relay and energize it with 12VDC... while holding both positive and negative (85 and 86) leads with two fingers (from one hand), disconnect power and if your hands are slightly moist, you will get a little jolt.

CG

 

[boomdriver]

Turned it around? If you turned it around and re-connected it then you just created a short circuit that will definitely blow the AC compressor clutch fuse and most likely the diode itself.

Those back-EMF spikes can be huge...most definitely enough to cause damage.

turned the plug around. that makes zero difference to what is nothing more than which end of a wire the current flows thru with nothing between there and the end.

 

[geekinavette]

turned the plug around. that makes zero difference to what is nothing more than which end of a wire the current flows thru with nothing between there and the end.

Ahhh ok, very true that makes no difference to the coil as it is not polarized...and the diode will turn around with it so will still be reverse-biased.

 

[boomdriver]

Ahhh ok, very true that makes no difference to the coil as it is not polarized...and the diode will turn around with it so will still be reverse-biased.

I had been running without a proper plug on the coil for awhile and was trying to get everything factory correct again. The replacement plug came from a vette salvager but when it did not work I wondered if it might have made any difference being turned around....had nothing to loose at that point. Had'nt discovered the blown fuse at that time...did'nt even consider that it could blow a fuse if the diode seemed to be good. (actually I was running breakers which compounded the problem with intermittent operation, fuses in the panel now, breakers in the console when needed)
Odd thing, the diode itself did not allow flow both ways, but it was the source of a direct short to ground causing the fuse to blow everytime. beats me...its gone, a/c works and I'm happy with everything except the forecast ! :eyerole

 

[TedC]

Diode 101 education. There are all kinds of diodes. One way current flow is common, but some are designed to flow current in both directions depending on voltage (ex. zener diodes act like standard diodes until you reach a specific voltage). Using a VOM to test diodes is just a static test, just like using a VOM to check fuel injectors. Proper way is using a voltage regulator and test across operating range.

I looked up the wiring diagram under compressor controls in the FSM (1989). The A/C compressor clutch diode is a standard diode based on the symbol (straight bar with triangle). The manual states that when the A/C clutch is de-energized, a voltage is induced in the clutch coil by the collapsing magnetic field. The diode provides a path for this voltage. If the diode is not connected, the induced voltage can damage the A/C control circuit in the blower module.