CJCzajka

96 Civic CX A/C non-operational, neither condenser fan nor compressor runs.

Looking for the A/C condenser fan connector diode specs?

At least what is a good bias resistance value? Mine is 3.5 M ohms and reverse bias of infinity. Most info I have run across indicates that a silicon PN diode usually has a resistance in the range of low to 10,000 ohms but some other types can have up to 10 M ohms. I am trying to determine if mine is good or not.

I got to this point by testing the two relays, fan, compressor, the pressure sensor and the thermostat. Grounding the thermostat pin 2 (running, A/C on) does nothing. The thermostat voltage at pin 1 and 2 is: A/C off 13.8 V and A/C on is 0 V. The implication is that the heater controller is grounding the circuit and pulling the voltage to zero. My suspicion is that the diode is bad by driving the circuit voltage unloaded but under load does not allow enough current flow to energize the fan relay and pull up the pressure sensor circuit to the ECM.

Would anyone think it a bad idea if I substituted a silicon PN 5-amp diode?

Any comments welcome.

Thanks
Chuck

ezone

Diode?
Look instead for why the AC would not work on a 20+ year old car. Start with some basics. Is the freon empty?

CJCzajka

The freon pressure is good. The pressure sensor has continuity (zero resistance).

With A/C off, the pressure sensor has 13.8 V at both pins 1 and 2. With A/C on, the voltage at pins 1 and 2 is 0 V.

Both relays test good and I also tested by substitution by swapping them. The fan and compressor run if I jumper pin 3 and 4 at the relay (respectively). The thermostat is working since I have 13.8 V at both pins 1 and 2 with A/C off (that is the thermostat is connecting the circuit between pins 1 and 2. The heater control board seems to be working by completing the condenser fan relay circuit to ground and turning on the A/C switch light (hence the 0 V reading on the circuit and pressure sensor circuit). I neglected to mention before that I also tested the ECM by substitution. That is why I am focused on the diode.

Thanks ezone for the response.
Chuck

ezone

You need a good wiring diagram now.

Upon activation, the AC thermostat provides ground to the condenser fan relay pull in coil......and you say that condenser fan turns on when you turn on the AC system, correct?
The diode in front of the condenser fan relay must be intact, as the 12v+ is coming through it to power the relay coil..

Done. Move to the next parts.

Under the same conditions as above, the AC thermostat is also providing the ground to the AC pressure switch, that switch's contacts ARE closed and this is now the "AC REQUEST" signal to the ECM (or pcm) according to the wiring diagram I see this should be RED/BLUE wire at ECM cavity A27 (or C5, depends on which version of the car you have) ........

and if the ECM is truly happy and decides it is really okay to operate the compressor---- it will then provide ground on terminal A17 (BLACK/RED wire) to operate the compressor clutch relay.



My scanner can show me the AC request and clutch control in the ECM data list, but I don't know what other scanners would be able to display that data.

CJCzajka

Have a wiring diagram.

The A/C thermostat completes the circuit through to the heater control board which, in turn, grounds the circuit. In my case the condenser fan does not run when the A/C switch is on.

I am assuming that the pressure sensor circuit to the ECM (diagram as C5, ECM connector 31P pin 5) drives the ECM when presented 12V+ and not at 0V. Can you confirm this?

At this point and because the condenser fan does not run but all the components in the circuit test good, I am presuming that the fault is in that circuit. The exception is my test of the diode with it's high resistance of 3.5 M ohms. When the AC switch is on, I see the entire circuit voltage go to zero 0V and grounding thermostat pin 2 does nothing (as I would predict). If the diode truly maintains that high resistance then that may let us see 12V+ when the AC switch is off (circuit not grounded) but not allow enough current flow to trip the relay when AC switch is on. That is why I have focused on the diode. Of course the diode may be some special type (e.g. cascading avalanche) that may lower it's resistance when a current flow begins.

Which brings us full circle to my original question about the diode specs. Or if someone has a good one handy and could give me a resistance measurement.

Again, thanks ezone for your help, it is appreciated.
Chuck

ezone

The condenser fan is supposed to run at the same time the compressor clutch is engaged. So BOTH are inoperative.....
No. The ECM needs to see the "ac request" line grounded before it will turn on the relay for the compressor clutch. Make sure you aren't just checking for zero volts, but that it's actually grounded.

If measuring at the ecm terminal c5 (if correct for your car)
AC switch ON = 0 volts (assuming thermostat ok, closed pressure switch, and wiring to the ecm are intact).
AC switch OFF = 12v (apparently provided by the ecm)
According to my diagram, grounding pin 2 is just duplicating the same ground the control panel was supposed to be providing to the thermostat.
Pin 1 of the thermostat (blue/white) is the one that leads to the fan relay and the ecm's AC request line.

Alternatively: Pull off the connector from the pressure switch and jumper the terminals with a paper clip or suitable wire. Now connect a grounded jumper to the paper clip, that should provide ground to the condenser fan relay and provide ground to simulate the ac request signal to the ecm which should then turn on the relay for the compressor clutch. (key on, of course....and I don't remember if this unit had to see the engine running before it would turn on the compressor, but I think it did not)
I can't find any spec or description for the diode. Not even a reason it's there in the first place.

Functionally, it must allow sufficient current for the coil of the relay
Test idea (after studying the diagrams) : Stick a dang jumper wire in place of that diode. Does the fan relay work now?


Dead tired, hope my brain got this right

CJCzajka

ezone, thanks for the help. Your input allowed me to better understand the circuit and suggested a way to test the diode. I searched through all my wall wart power supplies and found one that output 12V DC. I then connected it to the diode in series with the relay. The relay energized and pins 1 and 3 had continuity. So, both relays and the diode are now proven good.

I had tried jumpering the pressure sensor connector without success. I will try your suggestion of also grounding it.

I am going to retest all the circuit components by measuring the current flow at all the appropriate stages plus grounded at each component stage. This should allow me to prove component function better than just measuring voltage. For what its worth, my guess at this point is a bad current path (or ground) in/at the control board.

Alas my daytime highs are now in the 20's and low 30's, no garage and I am old with severe arthritis so I won't resume the saga until spring (barring any fluke days over 40).

Thanks again for the help. I'll be sure to update this thread when I continue/resolve the project.

Chuck

ezone

If it is that cold, static system pressure is gonna be too low to allow compressor operation most of the time!

Vapor pressure of freon very closely follows ambient temperature so in near freezing temp the pressure switch will be open, and ac is not needed in freezing temps

The engine might need to run a long time in freezing temp before theres enough heat transferred into the system to get the ac to even begin running, if ever