Hello Honda Civic Forum

I did search on my own for days, but I now need the wisdom of the forum to help me through a difficult issue I have with my Civic.1991 Honda Civic DX 1.5 Sedan w/ AC Compressor Issue. Many quick fix remedies seemed not to apply, namely swapping or replacing relays. Mine seem to work well when tested.

Recap: AC worked very efficiently for 17 years, then slowly lost ability to cool and clutch will not engage, and relays seem good.

Cutting to the quick: Why can’t I get a signal voltage to the control side of the compressor relay socket (placing a probe in the relay plugs) Here is what I did:

• Tested R-12 Low-side pressure reads 50+ lbs.
  
• Tested compressor clutch bypassing the entire circuit and going from positive battery terminal to positive connector on compressor (there is an in-line wire coupling just behind the radiator about a foot away from the compressor that I simply pulled out and connected that to the jumper lead--compressor side of the wire. That remainder of red wire goes to what I believe is the thermal protector sensor one side of it. The clutch does not seem that it will engage, only a suggestion of an engagement. You almost have to just feel with your hand and not look. It is that subtle.
  
• Tested the thermal protector, separately with needle probes, and I have continuity, so it is not frozen open to protect.
  
• 3 relays checked. Cooling fan relay, condenser fan relay, and compressor clutch relay. I removed them and read 107 ohms across the control side coil. When connected to low-voltage, approximately 10 V, the relay clicked and the continuity connection to the load side was observed on all three relays.
  
• Delay Control Unit (I think that is what it is--the component that disallows the compressor from too frequent on and off cycles. It looks exactly like a relay from the outside, but the contacts are all the same color, not one side copper and the other aluminum like the relays. The readings also did not behave like a relay either. Unsure how to test it, so I simply placed it back in the circuit. This component was positioned very close to the compressor right behind the radiator, about 6 inches from the compressor.
  
• Bypassing (jumping) the condenser fan relay causes the fan to run okay.
  
• Bypassing (jumping) the AC compressor clutch relay causes the vary faint feeling of the clutch vibrate only. Repeated connections with the jumper seem to only make it fainter.
  
• Bypassing (jumping) the cooling fan relay does not cause the fan to run. My guess was a heat sensor between the relay and the fan. I was bypassing at the relay.
  
• I recently replaced the fan motor and strongly believe that all the wire harnesses were reconnected property. It runs fine.
  

So, with the engine running, and the AC button fully depressed with the indicator light on, the fan on level four and the temperature all the way to cool, there is a reading at the controlside of the compressor clutch relay on the positive side at 13.6 V. The positive probe was placed at the relay socket hole and the negative probe to battery ground or negative. This reading was observed with the three relays mentioned above removed from the circuit and placed on the workbench while the probes were inserted in the corresponding sockets in the circuit wire harness/clip connector that would normally be connected to the relays. The confusing part is that this reading is only on the positive side of the control coil. It reads 13.6 V from the positive control side to ground. There is no voltage across the coil.

• This indicates that there is a possible weak or no ground in the AC system and may be linked to the Delay control unit for the compressor since it is the only component that connects the clutch to the control side of the compressor relay to ground to get a control voltage connected. (Did I say that right)? We need a voltage across the control coil.
  
• OR, open ground that is not closing (thus preventing a voltage across the control side of the compressor clutch relay) due to a pressure sensor or a sensor of another kind--like a closed ground depends upon a good signal coming from another section of the circuit. But I have 50+ lbs. low side R-12. Bad pressure sensor?
  
• OR, can a weak clutch prevent or mess with the relay, but that sounds like putting the cart before the horse.
  

The goal is simply to run the AC system with all the relays connected into the system, and read 12 V or greater going to the compressor (I can read it from the inline wire connector pulled apart and a probe placed inside that I mentioned earlier)

My takeaway is that I think for sure the compressor clutch is bad, because I should be able to see it jump and also hear a good click, and I do not observe either. But, do I have one problem like the compressor clutch or two, a deep electrical circuit ground issue?

The question remains: Why can’t I get a signal voltage to the control side of the compressor relay socket. If I (YOU) solve that, I will then perhaps have a green light to purchase new compressor, evacuate, remove, replace compressor, etc.

This image is from Haynes 84-91 civic manual. AC Circuit

 

Okay, I found a scrap of information that got me a few steps further. Here is an image of the scrap from a manual out there.

So, I did all the way to step 6, as you can see in the previous entry--going to the "next step." Step 7, I was unsure. I found he pressure sensor (looked horrible), got it off and jumped it. Started car with AC button depressed, fan on full and cool all the way down--Nothing, no compressor engagement. But they mention a set of wires that I could not see. "...Jumper Blue/Red and Yellow/White wires..." In Step 7 near the end of the first line of text. I could not see "Yellow/White" wires. Shouldn't a switch be the same color wire on each side--like a fuse? Or maybe they use a different color, in this case "Yellow/White" to depict which side of the circuit it is. "Blue/Red" is thermostat side, and "Yellow/White" is the diode side like in the schematic in the earlier post.

Before I closed the hood for the day, I did a continuity check on the pressure sensor as it was hanging free off of the high-pressure line probes (contacts). It read "open." Supposed to be "closed" right? Unless unplugging it renders it "open."

Now what? Thermostat?