Hey everyone, I'm having a hard time diagnosing an A/C electrical problem. Neither the compressor or either fans come on when the A/C button is pressed in the cabin. When I jumper Thermoswitch A, both fans and the compressor come on when it's pressed. I figured that had to mean that all the relays, fuses, and control module were all working properly and replaced the thermoswitch, but I still have the same problem. Can anyone shed some light?

Here's some back story that I feel might be related. AC was working fine on a roadtrip, turned it off cause it was too cold in the cabin. Later on I accidentally ran over a blown out tire that got sucked up into the front drivers side wheel well then spit out. I didn't realize there is a wire bundle up in there that could have been harmed. A couple hours later tried turning A/C back on and it wouldn't blow cold/compressor wouldnt come on. It was late and I didn't put two and two together. 300 miles later and the next day, saw that the wires had been laying on the tire which had rubbed through the insulation on many and frayed one of them. I repaired the wires that had been damaged (spliced in new sections of wire, soldered and heatshrinked, quality repair). Replaced the 15A condenser fan fuse underhood that was blown, checked the in-cabin fuse box for blown fuses but didn't see any, and figured this would fix it. Alas. Here I am asking to tap into the collective wisdom of all of you.

Is it possible to fry the radiator fan control module by any of those wheel well wires getting intimate, in a current sort of sense? It is confusing to me why shorting the connector for thermoswitch A would make everything work as it should, but the new switch plugged in by itself does not have the same effect (even when heated to operating temperature). I should also mention that while bleeding the cooling system, the radiator fan never came on, even after the car had been running for 10 minutes. I'm getting mixed signals from the car.

Anyway, Sorry for the novel. Thanks to everyone in advance. I truly appreciate the help.

-Phil

 

ECM normally enables compressor relay when a ground signal (low voltage) is received from ground path through AC Pressure Switch, thermostat, AC switch, and blower switch.

If AC diode were shorted, this might cause the ECM to enable compressor. You need the wiring diagram for AC to understand. Suggest checking AC diode separate from circuit. It should show low resistance one direction and very high resistance in other. If low both ways, this may be source of problem.

I would also check the wiring job to be sure all wires are spliced correctly (correct matches).

good luck

 

This is an interesting issue.

Some of the following is like what TexasHonda stated, such as checking the wiring job and the A/C diode.

When you push the A/C switch ON, and the heater fan switch is in position 1, 2, 3, or 4, a “ground” input is provided to the ECM/PCM through the A/C thermostat and the A/C pressure switch. The A/C compressor clutch relay is grounded by the ECM/PCM. When energized, the relay allows battery voltage to energize the compressor clutch coil which then engages the clutch.

Since you stated the compressor and cooling fans come on when you jumper the thermoswitch A, it may be supplying the "ground" input from the green wire through the A/C diode (which it is not suppose to) and then to the ECM/PCM. So, check the A/C diode like TexasHonda stated. In addition, there may be a possibility of an open in the Red/Wht wire coming from the A/C pressure switch; so, that the "ground" input is not going through to the ECM/PCM.

 

Awesome, those are great places to start, I appreciate both of your guys' input. This is my saturday activity, and I hope to have it solved this weekend. One more question: Do any of the A/C circuit wires run through that drivers side front wheel well? I have the A/C wiring diagram but I have never been amazing at tracing wires.

Thanks again

 

Part of the A/C circuit wires and the condenser fan circuit are on the left engine compartment wire harness, which are A/C compressor clutch, A/C compressor clutch relay, condenser fan motor, and condenser fan relay.

 

Okay, so I found the Diode and tested it with a multimeter. Really high resistance in either direction. So essentially it is acting as a blown fuse, and from what I understand of the wiring diagram it could be causing my problem. Just to confirm, is the diode honda part number 32146-SH3-003 ? I had a hard time tracking it down and I want to be sure.

Thanks everyone! I will update on whether replacing the diode fixed the problem.

 

That looks to be the correct part number.

Yes, let us know if it fixes the problem.

 

So unfortunately I bought the new AC diode and it didn't fix it. I realized that my multimeter (a cheap one) wasn't reading the low resistance across the Diode Resistance would climb rapidly until it got out of the range the meter could handle and would read overload for some reason. Just goes to show you, dont rely on cheap tools... D'oh!

Anyway, I went through the troubleshooting tree in the Shop manual for "AC System does not come on (Compressor and Both fans)" and everything checks out up until the A/C Thermostat. I grounded the middle wire (Blu/Red) with the thermostat plugged in to the connector and it didn't turn the fans/compressor on. Can I be sure that this is my issue? How does the AC thermostat work? I am thinking that essentially grounding the Blu/red wire is taking the place of the ground coming through the heater fan switch and the A/C switch right?

2nd set of questions, how difficult is the A/C thermostat to replace? I tried finding the process in the shop manual but was unable to. Do I have to evacuate the system? (I really hope not... )

Again, thank you all so much for your expertise and time! Working on cars is the fun part, troubleshooting can get frustrating.

 

Technical info. regarding how the A/C thermostat works:
The A/C thermostat is essentially a switch to control the A/C clutch cycling. The A/C thermostat for your model year car is principally comprised of a thermal resistor (thermistor) and a transistor (used as a switch).

The A/C thermostat has a three-wire connector.

For a transistor, current flow from Emitter to Collector and that flow is controlled by current applied to the middle terminal Base. The Blk/Yel wire coming through Fuse No. 8 (7.5A) from the under-dash fuse box is the 12 volt power source (+) which applies power to the transistor’s middle terminal (Base). (See diagram in the third image of my post above, Post #3 and this link regarding transistor to gain a better understanding.)
TRANSISTOR | JOEL

The 12 volt power going to the transistor Base, is also controlled by the thermistor. The resistance at the thermistor changes with temperature; so, the lower the temperature, the higher the resistance. If the temperature is below 37 degrees F, the resistance would be high, which would then limit current to the transistor Base; effectively cycling the A/C compressor off.

The Blu/Red wire would be the wire to supply the ground (-) when the A/C switch is ON, and the heater fan switch is in position 1, 2, 3, or 4. The Blu/Red wire runs through to the transistor’s Emitter.

If the ignition, A/C , fan switch is turned on, and the temperature at the thermistor is above37 degrees F, the ground signal should go through the transistor to the Collector (where the Blu/Yel wire is connected) and further through the A/C compressor circuit. This is assuming the thermistor and the transistor in the A/C thermostat is working properly.

Try this: Ground the Blu/Yel wire, if the A/C works with the car on, etc. , then the A/C thermostat may be bad. (The service manual has a test with the A/C thermostat removed.) If the A/C doesn’t work with the Blu/Yel wire grounded, then it is something else further up the circuit instead of the A/C thermostat.

The A/C thermostat unit itself sits on top of the evaporator (which you probably know, since you grounded the Blu/Red wire.) The A/C thermostat has a capillary tube that is inserted in between the evaporator fins. You may have to open up parts of the evaporator housing to remove the capillary tube if the A/C thermostat required replacement. You may want to see the posts in this thread regarding A/C thermostat capillary tube access. The refrigerant shouldn’t have to be evacuated for just the tube removal.
https://www.hondaaccordforum.com/for...s-40945/page7/

 

Ah okay, that makes sense. So the 12V power running through the thermistor to the transistor base usually allows the ground signal to pass through the thermostat and activate the A/C Circuit, but when the resistance at the thermistor increases as it gets too cold, the current to the base lead of the transistor drops below the transistor's threshold and the switch opens, disrupting the circuit. Man, it is so cool learning about how these circuits work. I am a mechanical engineer, so my electrical experience is basic, but enough to be able to understand when it is explained well. Thanks Redbull-1!

I will ground the Blu/Yel wire and just double check that it isn't something higher up in the circuit, then I plan on pulling the A/C thermostat out (the above thread you posted said on the 96 model the temperature probe was easy enough to get out of the evaporator without having to evacuate (phew)) and run the thermostat test out of the car with some ice water just to be certain.

I'm really curious if I could get to the circuit board inside the thermostat and replace any burnt out resistors/transistors/thermistors (which probably burned out when I had wires shorting together in the drivers side front wheel well). I think i'll give it a shot as the part runs ~ $70 new. If not, I will try the junkyard route and hope to get lucky. If not that, then I bite the bullet.

Thanks again RedBull and TexasHonda. It is much appreciated.