Re: 91 Civic AC Wiring

Now that spring is here and this issue has become more of a problem, I've had a chance to revisit.
There's an open circuit between the ECU and the high pressure switch.
I'm positive of this. There's a blue/red wire that runs from a diode to the ECU, and also from the high pressure switchvia a split. I've found continuity from the ECU to the diode, but it's open to the high pressure switch. The wire diagram shows nothing between the ECU and the switch, other than a S207 split where a blue/red wire runs to both the diode and switch.
Anyone know where I might find this S207 splitter? That's the prime suspect. This car has never been crashed, anbd no aftermarket devices wired in, other than a Crutchfield stereo with their supplied adapter.
Can anyone recommend a circuit tracer? Something that will allow me to track from the ECU backwards to the S207 junction, and beyond?
-RC

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You have no shop manual, and no proper EWDs, and yet you're "positive".
Plus the car's had its wiring monkeyed with by the "kids", which apparently has no significance to you.
I just scanned several pages from my copy of the genuine Honda manual for your car, and then I uploaded them to my site. You may find them here: http://www.tegger.com/hondafaq/misc/manual_scans/civic/91-civic_ac /
No idea what that "S207" is; Honda has no such designation. Connectors are prefixed with a "C", but "C207" is for the right headlamp.
All you need is a multimeter to troubleshoot this. And a proper shop manual. That the kids have messed around with the wiring is not something you should just shrug off. Honda wiring is dead-reliable, unless corrosion or tampering are manifest. Maybe the kids' butchery hasn't anything to do with your A/C problem, but I've seen so many electrical problems caused by such intrusions that I'd be inclined to consider the guilt to lie there until proven otherwiase.
Honda's shop manuals are just about the finest I've ever seen. I recommend you pick one up on eBay or Amazon, or some such place.
--
Tegger

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wrote:

Yes I'm positive. I've got excellent wiring diagrams printed up from the AllData discs.

The wiring was monkeyed with by my son, whom I've been training to work on cars. I double checked his work and it is sanitary.

Thanks for that upload. The troubleshooting flowchart is almost exactly the same as the one on the AllData disc, with one major difference-
At the bottom of page 22-6 the Honda manual flowchart splits to pg 22-8 or pg 22-7 depending on the symptoms. The Alldata flowchart has the opposite. If the compressor engages, the pg 22-7 flow chart is followed. If the compressor doesn't engage the 22-8 flowchart is followed. I believe the the Alldata is correct, and the Honda is incorrect. I know that's hard to believe!
I've been through the flow chart and using the Alldata version it says to try a different ECU. The Honda version says to replace the compressor clutch, despite it operating fine at the bottom of page 22-6 (huh?)
The ECU however functions properly if it sees a ground on pin 14 which is what is supposed to happen if the A/C switch is turned on at the dash. It isn't seeing a ground however because there is an open circuit between C285 and S207 (or a plain black dot on the Honda diagram).

Alldata uses the C prefix to identify connectors. They also use an S prefix to designate connections where multiple wires meet. The Honda manual uses an unidentified black dot. S207 idetifies the point on the wiring diagram where three blue/red wires come together.
On the Honda diagram just above the pressure switch is a wire designated blue/red 1. This is the wire with the open circuit. Blue/red 4 that runs from the diode to ECU is fine. The black dot above the pressure switch is labeled S207 on the AllData diagram.
Here's the circuit points that I isolated and tested-
1) Pin 14 of the ECU via C423 and at the ECU 2)A/C diode via C473 3)A/C Pressure switch via C285
I get continuity between pin 14 and C473. I do not get continuity between pin 14 and C285. I get continuity from the other blue/red wire at C285 (blue/red 2) to ground when the A/C switch is on (including continuity through the pressure switch).
Therefore the problem is somewhere in the blue/red 1 wire between C285 and the blue/red junction S207 (or black dot on the hHonda diagram). I get continuity from C285 to where blue/red 1 enters the bundle near the radiator. (Stuck a needle probe through the insulation to check.)

I've got numerous multimeters and a few scopes, and I know how to use them. :-) My background includes designing, building, and fixing custom prototype instruments in the med device industry, so I have a basic understandiing of electronics and wiring diagrams. My experience with cars however is only tech school and DIYer. Mind you, in 40 years of car ownership, I've never had to take my car to a shop, despite a habit of buying used cars more than 10 years old. I made good money in college doing tune-ups and minor repairs.

I didn't. My son and I reviewed his worked thoroughly.

And here's the rub. I agree completely with this statement. The car has never been wrecked and there's no aftermarket wiring hacks. No trailer wiring, no alarms, etc. The stereo kit was Crutchfield, which is plug and play. It plugs directly into the Honda wiring harness without _any_ hacking. The stock Honda speaker wires were used, no subs, amps, etc. were added. The auxiliary line that my son added is two RCA jacks plugged into the receiver, and fed out from the dash to an 1/8" jack on the passenger side. That's not the problem.

That's why I checked there first.
The C285 connector was also suspect, since I've disconnected it in the past when changing the radiator, to get a little extra wiggle room. The open circuit however is not in the connector proper since I get continuity from the wiring bundle to the terminal connectors.

I've got the Honda manuals for my 93 Del Sol and 94 Accord. I agree they're wonderful. For the 91 Civic the AllData disc (bought back when you could get it on CD and didn't have to go online) has also been great for basic info. It isn't nearly as comprehensive as the Honda manual, but it has an excellent wiring diagram.
I won't get into the family politics why I don't own the Honda manual for the 91 Civic. It's my wife's car and I'll leave it at that. ;-)
Meanwhile, I need to get a circuit tester that will allow me to trace the blue/red 1 wire from C285 to S207 (whereever that is). I have no expereince with this type of tool because I've always found the problem in an obvious location. Any recommendations?
In some ways it would be quicker to just pull a new wire from C285 near the radiator to C473 under the dash. I did this with a temprorary jumper and the A/C works. I'd rather find and repair the break in the existing wire though. I'm sorta picky that way.
-RC
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<mucho snippage>

I've never got into electrical troubleshooting to the point where I needed a circuit tester, so I have no idea.

Well that kind of nails the matter, doesn't it? It is indeed just an open in the wire.
Your narrative (which I snipped for space reasons) says you're _definitely_ several cuts above many people who post here, and way above me, that's for sure.
I think what you're asking for is stuff that probably only a seasoned and working Honda tech would know.
If you don't mind spending a few bucks, you can try a short subscription to Honda's Techinfo site: https://techinfo.honda.com The wiring diagrams there are far more extensive than anything in the paper manuals, but I don't know if they've expanded the '91 Civic's diagrams to contain the clickable hyperlinks that later diagrams have. I'm not in the US, or I'd check for you. Only US billing addresses may subscribe.
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wrote:

I disagree. You know these Hondas better than _anybody_ Tegger. That's why I'm here asking advice. :-)
I suspect that somewhere the break is due to something _I_ did. These things don't just happen, at least rarely in my experience. Usually it's something I screwed up (no auto tech to blame here, all the work has been done on this car by me for 19 years). Occasionally I find a defect... I'll post what I find, if I find it.
Thanks for the time you and others have taken to help me with this issue. Unfortunately it's not the only one I have with these old cars. I'll be asking some more questions in the future.

I'm such a cheapskate, I grimace at the idea of spending $10 for 3 days. Otoh, I don't mind spending extra for genuine Honda, because I feel Honda is a better value. Just something I have to get used to.
-RC
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I got back under the hood today and figured out the problem-
*#%$&? Squirrels!
I didn't think the issue was due to Honda quality. Ends up it wasn't any of my work either. I had replaced the radiator recenty so I checked that area carefully, suspecting collateral damage. That's when I noticed the tree rat damage.
They severed the wire inside the harness close to the low pressure switch. The varmint had not done any chewing, just a single bite or two that left the black plastic harness cover mostly intact.
We have neigbors that feed these nuisance animals, and any vehicle that sits for too long gets chewed on. This probably happened when the car sat for a week or so while the family was on vacation. One of my neighbors had his backhoe wiring completely torn up a few years ago. Time to once again practice rodent relocation.
I was able to solder splices into the blue/red wire with relative ease, considering the tight quarters. The A/C is works now.
-RC
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