91 Civic AC Wiring

I'm trying to figure out why the AC compressor on a 91 Civic isn't
engaging. There's plenty of freon and the compressor kicks on if I
apply 12V directly. I've followed a diagnostic chart from an Alldata
disc, and it appears the ECU isn't activating the relay. I'm dubious
however that the ECU is the problem.

I don't have the shop manual or wiring diagram, and the Alldata disc
makes no mention of the AC switch on the dashboard. How do I test if
the dash switch is telling the ECU to turn on the compressor? The blue
light turns on when the switch is depressed. I know the kids were
messing around in that area recently to install an aux wire onto the
back of the aftermarket stereo.

Can anyone describe how that part of the system works, and perhaps a
pin on the ECU to check for 12V or ground with dash switch on/off??

-RC

 

There should be a pressure switch close to your receiver/dryer that signals
the ecu to power the compressor. If you don't have enough gas in the system,
the ecu will not engage the compressor.

You have a leak in the system. Have it tested.

 

If, as you say "the kids were messing around in that area recently to
install an aux wire...", the first thing to do is check ALL the fuses under
the dash and under the hood to see which one they blew. Then pull the radio
and trace all the wires that were messed with and see where they tapped into
the wrong one. You will need a wiring diagram for this. You should be able
to find the service manual/wiring diagrams on line somewhere. Start with
www.tegger.com/hondafaq/

 

If you have Alldata, all the necessary wiring diagrams are there, along with
ground locations and trouble trees. It's also helpful if you have some
experience in this kind of troubleshooting.

 

First, a response to earlier messages-

1) I repeat, there's plenty of freon. I hooked up my gauges and
pressures are normal. That's the first thing I did. Compressor was
replaced by me about 1 year ago with a unit from Arizona Air, along
with receiver and expansion valve. Recharged with R12.

2) I checked the fuses. They're fine. The relay is getting 12V on both
the hot wires. That's part of the Alldata troubleshooting. It isn't
seeing ground from the ECU.

Back to the Alldata discs. I bought these back when the car was nearly
brand new (early 90s). I've dug around and can not find comprehensive
wiring diagrams on the disc.

I'll check again.

-RC

 

Followup. I found the wiring diagrams on the Alldata discs. I have
plenty of experience with this kind of thing. I'll try to report back
what the problem is.

-RC

 

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

 

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.

 

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

 

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.

 

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

 

Kind words indeed, but /I/ don't have a scope...

Not on a Honda, no. What tends to break wiring is age, vibration,
bending and stretching.

Wiring insulation does tend to get hard over time, and any vibration and
flex then gets concentrated at the point where the wires enter their
connectors, and that's when you get breakage. These can be really tough
to find when they're intermittent.

Would be nice, especially if you did find the exact location of the
break.

Older cars are full of problems; I have a love/hate relationship with
mine. Got a small fuel-leak right now. I think it's coming from the top
of the tank, but I don't have the time or the desire to drop the tank in
my driveway, so it's going to my mechanic on Thursday.

In spite of the issues with older cars, I still don't want a new one.
Mine (bought new) was boring until it got old enough to have stuff that
needed fixing. Yeah, I'm nuts. And probably masochistic.

For ten bucks you'll get the very latest and most correct diagrams
available. Mistakes are sometimes found over time, and the online
diagrams are most likely to be correct. If I was able to, I'd happily
buy a year's subscription.

 

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