98' Honda Civic DX A/C not turning on:

had AC systerm professionally recharged, held pressure and
system could be 'manually' started, compressor working, cold air...
but mechanic could not figure out why it didn't work by itself.
He tested and thought following good: relays, fuses, computer, compressor,
pressure switch, fans,

later I personally got service manual
section: trouble shooting: "A/C system not working"
1) fuse 56 & 17 -> ok
2) voltage OK between No.4 terminal of condenser fan relay & GRD
3) 12V OK between No3 terminal condenser fan relay & GRD
4) 12V OK between AC thermostat No3 terminal & GRD
5) 12V OK between AC thermostat No1 terminal & GRD
6) but when testing AC thermostat No2 terminal to GRD
(w/ engine running... both fans did run but compressor clutch didn't (!?)
chart simply asked if "both fans run AND does compressor engage?"
and if NO -> "Replace the A/C thermostat"...

so I was confused by that ambiguity (ie. half success/half fail)

so I started down
troubleshooting "AC compressor does not run" :
1) 12V OK between compressor clutch relay No4 terminal & GRD
2) 12V OK between ECM/PCM No17 terminal & GRD
3) 12V not-ok between AC pressure switch No1 terminal & ground
=> "Open in wire between condenser fan relay & AC pressure switch"

so thinking finally found problem, then
ran jumper from condenser fan relay wire under fuse box to
AC pressure switch BUT
did not solve problem... AC system still does not turn on ...

any guidance at this point would be appreciated....
(is it simply replace the thermostat?)

 

Does the engine rev up slightly when you press the button? That would indicate the ECU has received the signal to turn on the compressor. The ECU then checks that the rpm is OK and if it is, pulls the compressor relay wire to ground, closing the relay and starting the compressor.

The circuit goes control panel to thermostat to pressure switch to ECU to compressor relay. It's all negative logic, when the wire goes to near ground, the system comes on. Check if the voltage at the pressure switch goes from 5v / 12v to near ground when you press the button.

 

- no detectable change in engine RPM when A/C button depressed & lights-up.
- 4.7v was measured at pressure switch but I'll check tomorrow how this
varies with starting/stopping a/c
thanks