frank316

Ok guys I need some help. My A/C isn't working. Last summer I had the system recharged and worked fine all last summer, ran it a few times this winter just to keep it working. Go to use it last week and no dice. So far all the relays are good, fuses are good, power to both relays, condenser fan runs when the ac is turned on.
The compressor clutch will kick on and run when the 2 pins on the relay are jumped, the coil side of the relay has power but is not getting a ground signal. My understanding is that it gets this signal from pin a15 from the ecu, correct? pin a15 has continuity to the compressor relay but is not going to ground like it should. I tried a different ecu and that didn't work but i'm not sure if it was good, just odd that 2 wouldn't work. I don't understand what pin b5s purpose is or how it actually calls for the ac to kick on. I think that it should go low(less than a volt or so) when the heater controls call for the ac, well it stays at 5 volts whenever the key is on I think this is the problem, but I cannot figure out where this signal is actually coming from or how to troubleshoot any farther.
The pressure switch on the dryer is closed, as it should be, i tried jumpering the ac thermostat up under the dash, that didn't work.. Any trouble shooting tips from here? Thanks!

Ben

XKINGX

try putting a gauge on it... if it doesnt have refrigerant or enough it wont come on

pwebb0207

could be low system pressure. compressor will not be signaled on in this case.

Former User

What year Civic?

Former User

OP mentioned that pressure switch is closed.

frank316

It is actually a 99 with a jumper harness with an obd1 ecu. I know the ac worked as is last year so the wiring was right. pressures fine, with the pump hard jumped its like 35ish psi on the low side, I just can't figure out why the ecu is not sending the signal for the compresoor

Former User

If you unplug the pressure switch and ground the Blu/Red wire in the free connector, does the compressor clutch now engage?

frank316

No it does not

Former User

Does the Blu/Red wire lack continuity between the pressure switch connector and terminal B5 of ECU connector B?

If you ground terminal A15 in ECU connector A, does the compressor clutch now engage?

frank316

I'll have to get back with you on that tommorow, does the pin b5 at the ecu go to ground when the ac should be engaged?

technic05

Hi,

You say there is power to both relays, which is great, but do both relays send power to their designated loads?

Pull the connector on the AC Compressor. With the AC turned "On" check if it's getting voltage and start from there.

A relay can get power from it's control side, but if the relay does not switch internally it's a faulty relay. You can also try swapping relays. There's a small chance that even though you swapped ECU's its not a ECU issue.

Good luck.

technic05

Usually if something is sending 5 volts it's signal reference from a sensor of some sort. Try and research that pin to get a further understanding of how the AC system works electrically.

Btw what vehicle is this on?

Former User

ECU pin B5 supplies a reference voltage which, when grounded by pushing the A/C dash button, tells the ECU to increase the idle speed and to provide ground at ECU pin A15 (activates the compressor clutch relay).

frank316

So I should be able to ground pin b5 directly to kick on the compressor. If it does then start looking for an open in the wire between b5 and the pressure switch..sound about right? btw thanks for all your help!

Ben

Former User

Correct. That's where my troubleshooting suggestion was trying to take you:

Also do this:

frank316

cool, yes grounding a15 does kick it on, I just couldn't follow the wiring diagram to figure out where the signal for the ac was coming from..I'll test it to make sure grounding b5 turns it on then work backwards from there, but thats after class tommorow. Thanks again

Ben

SiR:Mallard

I have two additional questions to add to this topic:

1. If I disconnected the Pressure switch, I measure the following voltages:

Blue/White wire = ~12 Volts
Blue/Red wire= ~ 5 Volts.

This topic has thus far suggested that the Blue/Red wire with 5 reference volts is pinned to B5 and should kick a ground out of A15 when grounded out. Like the last person who tried to ground out the Blue/Red wire, it did not click over the relay when I tried it.

However if I ground out the Blue/White wire (measuring ~ 12 volts), it clicks the relay over right away. Why?



2. When the relay clicks over how much voltage should be picked up out of the single red wire leading to the compressor? When the relay kicks over I am picking up a slight fluctuation of voltage, but it's pretty minor.

Former User

Grounding the Blu/Wht wire will activate the condenser fan relay (click sound), not the compressor clutch relay.

As only the condenser fan relay (not the compressor clutch relay) is activating, the fan should run but the compressor clutch should not engage, consistent with the lack of voltage on the red wire that runs to the compressor.

As the compressor clutch relay fails to activate, test the wire that runs from this relay to ECU A15 for voltage and continuity.