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94 Accord A/C Relay Help

Old 07-19-2013, 04:56 AM
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Default 94 Accord A/C Relay Help

I have this 94 Accord LX. A/C is not operating. I can bypass the relay, and make the compressor come on. It works, and blows cold. It is full of refrigerant. You are suppose to be able to ground the red/white wire at the pressure switch, and have the compressor come on, but that doesn't work. I have some wiring diagrams. There are 4 wires at the relay. Blk/Yel 7.5a fuse inside fuse box, White 20a fuse outside fuse box, Red/Blu ground from ECM?, and Red to compressor. I have power at the Blk/Yel, no power at the White. Slight ground at the Red/Blu (dim sometimes, not bright, almost like 5v?), and of course ground for the compressor wire. If I give the white power from the Blk/Yel, and ground the Red/Blu the compressor comes on. The fuse in the box is good, but not getting power to the relay. WTF?
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Old 07-19-2013, 08:56 AM
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Default Re: 94 Accord A/C Relay Help

Post a schematic, those wire colors don't mean much. I assume that the blk/yel is power to the relay coil, and the white is power for the compressor that is switched by the relay contacts. The ECU can control the coil ground so that the compressor can be turned off during WOT.

You've already proven that the problem is not the relay, but is actually the lack of power on the white wire.
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Old 07-19-2013, 09:03 AM
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Default Re: 94 Accord A/C Relay Help

[ATTACH=CONFIG]168750[/ATTACH]
[ATTACH=CONFIG]168751[/ATTACH]
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Old 07-19-2013, 09:05 AM
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Default Re: 94 Accord A/C Relay Help

The A/C button is working, as it is grounding the wires at the thermostat in the evap box.

Not getting signal to the pressure switch.
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Old 07-19-2013, 03:23 PM
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Default Re: 94 Accord A/C Relay Help

You've already said the white doesn't have power. If you don't have power on the white wire, you don't have anything because the white wire is what supplies power to the AC clutch. Get that fixed first, and then see if you still have a problem.
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Old 07-19-2013, 03:30 PM
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Default Re: 94 Accord A/C Relay Help

The pressure switch completes a ground, you probably won't see a signal there, especially with a solenoid light probe thingy.. Why the thermostat control uses an NPN transistor inline with that ground I don't know... You shouldn't be poking around with anything but a voltmeter though, because you're not going to get much useful information from one of those shitty solenoid light probes. They work OK for 12V logic signals, like window switches and door locks, and to see if you have 12V power somewhere. That's the complete extent of the reliable information that they can give you though.
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Old 07-19-2013, 03:45 PM
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Default Re: 94 Accord A/C Relay Help

Even with power to the white wire, still no a/c clutch operation. Only with power at the white wire, can you ground the red/blu wire at the relay and the compressor will work.
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Old 07-19-2013, 04:17 PM
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Default Re: 94 Accord A/C Relay Help

Unplug the pressure switch and test continuity across the terminals. If the system pressure is in normal range and the switch is good, the switch terminals will have continuity between them. If the pressure is too high/low, you won't have continuity.

You can also unplug the thermostat (just to prevent damage if it's sensitive for whatever reason) and try grounding the red/whi wire and then the blu/yel wire (after the pressure switch is connected again). You really just have to start at one of the chain and work toward the other end. The easy place to start is the red/whi wire at the ECU, and then the blu/yel wire, and then blu/red wire. Each of those should work, and when one doesn't the component inbetween is the problem. Again, that assumes things like the system pressure is good, otherwise you might think the pressure switch is bad, etc.....

Think about the method here and think critically about the behavior you're seeing... It's a logic process.
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Old 07-20-2013, 05:46 AM
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Default Re: 94 Accord A/C Relay Help

I'm definitely not new to troubleshooting, but in this instance, more than one problem at once on the same part, is a little over whelming.

One thing I didn't think to check until right now, is one of the engine wiring harness connectors. A used motor was installed by someone else. I've already had to fix a connector at the transmission solenoid.

I've seen something similar on a different car.
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Old 07-20-2013, 07:29 AM
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Default Re: 94 Accord A/C Relay Help

Get the white wire fixed, and then try the things I've suggested. You'll find the problem if you methodically test each thing one by one. You're already off to a good start.
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