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E36 M3 (1992-1999) {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999


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Old Mon, Oct-30-2017, 01:46:15 AM   #1
LockDots
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Default Not your typical A/C (HVAC) question.

Weird AC question...


If the AC Compressor relay is missing, but the car is running with the AC turned on in the HVAC controls, can anything be damaged?
I'm thinking that because the compressor isn't on, refrigerant isn't flowing and the system overall isn't being used, despite being commended to be.


There's a reason behind this, which I'll explain later.
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Old Mon, Oct-30-2017, 03:16:15 AM   #2
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Default Re: Not your typical A/C (HVAC) question.

No. Your aux fan would be on all the time though (when HVAC a/c is on). I used to do this chasing horsepowers removing the a/c belt in the cold season.
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Old Mon, Oct-30-2017, 04:14:51 AM   #3
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Default Not your typical A/C (HVAC) question.

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No. Your aux fan would be on all the time though (when HVAC a/c is on).

…as long as the aux fan is known to be in good working order.



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Old Mon, Oct-30-2017, 04:46:40 AM   #4
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Default Re: Not your typical A/C (HVAC) question.

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…as long as the aux fan is known to be in good working order.



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Okay.
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Old Mon, Oct-30-2017, 02:47:23 PM   #5
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Default Re: Not your typical A/C (HVAC) question.

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No. Your aux fan would be on all the time though (when HVAC a/c is on). I used to do this chasing horsepowers removing the a/c belt in the cold season.
Ok, that's what I figured. I tested it quickly last night, just to confirm my expectations (which is exactly what you said).

I was asking because of a similar reason to yours. I've got my first Autocross event coming up this Sunday and was thinking of a workaround for keeping the fans on constantly throughout my run, without having to wire in a manual override switch.

I currently have my electric puller fan [temporarily] triggered off the radiator switch's low & high outputs. When I turn the AC on, there's a ground sent from the ECU to the AC compressor relay's input, which is tied into the radiator switch's output. I figured, as a quick and dirty trick, I could just pull my AC compressor relay when I get to the event, turn on the AC system to trigger both my puller and pusher fans at the same time, and then just put the relay back in when I'm done. This way I get constant maximum air flow without the drag of the compressor.

Once these two events are done, I'll wire in a PWM controller and manual override, but this should do fine for now.
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Old Mon, Oct-30-2017, 03:37:02 PM   #6
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Default Re: Not your typical A/C (HVAC) question.

The aux fan will not be on all the time. The aux fan is either triggered by the dual temp switch or the A/C's pressure switch.

If the compressor isn't running (as per you removed relay) and you temp's don't rise too high to trip the dual temp switch, with the AC being off, the pressure switch won't trip the AUX fan relays neither.

So nothing will happend, but a lit LED on the climate control display.
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Old Mon, Oct-30-2017, 04:16:57 PM   #7
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Default Re: Not your typical A/C (HVAC) question.

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The aux fan will not be on all the time. The aux fan is either triggered by the dual temp switch or the A/C's pressure switch.

If the compressor isn't running (as per you removed relay) and you temp's don't rise too high to trip the dual temp switch, with the AC being off, the pressure switch won't trip the AUX fan relays neither.

So nothing will happend, but a lit LED on the climate control display.
I don't know about that. It worked for me last night.

I removed the relay and started the engine. Then I turned on the AC (via the button in the HVAC controls) and was able to see both my fans turn on.

That being said, I didn't run it very long. Maybe 30 seconds or so. I'll have to try it longer tonight.
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Old Mon, Oct-30-2017, 05:12:19 PM   #8
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Default Re: Not your typical A/C (HVAC) question.

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I don't know about that. It worked for me last night.

I removed the relay and started the engine. Then I turned on the AC (via the button in the HVAC controls) and was able to see both my fans turn on.

That being said, I didn't run it very long. Maybe 30 seconds or so. I'll have to try it longer tonight.
That does surprise me as neither your temp nor pressure switch is activated without the AC relay.
The button on the climate control is just that, a button to activate the AC relay->comperssor.

I mean, I doubt your engine's coolant temp was triggering the aux switch.
You could unplug it to remove it from the equation.

Could US cars be wired differently?
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Old Mon, Oct-30-2017, 05:31:23 PM   #9
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Default Re: Not your typical A/C (HVAC) question.

You know what, since it's easy to check, I'll pull the relay from my car and will turn on the AC and see what happens.

If my fan kicks in, I'll dive into the ETM again. I only recall there being the 2 triggers I mentioned in my other post.

I've only once in 11 years had my aux fan kick in without the AC on and that was when it was idling for a long time in hot weather.
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Old Mon, Oct-30-2017, 05:44:51 PM   #10
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Default Re: Not your typical A/C (HVAC) question.

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Originally Posted by LockDots View Post
Ok, that's what I figured. I tested it quickly last night, just to confirm my expectations (which is exactly what you said).

I was asking because of a similar reason to yours. I've got my first Autocross event coming up this Sunday and was thinking of a workaround for keeping the fans on constantly throughout my run, without having to wire in a manual override switch.

I currently have my electric puller fan [temporarily] triggered off the radiator switch's low & high outputs. When I turn the AC on, there's a ground sent from the ECU to the AC compressor relay's input, which is tied into the radiator switch's output. I figured, as a quick and dirty trick, I could just pull my AC compressor relay when I get to the event, turn on the AC system to trigger both my puller and pusher fans at the same time, and then just put the relay back in when I'm done. This way I get constant maximum air flow without the drag of the compressor.

Once these two events are done, I'll wire in a PWM controller and manual override, but this should do fine for now.
You are over thinking it. Unless something is wrong, you aren't going to have over heating issues during an autocross run, especially not in fall weather. On a longer track sure, but a 30-60sec autocross run isn't going to cause any problems. I would just go enjoy the event, then do whatever switch when you have time.
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Discussing Not your typical A/C (HVAC) question. in the E36 M3 (1992-1999) Forum - {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999 at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)