Any Auto AC Experts out there?

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disneyd
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Any Auto AC Experts out there?

Post by disneyd »

The Miata's (1999) air conditioning is acting up and I wanted to get some opinions on where to start. Yes, I could just pull it out of the car and save some weight, but I kinda like having it available.

Anyway, I have a set of gauges, but I wouldn't say I'm proficient at knowing what they're telling me. So, any pointers there would be appreciated.

The AC will work at first, but will stop blowing cold air at some point. Then, 5 or 6 minutes later I will hear a hiss/flowing sound behind the glovebox (I'm assuming this is the expansion valve) and then the AC will work again for a bit, then repeat. The warmer it is outside and if I'm running a higher engine rpm (say cruising at 4k vs 3k) both make the problem worse (the AC quits working sooner and takes longer to come back online).

Any ideas? I'm going to put the gauges on it tonight and see if I can tell if the pressures are low or high, but I've never seen this set of symptoms on a car before.
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Re: Any Auto AC Experts out there?

Post by jcox07 »

Sounds like it is just low on freon. Any auto parts sells the cans of freon with the gauge on it and hook it up with the car running and ac on max high fan and if it does need freon you will know.
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Re: Any Auto AC Experts out there?

Post by MikeKelly »

Skip the gauges.

1- When was the last time it had refrigerant put in it? - Greater than 1 year - proceed to #2.

2- At idle, after 2 minutes (fan/AC on high) is the suction line + accumulator warmer than "beer on ice"? - If yes, proceed to #3.

3- Compressor cycling a lot + mostly off (AC on high + 2 minutes idle)? - If yes, proceed to #4.

4- Add 1 cam proper refrigerant to suction line (following directions on autozone kit). Proceed to #5.

5- Same checks as above, condition improved?


The front seal on automotive AC compressors is never "perfect" - Adding refrigerant occasionally is normal.

Never add more than a couple of cans (~2lbs) without asking additional questions.

Any "no" answers, stop + report back.
Last edited by MikeKelly on Thu Jun 23, 2016 4:59 pm, edited 1 time in total.
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Re: Any Auto AC Experts out there?

Post by MARKP »

Agreed with Mike. The symptom you describe sounds like low refrigerant.

On the same subject, different car, my 1993 Camaro's AC works for a few minutes but never comes back to cold. Adding refrigerant doesn't seem to help. Any thoughts on that one?
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Re: Any Auto AC Experts out there?

Post by MikeKelly »

MARKP wrote:On the same subject, different car, my 1993 Camaro's AC works for a few minutes but never comes back to cold. Adding refrigerant doesn't seem to help. Any thoughts on that one?
1993 Camaro + symptoms described = trash in orifice screen / and or wrecked compressor / and or moisture in system.

Most likely suspects.
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Re: Any Auto AC Experts out there?

Post by disneyd »

Youse guys were right. Low refrigerant due to a leaky schrader valve on the low side.

I think it was on the edge of tripping the low pressure switch so that's why it would go back/forth between working or not.

I had to add 8 oz of 134a (21 oz capacity) and it is nice and cold and stays running. Actually, I'm worried it may be too cold as I am seeing sub zero temps coming out of the center vents while cruising on the hwy. I am getting the shop manual approved temp of 38ish while holding it at 2k rpm in the driveway.

Too much charge usually decreases efficiency and makes them run warmer, right?


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Re: Any Auto AC Experts out there?

Post by oldmuscle69 »

MARKP wrote:Agreed with Mike. The symptom you describe sounds like low refrigerant.

On the same subject, different car, my 1993 Camaro's AC works for a few minutes but never comes back to cold. Adding refrigerant doesn't seem to help. Any thoughts on that one?
Expansion valve or THE leak, (maybe internal comp fail) did you put some of that crap in with the dye?
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Re: Any Auto AC Experts out there?

Post by oldmuscle69 »

A/C in my DD is also dead 306K I think I might to recover parts from a lower mile car and see how goes along with a few new parts.
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Re: Any Auto AC Experts out there?

Post by MikeKelly »

disneyd wrote:
Too much charge usually decreases efficiency and makes them run warmer, right?
If it's got an expansion valve, it shouldn't be too "ticky" about charge - A small overcharge will just end up as more liquid on the high side.

A big overcharge would run up head pressure and nothing good is coming from that.



You do have to be especially careful with overcharge on a fixed restriction (orifice) automotive system - That can cause liquid refrigerant to "flood back" to the compressor.

Multiple catastrophic problems there - washing the oil out of the compressor bearings / liquids can't be compressed / etc.


Given common automotive AC service procedure, it should be "completely totally unpossible" to significantly overcharge a noticeably undercharged system with one 8oz can of refrigerant.
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Re: Any Auto AC Experts out there?

Post by thrdeye »

Leave it to Dave to fix something more than you are supposed to.

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Re: Any Auto AC Experts out there?

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[PERSEVERING FACE]

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Re: Any Auto AC Experts out there?

Post by thrdeye »

My phone is bonkers

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Re: Any Auto AC Experts out there?

Post by SeanDrives4Fun »

Mike this the most knolagable post ive seen anywhere for a while! So... 02 miata says its good on charge by can, works for 10 min the starts off on off on i very quick itrerivals. Any ideas?
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Re: Any Auto AC Experts out there?

Post by MikeKelly »

Look for a flakey AC relay input (voltage fluctuating across coil) or output (voltage fluctuating on black wire going to thermal protector).

Check flakey voltage on the black wire from the thermal protector to the AC clutch.


Most likely suspects.

Most likely the thermal protector (flakey) - if the illustration linked below is correct (lower left of drawing).


http://www.miataforumz.com/1990-200-Mia ... System.pdf
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Re: Any Auto AC Experts out there?

Post by oldmuscle69 »

Is it fixed yet?
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