A/C Clutch cycles off/on above Idle.

Discussion in 'Fox 5.0 Mustang Tech' started by Madrid91, Jul 29, 2006.


  1. Madrid91

    Madrid91 New Member

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    In the HOT HOT HOOOOOOT Vegas desert. A/C is not blowing ice cold. Checked recharged system, but A/C clutch stays engaged at idle but only runs for a few second intervals if rpm is above 2,000 any ideas on cause/fix? Figure clutch is not staying engaged enough to move the freon.

    A/C was evac'd/recharged and converted to 134A as I though the system might have been low, but that wasn't it. All original parts and issue remains.:bang:

    92 5.0 GT 141K.
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  2. Madrid91

    Madrid91 New Member

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    92 5.0 Mustang GT
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  3. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Possible Low Pressure Cutout Switch problem. The LPCS is mounted on the accumulator/dryer that is the aluminum canister mounted on the firewall. You would need a set of charging gauges to tell what the pressure readings are on low & high sides to make an accurate diagnosis.
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  4. PuterAmI

    PuterAmI New Member

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    And one common problem on a Fox that causes these problems is the WOT relay. It is a small rectangular box that is mounted on the front of the passenger side shock tower. Check the wires leading into it. The insulation is usually burnt and this shorts the LPCS that jrichker is talking about.
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  5. Madrid91

    Madrid91 New Member

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    Thanks. Will check WOT Relay . I am running within range 28-38 low and 250-300 High (According to the 134 conversion instructions I found in the forum). Just got online today and didn't check the WOT relay. But did replace the accumlator/drier (line was included to compressor). Flushed everything re-oiled the Compressor and test at night was it was blowing about 40 degrees while driving, just the constant cycling is still happening when not on idle.

    Will let you know. :hail2:

    Final test will be to take it back out in the middle of the day to see if it can keep up with the heat...
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  6. Madrid91

    Madrid91 New Member

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    No luck on finding the WOT relay. Front passenger side Strut tower has Mass Air Flow sensor and back side has 3 cylindrical type relays that look good. Book is still packed in boxes so I don't have a diagram to follow. Does anyone have a police evidence photo of the WOT Relay?

    Thanks for the help.
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  7. 90mustangGT

    90mustangGT I felt sorry for girls because Founding Member

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    Have you checked high and low side pressures? The system could be overfilled, but if the problem recently started it probally isn't that. A restriction on the low side or not enough refrigerant will cause a pressure drop on the low side. If there is a restriction on the high side, then it will cause the pressures on the high side to rise too much and most likley the low side as well. Find out your pressures on both sides. Also, check to see if the switches are cutting off with a test light.

    The orifice tube is the most common location for a clogg. Not saying that is what it is.
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  8. PuterAmI

    PuterAmI New Member

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    That is very strange.

    I am assuming we are talking about the 92. Follow the wires back from the LPCS. They lead to the relay.

    You do know that depending upon ambient temperature and other factors that it should cycle, right? The LPCS turns the compressor off at a certain pressure seen in the low side.
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  9. Madrid91

    Madrid91 New Member

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    Yep Puter on the 92. Issue was that when it was R-12 thought was low, retrofitted to 134A with same history, the last weekend did a flush and replacement of the accumulator/Drier, re-oiled and recharged (134A). I just don't recall seeing a A/C when running at around 1,500 to 2,000 RPM cycle so often like every few seconds it is off,on,off, on (no more than 15 second intervals, air is getting cold but doesn't seem like it is getting as cold as is could becuase of compressor cycle. It all started with with me doing down the freeway a while back when I was heading back from California, and was running 85-90mph up loooooong hill, Engine Temp went to 3/4 in the normal range and the A/C started blowing hot. Didn't stop until temp when down to 1/2 mark in the normal temp range. Until then I never had recharge the system or given attention to what the compressor was doing, But it when I started checking and saw compressor cycle, have been working on it off and on ever since car never seems to keep up with heat when before than I remember having to turn the fan off of high because it was cooooooolllld.
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  10. Madrid91

    Madrid91 New Member

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    OK, low was 28, High was between 150 and 350 from idle to 2500 RPM.
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  11. Madrid91

    Madrid91 New Member

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    OK, think I found the WOT. It sits under the bracket for the mass air flow Sensor. Will take apart and check.
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  12. HISSIN50

    HISSIN50 "How long does it take to get help in here? SN Certified Technician

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    That sounds right. Your round plugs sound emissions-related.

    Your '92 might have the fuel pump relay next to the WOT relay as well (earlier on, the FP relay is under the driver's side seat).

    Good luck.
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  13. PuterAmI

    PuterAmI New Member

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    You would also want to verify that the system is being shut off at ~18-21PSI. Some folks forget that when converting from the Fox R12 to R134A.
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  14. Madrid91

    Madrid91 New Member

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    Yep, there is a second relay next to the WOT. Will have to hook up gauges to check again. Contacts looked clean as did wiring. Do you know if they kind of interval is normal? (A/C cylcling). Don't recall seeing the low end drop that low, just the high end moving around between idle and higher RPM.
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  15. Madrid91

    Madrid91 New Member

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    OK, puter, I looked at your conversion web page and saw the part about adjusting the screw on the LPCS. I never touched it (Used another confersion doc that I had found on the forum). So I presume it is sitting at the original settings of around 25. Which way to lower the pressure cut off clock or counter clock 1/4 for every 3PSI? In-law has the gauge and won't be there until this weekend. would like to at least make a 1/4 turn to see if it makes a difference. Thanks for the info.....
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  16. PuterAmI

    PuterAmI New Member

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    CCW lowers the cut-off pressure.

    Your cycling is indicative of a low charge. Gauges and a vent thermometer are the way to troubleshoot and fill an A/C system. Also remember that when you are close, only add ~2 ounces at a time.
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  17. Madrid91

    Madrid91 New Member

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    Agree Puter, but we have pumped a lot of R134 in this thing and the cycling never went away when running above idle, replace evaporator and still above RPM it cycles. If I leave it at idle it cyles normally. I am suspecting that because I didn't lower the LPC that I will continue to cycle above IDLE because the change in pressure with R134. What would you say is the absolute MAX amount of freon 134 you would Add to the system? Also, Looks like I didn't check for leaks and the system purged it all out this week while it sat. so I got to charge, check for leaks and charge back up. What would you say is the Optimum High and Low to be sitting at? Will try to do that tomorrow as well as see above idle where it is cyling at on the low end and adjust the low end that has not been touched.

    I just want to make sure not to overcharge......
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  18. PuterAmI

    PuterAmI New Member

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    See my writeup. Pressures in there.

    You really need to fix the leak first. Leaked out after a week is a big leak. The system should stay good for years. It could be that once you raied the high side pressure at 1800RPM, it has enough force to leak right back out and your low side starts dropping down to the cut-off pressure point.

    A Fox system takes a 42 ounce charge of R12 and oil total. That's 8 ounces of oil and 34 ounces of R12 (Freon). With R134A (SUVA), you want to only charge ~85%. I usually end up with ~26-28 ounces in a system once I know how much oil is in it.
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  19. Madrid91

    Madrid91 New Member

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    Found Leak (LPCOS) O ring. Only had to recharge some, system was still under pressure. Set LPCOS to turn off at around ~18-19 which made a difference was cutting off at around ~23-22. Charged to Low and High Settings as you said ~25-30 Low 225-250 on the High. Was cooling off at around 38-40 degrees at Idle. Haven't had a chance to get it out in the heat for a extended period but to see if it now can cool down and keep up with the Las Vegas weather again.....planning will let you know. Cycling has reduced with the adjustment to the LPCOS. What is the ideal RPM to see what it would perform at normal operational load RPM 1800-2200? When moving past idle to simulate normal easy acceleration, do you have a example of how fast the Low side will drop when you hit a little RPM but not revving it like you were trying to pass either? Just wondering how do you tell if your low is restricted at all and contributing to reduced cooling performance.

    Thanks for all the info I think I might be there, but can't pass on the great info. Your Docs were right on the money Puter......:hail2:
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  20. PuterAmI

    PuterAmI New Member

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    Thanks, Madrid.

    You would definitely know if there was a restriction. The pressures would shoot up/down (low/hi respectively) very quickly and your cooling performance would be terrible, even non-existent. You are getting decent temps (depending on ambient), so I would say that you do not have a restriction.

    I don't have a ramping of pressures. I've eyeballed it, but not made a good comparison. You could probably ask on the AC Forum to see if anyone has some insight.
    #20

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