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Thread: A/C Questions: Thermostatic Switches and Pressure Switches

  1. #1
    Garrett booster
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    A/C Questions: Thermostatic Switches and Pressure Switches

    I have an '85 Dodge 600 that uses a thermostatic switch on the low side (big) hose next to the firewall expansion valve. I was thinking of changing to an '86-'89 hose (the earlier one is no longer available and I'm restoring the system) but the newer hose has a fitting for a pressure switch in the that area instead of a place for the capillary tube of the earlier thermastatic switch.

    I'm far from being an A/C expert but know basically how systems works. These switches have the same general function, I believe, to tell the compressor when to cycle on and off. They both have two leads connecting them and both seem to be single pole, single throw switches.

    If I would replace the hose that has a thermostatic switch/capillary tube with a newer hose with a pressure switch, would the system work? Or are the electronics completely different? Why did they change?

    Thermostatic Switch:

    Pressure Switch:

  2. #2
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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    What I would do is pull up the wiring diagram for each type and compare where the "signal" goes for each type. My guess is it is a simple ground. If that is the case, then you can wire in the newer pigtail and fit the new type. The only issue I would worry about, and probably isn't an issue, is the sealing type/surface. I think they all used the same H-valve, though, so, shouldn't be an issue.

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    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    They serve two different purposes. The low pressure switch only kills the system when the pressure is below 20 give or take PSI. The thermostatic switch is supposed to cycle the compressor on and off based on the temp of the system so you don't freeze up the evap core. I bypassed the thermostatic switch in my minivan because I couldn't find a new one. I just had to cut two wires and splice them together.

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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    Thanks guys!

    Reaper 1 said "The only issue I would worry about, and probably isn't an issue, is the sealing type/surface."

    Not sure what you mean by that. Explain?

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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    I meant on the hose assembly to the H-valve. I don't think it's different, but I wanted to cover my bases.

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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    Reaper 1: The H valve (expansion valve) is the same on cars that use either hose. I'm replacing the H valve as it looks gunked up.

    I have a set of '85 service manuals so I checked the wiring diagram. It shows the thermostatic switch and also a low pressure cutoff switch in series with it. That''s screwed into the H-valve and is there on my car - I just didn't notice it.

    I have a service manual for '89 minivans but no wiring diagrams. The valve in question on the newer models they call a damped pressure cycling switch (Four Seasons and UAC call it a clutch cycling switch). It's function is to turn the compressor on/off depending on the pressure in the suction line and is hooked up to the compressor clutch. The '89 manual also says a low pressure cutoff switch is in series with the damped pressure cycling switch.

    So it seems like the difference between the '85 model and newer cars is that the '85 uses a thermostatic switch to cycle the compressor and the latter cars use a damped pressure cycling switch to do the same thing and is wired in the same way.

    I think at this point I'm going to clean up the outside of the original hose and give it a good internal flush. Then I'm going to check that no chunks of the inner lining are coming off in the flush process. If all seem well, I'll use it and hope for the best as it probably isn't a barrier type hose.

    Another option would be to have the original 're-hosed' but I'd have to investigate the cost of that and find a shop to do the job.

    It seems that I could replace the hose with a '86-'89 unit and use that 'damped pressure cycling switch' but these switches are hard to find. Maybe I'll grab one from a bone yard some day if I can spot one.
    Last edited by dodgeboy77; 08-09-2020 at 01:49 PM.

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    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    There was a pretty good discussion about making hoses in the big thread on AC. Seems like it takes about $150 to get the crimp tool and from there you can make your own with the stock ends if you need to replace the rubber pieces.

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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    I read that thread, and you're right, it had lots of good info. Since I only need one hose (actually two hoses in one assembly), I wanted to see if I could find a local outfit to do it for me for less than $150. I'm still not convinced an '86-'89 hose wouldn't work if I could find one of those pressure cycling switches.

  9. #9
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    Re: A/C Questions: Thermostatic Switches and Pressure Switches

    I just got off the phone with a guy from a big auto A/C firm in Trenton, NJ. He was very helpful. He said that though a hose in an '85 Dodge wouldn't be a 'barrier' hose, with the mineral oil soaked into the rubber, it becomes one. He also said it would be unusual if the inside rubber of the hose would be brittle and cracking off. So I guess I'll clean it inside and out and go with it. He said to use ester oil.

    As an aside, he mentioned that his firm has a lot of R12 available. He said a lot of guys with older, classic cars want to continue to use R12 so they have the original fittings on the system for car shows. I was afraid to ask what they charge for R12. ;-)

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