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Thread: A/C Diagnostics & Tools

  1. #1
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    A/C quit in my 1990 Westy one day after returning to Texas upon completion of a 7,000 mile trip. Had been having problems with proper idling with the A/C on, but had chalked that up to the van needing serious exhaust work (Tried to get it done on the road, but timing to order parts wasn't viable.) In response, last two days of the trip the wife and I had gotten in the habit of switching off the A/C whenever we came to a stop--not a difficult habit to acquire given we have always felt obliged to turn it down to keep from holding up lines at traffic lights in the city. When the A/C quit we were in motion (coasting toward a stop, I think), fan at high (4) and the compressor switch was probably set mid range.(Perhaps not "closely matched" as Cpt. Mike suggests, so might have tripped the high pressure cut-off switch? But then wouldn't the problem quit after the van had not been run for a couple hours?) Suddenly, nothing. No fan. Nada. I'm presuming the problem is not with the compressor since it was replaced about a year ago and when it died then the fan still worked, which is not the case in this instance. I've checked the easy fuses (1 and 12) and both are fine. I'm hoping that one of the other fuses listed in the Bentley may be to blame (wishful thinking?) but need help locating them:

    1) According to Bentley, "Fuse S" is behind the left (driver's?) air vent in the instrument panel. Presumably this is the vent just above the fuse box? Is it easier to drop the fuse box/harness or to remove the vent (I worry about breaking the tabs)? Also, where is this "Fuse S" referenced in the wiring diagram? I can't seem to find it looking over sections 97.143-145.

    2) Next, and again according to Bentley, there are two more fuses (Fuse S0 and Fuse S51) located "behind left rear 'D' pillar cover." Does this mean I need to remove the back cabinet to get to them? How difficult is this?


    If the fuses check out, what's the best way to proceed?

    Another obvious place to check are the switches which have probably gotten overused. Excuse my ignorance (I'm a extreme novice when it comes to auto mechanics) but is there an easy way to eliminate a switch as the cause?

    Lastly, I'm wondering if the problem might have something to do with the Idle Speed Stabilizer Valve (not that I would know what that is if I were looking at it!) or perhaps the Radiator cooling fan relays. Again, my apologies, but how does one test a relay?

    Appreciate any advice/suggestions. Would like to take a stab at finding the problem myself.

    Thank You,
    David
    Last edited by Capt. Mike; 01-15-2009 at 07:45 AM.

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  3. #2
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    To begin with a comment about your practice of cutting off the A/C at stoplights . . . Don't! This may have caused your problem. When you do shut it off, the system is at full pressure. It takes a while for the system to bleed down & equalize. When you then restart quickly, it is trying to start against a full head pressure and puts tremendous loads on the compressor & electrics. This may have blown your fuse(s) if you are so lucky that's all it did.

    If the A/C did not restart after a couple hours with engine off, the problem is NOT likely related to the high-pressure cut-off or fans. Check the compressor activation by watching the compressor clutch. If the system is activating the clutch, it will spin with engine speed (whole assembly turns, not just pulley). If the problem was the A/C activation of the radiator fan, the A/C would still work initially until if finally tripped the high-pressure cut-out.

    Don't get wrapped up in where fuse locations are by the Bentley text sections. In this case, the diagnosis section is written for '86 up; the wiring diagrams are for specific years and changed in '89. Add that mid-production changes were common. Trace your wiring diagram backwards to fuse locations if you can't find it quickly.

    "Behind the left D pillar" on a Westy means behind the vertical tunnel in the rear-most closet. Remove the shelves by prying out the upper retainers -- they are basically nails with a plastic head -- and then unscrew the two L-brackets holding the inner cabinet box in place. Behind there is a module that contains your big S-0 50a fuse which is an exposed melting link, not a regular fuse. Caution: This fuse becomes very hot and can cause a fire if you let anything touch it during operation.Be extra careful on reassembly of the cabinet that you don't allow any wiring or insulation to be near it.

    Switches are a low probability. You can independently test all but the thermostat switch with a volt-ohmeter. The thermostat switch would require artificially changing bulb temperature, but many (can't swear by VW's) bypass the bulb on maximum and would provide continuity at that setting.

    The idle-speed stabilizer senses the increased load created by the A/C in relation to the engine operation, but is not directly controlled by the A/C. i.e. the A/C pulls the idle down, so the stabilizer tells the CPU to adjust to bring everything back into balance.

    Relays can be tested with a volt-ohmeter or even a continuity tester (light). One of the better test lights for this function is kind that will clip onto a wire and pierce it just enough to get contact. You must understand the current flows from the wiring diagram and intended purpose. First test the activating circuit current. With the relay activated, then test the outputs. For example the A/C relay has two functions. One is to send power from the evaporator fan switch to close the relay and send the much higher power to the blower itself. A 2nd power input from the thermostat sends the much higher power to the various control units such as the compressor clutch. One is interlocked to the other -- you can't get fan power without the thermostat power being on.

    Relays are nothing more than electrical switches, using one power to open or close other circuits. Fortunately, the Bentley wiring diagrams show the connection numbers and internal routing.

  4. #3
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    Transferred from another post to consolidate similar topics.

    Air conditioning performance & specs

    rji Junior Member # 1488 posted 07-23-2001 11:33 PM

    i wonder if anyone could provide me with the hi/low pressure values at say 3000/rpm at ambient temp (say 80 f) for the a/c system on an '88 westy. i think the charge in the a/c is a little low, i think i may need to add 6oz. of r12 or so. also, does the a/c cool the van well in 90+ heat if operating correctly. my van takes a very long time to cool down. i posted these same questions the other day and suddenly my post is gone! still hoping for some info. thanks randy.

  5. #4
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    The reason VW doesn't "publish" all specs in the Bentley is the subject is big enough for its own manual. They DO publish the pressure switch setting. Murray Gold Seal, one of the biggest in A/C parts & rebuilding publishes a manual -- my old copy was P/N 209590 -- that does have tables. But it depends on system, compressor and many variables. It's way too big to post here, but I'll try to post a copy on my pics site linked from the home page.

    Done 7/24/01; look in Tech Drawings folder, title Murray A-C Pressures.

    The numbers vary hugely. The table is for a "typical" factory system at 2,000 RPM. With the oft repeated warning the numbers will vary from vehicle to vehicle. Table ranges of LP run 28-46 while HP runs 170 - 385. They are broken down by relative humidity & air temperature because of the combined effects of both on discharge outlet temps & head pressures. Reading the system to estimate "how many ounces" a system might need is nearly impossible.

    Since most only work on the A/C during the hot summer, I'll post that at 80°F, with 90% relative humidity -- common enough in my shop during the summer -- the table calls for an LP of 32-38 psi and HP 270-325 psi. I rarely get those numbers. I'm happy at LP <40 & HP >225. The corresponding outlet temp of 51-60°F is optimistic!

    In 'topping off' a normally healthy system, watch your sight glass on the receiver-drier. It should not "foam". On the other hand, if always clear, you've probably overcharged. I like to see the occassional bubble when RPM is changed. Some bubbles at idle is OK, dropping to just the occassional bubble as the engine stabilizes at 2,000 RPM, and then returning to the bubbles when dropped back to idle. (Bubbles in liquid -- not foam as on a beer.)

    The Westy A/C is at best, a crude system. The extremely long piping and high number of fittings means there will always be losses, even if just osmosis through aging hoses. A can a year is considered good! Especially since very few will follow the recommendations of running their A/C for a few minutes EVERY WEEK, even in winter, to keep the seals tight and lubricated.

    As to effectiveness, it's never been good. The outlet temp might be decent, but the ability to circulate from the rear to the front cabin is non-existent. Elsewhere on the site are numerous posts about circulation, shutting off outside air (always on in a Westy) and adding front cabin fans to assist. Even so, the rear of the Westy will always be much cooler than the front -- often to the point of discomfort.

    The reason your previous post is "gone" is because you didn't follow the Message Board Guidelines (nor this time). You were emailed the reasons and how to correct. Site administrators don't always have time to move posts for those that don't follow the guidelines.
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    Last edited by Capt. Mike; 01-15-2009 at 07:47 AM.

  6. #5
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    Help!!

    I can't seem to find the ac clutch relay. Bently say it should be behind the d-post. In my 86 syncro westy there is no cabinet back, just cloth, with the hoses and wires behind. I can't feel anything though the cloth so I don't think the relays and fuses are there.

    Anyone have any ideas?

    Icarus

  7. #6
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    Your shelved closet closest to the rear SHOULD have a second, removeable verticle section. The A/C relay is located behind that. The relay fits into a larger distribution box/panel that contains other components. Due to the high heat of the A/C's bar fuse, nothing can contact the distribution panel.

  8. #7
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    Hi,
    My A/C only seems to work with the fan on either #1 or #2. When I turn the A/C fan to #3 or #4 it shuts off.
    I don't know if it is connected, but it coincided with my alternator dying.
    I have an 89 westy with new compressor
    Any help is appreciated
    Thanks

  9. #8
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    Test the fan series resistance (pic Bentley 87.5 #87-91 with a standard volt-ohm meter. Cicuit 15, wiring diagram 97.138, shows resister -- the evaporator switch runs from it, so failure of one shuts off other for safety.

  10. #9
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    First, check your wiring between the switch and evap series resistance block. Previous owner could have got them disconnected or miswired. They should all be 12v live or near zero ohms in continuity mode; following those wires (BK/G, BK/W) Bentley 973140-7-9 will lead you to the block. W will lead you to the A/C relay.

    Then test voltage from the block between terminals 1 & 4 for low and 2 & 4 for medium. You should see significant drop in voltage live & increased ohms in continuity mode. Voltage between 3 (from relay) & 4 should also register a drop in voltage live or increase in ohms in continuity mode as it appears from the Bentley that there is still a resistor in the high mode. I can only hazard a guess but it may be to reduce voltage to 12v to avoid overloading the motor since the system normally puts out 14.5v charging during engine running.

  11. #10
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    Default A/C hot wires?

    Capt. Mike, you've cautioned us about the hot S0 fuse/link and the need to make sure nothing touches it - very important information in light of a recent minor burn in mine! I've also noticed that some of the wiring around the connection plugs can get VERY hot - too hot to touch. Is that normal? (That much heat back there me nervous!)
    With thanks.
    Last edited by Capt. Mike; 06-07-2010 at 12:12 PM. Reason: Email blocked

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