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Discussion Starter #1
Howdy all,
I do most of the work on our 7.3 Ex but... an A/C guy I'm not...hoping for a little advice. Please excuse the long post but wasn't sure which details would help. A few days ago we experienced an overvolt in the Ex; gauge went to 18+V and stayed there. Being on a DRT Texas hill country road we had to keep rolling for approx 5 minutes until we found a safe area to shut down. Long story short, after replacing the Alt and both Batts we discovered the A/C would not blow cold (it would before); front and rear fans worked but pushed ambient air.

After tinkering (summary below) I came to the conclusion that the pop-off valve at the bottom rear of the compressor manifold evacuated most of the 134a; the valve was wet and a mist was visible along the pass side VC, where it pooled in the bolt recesses the liquid was oily and green (the system was dyed/repaired back in 02-03 by the previous owner). So my questions are: would an overvolt condition somehow cause the A/C system to over pressurize? Perhaps if the high side switch terminal or associated wiring was damaged?

What should the voltage be at the high side terminal with the engine and A/C on?

Thank you in advance,
I had not planned on a major A/C refresh, but if that's what its gonna take so be it...it gets a little toasty around these parts...
Below is a summary of my "tinkering" with results:
1) A/C fuse good; A/C diode tested good but replaced with new from Ford anyway.
2) A/C clutch would not engage (air gap at .021).
3) A/C clutch did engage with jumping the low side switch.
4) High Side switch terminal voltage was 20-21 mV with engine and A/C on (yep, millivolts).
5) Low side pressure below 5psi with motor off and outside temp at 65-70F (tested with one of those cheap idiot gauges that come on the Autozone recharge cans, I could not measure high side).
 

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The answer to your voltage question is - the terminal at the a/c compressor should be battery voltage. So, around 13.6 volts with the engine running. The mV reading you are seeing is just induction voltages in the line. Not significant.

So- here's the answer to the pressure question. I don't believe in coincidences with automotive work, but we may have one here. I have racked my brain trying to come up with an explanation of how voltage can affect pressure. The best I can come up with is the dc evaporator fan would have been spinning faster which would have lowered your suction pressured. There might have been a mild drop in high side pressure as well. That wouldn't blow the high side valve. But, regardless of how it happened - here's where you're at:

1. Blowing the high side valve would have dumped the refrigerant. AND the oil in the compressor. That's the tricky part if you don't want to pull it apart. Normally, you would pull the compressor, dump the oil into a graduated cylinder and measure what was left. My guess is close to nothing. Then, you subtract that from what should be there and make up the difference. If you aren't going to do that, then I'd recommend adding 3-4 ounces of PAG 46 via an injector. The compressor usually holds 3-5 ounces of oil.

2. The fact that you still have positive pressure in the system (you said 5 psi) and that the leak was on the high side of the compressor tells us the system was never in a vacuum and wouldn't have drawn in non-compressibles (air). That's good news in that you could just fill the system without having to evacuate and vacuum. The residual pressure also shows the pop-off valve closed.

Now - that said - I wouldn't do it that way. First, you have to come up with why the system overpressurized. That would point to a restriction or lack of air over the condenser. The most likely restriction - especially in an FS-10 system - is going to be a clogged orifice tube. Your ex has an orifice tube up front an expansion valve in the back. I would use this opportunity to put in a new orifice tube. That will require opening the system, which would then require a vacuum to be placed on the unit before charging. I would also replace the high-pressure relief valve. It might be ok, but sometimes they get weak and are more prone to blow out or never completely seal right after the first discharge. You're looking for a Motorcraft YF52 valve. The rule of thumb is that anytime you open the system, you should replace the filter/drier. Considering how long your vehicle will last, I'd do that as well. You can get them on Rockauto for less than $30.

Where in Texas are you?
 

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Discussion Starter #3
RT, great info thank you! We're in the Boerne area.
Given your breakdown, I'm thinking a good set of gauges and vac pump, new high pressure valve, orifice tube, filter/dryer, and clean the evap; since I hate un-scheduled maintenance, I'm thinking a new compressor is good cya as well?
And just to clarify because I'm not familiar with the A/C lingo, the millivolt reading I got was from the high pressure valve/switch pigtail on the high press pipe, not the connector on the side of the compressor. If the high side pressure valve pigtail needs 13V while running, I should probably ohm that to ensure nothing got melted down. Would there be any reason to get into the condenser as well?
I recently went through and resealed the HPOP, Fuel bowl, all associated lines, turbo pedestal, every o-ring that was sweating dino, and the oil cooler... she still runs strong so if I have to get into the A/C, I don't mind doing all that should be done while I'm here.
Thanks a mil for your input.
 

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Well - I'm a bit too far away for you to run over and use my tools. Based on your mileage, I would recommend replacing the compressor. The FS-10 is notorious for "black death" - a condition where it chews itself apart and sludges the refrigerant oil. That's expensive because you end up replacing just about everything you can't flush. My first step would be to get the orifice tube out and inspect it. That requires a quick disconnect tool. I can take some pictures of where it is if you don't already know. Pull the old orifice tube out with a pair of needle-nose and inspect the screen. If there is minor debris, then I'd go ahead and order the other parts you listed. RockAuto tends to have the best prices. If the orifice tube is coated in thick black gunk, I think I'd recommend you go talk to the bank about a second mortgage and have someone fix it. Since you can't effectively flush the condenser or the evaporatorS (you have two) then, all that has to be pulled. All the lines have to be flushed. The expansion valve will need to be replaced. It can get overwhelming.

Hoping that's not the case. You can rent the pump and gauges from Autozone. To do this right, you'll need a refrigerant scale. Measuring the refrigerant in is the only way to get an accurate fill. If you do replace the compressor, make sure it is prefilled with oil. It should if you get any of them except the genuine Motorcraft service part. Skip the measuring oil part. That is a normal part of the routine when you open an intact system as it gives you an idea if the oil is distributed to other parts of the system. Your oil distributed out of the high-pressure valve. No way to measure that.
 

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I actually DO believe in coincidences. While I agree it seems that the high voltage incident caused the refrigerant system to vent, there are two things you cannot overlook.

The system not only has a pressure relief valve. The compressor clutch circuit has a "refrigerant containment switch" that electrically opens at 445 PSI and has a sole purpose of preventing a pressure relief event. The pressure relief valve opens at 550-600 PSI which if not defective or damaged would not open if the refrigerant containment switch does it's job.

High fan speed on the evaporator side would have it's greatest impact on the low/suction side of the system with regard to pressure. But let's say it did significantly cause the high side pressure to elevate. The containment switch is on the high side of the system and if it did it's job the system should not have vented.

The 7.3L does not have an electric condenser fan or an electric fan clutch. It has a basic viscous clutch which likely is weak... if you were traveling at speeds greater than 40 MPH the fan is not really effective anymore. Or so they tell us. At lower speeds a bad fan clutch would indeed cause the high side pressure to rise. Regardless, if the refrigerant containment switch had done it's job we would not be having this discussion.

My vote goes to the containment switch or the relief valve.
 

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So, FordDoc - if I'm reading you right, you're basically agreeing that there had to be a restriction or decreased air over the condenser as the only two causes of the high pressure? Right?

So, I agree checking your viscous clutch is a good idea. I think giving the condenser a good cleaning is also warranted. The new compressor will come with a new pressure relief valve. The high side switch is on the line going from the compressor to the condenser. That would be worth swapping. I still see no way that increased voltage could cause this.

Would you agree the best approach now is to replace the compressor, filter/drier, and the expansion valve - along with the high pressure limit switch?
 

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Discussion Starter #7
RT/Ford_Doctor, thank you both for the schooling on this and apologies for the late reply...been a looong day.
I'll have to find a quick disconnect tool and pull the orifice tube; I'm familiar with the location. I have a good set of gauges on the way, along with the above listed parts including a new Motorcraft compressor/clutch from Rock Auto. If it matters, our overvolt happened during acceleration from a near stop to 45 mph; outside temp was 35-40F, cab fan on 2 if memory serves. If I understand correctly, the refrigerant containment switch is internal to the actual compressor/clutch body? And the pressure relief valve, aka 'pop-off' is the one housed on bottom of the manifold bolted to the back of the compressor? Either way, a new compressor/clutch and pressure relief valve should take care of it, correct? The whole fan clutch/ speed/ high /low /pressure discussion is great info, it'll take me a bit to digest it. I'll post results on orifice tube.
Thanks again, y'all have been a great help.
 

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The genuine Motorcraft compressor is a service part and is usually shipped dry. You’ll need to ad 5 oz of PAG46 oil to that one. Verify it has no oil before doing this, but I’m pretty sure it will be dry.


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Discussion Starter #9 (Edited)
Thanks re the PAG oil. Attached are pics of the orifice tube, I got virtually no hiss when the pipe was disconnected, just a little puff...after seeing this I can't believe the A/C was putting any cold air out!
I'm a little afraid to ask but what's the verdict? Should I take out that second mortgage? The tan/white debris resembles cut hay or grass, no idea where that may have come from. Also, though it looks dark, it does not appear to be very thick on the screen when gently scratched with a pick.
 

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That's your restriction for sure, but that's not the black death. When you pull one of those, it is like tar. Personally, I would continue with your replacement, but I think I would have it evacuated in 6 months to a year and replace the tube again - just to catch any residual.
 

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Discussion Starter #11
You just made my day RT! Parts should be in next week, I'll update. And thanks a mil to you and Ford_Doctor for the input, its invaluable to be able to bounce unknown repairs off of those with more experience.
 

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OK - one more update. Most compressors have the pressure relief valve mounted on the compressor. I just remembered the FS-10 doesn't. It's mounted on the hose connector that mounts to the back of the compressor. So - you will need to order one of those (part number above) as well as the high limit switch (F5TZ19D594AA) to cover both of those as possibilities as to why the system didn't self protect.
 

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Discussion Starter #13
I've got the YF52 on the way, will add the F5TZ19D594AA - Thanks for the reminder!
 

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Discussion Starter #14
Apologies for the late update. Purchased new Motorcraft high pressure switch, high/low schraders, pressure relief valve, orifice tube, expansion valve, complete oring set, drier, and compressor w/clutch from Rock Auto. The compressor came pre-loaded with 7oz of Pag according to the tag. Cleaned the condenser and decided to pull the evaporator for a good cleaning as well. For those who might attempt the evap removal on an Ex with aux heat/air, read on.... for those who have a more stylish shortcut than this first timer, please reply because I found this job to be a giant PITA.
The aux heat and AC lines run between the evap case and passenger side VC/head. Problem 1: In order to physically get the evap/cover out, I had to separate the aux heater hose from the T-connection near the firewall and move the hose out of the way. Not sure if draining coolant is necessary because I had mine down for a water pump at the same time. Problem 2) the three screws on the bottom of the evap case are obstructed by the aux AC lines, they do not move and obscure your view of the screws. I ended up using a mirror to view and get wrenches on the offending screws... reattaching the screws was another dance I do not want to do again anytime soon.
Most of the schmutz on the evap (photo attached) cleaned off ok with warm soapy water and a stiff toothbrush, but I found a thin film between the fins which would not budge. Not sure what that film was but I decided sourcing a new Four Seasons evap from Autozone was the better part of valor (please see whining about PITA above).
Pulled vacuum for 2.5 hrs, probably overkill but with the aux AC I wanted to be sure. The system held vacuum for 2hrs so I called it good. After adding 7oz PAG and 68oz of 134a, one can hang meat in that rolling barn, both front and back. I never did identify the ultimate failure point which caused the pressure relief valve to let go. AC has held for a week now so we'll see...
Thank you RT and Ford_Doctor for your help and imput.

30f
 

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Great work. If you pull the tire and right front fender liner, those lower screws are right there. Sorry about that.


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Yes, good job and I am happy to hear you figured it out. This is why I still believe in coincidences. ;)
 

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Discussion Starter #17
Great work. If you pull the tire and right front fender liner, those lower screws are right there. Sorry about that.


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Gonna chock it up to payin' my AC dues....:cool:
 
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