skrap

I've owned the truck around 4 years. The records for the truck show the previous owner had it converted to r134a. I don't know of any other AC history. I assume it is the stock compressor.

When I bought the truck the AC wouldn't blow cold air. The local AC shop replaced the burst disc and filled off the air and coolant. It has worked off and on for the last four years (light use of the truck, not my daily driver).

I was hoping the intermittent cooling was something with the relay or cycling switch. In the process of troubleshooting the intermittent operation the burst disc blew and spewed all the coolant out. I assume that was the loud air blowing noise I heard.

I'm wondering if I should just replace the entire AC system at this point. I was thinking driver/accumulator, condenser, compressor, expansion valve, and all orings. Should the hoses and evaporator be replaced?

Does anyone have any advice on what to troubleshoot or replace? Any brand/store recommendations? I'm planning to do the work myself to save some money.

samiam4

Maybe you can help us terminology wise. What's this burst disc??

it is not uncommon to have sealing problems with the metal seals mopar chose over orings. When a system is converted to r134a. They change the oil Hopefully draining the compressor, so you add back 6 oz of comparable oil.
problem is these systems don't have high pressure switches nor electric fans triggered off freon pressure/Temps. Hence a converted system can hit higher pressure than r12.

You can try to add a can of freon and see if things work again? If your going to redo the system, be aware that hose sets and condenser are nla. So it takes effort to update coils with modern stuff. But you can get rid of the factory dryer and use something w orings.
When talking about these trucks. You need to be specific. Cummins powered had 2 different configurations. My experience has been only w the late cummins style.
Michael

skrap

Michael,

I agree, terminology is important so we can all be on the same page. My truck is a 92 w/intercooler. I believe it has the SD709 compressor. From reading through the forum, reading the FSM and looking under the hood the AC signal is feed to the PCM through the low pressure switch and cycling switch. Then the PCM determines to ground the W.O.T/A/C Clutch relay assuming there isn't a low idle speed detected. The only way I know to differentiate is with or without an intercooler and model year.

Cabin AC Switch -> low pressure switch -> cycling switch -> PCM 27 A/C Cycling Switch
PCM 34 W.O.T Relay -> Wide Open Throttle Cutout Relay -> Compressor clutch

What I called 'burst disc' the FSM calls 'HIGH PRESSURE RELIEF VALVE' - a safety device to avoid damage under high pressure conditions. I assume that is why I heard the loud air sound when I was in the cab of the truck. I assume that since that is what the AC shop said they replaced 4 years ago. It would be more correct to say I believe there was a massive leak that vented off all my coolant. It sounded like the auto air drain on my air compressor. It had a very distinct odor. The way the AC shop worded it, I made the assumption once it releases pressure it needs replaced. I'm not sure and shouldn't have assumed.

EDIT: The FSM says - 'The high pressure relief valve vents only a small amount of refrigerant necessary to reduce system pressure and then reseats itself'

I don't want to pay the high price of having the system rebuilt. I live in the hot desert and the summer is a bad time to take your vehicle in for AC repair as they are very busy. I want to learn how to do it on my own since I plan to keep my truck for a very long time. I will likely misspeak or call things the wrong thing. I have the paper version of the 92 FSM, and the PDF version of the 93 FSM. Some of the terminology is different from page to page and between the two manuals. I plan to refer to them often so I can learn.

Are there any other details or clarifications I can add to make this a valuable post for me and others to learn?

Thank you all for the support!

edwinsmith

I had my hoses rebuilt at an A/C shop. I replaced everything but the evaporator and added R134a. I've never been happy with it so I bought some reclaimed R-12 off Ebay along with 6 oz of Ester oil which is compatible with PAG oil and R-12. I also bought a new expansion valve and an new dryer. I'll post back when I get it converted back to R-12 like God intended.

Edwin

skrap

I've heard that R12 works better than 134a in the early systems designed for r12. My issue is the cost of r12 and what will the future availability and cost be. Also, the new compressors are setup for 134a according to what the documentation says.

If you remember report back your findings with r12 versus 134a.

I'm going to hook up some gauges soon and see what I can figure out. I'm pretty certain there is no coolant left (could be wrong). I did find a diagnostics scanner that will read live data from the PCM. I will be able to determine what the PCM reads for AC input. It is not hard to use a DMM on these trucks but nice to know what the PCM is reading without probing around.

Rug_Trucker

I am tagging this thread as eventually my AC is going to be repaired by me. Lots of other things that are a priority right now.

samiam4

I'm not sure on what parts are setup for R134a? The driers work with either oil just fine. Hoses should be improved barrier type which work with both systems. The problem with R134a conversions is keeping the high-side temperature down, so I believe they run a partial charge. Very much more finicky on charging correctly(be aware of this at home). If you are going to convert a system, I would take the compressor off support it with 2 bocks of wood and have the case drain open and drain the oil into a container and measure. Leave it overnight. You want as much of the old oil out as possible if your going back with ester. I though manufactures of compressors would only honor warrantee claims if pag was used ? Might want to verify.
I'd identify the electrical issues. Mine's a 1991.5 and I'm not sure the cycling switch is integrated or not with the computer.

It's more work and not the season to do it- but converting to a much more effienct micro-channel coil would be helpful in. aR134a conversion. The evaporator coils have come a long way too- I haven't looked but always though there was a way to fit a more efficient coil which would be smaller into our existing boxes with some close cell foam. Not there yet...

Rug_Trucker

I know the ex's 2001 brand new Silverado's AC sucked when I was at Monster Mopar. I think you need to just change out the drier for starters. I heard propane works good as a coolant if only that little flammability thing wouldn't be a concern!

samiam4

I think on propane it's actually a blend of propane and isobutane. That's for a mineral oil lubriant and won't work with his current conversion level.

I thought the pressure relief valve was the pre-1991.5 style. His information shows a 1992 dodge truck. I'm a bit confused.


Michael

Rug_Trucker

I don't need the details of converting to propane. Don't care as I would NEVER do it.

skrap

Michael,

I do have a 92 W250.

I'm going by the FSM. I haven't laid my eyes on the HPRV yet. I'm new to AC work. I'm learning as I go. Some of what I say might be wrong (I'm ok with being wrong and corrected). I need some place to start. I appreciate you taking time to read the post and help!



From the 93 FSM:

skrap

Last night I went and bought some gauges. This morning I connected them up to read the pressures.

With the compressor off the low and high pressure was just over 75psi.

The clutch is not engaging. Before the 'incident' (loud air purge sound that sounds like my big air compressor auto drain) the AC clutch would engage (intermittent).

I believe the PCM is happy.... well according to the ODB1 scanner reading from the 'data link connector' in the engine bay.

AC OFF:




AC ON:




I think my next step is to actually verify the AC relay is being energized and power is getting to the clutch.

edwinsmith

The cost of R-12 is quite a bit higher than R134a but IMHO it is worth it to have a decent A/C on a hot day in Death Valley. R-!2 requires mineral oil R-134a uses PAG oil. I use 8 oz of oil and it seems to be OK oil wise. Ester oil is compatible with both refrigerants which is why I'm using it. When I rebuilt the system for the second time, I took the lines off the compressor and ran the engine in idle to purge the oil out.

I have found that with R-134a and the stock High pressure relief valve at 600 psi the valve pops and there goes a bunch of coolant and oil. With the coolant fill below where the high side pops the low pressure switch will cycle (it shouldn't AFAIK) and it won't cool worth a darn. With R-12 the fill should be to the point where the sight glass is all liquid with no bubbles or foam. This isn't possible with R-134a and will pop the high side before then.

As to future availability of R-12 it will eventually be unobtanium since it isn't being made anymore. Hopefully I'll be dead before then and it will be the new owners problem.

Edwin

skrap

I think what you describe might be the problem I was having. It seemed like the compressor was kicking in and out as I would get intermittent cooling. I can't remember if it made a difference if I was driving or sitting at a light.

edwinsmith

Yes. When the low pressure switch cycles it thinks the system is out of freon and cuts the compressor off. Then the pressure in the system equalizes some and the switch cuts the compressor back on.