Recovery Machine Shows Less Refrigerant Than Expected on a Retrofitted Bus
You are servicing a 2012 school bus whose A/C system was converted from R-12 to R-134a years ago. The under-hood label is faded; the only legible text says 'R-134a, 4.5 lb charge.' You connect a recovery/recycling machine with a new filter-drier and calibrated scale. After evacuating the system for 45 minutes, the machine recovers 3.1 lb. The system held 28 inHg vacuum for 10 minutes with no rise. The customer states the A/C 'worked fine last summer' and no one has added refrigerant. Ambient is 70°F. The service port has a quick-disconnect fitting, and the Schrader core looks slightly oily.
1. List the plausible explanations for the 1.4 lb discrepancy and rank them by likelihood, explaining your reasoning.
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Ranked possibilities: (1) The system was undercharged before you arrived - a slow leak at the oily Schrader core is the most likely single cause; oil staining at a service port is a classic leak indicator, and a 1.4 lb loss over a season is consistent with a small Schrader leak. (2) The label charge is wrong - retrofits often used a reduced charge (typically 80–90% of the R-12 charge), so 4.5 lb may be a generic sticker, not the true spec. (3) Recovery losses - hose volume, non-condensable gas, or machine scale error can account for a few ounces, not 1.4 lb, unless the machine was mis-calibrated. (4) The system was previously 'topped off' with a sealant or wrong refrigerant, changing the effective charge. Confirm by leak-testing (electronic detector and dye at the Schrader), verifying the machine scale with a known weight, and checking the OEM retrofit spec if available. Do not simply recharge to 4.5 lb without resolving the leak.
2. The vacuum held steady at 28 inHg. Does that prove the system is leak-free? Explain, including what vacuum level and hold time you would require.
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No. A vacuum hold test at 28 inHg is not a substitute for a pressure leak test. Vacuum pulls fittings and seals together, so a joint that leaks under pressure can seal under vacuum. Also, 28 inHg at 70°F ambient is well below the ~29.9 inHg achievable, indicating either a weak pump, a leak, or moisture; a proper evacuation should reach 500 microns (about 29.5 inHg) and hold under 500–1000 microns for 10–15 minutes with the pump isolated. Recommend: repair/replace the Schrader core, then perform a nitrogen pressure test to the manufacturer's spec (often 150–200 psig) and a standing vacuum decay test with a micron gauge before recharging by weight.