Only 24 hg Vacuum

ykf7b0 on Wed June 17, 2015 9:18 AM

Year: 1976
Make: Chevrolet
Model: Corvette
Engine Size: 350
Refrigerant Type: R134A
Ambient Temp: 90
Country of Origin: United States

I have a new Mastercool 6cfm vacuum pump and Harbor Freight manifold gauges but I'm only getting 24 hg vacuum on a recently flushed system. (R134 conversion - factory 76 Corvette with new evap and desiccant) However, the system will hold this 24 hg all night so I don't think I have a leak. The vacuum pump was used a month ago on a Honda and did pull down to 29 hg. Please advise me on the next step I should be taking. Thanks!!

GM Tech on Wed June 17, 2015 9:27 AM

Do a test vacuum on your guage set- not hooked into your car- see if you get the 29 in-hg- if not, your guage may need to be recalibrated- or it is leaking.

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Cussboy on Wed June 17, 2015 9:49 AM

EXACTLY what GM posted.

ykf7b0 on Wed June 17, 2015 6:37 PM

Would this test be the same valve openings as pulling a vacuum only not connected to ac high/low ports? If so, then my Harbor Freight gauges are not calibrated. To calibrate can I simply remove plug on each gauge (high/low) and turn the slot screw? Thanks!

AC_Doc on Wed June 17, 2015 8:33 PM

Almost the same, except:

Make sure the valves on the manifold are closed.
Instead of the yellow hose hooked to the vac pump, connect the blue hose.

After the pump starts, Close the ballast plug on the pump (if it has one).

The Blue gauge now will read the vacuum provided by the pump.

[If the pump itself has a hose valve, close it. THEN turn off the pump.
Watch to see if the reading on the blue gauge holds.]

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Sometimes you must accept things at faith value!

Edited: Wed June 17, 2015 at 8:37 PM by AC_Doc

ykf7b0 on Wed June 17, 2015 10:54 PM