Chevy Express Dual AC Charge Level Correction

[Black Pete]

2006-ish Chevy Express, CCOT front / TXV rear. Marginal AC at idle. Pretty good at speed. Suspect years of minor shaft leakage has charge a little low.

What would be the procedure for measuring charge level? Have a set of gauges and also a thermocouple. On a TXV car I would check for subcooling at the condenser outlet.

On this one, not sure. Is subcool still valid in the same way? Another way might be to aim for zero superheat at the front evap outlet between evap and low pressure receiver under high load conditions front and rear.

Thoughts?

[JohnHere]

I wouldn't concern myself with superheat and subcooling on a MVAC system. Those measurements are more for residential systems. Checking the pressures won't tell you how much refrigerant is in the system, either. I would take it to a shop that has an RRR machine, have them recover the charge, and see what it weighs. If it's low, the shop can evacuate and recharge it while it's still hooked up to the machine. The specs for your vehicle call for 56 ounces net weight of R-134a and 11 fluid ounces of PAG-46.

[DetroitAC]

Our man Black Pete is clearly a refrigeration technician of some sort, if he knows enough to understand subcool.

Charge for a TXV system is determined by measuring subcool, but it's more complicated because we are looking for the "end" of a subcool "plateau".

A CCOT front and TXV rear system is different, reserve charge is held in the accumulator can. Charge is mostly determined the same way as a front only CCOT.

The normal procedure is to measure evap in temperature and evap out temperature, add charge in small increments (at constant high load conditions) until the evap outlet becomes colder than the evap inlet. This is more or less when the evap starts to overfeed and send liquid to the accumulator. Next, the reserve amount of charge is added, for a big system like yours that it's probably 150g or so.

Much simpler to just pull all the charge into a recovery bottle, weigh what you took out ( tare the bottle beforehand) and then weigh in what's needed to get to the charge listed on the sticker. Or just let the recovery cart push in exactly the charge specified.

Side note: a lot of those older vans have leaking rear lines/evaps, you can block off the rear. A lot of guys also block off the rear since only tools and gear riding back there and get better performance from their front unit.

[andrew vanis]

Charge for a TXV system is determined by measuring subcool, but it's more complicated because we are looking for the "end" of a subcool "plateau".

Could you please direct me to where this subcool "plateau" process might be detailed or can you elaborate?

I'm creating a custom system so there isn't a factory capacity spec.

I have a digital gauge that automatically calculates subcool for 134a (and superheat but can't use that on this TXV system). Conveniently it also charts on a cell phone so seeing a plateau should be easy.

Fwiw, it is a VW Vanagon.

Thanks!

[JohnHere]

Hey, andrew vanis...why not start a separate topic and reference this one? That way, any responses will be specific to your custom-system VW Vanagon, and maybe you can catch the attention of DetroitAC so that he can elaborate on the process as mentioned.