Problem understanding gauge readings

[michaelwoosley]

Hello group. I am an automotive mechanic but must admit that I'm very GREEN when it comes to working with automotive/truck A/C systems.

Today, I had a pastor from the local church ask me to look at his 3 cars as each one did not blow any cool air. One is a 2001 Mercedes 430CLK, the other a 2006 Saab (can't remember the model), and the third one was a 1990 Mercedes 420 SEL. The first one had pressures on each gauge the same (60/60). The Saab was yet another weird one as the gauges both showed low (about 10psi on each gauge) on hookup so I assumed that I should add refrigerant. With the engine running and system maxed out via the controls, I got a reading of 30 low/50 high. I was thinking that the high side gauge would eventually creep up to a normal level but it did not. As a matter of fact, the gauges showed a gradual reduction to 0 low/0 high with the engine running.

I'm hoping for some guidance on what I potentially did wrong, or what I need to do different. I also need help with explaining why my readings were the way they were. I'll do my best to ascertain what I think is wrong based of cheat sheets and limited research.

Based off the Nissen cheat sheet I downloaded a few days ago, it appears that the first vehicle has a potential bad compressor or a faulty ECV/MCV (whatever those are, Lol).

On the Saab, I cannot find ANY information to explain, except for the fact that the initial connect reading (static pressure), the readings were the same @ 10 psi each. I made the assumption to add refrigerant because the chart said if both gauges showed low, then the system was likely low on refrigerant. My feeling is now, that I may have misinterpreted "what low REALLY means". Also, I cannot find anything that would explain why the pressures on both gauges were dropping steadily to zero with the system running (I wasn't adding refrigerant at this point)

Then on the 90 Mercedes, upon hookup, both gauges were static pressured at 50psi and the reading changed to a negative value with the engine running with the low side at -10 and around 50psi on the high side. Again, I didn't see a scenario anywhere that depicted these readings so I'm at a loss again as to what might be wrong with this cars A/C system.

Again, I hope someone will take an opportunity to provide some GREAT training and allow me to cipher what the heck is going on with these readings.

Thank you very much and I cannot wait to hear from you all.

Mike

[JohnHere]

A few general observations to start: Based on your pressure readings, it appears to me that all three cars are empty of refrigerant or nearly so. One of them, the 1990 Mercedes, originally used R-12 refrigerant unless it was converted sometime during its life to R-134a. Do you know whether that's the case?

I would approach this by first giving each A/C system a close visual inspection for any obvious defects. If nothing turns up there, I would leak-check each system by evacuating, and seeing whether the vacuum holds. If it does, I'd add a few ounces of refrigerant (along with a small amount of fluorescent dye) to raise the pressures, and look for any leaks using a UV light and/or an electronic sniffer. With vehicles that old, leaks could be almost anywhere.

A word on charging: You can't do it--at least not accurately--using only a manifold gauge set. For proper charging, the refrigerant must be weighed-in using the appropriate equipment.

[Dougflas]

and do not rule out old leaky hoses.