Nissens AC Diagnostics

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Nissens AC Diagnostics

Post by Autosaver »

I went looking for some AC pressure, component temp chart(s) for diagnostics & came across Nissens.

Looks pretty good to someone like me who has just started in Auto AC as it answers questions (like pressure & temp ranges) that only someone with experience would know.

Interested to see what tbirdtbird & JohnHere think.

It's the first PDF link: ... nt=gws-wiz

It's the second PDF link: ... s-wiz-serp
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Re: Nissens AC Diagnostics

Post by tbirdtbird »

The first pdf could be improved by stating what percentage of the time each problem could exist. For example, at one point one of the causes is listed as too much dye in the system. Now, really how often will that be the case?

As far as the second pdf, without reading the entire thing, I have reservations already.
Take this quote:
"Before the test, make sure the system is in an operational condition. This requires a correct level of refrigerant charge, with at least 1,5 bar/25 PSI of static pressure to run the compressor. To determine that, static pressures of the system should be read before starting the engine. LP and HP pressure gauge indications should be very nearly to equal on both sides. Too low static pressure indicates a low system charge and probable leak that must be found and repaired.
In actuality, we pay very little attention to static pressures, they can be deceiving.

After the static pressures test is passed, operational pressures can be measured. For reliable outcomes, the vehicle must achieve its operational characteristics. The engine must get started, set to idling and maximum cold air production, and around 70% of the blow power must be set for the AC system. "
We do NOT use idle speed for checking any component of an AC system, and the fan must be set to max, not 70%. I am not sure that whomever wrote this up knows any more about AC than you do.

The first rule of learning AC is to have a thorough understanding of all the components and how they interplay. Often, this cannot be mastered all at once, but in pieces. AC is abstract, you cannot see freon and what it is doing, you rely on indirect evidence like temps and pressures. In a way, it is like learning electricity, another abstract. These are very different to learn than pure mechanical systems such as engines and transmissions.
I have maintained for quite some time, that if you want to improve at AC diagnostics and repair, then step back thru the many many pages of the forum. Read and understand every single complaint that comes thru and the answers from the pros. You will NOT find better pros than on this forum, period. Experts like ice-n-tropics, chick, bohica, Tim (the owner) also known as TRB, Cusser, Nick, JohnHere, Dougflas, Detroit AC, GM Tech, etc etc, are true experts with boatloads of experience, unlike any other forum could could wish for. These guys are brilliant. I apologize because I have inadvertently left off some important names.

I myself am not MACS certified like some of the others. My training is in residential and low temp refrigeration (down to minus 40F). You have to know what you are doing to get a freezer box to minus 40F. However, I have extrapolated my knowledge using many many different sources, including Ice's book "How to AC your Hotrod". Believe me, mobile AC is much more difficult than any other type of AC or refrigeration, there are way too many variables.
The categories of refrigeration are
Hi temp = 40F and above such as mobile and residential AC
Medium temp = -10F to 40F such as a resi refrigerator. For example ice cream MUST be stored at -10F or below because of the sugar content.
Low temp = -10F to -40F such as deep freeze of perishables
Ultra Low temp = below -40F. This requires a cascade system.
I have done all but Ultra Low. Mobile is absolutely the most difficult.

Start paging your way thru the archives here, a good winter project, you won't be sorry, It goes back 20 yrs.

And if you want to get the maximum help from us, please provide all the parameters we ask for, including vent temps, every time. You are there in person, we are trying to understand your system from a keyboard
When considering your next auto A/C purchase, please consider the site that supports you:
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Re: Nissens AC Diagnostics

Post by JohnHere »

Generally some good servicing tips from Nissens, and I agree with the exceptions and recommendations of what tbirdtbird said. Kinda' hard for those of us in the USA to relate to Celsius and Bar, though.

As for pressure/temperature charts, I hardly use them at all and wouldn't put too much "stock" in them. For instance, in one place they mention that at 85°F ambient, the low-side pressures should be ~50-55 PSI. At those gauge readings, the temperature of the evaporator coils will be about 58°F (14°C), and the air coming out of the center vents will be a few degrees higher...much too warm for any practical cooling. So those charts can throw you off, diagnostics-wise, especially for someone just learning about MVAC.

As you gain more experience, you'll develop a "feel" for what good and bad pressures are and be able to diagnose problems accordingly.

A few things to keep in mind:
—It has been said that more than 60-percent of MVAC troubles result from a low charge due to seasonal losses from the compressor shaft seal or other leakage points.
—Learn how to leak-check a system.
—System pressures can't tell you the amount of refrigerant in a system.
—A system should have its receiver/dryer or accumulator replaced before evacuating and recharging it.
—Always weigh-in the refrigerant from a well-held vacuum using a purpose-built refrigerant scale or RRR machine.
—Many modern MVAC systems have variable-displacement compressors as opposed to fixed-displacement compressors. Be sure to determine which type you're working on beforehand.
—Invest in professional tools, like those that this site's sponsor offers. They'll serve you well over time.

Good luck.
Member – MACS (Mobile Air Climate Systems Association)

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