MACS Service Reports - 2016 - JUN6

Tim Iezzi

So unless we get more data that "tunes" the test,
it seems to come down to this: if you want to use
the temperature drop method to detect a restricted condenser - including the sub-cool condensing
section - you'll likely have to give the full system
some time to go through cooldown before you take
the measurements, and even then, look at the entire system for other clues before you condemn the
condenser. With today's condensers and their tiny,
tiny passages, the result of plugging is likely to be
even more of a temperature drop than our longused rule of thumb. You might also try, on solenoid-valve variable compressors, (assuming you
have a solenoid valve electronic tester) dropping
the duty cycle to perhaps 50% before you make the
temperature drop test.

Don't Go Downstream Past the IHX

Figure 11: Chart shows readings on all the temperature sensors at 50%
compressor displacement, as controlled by the duty cycle valve.

Sanhua Group

And still another factor: that IHX in both the
high and low side circuits downstream from the
condenser assembly on the high side and the evaporator on the low side (FIG. 12). As we explained,
it also sub-cools the liquid refrigerant, so if you
clipped your temperature probe to the liquid line
past the IHX, there would be an even greater temperature drop. Of course, you should be able to
find a place to attach that probe pretty close to the
outlet line from the complete condenser/sub-cool
assembly and not bring the IHX effect into the picture, which would increase the total drop. By the
way, Iezzi's data included his temperature measurements across the Optima IHX, which were as
high as 19 °F. drop on the liquid side at 50% compressor displacement, just 4 °F. at 100% displacement. The increases on the suction side were even
greater, 28 °F. at 50% displacement, 31 °F. at 100
displacement. But at this stage we have no accepted test procedure for the IHX on any one car, much
less a rule-of-thumb procedure for all of them. You
can look at the numbers, but that's about it.
Yes, this business is getting complicated.
With older vehicles and fixed-displacement compressors, the 20-50 degrees F. numbers still look usable. ■

Figure 12: Illustration shows how the Internal Heat Exchanger is
plumbed into both the low-pressure and high-pressure sides of the system, to further sub-cool the liquid refrigerant on its way to the evaporator.

STILL ANOTHER (RELATED) ISSUE TO TEMPERATURE DROP TESTING
If you're going to measure the temperature drop across a
condenser, how you take the measurements can have a significant effect is our observation. If you use an infrared thermometer, switching from condenser inlet to outlet involves
a small time delay that could result in temperature changes
that are sufficient to throw off the test. And you have to be
careful where you take the readings-- a small slip can move
the target sufficiently off the condenser line to produce a
mistake. If you're going to use infrared, two thermometers simultaneously would seem to be the best approach.

June 2016

Further, they should have laser sighting so you see exactly
where they're aimed. And putting a wrap of magenta tape
on each location wouldn't be the worst idea. Naturally this
approach would require two technicians.
And finally, two infrared thermometers had better both be
accurate or at least read the same when checked at the same
location. Our experience is that there will be a difference of
at least a few degrees between them, so you want to know
the comparison, particularly if the temperature drop readings are close to the rule-of-thumb limit.

MACS Service Reports

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