MACS Monthly Newsletter - 2020 - DEC8

The Fans Are Running, And Running, And Running Too Fast
has a pressure transducer for A/C clutch control (Figure 12). The
next time he performed the test, he was able to include a check
of the transducer and confirmed what many technicians have
seen: an intermittent spike in the pressure reading. Replacing the
transducer was the next logical step, and yes, it eliminated the
problem.
However, there's another lesson to be learned in working on
the so-called manual A/C that's used in most of those F-150
models. The system may be called manual A/C, but the clutch
control has varied considerably over the years in the 2005-on period. In 2005-2006 for example, there's a conventional pressure
cycling clutch switch AND a high-pressure cutout switch with
both the 4.6 and 5.4-liter V8s. However, in 2007 Ford dropped
that type of circuit in favor of just a transducer for clutch control.
But if you're working on a 2008 model, don't assume anything,
even if you spot a transducer. For that model year there's also
a pressure cycling switch to control the clutch. Why? Maybe
someone wanted to separate the functions to improve reliability.
Fine, but by the next model year Ford was back to just the transducer and that design continues to today, although some overall circuitry changes have been made. The lesson to be learned:
check the wiring diagram for the specific model year, not relying
on what was true on the one previous or succeeded model year
edition. But you already knew that, right? n

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In really cold weather, electric fans should not be running at
more than low speed (if at all), or with an electro-viscous fan,
at a loping idle speed. But we've often heard from embarrassed
technicians troubleshooting a poor heat problem, even going so
far as to replace a heater core with no resulting improvement.
And at the end, they realize that while the fans aren't spinning
at the highest speed, they are turning a lot faster than would be
reasonable for the operating conditions.
The owner of a 2011 Ford F-150 with the 5.0-liter V8 came into
the shop on a cold winter morning with what seemed to be a
routine heating complaint. A quick check of the coolant temperature at the thermostat outlet showed a normal reading. However, the temperature at the heater inlet hose told the technician
there was a large temperature drop, and the radiator fans were
running at high speed. But then the fans dropped to a normal
low speed, or maybe they wouldn't. In some cases, the motorist told the shop, after a couple of minutes of driving, the fans
would go to high speed and stay at that high speed for the entire
drive. It's not surprising that this would reduce the heat output
sharply in cold weather.
In the shop the technician was able to duplicate the situation to
the extent of a high fan speed, but only for a few seconds. However, he saw that for that brief period, the A/C had cycled on. So,
he checked the wiring diagram, which showed that the system

Figure 12: Technician found the 2011 Ford F-150 A/C compressor was operating intermittently, which provided a diagnostic
clue as to why the radiator electric fans were coming on and sticking on, affecting heater performance.

December 2020

MACS Service Reports

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MACS Monthly Newsletter - 2020

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