MACS Monthly Newsletter - 2020 - FEB5

BLOWER DIAGNOSIS SHOULD INCLUDE WIRING DIAGRAM
When a blower fails, many technicians will check for
a blown fuse, as did this technician with a 2007 Nissan
Altima in the shop. He found a blown No. 22 fuse (15
amp) and where most technicians would simply install a
new fuse and hope for a fix, he wanted to see if the problem truly was with an ageing fuse or a problem with the
blower motor itself. So, he connected a fused ammeter
across the fuse holder to take a reading. He turned on
the blower motor and on low speed the reading was well
below the rating of the fuse. However, when he turned
up the blower speed to high speed (with the engine and
A/C on), the current draw rose to 19 amps. So, the blowing of the fuse was no surprise.
At this point, we almost can't blame him for replacing
the blower motor (but we do). That new fuse also blew
in short order on high blower speed, so he rechecked
the blower motor draw and the new motor also drew
19 amps. Did he reinstall the old blower motor, which
apparently was not the problem? Let's live in the real
world. However, he still had to find out the root cause,
because 19 amps maximum current draw for a mid-size
car's blower motor sounds reasonably normal to us.
Finally, in line with our long-recommended procedure (continually repeated in MACS Service Reports), he
decided to check the wiring diagram (Figure 7), which
shows a second 15-amp fuse (No. 21) in a series-parallel circuit. That fuse also was blown, and although the
blower would run on low speed with just one fuse, it
needed the electrical power through the second fuse for
higher speeds. Why didn't Nissan just use a single, higher amperage fuse? Well, we don't have access to Nissan's
electrical circuitry engineering department to be able to
ask. Many vehicles have a single 25-amp fuse to cover

www.nissan-techinfo.com

the blower circuit. The size of the wiring used may well
make the two-wire/two fuse approach, that also enables
a total of 30 amps coverage for the motor, even better. n

Figure 7: Nissan Altima wiring diagram shows that blower
circuit relies on power feed through two 15-amp fuses, Nos.
21 and 22.

BAD LUCK OR TECHNICIAN'S FAULT
tion line from the evaporator outlet was cold.
The temperature blend-air door seemed to be moving
normally and all the mode door responses also were what
was expected, so the technician thought the temperature
door might not be closing fully and the heater coolant control valve might be defective. The technician clamped off the
heater hoses but there was no improvement.
What to do now? An infrared thermometer check of the
heater hoses showed only a modest drop in temperature
across the hoses, which told the technician that there was
flow through the heater. What happened was that yes, the
heater coolant valve was stuck open, but the clamping of
the hoses was insufficient to stop the flow. Some hoses are
not soft enough to be clamped shut, particularly the shaped
ones. If you can't find places near the hose connections to the
valve assembly that you can positively clamp closed, you
may have to disconnect them from the valve and connect
them with a piece of pipe for a bypass of the heater. n

The toughest problems are those where an obvious cause
is quickly identified, where the technician fixes it (or thinks
he has) but really hasn't. In this case, an older Bimmer (2003
BMW 3 series with the in-line six-cylinder) is barely cooling
on a warm summer day. If you have a shop with a properlyfunctioning SAE J2788 recovery/recycle/recharge machine,
you can identify a low refrigerant problem, and in this case
the technician found the car had lost almost half of its 750gram charge. There may be a measurable leak or the leak
rate for the age of the car simply resulted in that amount of
charge lost.
The prudent approach is to install a full charge with a dose
of trace dye, and check the performance from there. In this
case the system quickly started to discharge nicely cold air
(41°F), but in short order the A/C register temperature was
up into the 50's and before long was over 60°F. The pressures
were normal (200 psi on the high side and 30 psi on the low
side). The radiator fan was running at what seemed to be
normal speed. The liquid line was hot and the low-side suc-

February 2020

MACS Service Reports

http://www.nissan-techinfo.com

MACS Monthly Newsletter - 2020

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2020