In reply to a message from ijlal sent Sat 19 Jul 2008:
It sounds as if you have inadequate airflow through the
coolant radiator when the car is slow moving or stationary
and / or when the A/C is running.
That may be because there is debris (leaves / straw / dirt
etc) trapped between the A/C condenser and the coolant
radiator. You should remove the top radiator support channel
On the other hand, there may be a problem with the cooling fans.
The V12 has an engine-driven viscous coupled cooling fan
behind the coolant radiator plus two electric auxiliary fans
in front of the A/C condenser behind the front grille. Those
electric auxiliary fans are driven through the fan control
relay module via a series circuit (slow speed) or in a
parallel circuit (high speed) depending upon the need for
additional engine cooling.
A twin thermal switch in the bottom of the coolant radiator
switches those fans on low speed when the coolant
temperature exceeds 187� F and switches them to high speed
if the coolant temperature exceeds 212� F.
The fans are also controlled by the air conditioning
pressure switches so that irrespective of the coolant
temperature switches, they are switched to low speed via a
single pressure switch when the A/C refrigerant pressure
exceeds 218 psi and to high speed via the ‘trinary’ pressure
switch when that pressure exceeds 290 psi to ensure maximum,
airflow through the radiator and condenser to enhance A/C
cooling efficiency.
You need to observe those electric auxiliary fans to ensure
that they are operating whenever the coolant temperature
requires slow or high speed operation or whenever you have
the air conditioning operating.–
The original message included these comments:
A 1994 xj12, I’ve changed the two thermostats because I though they
were the reason the car was running hot after running the a/c. So,
after that I drove about fifty miles, and the c/h needle still lean
to the hot side. especially when sitting or stalled in traffic,but
while running at about 50 mph, the needle drops downto the n
position. It’s the same way when I run the a/c. Is there any advice
for a novice? thank you
–
Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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