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2007 Views 7 Replies 5 Participants Last post by  Dan_UK_1984
Hi All - I have been having a bit of trouble with over heating, P38 4.6 I carried out a pressure test of the coolant circuit and each cylinder pressure test, this is an air line pressure test to see if the air comes back through the water to test for cylinder head gasket or cracked block, the cylinders were fine I did however show a leak on the radiator and have replaced this, but what I would like to do is to be able to control the two electric air con fans from an independent switch on the dash , but to leave the fans connected as they are for the air con, i.e just to put a supply to the relays when I want to what I need to know is do the relays receive a supply or do the receive an earth or is there a better place to connect a switch into the circuit, I do most of my driving heavy traffic and this is when the temperature can start to rise and to be able to control this rise would be a great help circuit diagrams would be appreciated
All the best Sillyboy East London UK.
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For what it's worth, I regularly sit in traffic at 35 degrees ambient with the a/c on full blast and the temp needle never passes 12 o'clock..... (touches wood quickly).

Just a thought before you start possibly upsetting the RR continuum by interfering with the wiring.
It's no good trying to correct a fault by blowing more air through the radiator.I would be inclined to check that the fan is locking up properly followed by the waterpump.
Temp sender units are common for failing after awhile, you dont mention year and model as its year related
Thermostat can also be failing
Another aerea is the push on electrical connector, after a few years it builds up resistance
Hi all, in reply to your question the vehicle is a 1995 4.6 HSE Range Rover known as the P 38, I bought the vehicle in June 2000, two days after purchase one of those rare 30degs days in England I blew both cylinder head gaskets the vehicle was taken back by the dealer and rebuilt including a new rad, thermostat and housing, but still overheated, Main dealer changed the water pump, viscous fan, and some sensors, LARGE BILL but still overheating, small R/R workshop advised engine change this was carried out MUCH LARGER BILL, but overheating was a thing of the past, until about three months ago, when I got caught in standstill traffic in the outside lane in the Blackfriers tunnel London, fortunately I always carry at least 5 lts of water with me ( had six R/R since 1984 so I always carry water ) it now appears a small pin hole in the rad is the culprit, the hotter the engine gets the more water loss, cold engine no water loss hence the pressure test was needed to revealed the problem, New Radiator, should be alright now for at least the next month or so !!! As for touching wood get your self a forest best of luck
Sillyboy (I own a P38 say no more)
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A Range Rover can sit all day in 30 degree heat with full load on the A/C and not even raise a sweat. To even think otherwise is ridiculous, you can drive through the Sahara and your temp gauge should never get near the red.

If you overheated on a hot day, it was not due to the ambient air being 5 degrees warmer than a normal warm day.

The viscous fan is constantly drawing air in, and is effective up to about 60mph - so it's almost the equivalent of driving down the motorway (when the clutch is fully engaged), this would be your first point of call. When the engine is hot, shut it down and see if the fan is hard to turn, it should be.

Your A/C fans work when the A/C is on, the speed depends on the pressure in the A/C system (pressure is relative to temperature) I do not believe they have a sufficient duty cycle to be used as a full time engine cooling fan. However, both A/C fans will be run at full speed as well as A/C compressor disengaged if an overheat situation is detected by the engine ECU (it gets its temperature reading from a different gauge to the one you see on the dash) So really your override switch is unnecessary.

We had a radiator replaced by the dealer a couple of years ago on the 2.5 BMW P38, just because at the time I was too busy to do it. They didn’t do the radiator hoses up at all, a mile down the road there was no coolant left which resulted in major overheating. They did not pay for the head gasket to be repaired! They said I should have checked the hoses myself!
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here we go again I've run the engine up to temperture removed front grill put A/c on full but no joy from the electric fans the fans are controled by the dual pressure switch, and two relays i've checked the relays both are good, can the pressure switch be replaced with out de-gassing the system Regards Sillyboy
The fans wont always kick in with the A/C on, it will depend on ambient temperature. Bare in mind that the viscous fan is still pulling the air through.

Go out for a drive for 10 mins, with the A/C set to LOW, then park up and have a look.

I know I said "Your A/C fans work when the A/C is on" but not all the time.

Dual pressure switch
The dual pressure switch protects the refrigerant
system from extremes of pressure and controls the
operating speed of the condenser fans. The dual
pressure switch is installed in the top of the receiver/
drier and senses receiver/ drier outlet pressure.
If the minimum or maximum refrigerant pressure limit
is exceeded, switch contacts open to disconnect the
power supply line between the ATC ECU and the
compressor clutch. The minimum pressure limit
protects the compressor, by preventing operation of
the system unless there is a minimum refrigerant
pressure (and thus refrigerant and lubricating oil) in
the system. The maximum pressure limit keeps the
refrigerant system within a safe operating pressure.

A separate set of switch contacts control the operating
speed of the condenser fans. Depending on the
refrigerant pressure, the switch contacts:

• Close to energise the condenser fan relays
and run the condenser fans in parallel (fast

• Open to de-energise the condenser fan
relays and run the condenser fans in series
(slow speed).

Single Pressure Switch
The single pressure switch controls the on/ off
switching of the condenser fans while the air
conditioning is on. The single pressure switch is
installed in the refrigerant line between the condenser
and the receiver/ drier. Depending on the refrigerant
pressure, the switch contacts:

• Close to energise the condenser fan
control relay and run the condenser fans.

• Open to de-energise the condenser fan
control relay and stop the condenser fans.
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