Air Con Temp Switch

Hi,

Could someone please help and confirm if I am right or wrong. I currently have a yellow dot air con temp sw fitted in the thermostat housing of my Discovery 1 1995 300 Tdi. At rest, (engine cold) the sw is open circuit, once the engine has reached a set temp the switch makes and switches on the air con fans to assist engine cooling, this is how I expect it to work. However, the yellow dot sw is the wrong temp range for my truck, it operates too early, it should be a green dot sw.

I have tried 2 brand new genuine LR part green dot switches (PRC 3359) from different batches and both are closed circuit and operate the fans as soon as the ignition is turned on (cold engine).

Should a PRC 3359 be open or closed circuit fresh from the box?

Thanks in advance

Mike

1995 Discovery 1 LHD 300tdi Air Con.

PRC 3359 is NC (normally closed) and is designed as a circuit breaker in conjunction with the other switch . ie if the engine s overworked and over heating it switches the air con off.The temp range will be correct it is just your car has lost some of its cooling efficiency , ie it is starting to overheat . I had a similar problem and I eventually found it to be the oil cooler thermostat was opening too late .You can buy other switches with different temp ranges , I think I used one from a vauxhall as a temp measure until I sorted it properly. The yellow dot one is a NO (normally open) switch and switches the fans on to increase the cooling but is overridden by PRC3359 at an even higher temperatureso the engine does not have to cool the AC condenser as well .Regards Jonathan

Hi Jonathan, thank you for your fast and detailed response.

Originally when I got the car the fuse for the fans had been removed because they ran continuously as soon as the ignition was switched on with the air con off. When I unplugged the original 3359 (front switch on thermostat housing) they stopped, assuming a duff 3359, I replaced it, but by mistake ordered a yellow dot switch, which cureed the problem but brings the fans on as soon as the engine warms up. I had assumed that the difference between yellow/green dot was only the temp range. Looks like I still have a problem with my air con fans coming on as soon as the ignition is on, and I now have 3 serviceable 3359s!! I presume that a green dot is the sw for my car and that a yellow dot is for a different vehicle. I am happy that the car is not overheating. Any ideas on why my fans run continuously would be most welcome.

Many thanks

Mike

Sorry Mike but it is not that simple some cars have one switch and some have two . The part numbers are PRC3359 green dot and PRC3505 yellow dot for a two switch car and just PRC 3505 yellow dot for a one switch car . Which is yours ?
Your does not have to overheat in the traditional way to cause this problem just run a few degrees hotter .
PRC3505 is 94°C / 100°C ie switches fans on at 100°C and off at 94°C
PRC3359 is 109°C / 115 °C ie switches the air con system ( compressor ) off at 115 °C and on at 109°C
The coolant thermostat is fully open at approx 88 °C so if your engine colant is running 12 degrees warmer than normal it will switch the fans on at 100°C and if it cools slightly but is still 6 or 7 degrees warmer than normal ie 94 to 95 °C the fans will never switch off .
This will not show on your cars temp guage as it will never look higher than half way at these values and in fact may actually look slightly below half way and no where near the red .
This slight loss of cooling effiiciency can be caused by , faulty radiator ,ie sludge inside,external fins caked with mud , leaves or corroded away . Same with air con condensor and blocking the flow of air to the radiator . Faulty thermostat both or either in the cars oil system or water system . Water pump starting to fail etc . Could also be a sticking relay in the air con /fans circuit . Possuible problems with leaky cylinder head gasket / cracked cylinder head ( also has other symptons ) etc . My cause was by the oil cooler thermostat opening after the water thermostat and it should have been before Regards Jonathan

Hi Jonathan,

Thanks for the info, I appreciate your time.

I’m having trouble finding good gen about my air con, I live in Spain and will need it in the spring.

On the thermostat elbow there are two switches, I had thought (read somewhere) that the rear switch is the temp gauge sender, am I wrong?

Just had a look and the plugs to these switches and funnily enough they are coloured yellow and green, could this relate to the yellow/green dot switches? if it does they are connected the wrong way round.

When I got the car the previous owner (a very good friend) reported that it was using water over a long run and then running hot, he was about to scrap it. He had owned it from new and had cooked the original engine several years ago, this rebuilt unit has only done 20k.

P gasket leaking, vent pipe from thermo housing to expansion tank found blocked, tappets too tight, about 3 litres too much engine oil in sump and the air filter was totally chocked. Water pump replaced when I did the p gasket, thermostat checked and I changed the oil stat as a precaution when I serviced the car. Removed the CAT and EGR and cleaned the intercooler, it now pulls quite well and returns around 33 to the gallon. Radiator now gets hot all over, I am running without the viscous fan and the temp gauge does not go above 9 ‘O’ clock posn. You might be right about it running a little hot as I am not convinced that the rad is completely clear. However, I have now done some long journeys, long steep climbs and city work around Madrid (air temp low 20s) without the needle budging or engine showing any signs of distress. This is my first LR and I have got most of my info to fix it from this site (fantastic), I’m really enjoying working and driving it.

I have struggled to find out info on Air Con, the wiring diagram in my Haynes manual has not helped. Any idea where I can find a decent one?

Mike

I think the coloured plugs should match up with the dots on the switches . The temp sender for the temp guage is just under the thermostat housing , there is only one wire going to it it is difficult to see as the air con compressor is in the way . The 9 oclock position for the temp guage seems too high , I can see both the white graduations on the little thermometer above the pointer and some space below the second line is still above the pointeron a summers day in the UK . Put the viscous fan back on . Most people who say you can run without it do not have an air con condenser in front of the radiator and live in cold UK not sunny Spain .It is your choice but I believe you have lost some efficiency in your cooling system and that is why the electric fans are coming on . regards Jonathan

Sorted, probably.

Once cleaned up, the plugs are clearly cloured green and yellow. Originally the plugs were cross connected causing the fans to run continuously. I incorrectly replaced the green dot switch with a yellow dot, still cross connected, the fans were then coming on as soon as the engine got up to temperature. Car now has a green and a yellow dot switch fitted, connected correctly and after a run the fans didn’t come on. However, it’s pretty chilly tonight, I will give it another blast at the weekend when it’s a little warmer.

I'm not convinced that the cooling is a 100% efficient, I am considering replacing the rad as a precaution. Jonathan, you raised a valid point about the condenser rad and fans not helping getting air to the engine rad. It was going back on for the summer, but I will refit the viscous fan sooner than later. -2 here this morning, so I can wait a little longer.

Next I will try the air con, it’s not run for at least 3 years, probably needs a re-charge and I will have the filter replaced at the same time.

Thanks for your help

Mike