Engine Overheating with AC on

Hi,
I am after some suggestions as to what could cause the following issue; after starting my engine it warms up and runs OK, but if I put my AC on it will start to get very hot after a few miles. It does not appear to be related to speed as happens in a traffic jam or at 60 mph. Also the auto trans overheat light came on when the engine got hot? It never went into the red only up near the H.

I have just replaced the head gasket, (head skimmed 12 thou and valves recessed 12 thou, home made solid copper 1.5mm head gasket fitted) water temp sensor replaced, other necessary gaskets replaced, three core plugs replaced. New coolant and engine oil and filter, also expansion tank cap.

There is no leak apparent from the water pump or P gasket area. The two electric front fans run and the main viscous fan appears to be working OK.
I can take the cap off the expansion bottle when at normal temp and the level remains where it should be, with no bubbles. I have just blanked the return from the thermostat to the water pump as there is no stat fitted. (no stat fitted since I have owned the car and has never been a problem before) This should help the water cooling as all flow should now go via radiator.

I am thinking that the water pump is the next area to attack. I thought the original reason for the overheat I suffered (and blown head gasket)was a core plug I found leaking and a expansion cap that was leaking when tested.

Copper head gasket route was chosen as this is the best head gasket you can buy! ( Well you can't actually buy one, you have to make it ) I last changed the gasket 4 yrs ago along with fitting a new head and I don't want to do it again. When I removed the old gasket it had lifted between the head and the center water passage and was beginning to push out towards the back on number 4 cylinder.
Any one else experienced this issue? I am particularly perturbed by the Auto trans overheat light, as I can not see any connection to the engine getting hot.
Cheers

Follow up:
Issue resolved. I removed the water pump and inspected it, nothing found apart from corroded long pump bolts. Bolts cleaned,greased and pump refitted with new gasket. Investigated autobox intermittent overheat light, switch found contaminated with water from draining radiator previously. No real issue, Auto box oil changed anyway as precaution. Oil looked fine just a bit brown and no burnt smell.

My next step was to remove the radiator for investigation. The radiator was found to be almost completely clogged, virtually no flow from the inlet across the radiator to the oil cooler side and back to the outlet. The water will always flow from the inlet to the outlet because there is a 5/8 dia hole in the baffle plate between them! Therefore the water will flow in and out of the rad unimpeded, even if the rad is completely blocked. In fact my engine had been dissipating all the heat via the engine block, thats pretty impressive considering my outside temp is about 35 deg C here in Brunei. No wonder my engine bay is always like a furnace.

Two things I discovered during this issue that are of note for anyone wanting to fit the ultimate cooling system, or modify their current system.
1) Remove the thermostat core for hot climates and blank the bypass hose from the top of the thermostat housing to the water pump.
2) Blank the by pass hole in the radiator, this involves removing the the inlet/outlet end of radiator and blanking the baffle plate thats splits the flow to force it through the rad on the top half out on the lower half. (two pass cross-flow radiator). Now all the flow goes through the rad without a third by passing it.

Land Rover in there wisdom designed the system to be able to cope with very hot climates, however they never modified individual cars that were being exported to these climates!

After previous issues and this latest issue with my cooling system and head gasket failure it caused, I have become a bit of an expert on these systems.
Therefore I will make a list of the common failures in order of priority to check.

Overheat List: Check when engine hot:
1) Are front two electric fans running? Should be running, if engine is very hot, try the A/C and see if they come on. If not check fuses and work through the electrical system after testing fans with direct applied power to ensure they are working. (There is a Fan Thermo Switch with two pins in the thermostat housing, there are two identical, one is for AC compressor)
2)Check the engine mechanical fan, turn off engine whilst hot and try to turn the fan, there should be some resistance, slightly stiff to turn, if not replace. Please note that as the unit works on centrifugal force when rotating, you will not feel any difference when cold or hot.

Overheating list: Check when engine cool!!!
1) Lack of coolant in expansion tank - Check for leaks, P-gasket at back of water pump, or water pump seal failure indication hole, are the first places to check. Check all hoses, radiator, core plug areas, [one at front of engine three on side under exhaust, one at back (can't see it with gearbox in the way)]. Tip, the front core plug usually seems to go first & they are not a problem if you always use proper coolant and change as required (2yrs).
2) Lack of coolant in expansion tank - After checking 1 above, possible head gasket blown. This may be indicated by water being forced out of expansion tank when engine is started, or by bubbles in tank when running. ( get a garage to check for exhaust contamination if you have one with the equipment). If you can not find any other leak this is probably the cause. I would highly recommend you remove cover and change your P-gasket and inspect water pump before pulling off the head. I would change the water pump whilst at it. Head gaskets are not hard to change but you had better find the cause first or you may be doing it again. Heads normally bow if they overheat. You can skim them, but recess valves the same amount is recommended. Fit a new Multi-layer Steel gasket (MLS) and new head bolts.
3) Overheats but no coolant loss from expansion tank, remove thermostat top blank plug, if no coolant visible, remove radiator blank plug. If no coolant is visible, the three way valve in the expansion tank line to thermostat housing and top of radiator is blocked. These are plastic, they can break and are easily blocked. Remove it and fit a brass T piece instead!
4) Indicating hot, coolant level OK - Check the thermostat is opening, start the engine when cold and feel the top hose to the radiator from the thermostat housing on top of the engine. This will be slightly warm to start with, when the thermostat opens it should be much hotter. it should open just before the engine gets to normal temp. Remove thermostat and test in boiling water, it should open. Just leave it out and try it if in doubt, or replace it, its cheap insurance.
5) Indicating hot, coolant level OK - Check the indication is working correctly. Use a electronic temp indicator with thermocouple wire,(ten quid on ebay) place the end of the wire (thermocouple junction, where the two wires are welded together) on the thermostat housing and check the temp, should be about 80-85 deg C once thermostat has opened and may drop a bit after this. This should indicate in the normal range, center of indicator on temp gauge. If your temp gauge is much different then suspect faulty indication. Change the sensor in the thermostat housing. (Small round single wire connector).
6) Indicating hot, coolant level OK - Checked above 5. Indication correct. Warm up the engine and check the difference in temp between the top and bottom hoses on the radiator, This should be several degrees (6-10) cooler at the bottom than top. the difference in temp shows how efficient the radiator is. If there is little difference then your thermostat could be shut or your radiator is blocked. [Please publish the temps if you try this and have a new rad fitted, I am estimating, as my rad was blocked when I tried this.]

Good check list raise: