Driving at low speeds the heater works fine. Drive at around 50 mph or above, the heater blows cold - slow down & you have heat again. This I believe is down to the control valves shutting off (you can feel the inlet pipes cool & then reheat).

Having replaced everything - including the control valves. I'm beginning to think that this is an ECU or sensor issue, causing the valves to shut & not allow the coolant to flow (Note:- I have fitted a New ATC unit).

Even tried driving with the control valve electrical connector disconnected ( theory behind this was that the valves would remain open). This was the same & just produced a fault on the IID check.

Tried running live values & everything appears correct. I did find a posting on how to check the electrical connector on the control valves, but unable to find it again.

Anyone else got a clue on this?.

Depends, just confirm your engine. Iv had similar but it looks like an air lock I had a small weep of coolant from my lpg vapouriser . All bled out and now seems fine.
I'm sure previously I had a faulty auxillary heater pump also causing the same issue. Its not your heating valves shutting down just the small electrical pump not working.

Hi Dave, I have the 4.4 AJ engine. No FBH fitted.
Bled system many times & No coolant loss. New auxiliary pump fitted & working, According to the W/S manual. The pump only works on low engine speeds & would account for the low speed heating.

This is the list of replacement parts :- Switch panel (ATC) Temp sensors, Control valves & aux pump, Thermostat,

I'm convinced it's an electrical issue. But struggling to find it!.

Iv got 6he m62 but I'm sure the heating system is the same, I had to remove the rad top hose in the end to bleed out the air locks I poured coolant down the hose hose direct to the thermostat and into the rad it's self. Then reconnected and final stage was to pull the small hose off the top of the rad that feeds back to the expansion bottle and blow down it several times forcing cooland and any trapped air to come out of the expansion cap hole each time topping up via the small hose again with coolant. It was a but all done now.

Have you checked your stat ?? And doesn't the jag engine have a factory fault with incorrect wiring to the aux heating pump ?

" And doesn't the jag engine have a factory fault with incorrect wiring to the aux heating pump ?"

Dave, Same set up as yours. Interested in your comment, What do you know about this? as I'm not aware of any wiring issues at all.

Trying to rectify this fault for years, It's just when I need to do any distance & speed in the winter, that it becomes a problem & now determined to sort it.
Just looked at the Jag forum & the issues appear to be when the pump fails & no heat at idle. I have the opposite, Heat at idle & low speed, No heat at speed. This happens quickly to cool & reheat, thus I think that the control valves are shutting off ;- Surely if the valves were open, you would still have heat being pumped around.

Paul.

I had the same issue. I had the FBH changeover valve rebuilt

Edit: just realised you don’t have a FBH! 1995 4.6 v8, green/tan (long gone)
2005 4.8iS X5, blue/black
2011 4.4 tdv8, black/black (gone)
2003 3.0d X5, Oxford green/black/manual
2021 p575 SVR, BR green/vintage tan

I wish I did, would have probably been an easy fix

There is a teck bulletin for the coolant pump where the wires were crossed over in the factory essentially making the pump work in reverse, I'm sure iv read it on here I'm just trying to track it down

Here you go:

https://tinyurl.com/2cuz9p49

Thanks Guys, But unfortunately wired correctly.
I did check the Control valves whilst I was in the engine bay & noticed that both connections were only receiving 2.62 Volts each (two values as it's the high line system). This remained constant despite low or high fan speed selected.

Trying to find what the current should be to the valves & wondering if this is causing the valves to partially close & made worse at high speeds?

I'm awaiting a new FSR & see if that has any effect

OK FSR fitted & exactly the same

Heater blows hot on maximum speed at less than 50 mph. Go over this & the temperature drops. Drive at a constant speed & then put your foot down the temperature drops, ease off & the temperature rises

Something is telling the control valves to close as soon as the engine or speed rises. I take it that this is coming from the ATC, but this brand new & was doing the same with the old unit.
Where does it get the vehicle speed & engine speed readings from?. Can this be adjusted with the IID tool?.

Update :- Heater works fine with everything connected - But temperature drops when revs exceed 1500 RPM (20 degrees C). Drop below these revs & instant heat again (26 /27 Degrees C).
Replaced the control valves with a blank & I get a constant low heat of 20 degrees C @ 2000 RPM.

Replaced the coolant valves, then disconnected the Aux pump & Viscous fan. Slight improvement constant 21.5 degrees C @ 2000 RPM.
Obviously the Aux pump is only there to circulate the heter matrix at low speed (which it does fine). Unsure how can I increase the high revs temeperature, possibly a higher thermostat (stnd currently fitted). Read a previous post regarding the same issue & replaced the water pump but to No effect!.

Haven't tried permantly wiring the aux pump yet, but this shouldn't be the case.

Any thoughts?.

I'm not familiar with the system but I read your thread with interest. It occurs to me that there must be another valve controlling water flow between the main circuit and the heater matrix circuit. Similar to the FBH I suppose but obviously different as you don't have that fitted. Cam

2011 TDV8 Vogue in Stornoway Grey

There are no other valves

Cheers Dave. Yes definitely no other valves. I have decided to replace the water pump & all hoses. Including the Oil mixer control valve, As this is all that's left.

I'm awaiting for the parts & a bit better weather

I'll report back once changed. Next problem now is finding a battery drain (Never ends)