77 XJ6 Heater Issue

Hello good people,
I have scoured the forums and cannot yet find a good solution to my current issue. I have a '77 XJ6C that doesn’t want to blow heat. A/C blows cold on all speeds, fans work fine. When I switch the system over to heat (via temp control) I get no fan speed unless it’s on the defrost setting. I hear the servo switch over, but no joy for heat. Both myself and my mechanic are currently stumped. Any pointers in the right direction would be GREATLY appreciated!

Many thanks
August

August,

My first thought would be a faulty temperature switch. The Delanair MKII system uses a temperature switch mounted onto the hot water pipes that feed the heater core. The intent of this switch is to prevent blowing cold ( unheated) air onto the occupants until the engine temperature rises sufficiently (~140f) to provide warm air.

This switch is located on the left hand side of the transmission tunnel behind the vent cheek panel. Look for the heater core pipes and then for a round disk ~3/4" (19mm) with 2 wires connected mounted to one of the heater pipes.

For testing purposes…you can remove the two wires and connect them together.

Hope that helps and is the first thing I would check myself.

Cheers

Gary

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I second Gary, August…

…the temp switch is bypassed in ‘full cold’ and in ‘def’ - so symptoms fit…

frank
xj6 85 Sov Europe (UK/NZ)
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Thank you Gary and Frank! Gary, if it is the switch, would the one you sell be a good replacement for it?

August

August,

I do not currently have anything that replaces that specific switch. I have not checked the usual Jaguar parts houses…but i think I can source an acceptable replacement if necessary. I will look into a source.

Cheers

Gary

Many thanks, Gary. I am by no means an expert on this stuff so I appreciate the help.

August,

At first glance…the thermostat in the picture would appear to work…normally open, circuit closes on rise and appears about the same style mounting. To be fair…i have not removed a temp switch for careful measurements…but this may be worth a try if your is indeed failed.

Cheers

Gary

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To elaborate, August; the switch is simply clamped to the heater core inlet hose, and disrupts fan control connections in ‘heating’ until coolant temps reaches around 105F.

It is not an essential part, just a ‘comfort’ one. As AC is turned out of ‘off’, the compressor engages and the air blown in will be very cold indeed, without heater core reheat. Basically, the same effect can be had without the switch by just shorting the switch wires, allowing all round fan control - and just not turn the AC on until coolant temp is ‘adequate’…

The switch shown by Gary will do - mounting similar. The only difference being that it connects at 140F; some 35F higher than the original. This will delay fan activation by some 1 - 2 minutes while driving, compared to the original. Which may not inconvenience you…much…?

Frank
xj6 85 Sov Europe (UK/NZ)
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Update, I’m a dummy. Turns out there was just a loose clamp that was letting some coolant out. Reservoir was full because it was past the reservoir. Upon taking the cap off, we realized the error. Simple fix. Blows hot. All is well with the beast.

Thank you kind sirs for your prompt and thoughtful responses, I appreciate it!