Greetings All,

After a recent query about the all too common no-heat condition, I got to thinking. The most common failure is for the individual valves inside the Dual Climate Control Valve (DCCV) to get stuck. If these valves stick closed, you won't get any heat from the heater core, no matter how high you set the desired temperature. It's also possible for the valves to stick partway, and you'd only get low heat.

The valves are spring-loaded open, so by default you get maximum heat should anything fail, as long as nothing is jammed. In the past, I've often recommended disconnecting the electrical connector from the DCCV. This removes any electrical command and the valves SHOULD spring open and give max heat. This is super handy for troubleshooting, so you can determine if you've got a control problem or a coolant-related problem.

The only downside is that connector is a booger to reach. It's buried between the hoses, and the connector has a little sliding latch that is difficult to release, too. There has to be a better way, right? I got to digging through the wiring diagrams, and sure enough there is.

If you have a pre-facelift car (1999-2002), remove fuse #1 at the Front Power Distribution Box:





For a post-facelift car (2003+), remove fuse # F32 at the Front Power Distribution Box:





In both instances, this removes the power supply to the DCCV. If you don't get full heat now (manually select HI on the control panel), the fan is running, and you've confirmed the coolant temperature is normal, then your DCCV is bad.

Courtesy of Gus, here's a blow-by-blow guide to replacing the DCCV:

JagRepair.com - Jaguar Repair Information Resource


Here are a couple of caveats if pulling the fuse for troubleshooting:

1) You will also lose power to the clutch on the AC compressor, which shouldn't be a big deal.

2) On V8 models, you will also lose power to the auxiliary coolant pump. The primary purpose of this pump is to increase coolant flow to the heater core with the engine at idle. To compensate, you'd want to rev up the engine a little to ensure adequate coolant flow. The pump also provides additional coolant flow during certain conditions such as an engine overheat, so don't leave the fuse out any longer than needed for troubleshooting.

 

Can I leave the #1 fuse out if it gives me heat. I haven't tried it yet as it's 430 in the morning. Waiting for some daylight. Thanks in advance for your time.

 

I would not leave the fuse out for any extended period. I offered that suggestion primarily for troubleshooting.

With that fuse removed, the AC compressor will also be disabled. Even during the winter months when heat is commanded, the AC is designed to run at a low level to dehumidify the cabin and limit fogging on the windows. The defroster might be able to keep the windshield clear, but you can expect fogging on the side and rear windows.

Until you can get the problem corrected (new DCCV?), you can manually increase the selected temperature until HI is displayed. Much like pulling the fuse, this also removes power to the DCCV and lets the two internal valves hopefully spring-load to the full open position for max heating. There's no guarantee the valves will actually open fully, as they are prone to jamming, but selecting HI is no different electrically than pulling the fuse as far as the DCCV is concerned. But on the plus side, the AC compressor will still run as intended to limit fogging.