Common "Fix" for bad Dual Control Valve?

Oh… I’d added this post to an old one but then thought I should try with a new heading.
Well well well! I can’t believe I’m back on this topic. some of my previous posts have related to heating or lack of it and cooling or lack of it. I’ve replaced various parts over time including the cooling fan after a recent engine overheating issue. I also noticed fluid residue around the crimped hose joints at the heater core end. This explained my slow loss of coolant. Anyway I decided to go with a new set of pipes and all is working well. No more leaks… but I had got used to the slow heat build up in the cabin and that was my “norm” Maybe 10 minutes of driving before any comfort in the cabin on cold days. With the new pipes fitted… almost instant heat!!! This made me investigate the old pipe set. How I was getting any heat I don’t know because under previous ownership and I guess to overcome a failed and constantly open dual flow valve and to save money They did this…

I guess it was some scrap they had handy… interesting that there is a thread inside?
Has anyone seen anything like this before?

I had my compressor and heater control valve replaced by an ac shop. Still getting hot air from some vents and cool from other. AC guy said it was probably the solenoids controlling the air flow deflectors and suggested an electrical specialist. This doesn’t seem likely to me. I suspect that the wire bringing voltage to the control valve is defective. So one side may be working and not the other. Since that is, technically, an electrical issue not sure I’ll be able to talk the ac guy into fixing it. Wonder if that would be considered standard or would only an exceptionally careful mechanic have tested to make sure the valve was getting voltage while he had access to the unit? I guess it could also be the module which is actually a lot easier to get to than the wire.

Maybe that “fix” was done by someone with no access to the Internet or factory service manuals. If it was on a V8 S-type, it hardly makes sense unless the person was on the most slender of budgets - the work and labor to get to the DCCV is so extensive, it’s almost crazy not to replace it. I wouldn’t be surprised it the cooling system was filled with the cheapest coolant available too, as opposed to the factory specified Dex-Cool equivalent.

I just replaced my DCCV. For a long time I wondered why I had poor defogging on the left side of the dash.

After developing a coolant leak, I traced it to the DCCV and replaced it. Suddenly, I had nothing but hot air coming out! Upon closer examination, I could see that on one side (the left coolant channel) the valve was stuck in the closed position, explaining why I had poor defogging on that side. In the moderate temperatures where I live, the loss of heat wasn’t noticed, as the right side easily kept the temps where I wanted it.

The constant hot air was because the solenoids to shut the valves were not being energized. They default to the open position and need to be energized when the A/C is engaged to shut off the hot water flow.

Measurements of the connector going to the valve showed voltage but little amperage. So I concluded it was probably the climate control module (which also supplies the current to actuate the valve.) It seems that the climate control module has no electrical current overload protection, so if a stuck valve draws too much power from the module, the power output on the module can fail.

Thus, not only test for voltage but also amperage to the DCCV.

So I sent off the module to be repaired ($250USD vs. $1000 for a new module) and this fixed the problem. The fix is also supposed to have current overload protection that will shut the module down if current draw exceeds a threshold. So all is well now.

But always check the DCCV first and replace it if neccessary before buying a new climate control module, or you could be buying a lot of $1000 parts repeatedly.


Thanks Dave. Yes mine’s a 2002.5 S Type 4.2.

The guy I bought it from said he was from “a Jag family”

The service manual was well maintained in earlier days but knowing the reputation of the garage

whose stamp appeared in latter days I was disappointed but not surprised at some of the “work” I discovered as I was

getting the S ready for our RWC (Roadworthy Certificate) I’ve often thought that the stamp may have been borrowed

and filled in to enhance the sale?

I had also replaced the coolant control valve early days……but I have to admit that the low cost of the aftermarket sucked me in.

Sadly that was not my only experience with the “good looking” dollar saving parts….

See my earlier post Aftermarket Parts!

So… having been around the circle I’m probably going to do the DCCV again but this time with a Bosch part!

I guess they chose the heater core end to plug due to the connection being a simple hose clamp and not the quick fit connector at the valve end?

Thanks so much for the technical info and details of the DCCV operation.

Much appreciated.


Thanks Richard. I always appreciate a reply.

I’m just an average bloke with plenty of tools and a cheap diagnostic reader.

I know that correct diagnosis is the key to success in every job we tackle but if it’s not a fuse I’m in uncharted territory☹

That’s why I love Jag Lovers as someone is likely to have the knowledge or past experience to share.

I would highly recommend a genuine Bosch replacement for the DCCV if you’re going to replace it.

Maybe it’s an easy job from underneath if you have a hoist, but I went in from the top which means removal of the top hose, coolant tank etc.etc.

Anyway good luck with your project and have a look at the recent reply from another Dave “bdragon”

Kind regards