Is 1A big enough? (*)
My new DCCV (not yet fitted) reads just over 14 ohms when cold. That's over 1A when the alternator is on boost (over 15V for a while).

(*) engineer-speak for "probably not"

I don't know how many people realise the alternator is programmed to put out different voltages (it's in JTIS if you're inclined to read)...

 

Good point. So the DCCV should normally operate at 13-15 ohms and voltage should be between 12v-15v normally. I should put in a 1.25A fuse. A 1.5A or 2A should do the trick.

 

Yes, there is a way to do it, maybe a little easier.

On your car, there a connector just below and to the RH side of the radiator, accessible from under the car. It's clipped next to the big cooling fan power connector. You can separate that connector and using the DCCV feed wire as a common, measure the resistance of both valves. That's for all S-Types 2003 and later.

For 1st generation S-Types, the connector to use is clipped to the fan shroud just below the MAF Sensor, also clipped next to the fan power leads.

That will save all that digging to get to the DCCV itself.

Cheers,

 

In practice I've found the resistance you want to see is 15-17 ohms. I start throwing the valves out if they get below 15 ohms. If they get below that figure, somethings going on and I won't trust it.

Cheers,

 

Well, the repair is complete and all is working as it should. The DCCV valve replacement was easy (except getting the wiring harness off the DCCV), the coolant bleed process was not difficult and the CCM now has a jumper and a glass 2A fuse replacing the burned trace from the pin to the Siemens BSP 78 chip marked as IC7. If the DCCV goes again and the resistance in the circuit drops from normal 14-18 ohms to 6-7.5 ohms, the fuse should blow and save the CCM from damage - maybe not as elegant as the circuit protection that some of the rebuild guys put in, but I can replace the fuse in the CCM easily if this happens again and I've saved another $150-$300 on a CCM rebuild. That is, if it works!
To be expected, but still shame on my Jaguar dealer for quoting $2700 for wanting to source a new CCM ($1500), Jag DCCV ($400) and then about $700 labour... This entire process took less than 2 hours and cost $250 at most - and if I'd have shopped around for a better deal on the DCCV it might have cost under $100.
Thanks to everyone who helped me with this, especially xjrguy and jagv8! I've overcome my fear of opening the bonnet and saved myself a small fortune in the process.

 

Good result! Thanks for the follow-up.

 

Can definitely use a diode as its one directional

 

So is the current. Either the diode is always conducting or the circuit is disabled so what is it doing that's useful?

 

The direction of the current A to B the direction of the short circuit is from B to A ... Yes I would use a fuse aswell. Not a 1amp... A 3amp inline but I was merely saying I have seen the thread to which the writer referred to and a agree a diode can be used.
Vern

 

There's no such short circuit in the opposite direction.

 

I know it's a stretch asking,,, but would this be true for a 2004 Jaguar XK8 XKR? Begging, lol. Be gentle