I haven't driven the XJ12 much this summer, and it hasn't been an exceptionally hot summer here in Chicago, but the other day I noticed the temp gauge at 12 o'clock instead of the usual 11:30. Did a little inspection and realized the two secondary cooling fans in front of condenser don't come on even when this hot condition exists.

Checked the two 30 amp fuses. Checked good.

I noticed the wires going into the radiator temperature switch looked a little frayed at the harness and when I unplugged the harness two of the wires broke. So I cleaned the ends of the two non-ground wires and joined them, by-passing the switch. Still no cooling fans. I swapped out the fan relay with two different relays from running XJ6's and still no secondary cooling fans.

The relay is getting fused power from both fuses and ground at the relay checks out.

Am I missing anything here. The XJ12's appear to have a simpler relaying system with the only input coming from the radiator temp switch and not the additional two pressure switches like the XJ6.

Thanks for any help...

 

The radiator cooling fan control is the same on both the AJ16 and V12 engine with the exception of the A/C commanded on requirement . The large white fan control module relay is located under the car in line below the air filter . To verify that the 2 fans and control module work you can release the radiator temp switch connector ( radiator aft left lower location ) and take the white wires one at a time to ground ( which is what the switch does with the black wire ) . Paper clip works and is a low current control circuit . One white wire to the black will give you one fan and the other white wire will give you both . You can bring the right fan connector over to the left for troubleshooting if the left doesn't come on . Look at the ground studs below the air filter . Be careful of running fan . Someone menioned the a X308 fan will work on a X300 with modifications . My fans were working fine so I took them apart anyway and they have very large long wearing brushes and a LRC noise filter .

 

Thanks for the troubleshooting advice. It seems I'm getting closer to a solution. I have voltage on each of those white wires. when I ground one of them the left cooling fan turns on, so a repaired harness plug to the radiator switch is in order. When I ground the other white wire I hear the relay click but no right side fan. When I combine both white wires and ground, again only the left side fan runs.

Maybe a bad right side fan motor?

 

Bring the right connector over to the left , secure loose wiring to not get caught in fan . Then perform each white one at a time to ground . This will verify you have enough power on the right connector to run the known good left fan motor . The White / Blue wire should be on the connector location opposite side of the Black on the outboard edge . This also verifies the relay is operating . A relay can click as it pulls in but not carry enough current to run a high current device as one mode of failure . Was the XJ12 a resent purchase that had my eye ?

 

I haven't yet had time to do more troubleshooting. On a hunch, found and ordered a cooing fan assembly off an XJ12 6.0L in case I have a bad fan motor. I'm not sure the harness plug to the radiator temp switch is salvageable as the wires a broke right at the back of the connector. Same connector as the XJ6 so I should be able to round up a pig-tailed replacement.

I've had the XJ12 since the new year. The only other issue I'm having is a secondary air injection intermittent fault. I'll have to get it sorted before my emissions testing comes due...

 

You can pull out the wire seal grommet and soldier the wires to the stub wires left over ( stripped of insulation with a Xecto or razor knife ) . You won't be able to get the grommet past the soldier repair so you can seal it up with some non vinegar based silicone sealastic ( corrosive to wires ) . You can do this for trouble shooting before your final repair of the connector if it gets you down the road . The XJ12 you have is likely the one I was looking at , everyone has to have a dream . If you are not going to use the fan frame that goes across the whole radiator opening I am in need of one for the previous owner of mine did not repalce the original during front end damage repair . As to getting advice on the V12 ( engine regulation is very different ) you would probably get some more input from the XJ ( S ) population , but you can still ring out the pump and relays . The ECU comes in 2 versions , the later years not being Lucus . Do you have the Wiring Schematic for the X300 and the V12 Engine Managment Manual ( starting page 193 ) ? Get to cut the grass now .

http://www.jagrepair.com/images/Trai...20-%202000.pdf

Thanks , Parker

 

Thanks for the link Parker. I do have the electrical diagrams but the diagnostic manual will help I'm sure.

My XJ12 is a carfax one-owner and came from a collector in the Chicago area who liked "last of" editions of cars. His wife drove it and it's in excellent condition. I swapped the original 20-spoke chromes for 18" XJR Asteroids, and removed the 24k gold-package trim that was added by this owner. I initially planned on flipping the car, but while driving it home for the first time, I knew I'd regret not keeping and driving her. Other than the fuel mileage, she's the ultimate highway driver.

 

Maybe someday you could flip it to me at 60k miles it ts a good find , I have to settle on a 110k mile inline 6 but I do have plans on refining the engine out to 300hp normally aspirated . Keep in mind as you own it the suspension is stiffer then the standard X300 including the VDP so the shocks and suspension bushings are stiffer as well as a different transmission . You can modify the ignition system over to solid state before the big bucks for even a distributor cap . You can bring it down softer but you compromise handling .