Repairs to Jaguar XJS Facelift Headlight Logic Module - DAC6902

[mwsmith29]

When putting my XJS back into my garage I realised there was a fault with the headlights. The dipped beam had gone very dull, so I switched to the main beam and I found this to be OK. As both dipped beam lights were faulty, I focused my fault finding on the Head Light Logic Module.

Some years ago, I had repaired this module when the lights were failing in a similar way. I cleaned the internal contacts on the two relay assemblies and full headlight brightness was restored. I expected to encounter the same issue and maybe have to replace the relays if the problem was repeated.

I removed the module from its under bonnet mounting bracket and detaching it from the vehicle harness. Then with the module on the workbench I scraped away the silicone sealant on the base of the module. The two moulding clips that secure the cover to the printed circuit / connector block were prised apart with a small screwdriver and the printed circuit board lifted from the cover for inspection.

I detached the springs on the two relays and stored them in a safe place. I then lifted out each of the armature assemblies, see photo. I was surprised to find the silver contacts in a good general condition, only a small amount of contact pitting and carbon deposits.

I decided to check the PCB in more detail to see if there were any other issues that might affect the modules operation. The large electrolytic capacitor seemed in good visual condition There was no evidence of leak out and it passed a basic multimeter check. All the other components looked good also. However I noticed some distortion on the top of one of the two BC337-40 transistors so decided to check further. (See photo).

Using a solder pump, I de-soldered both transistors and removed them from the board. A look up on google identified the transistors as NPN types and a further google look up gave simple instructions on how to test them with a digital multimeter. The transistor with the distorted top failed the test but the other passed. (I have had faults caused by BC337 transistors on other modules on my car namely the speed interface and seat control modules so I already had spare transistors in my possession). Replacement transistors are available from the usual on-line stores and are not expensive. It should be noted that all the electronic components on the PCB are still readily available with the exception of the relays. It is not a complicated module so if you are not confident at fault finding yourself, you can take the module to any electronics engineer or tech guy and they will easily identify faults.

I decided to replace both the transistors with new ones which I soldered in place. I cleaned the relay contacts using a fibre pen and very fine abrasive paper (2000 grit) and reassembled the relay springs. Finally, I reassembled the module and fitted it back to my car ready for testing.

I operated the module a few times by switching the lights on and off and then switching between high and dipped beam. This was to allow the relays to bed in before a proper visual check is conducted. The test was successful which was both gratifying and soft on my wallet !

[davidr]

Well done - a good fix!
I'd love to publish that in the magazine with your permission

[steve_m]

Well done that man!

[ptjs1]

Excellent job, Mr. Smith!

Paul

[andypryce]

Most impressive, I am sure a Jaguar main dealer would have done the same for you

[AndyF]

Excellent work! I love seeing how you have tackled this - and then writing it up in such a way that others can follow. That is exactly what this club and forum is all about.

Take care,

AF

[ng4xjs]

Wish I had thought of that before splashing out on a new unit. So what about the ant slosh module for the fuel gauge. How do I fix that please?
Reads correctly initially and then quickly drops proportionately towards empty,

Nick

[ptjs1]

The Anti-slosh module problem is usually caused by the capacitors "leaking". I've got mine out of the car at the moment and intend to try and do a repair if I can get someone to identify which capacitors I need.

Paul

[ng4xjs]

Hi Paul - Do let me know the outcome. I've lived with this for some time so not really a personal issue. I have contemplated a replacement but so far resisted.

Nick

[mwsmith29]

I’ve not looked at that module.
Maybe you should start with a new post specifically for it. Opening the module up and add photos to help.