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Well, I learned something today


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OK. S-Type. When you press the brake pedal, the driver's side brake light fails to illuminate, the driver's side fog light illuminates and the driver's side tail light glows dimly. In addition, there is a tail light bulb failure warning on the instrument panel.

That somewhat complex little problem has vexed me for two days, so much so that towards the end, I was losing the ability to diagnose logically. I just couldn't nail it down.

It really is true that sometimes you can't see the wood for the trees.

Of course, it didn't help that the rear lights on the S-Type are all negative side switched, and the fact that we had just fitted the tow-bar electrics which worked perfectly.

"It's an earth fault" I said to myself and indeed it does have some of the classic similarities but I just couldn't get a bead on what the problem was.

Thanks to Joe Dot Com who offered invaluable advice and an inspired synaptic link from Denis, things began to make sense.

Did you know that the rear light clusters are not only negative side switched but that the 12 volts to the cluster is actually two separate supplies?

Thereby hangs the gist of the solution. What had happened is that one of the 12 volt supplies had gone down and as a result there were some weird and wonderful paths through the bus-bars within the light cluster. A sort of positive earth fault I suppose :winkiss:

Anyway, after almost two full days of getting nowhere I now have another little foible regarding the S-Type stored away for future use.

We also successfully diagnosed the front foglight problem on Denis's car as well :yahoo:

Thanks go to Stacey as well for the endless cups of coffee and for listening so politely to the almost constant mutterings from yours truly.

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Hi Paul,

Very interesting and thanks for sharing, do you know if this is unique to the Stype or could this apply to other models ?

I am always baffled by manufacturers to complicate electrical setups, or maybe there is an important reason for the dual switched polarity. Reminds me of my first car VW Beetle which had a 6v reversed polarity electrical system, as you can imagine it was hopeless and soon modded to a 12v system. Got me thinking how handy some type of electrical workshop would be just to give us an insight into our cars electrical systems.



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I wonder if if that was the reason why Front Fog lights were not fitted to the S Type 2007 Model?

I wondered why mine hasn't got front fogs when most other options are fitted. Now I know [emoji1]

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I don't think it bears any relationship to the front fog lights chaps. If the front electronics module is the same as mine, front fog lights could easily be retro-fitted as far as the wiring is concerned. Not sure if the lower bumper is capable of holding the fog lights though.

The only reason I can think of regarding the two power supplies is perhaps so that in the event of one supply going down you would still have some rear lights, for example, in the case above, where the fog light appears to mimic the brake light.

Just a surmise of course.

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You are probably right, Paul.

I thought about having the Front fog lights fitted, but I do little night driving and the headlights are pretty good on the S type.



P.S.  Have you seen the post about the Jaguar Spares event at Stoneleigh?  I might go unless SWMBO has something else less interesting planned.

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