I'm having trouble with the headlights and the auto function which seems to be caused by the main lighting switch on the steering column. Car is going in next week for investigation.

So I opened my JTIS disc on our PC and had a look at the wiring diagram for this subsystem. Well, it appears I am way out-of-date. I expected to see switching via separate wires for 'Side' and 'Headlights', plus no supply for 'Off', and a separate feed for 'Auto-lights' to the light detector. Simple me !!

It is nothing like that. The whole system seems to rely on varying voltages over signal wires connected to the instrument cluster, and then via the Front Electronic Module to switch the lights. The main lighting switch on the column contains 13 resistors which are switched in and out in groups !

There looks to be a reference voltage input on a Black/white wire. This then supplies each of: -

- main beam
- main beam flash
- LH and RH turn lights
- autolamps
- exit delay (3 settings)
- side lamps
- headlamps (dip)
- off

There are four signal wires coming out: -

- White - main beam flash and main beam (3 resistors 220, 270 and 470 ohms, latter two switched)
- White/purple - turn signals (as above 3 resistors)

next two are via a 56 ohm resistor
- White/green - exit delay and autolamp (3 resistors 180, 270 and 470 ohms)
- White/red - off/side/dipped main lights (as above 3 resistors).

Even the 'Off' setting supplies a voltage to the system, via the 56 ohm resistor.

Are there any experts who can explain what is happening in simple terms ? I presume the voltages on each signal wire are used by a logic control to work the lights. It seems a complicated way to turn lights on to me.

 

wow, way over m.y head...sorry An EE may be the only person to explain that one to you. Can't you just test the theory of swapping out the lighting stalk?

 

It's complicated but it reduces the number of wires in the column.

The entertainment system controls on the steering wheel of the X308 works the same way. It only needs two wires to control an array of signals.

The audio module can read the voltages and translates that into a command to the audio module. For example click the volume up button. The audio module sees a X volt signal and maps that into volume up. If you hold the button, it is also measuring the duration and understands that it is continuous up until the voltage falls.

 

I got one of the wire colours wrong, it is White/blue, not White/purple.

To H20boy, Yes, I have authorised a new switch if necessary, they are around £140 here in the UK, (plus our 20% VAT for our wonderful government !!), Its about a 20 minute swap time.

 

I'll try to keep it as simple as possible for you

The steering column switches (stalks and buttons) all use voltage dividers
Each of these are setup so that a unigue voltage is present on the output depending on which switch is on
The modules use these voyages to decode which switch is pressed and therefor what to do
As was said before you can get all the signals now on just 4 wires. two for the stalks one each for the steering wheel controls

Hope that helps. Let me know if you need more as i'd need to think of an easier way to explain it

Cheers
34by151

 

Huh???

 

Yep, voltage dividers.....

Amazingly my '92 XJS has a similar set up for the cruise control switch. It states in the electrical manual that "SET" gives 100ohm resistancee, and "RESUME" gives 300ohms.

During my cruise control troubleshooting I probed the switch and found it to be exactly as described.

I suspect my garage door opener switch utilizes similar technology because it has 3 separate buttons (open/close, light, lock) yet only 2 wires going to it.

Cool stuff......

 

Yes, 34by151 has confirmed how I thought it worked. However, it does seem an awful lot to cram into the small space on the end of the stalk. 13 resistors plus the make/break of the individual switches. I have to say, I'd behappier with a separate lighting switch on the dash like my old XJ Series 3 had, plus my last car, a Rover 75.

 

Think of it this way

If you have 12V and 2 resistors of the same value in series you would have 6volts between the 2

If the first one was 2K and the second 10K you have a different ratio and you would get 2 volts

if you add the 2 resistors together 12K and divide volts by resistance you come up with 1ma or 0.001 amps

Ohm's Law V=I*R
V= Volts
I = Amps
R = Resistance

In you would never use 12 volts in a car as its not a stable voltage. Typical is a regulated 10, 7 or even 5 volts

PS Fraser, the controls in the R75 uses the same system, well mine does anyway
Yes I have an R75 tucked away, (royal blue perl, sprintex supercharged v6) every time the Merc fails the trusty R75 comes out to play

Cheers
34by151

 

I loved my Rover 75, and would have kept it if I had the parking space, as I didn't need its sale money to buy the X350. IMHO it was one of the best cars ever produced by the British motor industry; better than the S-type, anyway.

 

Well car was with the shop Tues - Friday this week, got it back around midday today, and the upshot is they disconnected and cleaned all loom connections, checked the pins, and have replaced the column lighting switch.

So far, in the early darkness of our November in the UK all seems well, I have not seen any winking or flickering so far, but that's only one evening gone ! However, I pulled out the wiring diagram for the HID lights from my JTIS CD, and between my left hand and the headlight bulbs I want to switch on there are only two moving parts, (1) the column switch, and (2) the dip beam relay, so if the fault reappears a new relay will be next, but I'm keeping my fingers crossed.

Car is driving beautifully at the moment. Maybe I shouldn't have said that !!

 

And..after more than 3 days?

 

All continues well, with no flickering or 1/2 second loss of headlights during night hours, and no funnies during the day on AUTO. This latter problem was that the lights would come on momentariliy whilst driving in daylight, so great danger of somebody mistaking one's intentions. Just have to see how it goes now.

Here are some pics of the switch. First off the switch had been "accessed" at some point, because part of the body moulding that takes one of the securing tangs was broken off. ONce I got inside, nothing untoward appeared. I do have my doubts, however, on the tiny plug on the end of the stalk where the four wires connect to the circuit board.
The third pic of the cover shows the missing corner where a securing tang clicks in. So Charlie Allthumbs obviously got to my car at some time in the past, probably looking for the problem I had, but didn't swap out the switch.

 

Not sure what the part cost, but as you have it why not repair it and keep it as a spare

Get some scrap plastic.
Take out your demel and carve out the missing bit
To plastic weld it on;
Put superglue in the join then cover in baking soda.
This gives an instant permanent join
You can brush of the baking soda right away as it all will set instantly.
Now you can sand back any excess with wet and dry or the demel

Cheers
34by151

 

Nope, its going in the bin because the broken case is not the problem, it is the intermittent faults somewhere in the rotatiing switch in the arm, or on the circuit board, that are the problem.

Switch cost £124.91 (= $198). Of course there is our viciously high VAT tax of 20% to add on to that.

Last bill of 2012 ? Lets hope !!