Testing micro switches in the brake light switch

[Dave1941]

I am having the micro switches in my XK8 brake light switch replaced. How should they be tested before I commit to refitting the switch to the car? Does anyone know where I can buy these 2 micro switches for an XK8 2005? Thank you.

[Astromorg]

When the brake pedal is pressed, one switch should change from Open to Closed to enable the operation of the brake lights and the gear selector lever solenoid interlock (If in Park) (White/Blue and Orange/Green wires) and the other should change from Closed to Open to cancel cruise control (Blue and White/Blue wires) This info is also passed over the CAN bus to inform the ECU. Before replacing the assembly, connect the wiring and with ignition on, operate the switch by hand and see if the brake lights work and the gear shift solenoid releases. Testing cruise control will have to wait until you're on the road unless you have access to a suitable diagnostic scanner that will show the switches operating.

A suitable replacement switch would be this one from RS Components; part no 682-1557. Just put that number into the search box at the top of RS C's website and you'll get all the details so you can check its dimensions etc. Plus RS C do a great, next day free delivery service, with no minimum charge.

When the switch is assembled, ensure that the ratchet arrangement in the white plastic part of the switch is opened out as in the second photo, not pushed into the housing as in the first one. It will self adjust if necessary when the pedal is operated.

[Dave1941]

Thank you for replying - I have a spare brake light switch into which I am having the micro switches replaced. I want to be sure it works when the job is done before I re-fit it to the car. How can I test the switch to determine that it is aok?

[aaj]

Hi
the small PCB is marked SW1 & SW2
SW1 is for Brake lights
SW2 is for Cruise Control
There are 2 Units for the XK8/R (X100)
LJB 6420 AB (NON-CRUISE) - 2 wire
LJB 6420 BB (for Cruise Control) - 4 wire
X100's without Cruise Control can use either witch Unit but X100's with Cruise Control MUST use the 4 wire version.
All 4 wires are Black
Both units use the same plug & socket
The outer 2 wires are for the Brake Switch
The inner 2 wires are for the Cruise Control
With the Switch Body removed from the car (as the photos provided by Astromorg above):
Switch SW1 (2 outer cables) forms a closed circuit at rest (current flows), when the white cam activates the Switch the circuit breaks
Switch SW2 (inner 2 cables) forms an open circuit at rest (no current), when the white cam activates the switch it forms a closed circuit
SW2 is the microswitch you can see in the photo (SW1 is behind SW2)
As Astromorg has said, the ratchet on the mechanism MUST be reset prior to fitting the Switch Unit in place or it may will not work as expected.

Regards

AllanJ

[Dave1941]

Thank you all for this information. I bought two new micro switches and had arranged for them to be soldered, but before that was done the switch was tested by my expert and found to be working! There is nothing wrong with it. Can you give any other reason for the warning messages re lights and cruise control appears on the dash. Incidentally, the diagnostic check points to the brake light switch.

[Astromorg]

Yep. That happened to me. Mine was caused by a dry solder connection between the circuit board and one pin of one microswitch. I re-soldered all the pins and have never had a problem with it in the nine years since.

[Dave1941]

Are you saying that you re soldered all of the pins in the micro switch, or the circuit board. If it is the circuit board, where exactly can I find it?

[aaj]

If you look at the photos Astromorg has supplied there is a thin dark green board above the microswitches.
There are the 4 solder joints for the wires & three solder joints for each switch (although only 2 of each switch are used electrically).
The Board may be removed by lifting the 2 ‘tabs’ above the Board & pulling towards you (as you look at the photo). Sometimes there is a ‘blob’ of Silicone Sealant holding the Board to the Switch Body ( light green unit in photo).
Jaguar Electronic/Electric Units are notorious for dry joints therefore I would re-do all the solder points to be sure.
AllanJ

[Astromorg]

Thank you Allan

[Dave1941]

Let me get this right, even if the switch test shows that it is working there can still be a dry solder in the micro switch.

[aaj]

Depends on where the switch is tested (you can test at the pins &/or at the cable plug)
The dry joints are not within the switch but where the switch connects to the PCB (Dark Green Board)
Dry Joints can show fine one minute & fail the next (and can be intermittent)
For the sake of a few seconds with a soldering iron it's worth doing each connection
Be careful when removing/replacing the PCB as it's easy to damage the 'arms' of the microswitches making them u/s!
If you are not comfortable doing this then your local TV repair man will do it for a few £'s (assuming you're removed the brake light/cruise control switch unit from the car)
AllanJ