I mistakenly hooked my feed wire for the fog light switch to the head light terminal. When I turned on the headlights and the fog lights all the lights went out. With a test light I traced power back through the solenoid, through the ammeter to the light switch- nothing! After I take out the switch can I repair it or just order a new one? I’m OK repairing mechanical things but not so hot on electricity. Thanks for any advice you can give me. John
Maybe not a bright question (pun intended), but is the fuse intact?
I successfully repaired the head light switch on my '38 SS.
The central nut is soldered so you have to un-solder it, then it comes apart.
The terminals were very dirty and caused the switch to get very hot, which caused the spring to relax.I cleaned the contacts and stretched the spring to give the proper force, put it back together, and now it works fine.
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John,
Don’t complete understand what you exactly did. You apparently have the optional fog lamps on your car including the fog lamp switch. Are these the original (optional) Jaguar parts or something else?
Jaguar used the Lucas SFT700 fog lamps and the “push pull” Fog Lamp switch Lucas 31228. See below.
Regarding the lighting switch, your XK 120 DHC probably has the final version as used by Jaguar for the XK 120 production which is Lucas switch 31428/A. See pics below from top and bottom.
Now where did you connect the cable to the Fog Lamps switch? Terminal 3 or 4 is connected to the 12 Volts supply and the other terminals are used for the wires to the dip switch and the dashboard lighting after you’ve turned the light switch on.
But even if you have both the head lamps and fog lamps on, the total power consumption will be under 200 Watts (2 x 45 W + 2 x 48 W) corresponding with a current of 17 Amps. I doubt whether this would lead to a “fried” light switch.
Like Lewis mentioned: have you checked the fuses? This can be the fuse for the supply voltage to your switch (see fuse in the Control Box RF 95) or one of the fuses in the Fuse Block (red wires to the head lamps). I noticed there’s one 15 Amp fuse in there, that might have blown.
Bob K.
Lew, Rob and Bob thanks so much for your quick and very helpful responses. My fuses were all okay visually and continuity. But you led me to take a closer look where the wires from the fuse box went. I found I had power to terminal A1 but not A on the voltage reg. The fuse was good between them but when I turned the VR over I found the screw on A was loose. Tightened and everything light up again. Sure beats taking the dash apart and trying to fix the light switch! Rob, so I’ll add your very clear directions to my notes for future needs.
Bob, my light switch looks like Rob’s on the back and the one in my workshop manual so no terminal that I can hook my power line to if I don’t want the headlights to come on also. Any thoughts on where I can connect the fog light power wire?
My fog light switch is a reproduction but looks like yours but has three terminals, I assumed 2 of them for the 2 fog lights.
Again thanks for your guidance, John
My apologies, my lighting switch is not like Rob’s. It is more like Bob’s (center post) but 5 terminals. After crawling under the dash I realized I replaced the switch 4or5 years ago and forgot. A very long senior moment. Anyway I had the line to the fog light switch on the same terminals as the red wires to the lighting switch. John
Sorry for being such a pain. I think I figured it out. I attached the power line to the fog light switch to the terminal on the fuse box which holds the two red/green wires and I get fog lights and no headlights on the parking lights position on the lighting switch. John
John,
If you have a 5 terminal switch you might have the XK 140 switch Lucas 31524 (see picture below).
Do you have 3 positions or 4 positions on your switch? If 4 positions, you might have the switch with the extra Foglamp position.which would make the wiring very easy and you no longer need your separate Foglamp Switch.
Bob K.
Bob, 5 terminals but only 3 positions. Can’t remember who I bought it from but probably SNG or Moss. Anyway it works fine with the separate switch. Thanks for all your time and expertise. John
Rob R. and Bob K. Here are my comments on your photos of XK120 head light switches. Rob’s switch is the early type, which is easily identified by the chrome “horizontal” dash board switch lever.
Sometime in 1952 (I think) the early switch was replaced by the three-position Lucas PRS. 5 head light switch. I believe the PRS. 5 switches continued to the end of XK120 production at least for XK120s not equipped with fog lamps, and they are identified by their offset shaft and six terminals, of which, only two of the terminals were connected to the wiring loom. These switches are also identified by the later “vertical” chrome dash board switch lever.
Bob K’s photo’s are of the later center-shaft PRS. 7 switch. I have my doubts whether this switch was commonly supplied to the XK120 although Viart says it was. There was also a four-position PRS. 5 used with fog lamps, and I used to own one that I sold for use on an XK140 MC roadster I helped restore. At that time, I also had a four-position center-shaft PRS.7 switch that we found had slightly too close of clearances behind the dash to fit the cutouts in the XK14O metal panel. So we used the PRS.5 four position switch instead.
There was also a Lucas three-position offset shaft switch with only five terminals. It’s hard to read the stamped part number on my example, but it appears to be 313036, (The last digit might be an 8).
This switch does not have the elongated through-the-dash threaded case found only on the XK switches. However, the internal electrical internals of this switch can be substituted for burnt out electrical internals in the elongated threaded shaft six terminal PRS. 5 switch. Of the five terminals on this switch, the two necessary for correct operation of the sidelights and head lights are present. This may help explain why the aftermarket head light switch noted by (Grapevine) John has only five terminals.
Mike.
We had an earlier discussion on the “later” light switches on the XK 120, as specified in the DHC Spare Parts Catalog… For the article: see here:
I cannot proof that the DHC light switch was also used on other models (including my OTS) but it’s also not impossible.
Reading what John mentioned regarding the purchase of his light switch, there are many options as to what type it actually is. But as long as it does what it should do…
Bob K.
Bob, I see we also explored the PRS.5 and PRS.7 switches a year ago.
The only other thing I can contribute is the PRS.5 switches are repairable where the PRS.7 switches are not. When opened up, the PRS.5 switches contain a half dozen or so individual pieces than can be cleaned of tarnish and made to work as new. Sometimes there are broken brass connectors between the terminals, possibly from overloading. Those connectors can be successfully repaired with solder.
The PRS.7 seem more robustly built, but the internal components are incased in a one-piece plastic housing. I don’t see any way of servicing the internal switch components of a failed PRS.7.
Just so there is no misidentification, the picture I posted earlier in this thread was of the switch in my '38 SS, which is the S.37 type,
which is also found in early 120s including my Nov '51 FHC 679187. Only 3 wire terminals.
As this thread began on the subject of fog lights, it may be of academic interest to this group to know that in '38 they were using the PS.7 switch for fog lights.
I suspect the posts by Bob K are the closest to what you have and are working with. I will not touch most of what has been said but I want to say in my experience I have had the later type fail internally. Most people today now use a relay to avoid excessive load going through the switch. I saw a few push pull type of switches in the above posts. Over the years I have used a few new repros and they have all failed. A few old 120 switches held up better but I still insist this is not to be counted on w/o a relay. Relays are cheap and fairly simple to install. Try to find a 4 position switch. I want to point out that I have seen some schematics of the headlight switch that are not correct. #4 is the correct input, but after that don’t be overconfident in what is suggested.