Repurpose of Unused Heated Rear Window Switch as Starter Disable Switch

Follow a suggestion by @Big2Bird in another email string I modified my 1969 E-Type FHC today to repurpose the unused Heated Rear Window Switch to act as a Starter Disable Switch by adding two new wires. The first wire went from the C1 pin on the Lucas 33252E Starter Relay and connected to the #1 pin on the Heated Rear Window Switch after I removed the White/Red wire from the C1 terminal. The second new wire went from pin #2 on the Heated Rear Window Switch to the White/Red wire female connector leading to the starter solenoid. I did not modify any of the existing car wiring. In addition to building up the new wires and soldering in the appropriate male and female connectors with shrink tubing, I also disassembled and cleaned the electrical contacts inside the unused Heated Rear Window Switch to clean up 54 years of gunk, and disassembled and cleaned the electrical contacts inside the Lucas 33252E Starter Relay which were moderately tarnished. The attached pictures show some of what I did.

Picture 1 shows the Starter Relay and wiring before the modification. Picture 2 shows the electrical contacts inside the Heated Rear Window Switch cleaned up and ready to reassemble. Picture 3 shows the new Red and Black wires going to positions 1 and 2 on the Heated Rear Window Switch. Picture 4 shows the routing of the new wires beneath the glove box. Picture 5 shows the Heated Rear Window Switch in the “ON” position enabling a start.
I drove the car twice today after making this modification and I am pleased with how it worked. With the Heated Rear Window Switch in the “OFF” position, the starter solenoid is disabled and the car will not crank or start. With the Heated Rear Window Switch in the “ON” position the starter cranks the engine and engine starting is normal. These wiring changes are not obvious, are easily returned to their original configuration, yet I now have a minor theft deterrent. All I have to do is remember to turn on the Heated Rear Window Switch before starting the car. :smirk:



Excellent modification!

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I have a hidden switch on my car interrupting the fuel pump wiring. Easily reversed. However, one day my brain will experience more befuddling than reality in its everyday life, and by then someone else will have to have been told, and shown what I did. Same with the permanently wired in alarm, and soon with the wired in GPS tracker.


It’s more easy than you think, especially with the looms for later cars or compatible with automatic gearbox cars. These will have wanted to inhibit starting if not in Park or Neutral, so used a black/green wire from the starter motor relay in the engine bay and brought this back to the autobox gear lever in the cabin. Instead of directly grounding the starter motor relay winding with a black wire, it is possible to ground it via unused black/green which is brought to the switch for onward earthing from there.

You can even substitute in the 2way switch with a 3way switch and still use the second position for an Otter override as the middle position starter inhibit is only ever needed momentarily when the starter motor needs engaging. This means the spare switch can multitask.

You can see this starter “option” on the s3 wiring diagram, a copy of which is on the xkedata website.

kind regards

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I’ve been using my E-type’s rear defroster switch for a theft deterrent for years. I don’t want a thief to know anything is amiss, such as not being able to crank the engine. I just wired the switch to ‘make’ or ‘break’ the circuit to the coil. They can crank and even get gas, but no spark.

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I used mine to control the fuel pump. When not pushed or in the same ,not “on” position as the others, the pump will not run.
I decided to control the fuel pump because its not fused and there is no way to kill it with the ignition on (at least on an SII).
Good for trouble shooting and as an added safety feature.

When I added Pertronix module to my car years ago I did something similar. I got a long length of red wire and spliced it into the Pertronix red wire so I could run it to a hidden switch in the cabin. I did not use the rear defroster switch though. The first switch was pretty light and did not last long so I then got a heavy toggle switch and it has been good for 10+ years now. No holes or splices into anything original to the car so it all can be easily removed. Now it will just crank like an old Jag that does not want to start plus I have an easy way to turn off the Pertronix if I need to have the key on without the engine running.

68 E-type FHC

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One possible benefit of the fuel circuit is the car actually will start and run, they might get 100 yards down the street and it conks out.

This will spook them

I had a car a stolen and abandoned in the middle of the road with the doors open,
as it had a fault that caused stalling, and needed a fiddly procedure to restart

Not much time to fault find in that position in the middle of the night

This particular vehicle was stolen a 2nd time, this time on a trailer, and permanently, which (unfortunately) would be a consideration were I to be a potential E-type thief

Stolen plates and towies high vis, dudes E-type broke down again

I too wired the fuel pump to this switch.


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Hiding in plain sight is best.

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Here are some more pictures of the starter relay (before and after I cleaned the tarnished contact points with 1000 grit sand paper) and Heated Rear Window Switch when I disassembled it but before I cleaned up the contacts with some solvent and a small file.
The Starter Relay operated properly despite the tarnished points. The Heated Rear Window Switch did not work at all due to the buildup of old grease and dirt covering the contacts. The disassembly and cleaning fixed the Heater Rear Window Switch.


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In additional to an actual “disable” switch, put a “fake” disable switch up under the dash and hanging down where it is easy to see and get to, with wires extending up and behind other wires, then strap/tape those wires so they cannot be easily pulled out.

A person going to steel the car will see the “fake” disable switch, and play with it, thinking that is supposed to let the car start. Time is not on the car thief’s side and that may help encourage them to give up and move on to another target car.

The ‘time waster’ switch is in plain sight and not hidden, the real disable switch is hidden in plain sight.

Like putting three deadbolts on a door and only locking two of them - the thief ends up locking one while unlocking the other two.

I still think the best is a ground interrupt switch, down by the clutch pedal, American side.

That way, the car can’t be driven 1 yard, much less 100, which is plenty of time for them to screw up very expensive metal.

Good idea, and that would be the one to hook up to the electronic siren, which can have amusing results (It can be wise to fit a master switch to disable the siren)

You are probably right about the 100 yards bit for sure

What form does your ground interruption switch take ?

I have had many parts stolen, even a theft targeting specific Jaguar parts from some low dog

It is my wish to build a pit on my rural property, with sharpened steel rods in the bottom, throw in a few local deadly snakes, camouflage it, surround it with eminently stealable parts, then fill the pit in when I go there

Alas, one can only dream (if its good enough for Indiana Jones :grinning:)

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Really simple: it was a heavy amperage cut-off switch, hidden up on top of the drivers foot well, just to the upper left of the clutch pedal.

It was behind all the stuff that’s underneath the heater box, and the ground from the battery went straight to it, then to the standard position for the ground strap on an E type.

You reached down, turned a little key to cut the ground, and no amount of hot-wiring, no amount of jumpering could start the car. Matter of fact, my dad used it as a test to see how good prospective mechanics’ troubleshooting skills were, and few figured it out.

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I also wired the fuel pump to the switch. Love it. When pump is off, the switch is in the same position as all the remaining switches that are off so nothing looks different. I get in the car, turn the key, hear nothing. Press bottom of the switch, see the white on top of switch & hear the pump activate.
Did it decades ago, & if I remember correctly did it all with the fuse box panel down, 2 jump wires, & a test light.
Michael Caro

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On an SII you probably had to remove the hardura under the glove box to access the connection for the wire feeding the fuel pump. On my SII I could not locate the wire feeding the fuel pump in the drop down panel.

I know exactly how far the car can go with the fuel pump turned off. Only a small panic since i didn’t make it to a major road.

Pretty sure it’s a white cloth covered wire on fuse # 7(maybe 6).
Pulled it off & cut off the female clip. Soldered a jump wire there. On the other end
of the jump wire I placed a female clip & attached it to the farthest away, passenger side male tab of the dash switch. (Definitely).The 2nd jump wire is attached to the farthest away driver side male tab on the switch & attached to the other side of the fuse box. Can’t remember hot vs. ground. Can’t remember which tab on which fuse I placed 2nd jump wire 6 or 7.
Been decades; car is on lift W/another below & battery shut off. Best I can do @ the moment. I’m positive I did this from the fuse panel from the drop down door.
However, 20 year everything restoration. Hardura down, A/C down engine & tranny out, front suspension out, et. al.

P.S. can anyone tell me how to highlight appropriate words in another’s post?
I forgot & can’t do.
Michael Caro