Odd issue noted with lights on SPF.

[Glenn Peterson]

This weekend I moved a new toolbox in front of where I park my GT40 which gives me a reflection I haven't normally had in the past. It was after dark when I pulled the car back in but I didn't have the headlights on because I was just pulling up about 10 ft and could clearly see. As it turns out, however, the low beams came on when I hit the brakes (something I assume has been happening all along).

On Sunday I did some experimenting and noted that if I turned my dash lights on (they are generally off) then the headlights only come on enough to barely glow. I also found that with the dash lights switched off and the fog lights switched on, the fog lights also come on with the brake lights.

I have not monkeyed with the switches or electronics beyond running a hot wire for the hydraulic front lift kit to the ignition "on" post.

Is this normal? I can't seem to think of the reason for it, but I've never had a GT40 before so......

 

[Rick Muck- Mark IV]

You need Blas's wiring diagram. It appears there is a "backfeed" somewhere. Was your car completed by a shop? If so, the electrics should have been checked and this discovered and corrected before you got delivery of the GT.

 

[Bob Putnam]

That type of backfeed is usually caused by a bad or missing ground somewhere.

 

[Stephen Ducker]

The brake lights seem to be a common factor in most of your problems. They seem to be earthing through other circuits, ie the headlamps, creating a backfeed as Bob suggests.
Make sure the correct bulb type is fitted & then check the earth/ground on the bulb holders.
(ie start simple ! then move onto the other earth/ground points)

 

[BenL]

That type of backfeed is usually caused by a bad or missing ground somewhere.

Definitely sounds like a bad ground/earth. This is a very common problem with Elans and other old Lotuses.

With a fiberglass body, the only ground the light fixtures have is the ground wire (the GT40 body doesn't provide a ground like a normal car with a steel body). If you don't have a functioning ground wire for the brake lights, the electricity will use ground connection within the fixture to go through one of the other bulbs in the fixture and then follow its currently unpowered wire to the front of the car which goes through the bulb in front, providing power to the light when the brake's electricity exits through the front light's ground.

This is why when you bring your car in for an annual inspection you'll be asked to step on the brakes while you have the turn signal flashing or headlights on. If the other lights in the fixture are on, then there isn't an alternate path for the brake lights to use to find a ground.

 

[Glenn Peterson]

You need Blas's wiring diagram. It appears there is a "backfeed" somewhere. Was your car completed by a shop? If so, the electrics should have been checked and this discovered and corrected before you got delivery of the GT.

Mark IV,

It’s a Superformance so it was checked at the factory, etc but to be fair when you check light function you have someone at the front to observe all of the functions and then they go to the back to check all of the function (or vice-versa). Doing this would not reveal this problem UNLESS you happened to have a 3rd person standing in front observing “just in case” when the back was being checked and I don’t think that is how most folks check these things.

Using the usual two person method, this car passed all light checks without a hint of trouble until a shiny tool box was as moved to the direct front and made the anomaly visible.

It turns out that the FACTORY swapped two wires around inside the passenger rear tail lamp assembly, so this is not a fault of the shop that finished the car.

Having said that, I want to give a HUGE shoutout to Merlin Pogson of Legendary Racecars in Calvert TN who read about this and reached out to me privately. He had a planned trip near me in Houston TX and this morning he was in my driveway working this problem.

He went above and beyond to fix a problem that he didn’t cause and public praise / recognition certainly isn’t enough, but he’s getting it just the same.

So in case anyone else has a similar issue with their SPF, check those tail light leads at the bulb!

 

[Rick Muck- Mark IV]

Glenn,

Glad Merlin was able to resolve the issue for you. He has a lot of experience with SPF and often knows "what the issue is". I have known Merlin for a long time.

Regardless, if the car was built out by a shop, it is on them to have checked all of the vehicle functions. Now you can never be 100% as it is not unusual to have something pop up later. I had 8000 miles on one of my GTs with no electrical issues. I sold the car and 500 miles later a wire pulled out of the fuse block and bricked the car. No one's fault, stuff just happens sometimes.