550 no brake lights

Usually there is a grounding issue if you have issues like that.

Anyway, first check for voltage at the light connectors:

Image Unavailable, Please Login

The red wire is the brake wire. You'll need someone to push on the brake pedal (with the key turned on). The red/black wire is reverse. The black wire is ground. Check continuity to a good ground on the chassis (let us know what the resistance value is). It should be close to zero ohms. Unfortunately, the wiring diagrams don't show the location of the ground for the brake lights.

If no voltage at the lights, you'll have to check the switch. Can you take a photo of the wiring on the pedal switch? The wire colours disagree with the Ferrari wiring diagrams, but I suspect the blue wire is key power from fuse 22 and the two red wires go to the brake lights and ABS system. Remove the switch plug and check for voltage on the blue wire (key on). You may also be able to check across the switch pins for continuity with the brake pedal pushed. Should be zero ohms.

 

That doesn’t make sense at all. I don’t know what you’re checking.

Remove the tail light plug and check the red wire for voltage, not resistance. I mean check the plug half that has just come off in your hands. You’re looking for 12 volts between the red wire and the chassis/body with the brake pedal pushed.

Then, in resistance mode, check between the black wire on the plug and the body. If that is 37 ohms, that is a big fail. Is it the same on both sides of the car?

What do you mean by “the ground is on the battery”? The ground for all the tail lights should be somewhere in the trunk/boot.

 

Isn’t there a metal piece on the trunk latching mechanism?

That still seems rather high. Good grounds should be 0.2 ohms. One ohm is reasonable. Is the battery at the front or rear of the car? Did you use a long piece of wire as part of your test equipment? Maybe your test lead connections aren’t the best? Or maybe your battery terminals need cleaning.

So that’s a fail also. Was the key on?

 

Sometimes if you use cheap crocodile clips on test leads, they can add too much resistance. Try measuring the resistance of the test lead.

By the way, is "bean town" Boston? I don't know if your car is a USA car or Europe car. They have different wiring.

 

Well, if we go by your mechanic's report, the brake lights come on with the turn lights, so you may have two issues. Of course, he may have been seeing some kind of reflection off the turn lights. Do the brake lights come on with the turn indications?

Do you mean pull the plug off or pull the switch apart. I think most people just replace them because they're relatively cheap.

Anyway, if you have a bit of wire like a test lead, you can always short out the blue wire to the red wires on the connector to see if the brake lights come on. i.e. with the key on.

By the way, if you push on the brake pedal, is there an air gap between the switch plunger and the brake pedal arm?

 

Make sure you have enough travel on the brake pedal before you go adjusting or replacing the switch

Image Unavailable, Please Login

I don't recall seeing a switch clearance value for the 550.

 

Can you tell if this 2 inches is enough for the switch plunger to extend fully?

 

I'm wondering if other 550 owners can chime in here. Is the brake pedal supposed to be this hard. How much does the behaviour change with the engine running?

Have you pressed the pedal so much, it's just firming up?

If you can adjust the switch to clear the brake pedal when the pedal is pushed in 2 inches, this will be a temporary fix until you can establish whether the brake system is operating normally? I guess you'll need a couple of suitable spanners (or you might be able to get by with one).

Ouch... I see that the workshop manual says that the brake pedal rod should never be adjusted. It says that you have to replace the whole master cylinder brake booster unit (not just the cylinder) as the travel is set up at the factory.

 

I don't know if the paint is there to show the switch has been adjusted or to show if the switch has moved from that adjustment. Hopefully you don't have to fight Loctite or some other sealant during removal.

 

That seems to implicate the brake switch, don't you think Ian @Qavion?

 

I should probably climb up on a ladder and look at mine because I'm probably confused, but why is that relevant? The switch should close almost immediately upon starting to press the brake pedal?

 

That’s the primary issue. It’s just the other odd behaviour that can’t be explained by a faulty switch.

Understood. I was just trying to determine if there was any clearance at all. If the switch doesn’t disengage from the brake pedal when pushed, you know the adjustment is bad, but you don’t know if the switch is also bad.

Normally a value is given in the workshop manuals for the amount of push you should have on the switch plunger with the brake pedal released. You don’t want it to be too sensitive.