Have had my '95 V12 convertible since May, always had a noticeable voltage meter drop when applying brakes. I did the following to find the fault, with no appreciable change:

• Pulled brake/taillight bulbs
• Disconnected bulb fail sensors
• Cleaned battery terminals, cables and main ground point in trunk
• Pulled and cleaned all circuit wire connectors

Only thing that made it stop was pulling the fuse for the brake lights ("Stop lights"), 15A fuse #19 in Right-hand Side Rearward Fuse Box.

Where to look next? The brake light switch and wire connector above the brake pedal? My understanding is that there is no brake light relay in these facelift cars.

 

This voltage dip is more of a 'feature' than a problem. I've owned no less than 5 face lifts and they all do this. It's when they don't do this that I know something is wrong. For example I recently blew apart a 1995 for paint and when I didn't see the gauge dip I knew my stop lamps were out. Sure enough I forgot to plug in the harnesses. I suspect feedback through the lamp warning unit changes the ground potential for the gauges causing what appears to be a voltage dip.

Here is what you should do... Buy a cell phone charger with a volt meter like the one shown below and monitor the actual voltage. It's good practice on any old car.

There is a relay for the stop lamps and its located on the left hand side of the boot, see below.

 

Thanks for the quick and detailed response.

I pulled the left and right inner trunk panels during my testing. Also installed fresh 7528s pending a decision on LED replacements. In the left rear wheel arch area only found the ABS relay, CD changer and what I assume is the audio amplifier. Maybe I need to look behind/above the spare tire?

Also, I have a Bluetooth adapter with voltage display in my cigar lighter. If I keep my foot on the brake it will show a slight voltage drop, but not one as sudden or large as the dash volt meter.

BTW, I also cleaned the connector that feeds the dash gauges, read somewhere that might help.

 

...and still no sign of the Stop Light relay.

Removed the spare tire and looked behind its trim panel.

Could it be under the CD player? Behind the metal panel holding the trunk lid support?

Location specifics or photo much appreciated!

 

There are a number of relays on the left hand side inner wing, well forward near the wheel arch. Very difficult to see. Use a camera phone. The anti-slosh module etc are down there.

Paul

 

The relays must be pretty far up inside the left hand rear wing, as I used my phone and my telescoping mirror to no avail.

Given their location, how do you access the relays? Pull out the rear seat or other interior panel?

 

I can just get to mine. However, some owners remove the black brace panel by the start mount to get to them.

Paul

 

You mean remove the black metal support panel for the trunk strut? Circled in red in the photo below? Not my car, but similar.