I have a 2000 Jeep Grand Cherokee Laredo with a 6cyl engine. Recently, a
friend of mine told my my rear brake light was burned out so I bought a
new lamp and replaced it. It was then that I noticed that the lamp
wasn't actually burned and the same problems persisted with the new lamp
(and another second new lamp). Here's what happens....

When the parking lights are on, both tail lights come on fine, both with
the same intensity. If the tail lights are off and you press the break
pedal, both brake lights work (although the one on the driver side is
slightly dimmer than the other one). If the parking lights/tail lights
are on and you depress the break pedal, the lamp on the driver side goes
off completely. The one on the passenger side works fine.

I checked all fuses and relays (could not find a relay for the break
lights, only one for the tail/parking lights). I could not find any
ground either....sometimes a bad ground can do freaky things. Any
suggestions or someone experienceing a similar problem? Thanks!

Regards,
Dave.

 

Bad ground on the light fixture. It grounds through the bolts.

It is stealing the ground through the running light which is why the
running light element comes on with the brake switch. The running
lights are wire grounded, not fixture grounded.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

David Baldacchino wrote:
>
> I have a 2000 Jeep Grand Cherokee Laredo with a 6cyl engine. Recently, a
> friend of mine told my my rear brake light was burned out so I bought a
> new lamp and replaced it. It was then that I noticed that the lamp
> wasn't actually burned and the same problems persisted with the new lamp
> (and another second new lamp). Here's what happens....
>
> When the parking lights are on, both tail lights come on fine, both with
> the same intensity. If the tail lights are off and you press the break
> pedal, both brake lights work (although the one on the driver side is
> slightly dimmer than the other one). If the parking lights/tail lights
> are on and you depress the break pedal, the lamp on the driver side goes
> off completely. The one on the passenger side works fine.
>
> I checked all fuses and relays (could not find a relay for the break
> lights, only one for the tail/parking lights). I could not find any
> ground either....sometimes a bad ground can do freaky things. Any
> suggestions or someone experienceing a similar problem? Thanks!
>
> Regards,
> Dave.

 

"David Baldacchino" <dbaldacchino@ev1.net> wrote in message
news:3F3AFD30.1040505@ev1.net...
> I have a 2000 Jeep Grand Cherokee Laredo with a 6cyl engine. Recently, a
> friend of mine told my my rear brake light was burned out so I bought a
> new lamp and replaced it. It was then that I noticed that the lamp
> wasn't actually burned and the same problems persisted with the new lamp
> (and another second new lamp). Here's what happens....
>
> When the parking lights are on, both tail lights come on fine, both with
> the same intensity. If the tail lights are off and you press the break
> pedal, both brake lights work (although the one on the driver side is
> slightly dimmer than the other one). If the parking lights/tail lights
> are on and you depress the break pedal, the lamp on the driver side goes
> off completely. The one on the passenger side works fine.
>
> I checked all fuses and relays (could not find a relay for the break
> lights, only one for the tail/parking lights). I could not find any
> ground either....sometimes a bad ground can do freaky things. Any
> suggestions or someone experienceing a similar problem? Thanks!
>


You have lost a ground to the lamp that does not work properly.

The brake lights are getting a ground through the running lamp circuit, but
when the running lamps are on, the ground isn't there. The lights, brakes
and running lamps, should have a dedicated ground at each location. The
Cherokees seem to have trouble with the ground of one of the rear lamps
failing. I don't know what the actual failure is, but I would begin my
search by looking at the lamp socket itself, then following the ground leads
back to whereever they are connected to the body. On the TJ, YJ, and CJ, the
ground is provided by one of the bolts that holds the lamp housing to the
back of the vehicle. The WJ and XJ should be a variation on the same method.
Notice the inside of the housing, if it can be opened, and there will be at
least one of the mounting screws/bolts that has a metal strip, this strip is
what provides the ground. Make sure it is clean, and that the fastener is
clean around the body.

 

"Mike Romain" <romainm@sympatico.ca> wrote in message
news:3F3B93BA.371D97F9@sympatico.ca...
> Bad ground on the light fixture. It grounds through the bolts.
>
> It is stealing the ground through the running light which is why the
> running light element comes on with the brake switch. The running
> lights are wire grounded, not fixture grounded.
>

What is really happening, Mike, is the ground that is missing from an
individual light is supplied by the other lights in the system. This is only
true for the running lights. When the brakes come on, the brake lights get
the ground through the running light filiment, not the brake light filiment,
but only when the running lights are off. When the running lights are on,
and the brakes come on, the brake lamp circuit provides voltage to a path
that was formerly a false ground. Because the running lamp now has 12v on
both sides of the filiment, it goes off.

You are right though, but for a slightly different reason. The defective
ground is replaced by a route through the wires, so the running lights
remain functional, kinda.