Unusual electrical question

[C.J. Roberts]

Here's what's happening: When I have the lights on and hit the brakes, my odometer dims, and my dome light won't turn on from the MFS. It doesn't affect anything else, The rest of the lights work, the brake lights work. Nothing goes off when this happens.
I initially thought that the MFS was bad, but replacing it did nothing.
It's not that much of an issue, but I'm curious as to what the cause might be. My second thought, (after the MFS), was that there's a bad ground somewhere, but since nothing else seems to be affected, I don't see this as likely.
I do have the #4 fuse removed because of the top off, but again don't see any connection to that.
Anybody seen this before, or have suggestions?

 

[macleanflood]

Could try removing brake light bulbs one at a time to see if anything changes.

-Mac

 

[Zorba]

If nothing else turns up, its a bad ground.

 

[C.J. Roberts]

Could try removing brake light bulbs one at a time to see if anything changes.

-Mac

I doubt that it would change anything. The brake lights work whether the lights are on or not. I keep circling back to thinking that it's a ground issue, but I've no idea where or why, since everything works normally with lights off.
Looking at the light schematics, I can't see anywhere that the odometer illumination has anything to do with the dome light OR brake lights.

 

[C.J. Roberts]

If nothing else turns up, its a bad ground.

My thoughts too. But where? And why only those systems? And why ONLY with lights on?

 

[jonny jeep]

If there is a bad ground at the rear or corrosion in the bulb socket it could be causing enough voltage to leak back to the cluster via the tail light filament to make the cluster think the lights have been turned on, hence the dimming when braking.

Fuse 4 being removed shouldn't stop the dome light coming on with the switch. Try refitting fuse 4. If the dome light works with the door open, the ground path via the multi-function switch is bad. If the dome lights still don't come on the power isn't getting from the IOD fuse under the hood.

 

[C.J. Roberts]

If there is a bad ground at the rear or corrosion in the bulb socket it could be causing enough voltage to leak back to the cluster via the tail light filament to make the cluster think the lights have been turned on, hence the dimming when braking.

Fuse 4 being removed shouldn't stop the dome light coming on with the switch. Try refitting fuse 4. If the dome light works with the door open, the ground path via the multi-function switch is bad. If the dome lights still don't come on the power isn't getting from the IOD fuse under the hood.

I'm pretty sure that the tail lights aren't part of the problem. If they were, I'd think that it would happen with the turn signals too, which it doesn't.

 

[DaYooperTransplant]

I have a very similar issue. Although, without the headlights on when I apply the brakes my odometer completely goes out. When the lights are on, it only dims. Haven’t noticed if it affects my dome light but I have a 97’ so no MFS, it works off the headlight switch.
Been like this for sometime now and I figured it was a ground problem. I’ve went through all the grounds I can find and cleaned them up, but no luck.
I’ll be following along this thread with hopes someone has an answer.

Edit to add: I thought maybe it could be the brake light switch on the brake pedal arm, but didn’t see how that ties into the dash on the schematic. But I’m not that great at reading those things.

 

[C.J. Roberts]

If there is a bad ground at the rear or corrosion in the bulb socket it could be causing enough voltage to leak back to the cluster via the tail light filament to make the cluster think the lights have been turned on, hence the dimming when braking.

Here's another question: What makes the odometer dim in the first place? If it's because it detects a current draw, it might be that since I have a secondary wiring harness on my headlights, they draw only enough wattage through the MFS to activate the relay. Couple that with the fact that my tail lights are LEDs, maybe it only draws enough wattage to dim it when I put on the brake lights.
Thoughts?

 

[Zorba]

My thoughts too. But where? And why only those systems? And why ONLY with lights on?

Because its "FM" - Fucking Magic. When the problem makes no sense whatsoever, its a grounding problem 99.9999999999999999% of the time!

 

[CharlesHS]

To either rule them out or confirm they are the problem; I would inspect the ground connections below the dashboard in the drivers and passenger side foot wells, the ground connections under the lower dashboard speaker mounting screws and the ground connection behind the climate control bezel attached to the steel dashboard cross brace.

 

[Wranglerfix]

It always comes back to the basics. Check grounds, make your battery connections shine like your Sunday school shoes and double check anything hooked up aftermarket.

 

[jonny jeep]

There's nothing in the FSMs that describes the dimming operation of VFDs. Looking online, this is done by altering the duty width of the drive pulses, so must be IC controlled.

Since the dimming only occurs when voltage is detected on the dash dimmer input from the headlamp switch, there must be some voltage leaking to that circuit when the brake lamp is on. My tail lights are LED and it hasn't affected my odometer, so I don't think your LEDs should be the cause. If LEDs were a factor I'm sure we'd see more reports of this thing happening. I also upgraded my headlamp wiring before I went to LED headlamps so that shouldn't affect anything as I see it.

The places I figure the voltage leak could happen are at the rear brake lamps (bad ground, socket or trailer wiring), at the multifunction switch, or bad wiring between the multifunction switch and rear lamps.

If the rear lamps were unplugged and the dimming still occured, it would indicate the 'leak' is at the multifunction switch or wiring.

 

[C.J. Roberts]

There's nothing in the FSMs that describes the dimming operation of VFDs. Looking online, this is done by altering the duty width of the drive pulses, so must be IC controlled.

Since the dimming only occurs when voltage is detected on the dash dimmer input from the headlamp switch, there must be some voltage leaking to that circuit when the brake lamp is on. My tail lights are LED and it hasn't affected my odometer, so I don't think your LEDs should be the cause. If LEDs were a factor I'm sure we'd see more reports of this thing happening. I also upgraded my headlamp wiring before I went to LED headlamps so that shouldn't affect anything as I see it.

The places I figure the voltage leak could happen are at the rear brake lamps (bad ground, socket or trailer wiring), at the multifunction switch, or bad wiring between the multifunction switch and rear lamps.

If the rear lamps were unplugged and the dimming still occured, it would indicate the 'leak' is at the multifunction switch or wiring.

When you say you upgraded your headlight wiring, how did you do it? I have one of the wiring harnesses that is used when you use a higher wattage bulb, so that the only thing the MFS does is switch a relay, thus reducing the wattage going through it. Between that and the LED tail lights, I probably don't have enough wattage going through the MFS to trigger the dimming until the slight wattage of the brake lights is added to the load.
After reading everybody's input, I've pretty much decided that it's nothing to worry about. Next time I'm working under the hood on something, I might just unplug the headlight harness and plug the headlights in normally just to test the theory.
I appreciate everybody's input. I love this forum and everybody on it. Well, except for maybe Zorba and Boogie Man.

 

[jonny jeep]

I wired in relays and heavier gauge wiring, switched by the original headlamp wires. It took the voltage at my headlamps, with engine running, from 11.8V to 13.8V.

If anything, with headlamp wiring upgraded and LED rear lamps, there should be less voltage drop at the switch, so the voltage on the dimmer circuit should be fine for normal dimming operation.

 

[jonny jeep]

In fact. In later years where the cluster/dash lighting is controlled by the cluster itself, I would think the voltage on the dash dimmer feed is just used as a reference voltage so it doesn't really require any amperage to pass through the circuit.

 

[C.J. Roberts]

I wired in relays and heavier gauge wiring, switched by the original headlamp wires. It took the voltage at my headlamps, with engine running, from 11.8V to 13.8V.

If anything, with headlamp wiring upgraded and LED rear lamps, there should be less voltage drop at the switch, so the voltage on the dimmer circuit should be fine for normal dimming operation.

That's what my harness does. Maybe my initial description wasn't as clear as it could have been: the ONLY time the odometer dims is when the brakes are on. Otherwise it's on full brightness, as though the lights were not on. That's why I'm saying that the brake lights are adding just enough current draw to dim the odometer. Does that make more sense?

 

[jonny jeep]

I guess I kind of misread things earlier. I have been looking at this in the evenings mainly with beer involved.

I still feel there must be some leaking between the brake lamp and marker lamp circuits though.

I'll have a look at the 02 wiring tomorrow and see if anything stands out.

 

[jonny jeep]

Nothing stands out in the FSM. Brake and dash dimmer wires are well separated in the multifunction connector, so there's no way brakes should alter the dimmer voltage there.

I think I'd try and stick a thin wire into the cluster connector at the dimmer feed and use it to see if the voltage at the pin changes when the brakes are used. If it did, I'd unplug the rear lights and see if it still does it. The only thing that might gain is knowing whether it's caused by the rear lamps or not. As long as everything else works as it should, I guess like you've said it's nothing to worry about.