Interior light issue

[skippy3k]

My interior lights have been working intermittently, and now they pretty much don't work. When I hit the button for the interior lights, they come on so I know the bulbs are good. but when I open the door(s), the lights don't come on. I've removed all the switches and cleaned them up, as well as where they mount in the door pillars.

But here's the interesting thing; if I manually pressed the door switch, I can hear a faint "clicking" sound under the seat as if a relay is being activated. This is telling me the switches are doing their job, but I can't find anything in the ETM off of RAVE that tells me about a relay.

Is their another component of the interior lights that I am missing? Normally I wouldn't mind, but it is creating havoc with my alarm system...if I lock the truck, the alarm has gone off in the middle of the night because (I assume) the interior light has been activated and the truck thinks a door was opened.

Thanks.

 

[WILL.TILLERY]

one of the switches is bad

 

[skippy3k]

Is there a good way to find the bad switch, or is it one of those situations where I might as well replace all of them? I believe there are six, correct? 4 doors, rear hatch, and hood.

EDIT - I just removed all the switches, hooked them up to a voltmeter and a simple circuit and tested. When pressed, the voltage goes to zero. When released, it completes the circuit. Not sure if this is the best test, but to me it shows they are working....?

 

[az_max]

isn't there a BECM in there too?

 

[skippy3k]

On a RRC? I sure hope not.

 

[WILL.TILLERY]

no becm.... there is an interior light relay/timer

 

[skippy3k]

no becm.... there is an interior light relay/timer

Aha! Where is it? I've searched the ETM without finding it.

 

[skippy3k]

More on this. I've learned that they did away with the courtesy light relay after '94. The '95 models had the courtesy light delay controlled by the MFU, along with the wipers, defrost, etc. I also did a final test of the door switches, and found that I could trigger the alarm by opening each door and rear hatch, so the switches are doing their job. But when I ran the MFU diagnostic procedure, I got very negative results with the interior lights, rear wiper, and front and rear defrost. Which is interesting, because the rear wiper works sometimes, the front defrost goes on by itself, and the rear defrost is intermittent.

My theory is the MFU is failing. Anyone have experience with bad MFU's? Is it somewhat common?

 

[WILL.TILLERY]

Sounds like you found it. the MFU is on the back of the fuse box...Drivers side knee panel

 

[Ed Cheung]

I had the same issue, plus it affect the clock on my 98 DI. Every time I open the door, it will reset the clock to 1:00, it works like a timer now....I checked the rave, all door switches route back to the MFU, and the interior comes from the MFU soI guess the MFU is the center of the problems. That thing is like 100GBP, so I will replace it when I really had enough of it.

 

[TigerDan]

Now I know why you need the MFU Scott...and yes, I do have one, just been swamped the last coupler of days and haven't had the chance to tell you.

But I'm having a similar problem with the '94 LWB I just picked up, and I don't believe it has an MFU, or if it does, it's sure not in the same place as the '95.

My courtesy lights come on when I open the door, but then stay on and don't time out when I shut the door so I have to pull the fuse. And I can hear the relay click from under the driver's seat like you mentioned when I manually operate any of the door switches...but the strange thing about it is, I can also hear what sounds like a solenoid clicking from under the truck at the same time. Not to mention some other problems, which are going to get their own thread...

 

[skippy3k]

Dan, do you still have EAS on that truck? That might be the other clicking sound you hear. But for a '94, I think it's a lot easier for you because you have a courtesy light delay relay. I would swap that out first.

 

[TigerDan]

It's had the coil conversion done, but they only removed the airbags, everything else is still in place. (For now...) But the clicking I hear underneath seems to be coming form the area of the tranny/T-case. I guess I need to have someone fiddle with one of the door buttons while I'm underneath tracking the source of the sound. Just seems strange that the sound is triggered by a door button (and does it whether the dome light fuse is in place or not.)

 

[sven]

All EAS trucks have a delay relay that keeps the EAS active for a certain amount of seconds after a door is shut. Thats probably is what you're hearing. Id just remove it.

 

[skippy3k]

Argh. So I replaced the MFU, and PT was right...it's hardly ever the MFU being faulty. I know electrical problems isn't the favorite topic to diagnose here, but hopefully someone can point me in a new direction. Here is what is happening;

1. Interior lights don't work, but will turn on by pressing the button.
2. Door switches work, because they will trigger the alarm when the door(s) open.
3. When you press the front heated screen button, the rear heated screen button lights up and you can't turn it off until it times out or you shut off the engine.
4. When you press the rear heated button, it turns on...sometimes.
5. Rear wiper works, but not when the rear heated screen is on. (By design maybe?)
5. The MFU diagnostic test doesn't always work.

All of this seems to point to the MFU, but even after replacing it I get the exact same symptoms. Is there anything else that involves all of these components that I can check out?

 

[az_max]

Where does the MFU ground?

 

[Roving Beetle]

Typically when buttons are pressed and something *else* lights up - it means it's finding a ground path another way than where it should be going.

My guess is you have a ground loose/broken or dirty/corroded.

Not sure what to tell you but to start digging, starting near the MFU and the big ground bundle on the firewall near by.

Rear wiper should work fine with the heated window on. Again, my guess is ground issues.

 

[skippy3k]

Grounds....ok, I like that theory. At least it's something I haven't looked into yet. I will check it out and let you know.

 

[92rrrandall]

One thing that i can tell you which will save some time on this problem:

The way the light switches in the door jams turn on the interior lights is by grounding the circuit. In other words, the steel that the door switch is screwed to is the ground for whatever lights turn on when that door is open. Or...the switch is the last thing in the circuit before the body grounds the lights.

One guess would be that the wires in the switch panel are mixed up a little. Another guess would be that there is a ground in the rear of the car that is not good. Another guess is that the neg battery cable is not connected very well to the battery or chassis.

One simple test would be to remove the switch from the door and connect the 1/4" wide tab to the neg terminal of the battery. If the lights do not come on with that certain ground then something is wrong with the light circuit, not the switch or the ground.

Randall

 

[skippy3k]

I figure I would update this thread for future reference. Things got worse. One night my alarm went off at 3am. I spent the next day tracing grounds and still couldn't figure it out. So I decided to pull the horn relay so that at least it wouldn't wake my neighbors up and instead just treat them to a light show. This worked great until last week when some asshole didn't defrost his windows and merged right into my lane...and there I was, without a horn. This had to end.

So after almost 5 months, I decided to take another stab at the problem. I traced each ground. All were good. I traced wires going to and from fusebox (the best I could) to ensure none were broken. I pulled the fusebox and took it apart. There I found significant corrosion going into the connector for the MFU. (See photo below.) Aha! That has to be it, since all functions that didn't work are MFU related. I cleaned it up, re-assembled and put the fusebox back together.

It still didn't work. Completely frustrated, I started to push the fusebox back into place and quit for the day (again). Suddenly, I notice my interior lights working. I opened and closed the doors and made the lights go on and off just to feel good for awhile. I then continued putting the fusebox back into place and noticed the lights were off. Then on. Then off.

Aha again! Bad connection. I wiggled every wire going into the box until I narrowed it down to one specific connector going into the back of the fusebox, then the specific wire. It then took me 30 minutes to figure out the exact way to apply pressure on the connector to make the lights work on demand. Now I needed to decide was it the connector or the fusebox? Since there isn't much room to work, I made jumpers for all 7 wires in the connector. I could apply pressure on the brown wire at the connector end and the interior lights would still work. Applying pressure on the brown wire at the fusebox end make them fail. It was the fusebox.

I got a replacement fusebox and 15 minutes later was not only greeted with interior lights but also the nice "ding ding" sound when you leave your headlights on. Bonus. Heated windshield works too.

Who knows what failed in the fusebox since it seemed pretty simple. Just copper runs going to and from wire connectors. No circuit board or anything fancy. It probably was a ground within the fusebox itself. But now I know that when a fusebox fails, it just doesn't "fail". It can give mysterious symptoms.