MFU from 97 Disco 1 - interior lights won't swich off

Smell = electronic burnt

Suspect failure - top left transistor, which is a P55N06. Plastic melted on case adjacent to transistor from long term heat. This device is the only high power device on the board, and is probably used to switch on the interior lights and slowly dim them when turning off.

The transistor has no heat sink. IMHO this will lead to failure over a long period of use. A small soldering iron can be used to swap this one out ($8 on line). If I were to do this, I would add a small heat sink.

For the rest of us, perhaps we should consider using fewer bulbs on the interior lights, or lower amp drain ones. Some of the new LED replacements offer a lot lower drain (like 30 or 40 milliamps vs 830 for a 5008LL cargo light bulb). The cost of an MFU, and the "fun" of remove/replace the fuse panel, might make the bulb change worthwhile.

And for the curious, the interior photo of the MFU shows that repair of the "surface mount technology" is beyond what many mechanics want to deal with - just snag one from the next run to the salvage yard.

 

Good job.
Thank you for this.
What did you check first before figuring it out to the be multifunction control unit?

 

Checked every fuse on panel and under hood. MFU has multiple fuse inputs.

Checked doors to see that they tripped alarm. Later, when I studied the electrical troubleshooting guide I saw that the alarm module was separate.

In repair of the rear wiper I discovered that one door had a slight ground, which was enough to make the MFU inhibit the rear wiper, but not enough to make the alarm not function. Swapped MFU with a used one that I got a couple of months ago in the boneyard. Many other items have been ahead of dome lights on the list of repairs needed, just finally got back to lights.

Looked at the troubleshooting guide and decided since all three interior lamps went through C277 connection I could maybe hook up some relays to get one door to activate dome lights. That same schematic shows the MFU as the main source of grounds to the three interior lights.

 

Questions:

Where is the MFU located? Was it difficult to swap out?

I looked at the pic, but was unsure of what you were attempting to show, could you add an arrow or something to it?

I usually switch my interior light off at the panel just because i tend to leave the doors open a lot when I an doing things either to or around my Discovery.

 

The MFU is attached to the back of the fuse panel to left of steering wheel. It takes two bolts to remove, and twisting the wire harness to turn it so you can get to the module. Best done with battery wires off, you are close to all kinds of things that should not be shorted out.

In the picture, the rectangular transistor is at the top left, with three wire leads coming out and soldered to circuit board is the problem. That unit can handle 50 or 60 watts of continuous power, if it is on a good heat sink. In free air it is rated for about 1 watt. Each dome light bulb is about .700 amp, and the rear cargo lamp is about .800 amp. So that would be a little over 2 amps, times 12 volts, and you have 24 watts. The transistor is attached by a melted plastic stud in the case, so it is not in "free air". However, it is enclosed in an unvented plastic box under the dash of a vehicle, so it is warm to start with. Mine actually melted a spot in the plastic case beside the transistor.

An average skilled technician would replace the transistor, I would cut the leads off at the case, not the circuit board, and solder the new one to the "stalks".

The transistor is used to to turn on the lights and provide the dimming feature where the lights don't just click off like with a relay.

IMHO such a design will be prone to fail after repeated use, even though it is only on for 8 minutes or so. The cost of the module, and the time to grab one from the junk yard (hope it works), or get from other sources, add up to some dollars. One way to extend the life of existing modules is to switch out the three interior bulbs to LED, which draw only 10% of the power of the regular ones, or less. I noticed that once of the banner ads for this forum is a company that makes such lights that snap right in, no re-wiring needed.

I put in a 12 LED small square panel in the front dome light, it is way brighter than stock, makes it easy for old guy like me with bad eyes to see the topo map.

 

P55N06 has a 20 milliohm on resistance. Given the current draw of all of the interior lamps it's probably generating less than a watt of heat.

Digi-Key stocks the part:

Invalid Request

 

that is correct. power consumed by the load is not the same as power dissipated in the transistor,
in this case power is the load current squared times the on redistance, about 0.1 watt. if you're doing the VI method remember that V is not 12v, but rather the voltage drop across the transistor when the load current is flowing through it, V=IR. the thing about Ohm's law is. no matter how you slice it it works every time

 

OK, perhaps the root cause is something different, but that transistor with no heat sink melted a hole in the case next to it. Maybe because of a shorted bulb. Maybe the dimmed bulbs, which draw a lot more current with lower volts (try "warming up" an incandescent lamp while monitoring current). Since I put in LEDs no problems.

Despite claims by the Borg, Resistance is not futile. It is volts divided by current.

 

I just had the same issue on my 97 Discovery SE. With the same burnout out part as pictured in the first post. Found out because my battery was inexplicably drained one day which led to investigating why the interior lights stayed on constantly.

I found that if you switch the front light to off, and remove the rear seat overhead light and cargo area light the battery stays charged fine for at least one week. However, there is a light around the key tumbler that is not easy to turn off so eventually the battery will die.

I went to a junkyard and pulled an MFU off of a 96 SE7. Instead of being tan in color it was a black plastic case, but I threw it on my car instead. It had the same connectors and mechanical interface. It worked great.

Cost me $7.00.

To elaborate on how to remove the MFU... you remove the two 8mm bolts off of the fuse panel under the steering column. I also removed the two satelite fuse buses, a big relay and what I think was a buzzer so I could rotate the whole fuse panel better because the MFU sits on the back of the fuse panel. I tried accessing from the back side, but there is not good access, you must go from the front side of the knee bolster. Once you loosen everything up and free some space around the fuse panel you can twist it around, pop back side connector off of the MFU, then pop the two mechanical tabs off the LH and RH side of the MFU releasing it from the panel. There is still one connector holding it to the fuse panel and you just rock the MFU as you pull it out. It wasn't as tough as I thought it would be. Just be patient and be careful. Removing the junkyard unit was easy because I just went in with some side cutters and cut all of the wires to the fuse panel.

I found some interesting subtle differences on the 96 vs 97 MFU functionally. The 96 turns on the fog lamps when any door is open. I don't know why they would do that. Also my intermittent front wiper works better... I used to have to switch to the second wiper position then back to intermittent to get the intermittent to work, but now I just click once to intermittent and it starts right away.

I hope this helps somebody else with the same issue out there!