Hey everyone, the brake lights on my 1987 300d mysteriously stopped working when I took it in for a safety inspection ! Maybe they were broken before, it's hard to know for sure , since the car has been in and out of the garage so many times since I bought her, I though I had almost all of the bugs worked out, when this brake light failure happened. I tried replacing the bulbs to no avail, I checked the fuse on both sides with a multimeter, and I checked the switch on both sides and it is good. I know there is power before and after the stop lamp switch, but the wiring harness becomes very complex after it goes under the dash, I feel like I might be missing something very obvious, but don't really know what to look for under the dash, the situation is probably worse than normal , because I can tell lots of other people have been poking around under the dash. I tried tracing the wires from the dash to the rear, but the colors do not seem to match up, and the wires are covered with a rubber sheathing and/or wrapped with a gummy cloth electrical tape. If anyone else has had this problem I would really like to know what they did to fix it. The chassis is a w124 chassis, the engine is an om603 (I just had the cylinder head replaced with the newest design, old one was cracked) and the engine has been convereted to run on vegetable oil (the system is a greasel system). If I could get the brake lights to work, collecting the vegetable oil would be a no brainer especially since diesel is running about 3 bucks a gallon now ! Any help is greatly appreciated.
1987 300d brake light , stop lamp , not working
- Thread starter williamf2
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The stop lights normally have a live going to the bulbs, and the brake switch gruonds and completes the circuit. To verify this, take out one of the bulbs, if you have 12v on one of the contacts, then the brake light switch is faulty or one wire is not going to ground. this can be double checked by putting one test lead onto the bulb holder connection that does not have 12v on it with the bulb out, switch the meter to ohms, and the other test lead to chassis (ground) and have someone press the brake pedal, should be around zero ohms or bleep.
Malcolm
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This being the case then the lamp holders are not grounded.Ian Brown said:
Malcolm
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Brake light switch supplies the power
I have looked at the wiring schematic, and know from testing that the brake light switch, or stop lamp switch (as it is often referred to) supplies the power, it is very easy to verify this by testing with a test lamp, or using a multimeter on the dc voltage setting. power goes to the switch, and the switch is open, until the brake pedal is pressed, at that point the switch completes the circuit, and the brake lights are supposed to illuminate, the ground is not an issue, I know beause the lamp cluster shares the same ground, and all the other lights in the cluster are working, I removed the brake light bulbs to look for corrosion etc. but they were all very clean, they just are not getting power to the cluster
I'm totally bummed, if I knew someone that was an electronics wizard, wi ouwld gladly pay foir the repair, but more often than not, if I cannot easily fix it, neither can the shop, and thus they charge accordingly. Is there some special diagnostic tool that could help me find a break in the wires, without having to tear them all up from front to back ? Any help is greatly appreciated.
I have looked at the wiring schematic, and know from testing that the brake light switch, or stop lamp switch (as it is often referred to) supplies the power, it is very easy to verify this by testing with a test lamp, or using a multimeter on the dc voltage setting. power goes to the switch, and the switch is open, until the brake pedal is pressed, at that point the switch completes the circuit, and the brake lights are supposed to illuminate, the ground is not an issue, I know beause the lamp cluster shares the same ground, and all the other lights in the cluster are working, I removed the brake light bulbs to look for corrosion etc. but they were all very clean, they just are not getting power to the cluster
I'm totally bummed, if I knew someone that was an electronics wizard, wi ouwld gladly pay foir the repair, but more often than not, if I cannot easily fix it, neither can the shop, and thus they charge accordingly. Is there some special diagnostic tool that could help me find a break in the wires, without having to tear them all up from front to back ? Any help is greatly appreciated.
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Sorry about your electrical problems ,but I wish I also had your problem with diesel prices , try almost £5/$8.5 gallon in the UK at present !!! :-( :-( :-(williamf2 said:
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Look here, http://www.detali.ru/cat/oem_mb2.as...GM=717.430&CT=F&cat=501&SID=54&SGR=040&SGN=03 , item 101 is a lighting control module ,is it possible the brake light system passes through a relay in here.
Geo.
Geo.
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lamp control unit ?
I suppose it may be possible, I will admit that the schematic is a bit confusing, but I didn't see a lamp control relay or unit mentioned on the schematic, has anyone ever fixed thier brake lights ( stop lamp) by having a relay replaced ? The big unit that ties into every lamp isthe bulb failure monitoring unit, I couldn't find it, or if I did I didn't recognize it. Does anyone know where it is located, and if so, could a failure in that unit cause the lights not work ? It would be a little Ironic that the bulb failure monitoring unit, causes the brake lights to stop working right ?
Will
I suppose it may be possible, I will admit that the schematic is a bit confusing, but I didn't see a lamp control relay or unit mentioned on the schematic, has anyone ever fixed thier brake lights ( stop lamp) by having a relay replaced ? The big unit that ties into every lamp isthe bulb failure monitoring unit, I couldn't find it, or if I did I didn't recognize it. Does anyone know where it is located, and if so, could a failure in that unit cause the lights not work ? It would be a little Ironic that the bulb failure monitoring unit, causes the brake lights to stop working right ?
Will
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I know the fuel prices in Europe are much higher than they are here in the US. Just out of curisoity how much would you pay for a gallon of cooking opil at the grocery store ? Right now we can get 4.5 gallons for about 13 dollars, so it's almost break even now fo me to go buy vegetable oil at the store.
Will
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The lamp module is only for the headlamps, the fuse for the stop lights,is it before or after the switch, if after the switch, then the fuse holder could be faulty, this is a common fault where the fuse losses contact with the holder. measuring on top of the fuse will tell you nothing.
Malcolm
Malcolm
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Fuse holder ?
Dear Malcolm, I tried measuring above and below the fuse, i will look at it again, clean all contacts, and replace it with a new fuse, I doubt it will rmedy my problem though, wishem luck, I will report back
Will
Dear Malcolm, I tried measuring above and below the fuse, i will look at it again, clean all contacts, and replace it with a new fuse, I doubt it will rmedy my problem though, wishem luck, I will report back
Will
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William, you have the cables in the back for a tow hitch, coloured Violet and red, these are the brake light cables for the tow hitch, mabe you could use those.
Malcolm
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fixed the elusive brake light problem sort of ....
The problem is fixed, I'm not sure what I did to fix it, but I am certain that the problem related to the bulb failure monitoring unit. My guess is that some of the connection were not tight. My friend Dave at AA truck and auto (they sepcialize in diesel engines, and dave used to be a mercedes mechanic) really helped me out with this mystery. The first thing that really made a difference was the wiring schematic from mitchell repair , the schematic from the service manual (on cd) was very vague and/or I had the wrong schematic, Dave printed out the schematic from the mitchell repair database (they sell thier info services to car repair shops) The better schematic showed the fuse, the stop lamp switch, and the "lt monitoring unit" as being between the battery, and the brake lights. I had already tested the fuse, the switch, and the ground for the brake lights (stop lamps) and all were fine. When Dave and I looked at the new schematic it became obvious that the lt monitoring unit was responsible for transferring power between the stop lamp (brake light) switch, and the wires that lead to the brake lights. The lt monitoring unit, or bulb failure monitoring unit as it is sometimes called is behind the main fuse panel underneath of the cover for the relayes etc. When you take the cover off you see, several small square metallic relays, 2 red fuses, and one huge black rectangular box, this box is the monitoring unit. You will need to pull it out, and wiglle it to expose the pin scokets beneath, and allow you to remove the two pig tail sockets on eaither side of the monitoring unit. There are three sets of pins, there is main block of 16 pins (in the middle beneath the monitoring unit) there is one set of 10 pins, and one set of 8 pins, both on pigtails that plug into the bottom of the monitoring unit. The schematic shows that power from the brake switch flows through to pin 15 in the monitoring unit, so we pressed the brake pedal down, and used a test lamp to verify that power was going to pin 15, and it was. Then the monitoring unit is responsible for passing the pwer from pin 15 to pin 11 (for the high mount brake light) and pins 1 and 3 (of the group of 10) pin 1 is for the left stop lamp, and pin 3 is for the right stop lamp. So we removed the monitor unit, and carefully jumpered pin 15 to each of the brake light pins , and the brake lights lit up, telling us that the problem was within the monitoring unit, I opened the unit to look for any visible component damage, and did not see any, it was a fairly solid state unit, nothing looked intentionally replaceable, there were no burn marks etc, so I just put it back together, and magically the brake lighst started working (maybe the connections were loose ?) Either way this unit controls every exterior lamp , if you have an exterior bulb failure on a 1987 300d and the obvious things are ok (fuse, switch, bulbs etc.) then I would suspect teh control unit, even without the great schematic you could get an idea for each pin by using a test light, and turning the lights on, one at a time and checking each of the pins with your test lamp, this will tell you what pin the power enters in on for each light, if you are getting power to the incoming pin, then use a continuity tester to determine which pin is the outgoing pin ie in my case the brake lights are messed up, so I hook the cable from my continuity tester to the positive terminal of the brake bulb holder (you may need to use electrical tape to keep it from shorting to ground) and then use the probe end of the continuity tester , to determine which pin is for the light the other end of the tester is hooked to. Of course you could probably go to the nearest service shop that uses mitchels and pay them 20 bucks to print out the wiring diagram for your car. Either way one you know the incoming and outgoing pins, jumper them with a small piece of wire, if the light comes on, then the problem is in the monitoring unit, I imagine it is an expensive part, luckily for me it all worked again after I reassembled it. Hope this helps anyone else out there who is as miffed as I was with the external bulb failure problem. If anyone has any questions please feel free to e-mail will@willfagg.com Hope this helps.
The problem is fixed, I'm not sure what I did to fix it, but I am certain that the problem related to the bulb failure monitoring unit. My guess is that some of the connection were not tight. My friend Dave at AA truck and auto (they sepcialize in diesel engines, and dave used to be a mercedes mechanic) really helped me out with this mystery. The first thing that really made a difference was the wiring schematic from mitchell repair , the schematic from the service manual (on cd) was very vague and/or I had the wrong schematic, Dave printed out the schematic from the mitchell repair database (they sell thier info services to car repair shops) The better schematic showed the fuse, the stop lamp switch, and the "lt monitoring unit" as being between the battery, and the brake lights. I had already tested the fuse, the switch, and the ground for the brake lights (stop lamps) and all were fine. When Dave and I looked at the new schematic it became obvious that the lt monitoring unit was responsible for transferring power between the stop lamp (brake light) switch, and the wires that lead to the brake lights. The lt monitoring unit, or bulb failure monitoring unit as it is sometimes called is behind the main fuse panel underneath of the cover for the relayes etc. When you take the cover off you see, several small square metallic relays, 2 red fuses, and one huge black rectangular box, this box is the monitoring unit. You will need to pull it out, and wiglle it to expose the pin scokets beneath, and allow you to remove the two pig tail sockets on eaither side of the monitoring unit. There are three sets of pins, there is main block of 16 pins (in the middle beneath the monitoring unit) there is one set of 10 pins, and one set of 8 pins, both on pigtails that plug into the bottom of the monitoring unit. The schematic shows that power from the brake switch flows through to pin 15 in the monitoring unit, so we pressed the brake pedal down, and used a test lamp to verify that power was going to pin 15, and it was. Then the monitoring unit is responsible for passing the pwer from pin 15 to pin 11 (for the high mount brake light) and pins 1 and 3 (of the group of 10) pin 1 is for the left stop lamp, and pin 3 is for the right stop lamp. So we removed the monitor unit, and carefully jumpered pin 15 to each of the brake light pins , and the brake lights lit up, telling us that the problem was within the monitoring unit, I opened the unit to look for any visible component damage, and did not see any, it was a fairly solid state unit, nothing looked intentionally replaceable, there were no burn marks etc, so I just put it back together, and magically the brake lighst started working (maybe the connections were loose ?) Either way this unit controls every exterior lamp , if you have an exterior bulb failure on a 1987 300d and the obvious things are ok (fuse, switch, bulbs etc.) then I would suspect teh control unit, even without the great schematic you could get an idea for each pin by using a test light, and turning the lights on, one at a time and checking each of the pins with your test lamp, this will tell you what pin the power enters in on for each light, if you are getting power to the incoming pin, then use a continuity tester to determine which pin is the outgoing pin ie in my case the brake lights are messed up, so I hook the cable from my continuity tester to the positive terminal of the brake bulb holder (you may need to use electrical tape to keep it from shorting to ground) and then use the probe end of the continuity tester , to determine which pin is for the light the other end of the tester is hooked to. Of course you could probably go to the nearest service shop that uses mitchels and pay them 20 bucks to print out the wiring diagram for your car. Either way one you know the incoming and outgoing pins, jumper them with a small piece of wire, if the light comes on, then the problem is in the monitoring unit, I imagine it is an expensive part, luckily for me it all worked again after I reassembled it. Hope this helps anyone else out there who is as miffed as I was with the external bulb failure problem. If anyone has any questions please feel free to e-mail will@willfagg.com Hope this helps.
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What a nice way to spend the day and really get to know your car, so pleased that its working now. With all of the work involved, it is nice to find a definite fault. Nothing worse than a intermitant fault, thanks for posting the answer.
Malcolm
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Cooking oil
Not sure on UK prices for cooking oil. It's legal to run a car on cooking oil here but - and it's a big but - you have to account to HM Customs & Excise and declare the amount you use. You then pay duty on it at the prevailing rate
Amazing really that you could grow a field of rape, harvest it for oil, use the oil and it's yours free & clear. But put it in a car and you owe the taxman!
Nick Froome
www.w124.co.uk
williamf2 said:
Not sure on UK prices for cooking oil. It's legal to run a car on cooking oil here but - and it's a big but - you have to account to HM Customs & Excise and declare the amount you use. You then pay duty on it at the prevailing rate
Amazing really that you could grow a field of rape, harvest it for oil, use the oil and it's yours free & clear. But put it in a car and you owe the taxman!
Nick Froome
www.w124.co.uk
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This used to be the case if you had a water wheel or wind turbine to produce electricity.don't know now that we are all going greenBolide said:
Malcolm
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Television,
The taxman now pays you 3.5p for every unit of electricity you produce from a renewable source under the ROCS scheme(Renewable Obligation Certificate) ,whether you use it yourself or sell it.Why do you think all these windmills are being built !!!
Geo.
The taxman now pays you 3.5p for every unit of electricity you produce from a renewable source under the ROCS scheme(Renewable Obligation Certificate) ,whether you use it yourself or sell it.Why do you think all these windmills are being built !!!
Geo.
