83 300sd Brake Lights Blown Fuse help please
 

Hello all,

I'm a newbie to this forum so first let me thank all for creating and maintaining. I have a 83 300sd

Issue: Fuse for brake lights continues to blow. I can change the fuse and in the driveway do everything to try and make it blow with no result and the brake lights work no problem. I take it out on the street and fairly soon when applying the brakes the fuse will blow. The tach, fuel gauge and temp gauge are on the same fuse. Couls it be the brake switch or is it most likely a grounding out wire somewhere?

Thanks for any suggestions you might have, and thanks again for the awesome forum!

Craig

Aftermarket radio? If so, maybe a messed up wire with the radio, the antenna or an amp, if there's one in the trunk.

It blows with the brake, though? Not a power window, or a turn signal action? You're not messing with cruise control are you?

Triple check you also have the correct type of brake bulbs. If you use an HD bulb they can partially melt your socket.

Welcome to the forum.

Tip: if you're working with a fuse, ID the fuse(s) you've worked on because sometimes there is more than one fuse for a circuit. E.g. windows have a fuse for the relay, and a separate fuse for window motors.

Yak, thanks for the response. Original radio with a non powered fixed antenna. At first I thought it was caused by using the turn signal switch but now I'm sure the fuse only blows when I apply the brakes. But not all the time, can't duplicate in the driveway. Cruise control hasn't worked in years. I looked at the bulbs and I'm not sure if they are the right ones but I'm on my way to get 2 new correct bulbs. I just drove about a half mile and couldn't get it to blow. The fuse in question is the 8amp in socket 12.

Proper fuse size should be on the paper insert in your fuse box cover.

More Clues
 

Another clue to the investigation! Sometimes when I start the car for the first time in the morning the wipers come on all by themselves. They wipe a couple of times then stop. I put in new correct bulbs late yesterday and drove a little and the fuse didn't blow. When I drove it this morning it took a couple of brakings the then the fuse blew again. Any suggestions??

they say to use German bulbs

Perhaps you have a wiring issue with the turn signal switch. Perhaps you should take the carpeted panel off and look upwards to where the switch connects in to the harness, maybe something is a miss there.

JamesDean thanks for the response. Good idea, I've been suspect of this because several years ago the combination switch was replaced and I thought just maybe something was going on there. That might explain the wipers coming on randomly. Could a guy unplug that combination switch and then see if the brake lights blow the fuse?

I dont see why you couldn't unplug it and see what happens.

Don't do it at night, as I think you might lose regular headlamps. I recall someone saying that they'd lost low-beams due to a bad switch.

Ok, I removed the panel under the dash yesterday thinking but unknowingly that the problem might be the brake switch. So just a few minutes ago I disconnected the combination switch plug and put in a new fuse and drove down the street. By about the 3rd braking the fuse blew once again. Would it be safe to say the problem is not the combination switch? Could it be the brake switch? It's sure looks fun to change that thing!!

Hm, well yes I think you've eliminated the combination switch.

If you're in a low-traffic, quiet residential area (I live in a one-entrance developement with no traffic) I'd suggest unplugging the brake lights in the rear and driving again and seeing if the fuse blows.

Ok, I took your suggestion (thank you) of unplugging the tailights. I unplugged both taillights and drove around my quiet area and did not blow a fuse! Does this mean I could eliminate the brake switch as an issue? Then I hooked up the drivers side tailight and drove around and no blown fuse. Then I dsiconneted the drivers side and hooked up the passengers side and could not get the fuse to blow! I thought I might be able to isolate the problem to one side or the other. Maybe that is over simple thinking!!

American bulbs are bad for that I've heard.I replave all my bulbs with LEDs.lower wattage,and brighter.The europeon bulbs are called osram.

So now the question is this: Is the problem in the tail-light or did just removing the additional 3.5 amp load (brake lights) from the circuit just allow whatever is causing the problem to draw current and not blow the fuse.

The FSM says:
http://i.imgur.com/jO8qWl.jpg

I've never heard of this.

I've always used Sylvania or Philips bulbs in my cars, more accurately whatever the parts stored had in stock.

Sylvania/Osram are the same company.

If you notice the bulbs the Osram have lower wattage,where as the others higher wattage.Thats why I went to LEDs lowest wattage easier on alternator and battery.I was blowing fuses with the other bulbs

Sylvania/Osram's replacement guide (Lamp Replacement Guide View All Bulbs) says to use 7506 bulbs which are 21 watts, thats what I've always used. This is the wattage the factory used as indicated by the housing. Wattages are stamped on them IIRC.

Well, what to do, what to do! What is the FSM? The fuse box cover in this car says nothing about Anti Lock Brake System or Anti Theft Alarm System. There is some what appears to be some aftermarket alarm system stuff under there. I wonder if this might be an issue? I wonder if a guy could disconnect all that stuff? Am I at the spot where I have to take it to someone? I'd rather not of course!!

The FSM = factory service manual.

Its free from Mercedes now:
Untitled Document

They didnt put ABS on until 1985 so you wouldnt have that, I dont think you'd have the alarm either. But if someone added something, I would suspect that immediately. You should disconnect the power line going the after market stuff, perhaps that is the problem.

Disconnecting the brake lights only alleviated 3.5 amps from the circuit. When you hit the brakes adding 3.5 amps put it over the 8amp fuse's limit.

So something else is on that circuit drawing more than the factory intended.

Good News!! I went under the dash armed with a pair of wire cutters and carefully removed all the aftermarket alarm stuff. I wasn't completely sure but you can tell the original work looks alot different than this cobbed up stuff! So I got rid of it, hooked up the taillights, combination switch and took it around the neighborhood. Could not get the fuse to blow so I thought if anything will make it blow (Murphy's Law) going out it traffic will! Voila! NO blown fuse!! Woo Hoo!!! True test will be tomorrow morning but I'm optimistic. So JamesDean, thank you very much for your help!! Also, thanks for the FSM link. This will come in handy. Thank you oldsinner111 for your useful input also!

Always glad to help. Glad it worked out for you. Let us know how tomorrow morning goes as well!

Spoke To Soon!
 

So, yesterday after thinking we had solved the problem I drove around a lot. I drove to pick up my daughter from school and everything is fine. When I get home I put the under dash back together, I put the trunk back together and I revel in the accomplishment! Few hours later I take the car out completely confident and within a few brakings the fuse blew! I felt like crying! What a surprise and I can't believe it! I put in another fuse and it continued to work just fine for probably 15 miles of stop and go.

Just went out and within 3 brakings the fuse blew again. Not sure where to go from here. Any suggestions?

Have you done anything with cruise control lately? If you dont use it or it doesnt work, disconnect the amplifier unit. Its located under the driver's side dashboard. Left of the brake pedal. It has a 14-pin connector on it. Its on a bracket with a smaller module. Maybe your unit has failed catastrophically somehow. I've not encountered this before but its as likely as a candidate as any.

Are you sure you completely disconnected the aftermarket alarm unit from the car's power ?

Hi JamesDean, Thanks for hanging in there with me! I just crawled under the dash again and pulled out the remaining aftermarket alarm stuff. I actually went inside the fuse panel and removed where they were connected in there. I thought the same thing that maybe I hadn't got everything. The cruise control hasn't worked for years and years so I'll disconnect it as you suggest. I was wondering if any of those dash instruments on the same circuit could cause problems. The oil pressure gauge has been intermitent for a couple of years but has never been a problem. Thanks again!

My 300SD's cruise units have been out for years as well. I've recently gotten into repairing the units actually, if you're interested send me a PM on here.

I'm not sure about the gauges actually.

What we might want to do is get a multimeter and set it to measure current and stick it where the fuse would go and see just how much current is flowing through that fuse.

I can do the same on my 300SD and we can compare numbers.

Ok, I really need to get a good multimeter. The one I have is not even useable. Are there any suggestions on a good all around meter for automotive work? Does the oil pressure gauge use any electricity? When I turn the key on and not starting the engine the oil pressure gauge pegs to the top and stays there until the fuse blows and then about a minute later it drops back down.

I have a Tripplet 9045 meter. But I bought that mainly for my electronics work. Its a really nice meter. Does all sorts of stuff, most stuff more expensive Fluke units do too. The Tripplet was around $80 I think.

For automotive work, the most basic of meter will work. I've got two of these $2 unit, http://www.smcelectronics.com/PICTURES/DVM1-L.JPG They cover 99% of what you need for the DIYer.

As for the oil gauge. I'm fairly certain stuck at 3 means something but I cant remember what exactly.

x2 on the meter JamesDean suggested. They are plentiful on ebay for around $10 and being digital take a fair share of abuse (read that as being used by DIY'ers who know very little of meter use)

There are just a few meter things to know.

Do not every attempt to measure resistance with any power on the circuit. Doing so will usually result in the immediate destruction of that R reading position. Such as the 200 which is the most common scale we use.

Nice to check the meter by shorting the leads which should result in a 0 reading. If it doesnt note what it is .02, .03 so that when you check glow plugs you can subtract the shorted reading.

Voltage is taken with the Black lead, Com, sitting on ground and the Red to the area voltage is being checked like a fuse. Measure the DC volts at one end of the fuse ... 12 volts ... move to the other and read 0 volts you have a bad fuse.

Note that Resistance and Voltage readings are taken with the Black pushed into the common and the Red into the middle location.

Current readings are taken with the Red moved to the top position. A good rule of thumb if you arent used to reading current always start with the largest range ... in this meter 200M.

Measuring current is a tad more difficult in that the meter has to be inserted into the circuit usually by cutting a wire or a trace.

Sound advice, just some additions:



The Cenn-Tech unit has a 10A position just below the 200M position. Also the unit is fused to 200mA most circuits would pop that fuse easily.

I'm not aware of the 10A port being fused.

Additionally, the easiest way of measuring current on these cars is to get some alligator clips and clip the lead into the socket or onto the pin and just use a jumper wire to complete the other pin/socket connection.

Hopefully we can track down your problem!

Ok, it's clear that in reading the response by TnBob and by the way, thank you, I don't have a clue what this all means. Thought I might go to Sears or Lowes and pickup a meter and maybe then some of this will make more sense. Are those Cenn-Tech units from Harbor Freight? I'd like to do some more work on this tomorrow. Thanks again for the input. I was definitely feeling lost in space today but maybe a few tests can bring me back!

Meters aren't too difficult. You'll rarely try to read current in cars, mostly voltage and continuity. But you do have to respect electricity or you'll end up trying to weld your test lead to your car or your battery at some point. That's never happened to me...

If in doubt, post a pic of the controls of whatever meter you buy, and someone will chime in with settings and lead connections.

I went to the posted startek site. Fuse 12 in your car is busy.

I'd disconnect the cruise completely, possibly unplug the turn signal relay, and look under the hood for chafing or loose wires. There are a few more pages of gauge connections, but I think the pdf covers the likely culprits for your car.

Yes, the Cenntech ones are at harbor freight. Get it there.. I saw the same exact unit (just different "brand" sticker) at Advance Auto for $20!

Here is an idea:
Monitor the current at the fuse as I've indicated and unplug devices that are connect to is as indicated by Yak's PDF and see how the current does. If you see a significant drop, perhaps thats your culprit and if everything is unplugged well then something aftermarket is connected in or you've got a bum wire somewhere

Summary
 

Yak, thanks for the info and the link. It will probably help out. I wanted to summarize where I stand so far with tests and observations:

I cannot get the fuse to blow applying the brake sitting in the driveway.
Just driving slow around the neighborhood the fuse won't blow.
Seems to only blow the fuse after being up to about 30 mph.
The fuse will blow with the combination switch disconnected.
Fuse will not blow with tailights disconnected.
A red fuse will blow also even though it's twice the size.

I disconnected the cruise control thing under the glove box but not the actuator. Where is the actuator located? I'm curious and suspecious about the warning system stuff because it doesn't seem like any of that stuff works at all. Also, where is the turn signal relay?

Thanks again all for the input and help!! Very much appreciated!!!!!!

Thanks for the summary! That's very helpful.

The actuator is connected directly to the amplifier under the dash, so effectively its disconnected.

Try this, look in the driver's footwell. Remove the carpeted piece thats on the center console. There is a connector down there that connects backup tail lights and speedometer signal and a few other things. See if anything is a miss there.

On the 560SEL, that connector was taken apart and we had no speedometer or rear backup lights.. that was the culprit.

The only thing I can think of that change when you go from standing to 30+mph is the speedometer input signal. This signal is generated by a sensor on the tail of the transmission. I believe its an inductive pickup sensor for your car, hall-effect for newer. Perhaps it is grounding out somewhere.

You're clearly taking down more than 16A when it goes. Something is grounding out when it shouldn't be.

Another idea might be the warning relay, I believe its right next to the cruise control box.. I've read things can fail inside those and you lose the buzzer..

Finally, double check your tail light harnesses, just make sure they look clean and you dont see any wires touching each other. You should be able to connect your new meter to ground and the 12V wire on the harness and verify there is no continuity.

Just some ideas.

JamesDean, I see a meter on HF website just like the one you had a picture of for $3.00! That seems to good to be true but I'll go fetch one up in the morning. I'm from Idaho so maybe I could get you to go reeall slooow and explain how to set up the meter on the fuse. I guess the car would be running?

Certainly, its very simple.

Set the meter to 10A (about 5-o'clock)

Connect the probes appropriately (black to COM, red to 10A).

Put one lead (doesnt matter which) on the top copper spring clip where the fuse would sit, and the other where the bottom of the fuse would sit.

You are simply replacing the fuse with the meter. For the purpose of this experiment you don't want to let them sit too long, as the meter is unfused, just long enough to get a reading. A few seconds, the number should steady out to something.

I'll check my SD shortly and post a picture for reference.

JamesDean, the warning relay is interesting because there's never any warning buzzer for anything. The sensor on the tranny is an interesting idea also. I'm looking forward to trying these ideas out!

Hm the warning relay is interesting. When mine was ON, I had a .750 A draw on the circuit, other wise the circuit only drew what the picture shows:

Hope this helps when you go to test yours.

http://i.imgur.com/oLCzQl.jpg
http://i.imgur.com/iekval.jpg

Those large black relays you see aren't factory, I added them in for the keyless entry system, don't mind them.

I assume the meter also has specific settings for current with the lead positions - if so, make sure the wires are plugged into the correct places.

Measuring volts, or ohms, you might need to move the wires.

A tiny bit of meter theory:

Measuring volts, the electricity doesn't flow through the meter - it's an open circuit.
Measuring amps, it does flow through, so it's a short circuit.
Measuring ohms, it needs a little power to measure the voltage drop, so it ADDS current.

As long as you know what you're trying to measure, you can just read the face of the meter to set it up. Sometimes it's useful to get a sanity check by measuring a known quantity; e.g. "am I using this right? Is it measuring correctly, or at least what I'm looking for?"

That $3 meter is the correct one. It will eventually need a new 9V battery if you do much resistance checking.

At $3 consider getting 2 meters ~!~

Initial Meterings
 

Ok, got the meter. I set it on 10a, removed #12 fuse and tested. Here are the numbers:

Key on engine off = .64
Engine running = .62
Engine running brakes on = 4.01 max
Engine running, brake on, left turn signal on = 6.50 max
Engine running, brake on, right turn signal on = 6.89 max

I went under the car and found the single wire going to the back of the tranny and at first I thought it was rubbed raw on the crossmember but I don't think it is. What happens if this is disconnected?

Do I need to remove the center console stuff to get that carpet off the side of it? I took the floor carpet out but was afraid to yank to hard on that side carpet.

I disconnected a plug that was to the left of the cruise control amplifier, is that the warning thingy?

Thanks to all for the guidance and help!

Thank you Yak for the Meter Theory info! That's helps and it will be really great for me to get a better understanding of this stuff!

These numbers all sound acceptable. Each tail light / turning light is about 1.5 A draw each.

That is probably the speedometer sensor wire.

The center console carpet is held in by a screw in the vicinity of the gas pedal. If you've removed the knee panel, remove the screw and the carpeting should be able to be removed.

It should have been to the right of the cruise control unit, they share a bracket.

On the far right side under the glove box there is only one box on that bracket. Maybe I should get a photo.

Ok, removed the the carpet on the drivers side of the console. There is an odd looking coiled white wire in a deteriorated sheath that goes out the side of the tunnel near the tranny. I saw it while underneath also and it doesn't go to anything. Small gauge white wire, very strange looking.

The cruise control amplifier is located under the driver's side dashboard. Just left of the brake pedal. The warning box is like 2-3 inches to the right of it.

Thats the evaporator drain tube. If its deteriorating, you should consider replacing it. Water flows down through there and if you find a puddle on your floor in the car, you'll know why. I'm not sure why they used that type of hose. Weird stuff.

My Error
 

Ok, sure enough I had unplugged the stuff under the glove box! So, I unplugged the two units left of the brake pedal. That would be the correct cruise control amplifier and the warning box. Put in another fuse, drove it up to 25-30mph and the fuse blew.

Try that plug at the right of the gas pedal, look at it see if anything looks amiss.

I think this is the one you just mentioned

Maybe this is something. Under the carpet that is on the drivers side of the console the are two different plug in type connectors 8pin think. One of them closer to the seat and pretty low on the tunnel. The second one is right under the air duct and right up against that wierd evaporator drain hose (very weird stuff). I unplugged this thing and there was this brown gooey substance that had puddle to the low end. It's all around the bottom 2 pins and feels like it's also inside the female side of the connector. Is it possible to removed the female side from it's bracket so I could clean all that out. Since it's tied right to that weird hose I wondering if it has gotten wet??

That might be something indeed! That second one was the one I'm was talking about previously.

If your evaporator drain hose is leaking water into the cabin then it very well could have gotten that connector wet. Disassemble it and clean it up. You might have very well found the problem.

It is possible. I believe you can pry it out with a screw driver, or perhaps move it in one direction or another and it will unclip. I cannot remember.