83 300sd Brake Lights Blown Fuse help please - Page 2 - PeachParts Mercedes-Benz Forum

ShadeTree300SD · #1 03-14-2012, 01:58 PM

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!!

Holmesuser01 · #2 03-20-2012, 06:58 PM

Crazy question:

Has the brake light wiring been mucked with? any of the wires damaged? On some cars I've seen, the tail-light wiring passes under the side plastic panels in the trunk. Maybe one of yours is getting wet somehow, or maybe the wires have been crushed? Something to check.

JamesDean · #3 03-14-2012, 01:25 PM

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:

JamesDean · #4 03-15-2012, 11:46 AM

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 ?

ShadeTree300SD · #5 03-15-2012, 12:17 PM

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!

JamesDean · #6 03-15-2012, 12:22 PM

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.

ShadeTree300SD · #7 03-15-2012, 02:07 PM

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.

JamesDean · #8 03-15-2012, 02:44 PM

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.

TnBob · #9 03-15-2012, 07:13 PM

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.

JamesDean · #10 03-15-2012, 07:45 PM

Sound advice, just some additions:

Quote:

Originally Posted by TnBob

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.

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!

ShadeTree300SD · #11 03-15-2012, 08:10 PM

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!

Yak · #12 03-15-2012, 09:27 PM

Quote:

Originally Posted by ShadeTree300SD

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.

ShadeTree300SD · #13 03-15-2012, 10:20 PM

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!!!!!!

JamesDean · #14 03-15-2012, 10:06 PM

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

ShadeTree300SD · #15 03-15-2012, 10:30 PM

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?

Thread Tools
Show Printable Version Email this Page
Display Modes
Switch to Linear Mode Hybrid Mode Switch to Threaded Mode