'80 300SD: Need encouragement (and advice)... about to remove instrument cluster

[WrightSounds]

Hello Hello, it's me again. I really didn't think I'd be bugging y'all so much but this car is like a rubik's cube 

Here's what happened:
I touched the dimmer dial for the dashboard/cabin lights last night and the lights for the heat/ac controls and the lights on the instrument cluster went out.

I've read plenty about this over the last hour and I've come to a conclusion that the dimmer pot/rheostat is toast. A good sign of this is that the instrument cluster lights and the lights for the heat/ac controls use two different fuses, right? So for both of them to die at the same time is probably because of the dimmer pot/rheostat failing therefore stopping the current from getting to where it needs to go. I've turned the dimmer from end to end in hopes of "cleaning" it from residue so it makes contact again but no luck. I've also tried to find a "sweet spot" but no luck.

I've read about bypassing the dimmer pot and that sounds like a plan to me. I've always had the dimmer turned all the way up and only touched it last night to see if the lights would go any brighter. So this involves removing the instrument cluster. I've found the instructions in the Library's manual: http://handbook.w116.org/Chassis/54-250.pdf and understand its just held on by a rubber strip. I also found a thread in this forum with pictures of the removal but I can't find the thread again  (help)

Basically, I'd LOVE to know all the silly things that could go wrong or things to look for so this hopefully won't turn into an 8 hour test of patience.

-Are there any tricks to removing OR re-installing the cables (speedometer, oil pressure, etc) to the cluster? Are the oriented a certain way? Are there clips or locks or levers or ?? that hold them in?

-I've seen where a lot of people remove the steering wheel before removing the cluster panel. Please tell me I don't need to do that 

-Does anybody have a pic of the dimmer pot/rheostat from back? Are the pins accessible so I can solder a jumper between the outer two and bypass the dimmer?

Can't think of anything else right now. My car is a 1980 300SD.

[JasonP]

Be sure to check your fuses first, you might be able to bring back the lights until you replace/bypass the rheostat.

I have never removed the instrument cluster, but I have replaced some blown fuses. The fusebox is in the engine bay, driver's side corner against the firewall. If you are lucky, when you open it up you will find a) spares, and b) a listing of which fuse controls which device.

A long-term goal would be to replace the fuses with brass. Apparently, Mercedes messed up here: they have brass fuse holders and aluminum fuses. The dissimilar metals corrode over time, and lead to bad connections. MercedesSource knows all about it - http://mercedessource.com/node/48.

[WrightSounds]

Thanks again Jason! I checked out the fuse box. The paper showing what each fuse is connected to is there but I don't know what the heat/ac controls are actually called?

Fuse 4 says "Instrument Cluster" along with A LOT of other things. Are the heat/ac controls part of the "Instrument Cluster"? I thought I read that they're connected to different fuses? I know the shift indicator light is on a different fuse (#5) then the instrument cluster.

Here is what my layout is: http://www.benzworld.org/forums/attachments/w116-s-se-sel-sd-class/395236d1315110625-fuse-layout-300sd-1979-80-116300sdfusechart.jpg

Here is my SUPER dumb question(s) of the day. Are the fuses really just color coded? Fuse four is a WHITE fuse. If there is another extra WHITE fuse can I assume (ugh) that its the proper fuse. There seems to be little to no writing on the fuses. Can I visually inspect if the fuse is shot? And if one of the things that the fuse is connected to works, say, the turn signals, then I can assume (ugh) that the fuse is good, no?

A long-term goal would be to replace the fuses with brass. Apparently, Mercedes messed up here: they have brass fuse holders and aluminum fuses. The dissimilar metals corrode over time, and lead to bad connections. MercedesSource knows all about it - http://mercedessource.com/node/48.

Looks like I definitely have a mixed bag of aluminum and brass:


Should I install the spare WHITE fuse into slot #4 and see what happens?

[JasonP]

Can I visually inspect if the fuse is shot? And if one of the things that the fuse is connected to works, say, the turn signals, then I can assume (ugh) that the fuse is good, no?

Yes, they are color coded for amperage. If you place the wrong fuse in, you could melt some wires and possibly start a fire. If the circuit calls for an 8 amp fuse, and you put in a 16 amp fuse, then 8-15 amps could possibly surge through and blow everything out. Your sheet should have a legend on the bottom. Here is mine:

http://forum.w116.org/mechanicals/i-begin-tackling-the-acc/msg97037/#msg97037

If something on the circuit works, then yeah, your fuse is good. You visually inspect, or you can take it out and check for continuity (using a multimeter), or you can just twist it back and forth to reset the connection through any corrosion that might be there.

It very well could be your rheostat. This is just a good first-check in troubleshooting electrical problems. Every one I have had has ended up being a fuse.

[JasonP]

You're having daily adventures. Fun, isn't it? A year from now you will be an expert. 

[WrightSounds]

You're having daily adventures. Fun, isn't it? A year from now you will be an expert. 

I know! I'll quit my job as a guitar tech and become a Mercedes tech (strictly for 1980 300SD models... that are just like mine... and that have only had the same problems as mine  )

[WrightSounds]

Your sheet should have a legend on the bottom. Here is mine:

http://forum.w116.org/mechanicals/i-begin-tackling-the-acc/msg97037/#msg97037

Yeah, we have the same layout. So see how #4 on the legend is a White 8A fuse? Doesn't my #4 in the pic look like a faded Red fuse like all the other red fuses except two that look recently replaced? I have a spare White fuse (see pic) and I feel like I should put it in slot #4 regardless if that's the prob or not 

[JasonP]

I would. Looks like someone put the wrong fuse in. Maybe that is why your rheostat fried. Since 8A is less than 16A, I do not see a down side in replacing #4 with the white.


... Or you could make guitars out of Mercedes parts. I am pretty sure they would last longer. 

[WrightSounds]

I would. Looks like someone put the wrong fuse in. Maybe that is why your rheostat fried. Since 8A is less than 16A, I do not see a down side in replacing #4 with the white.

I'm about to drive to work so maybe I'll do it when I get HOME 

  ... Or you could make guitars out of Mercedes parts. I am pretty sure they would last longer. 

Don't give me any ideas! Very little time as it is 

[WrightSounds]

Well, after I started my car and realized my blinkers didn't work, I checked fuse #4 and it was definitely blown. I then replaced it with the proper White 8A fuse and that corrected the blinker problem but not the dashboard lights. So it looks like the dimmer pot needs to be bypassed which takes me back to removing the instrument cluster.

Like I said, I've read all about it... any clues or pointers for things to watch out for when removing and re-installing it from people that have done it? And I still can't find that thread from here where the member took very nice pics... any idea?

[jbrasile]

WrightSounds,

To remove the instrument cluster:

1) Cover the top of the steering column with a soft towel to avoid any scratches
2) Carefully pull the cluster towards you from one side and then the other, you can use two fingers to "grab" the inside of the instrument binacles, be gentle and careful not to damage the wood.
3) Once the cluster is loose, you need to disconnect the oil pressure line from the back, it is a 10mm nut and you can reach it by tilting the inner portion of  the cluster , you will see the nut once you do that
4) Reach from behind and disconnect the speedo cable
5) Once the oil line is removed you should take out the main round plug from behind the combination gauge
6) There will be a cruise control connector to the speedo as well as a clock wire (RH side)  and another wire (blue I think)  connected by itself near the oil line

Installation is reverse, to make re-connecting the speedo cable easier, loosen the cable tie that holds the cable to the floor, you should be able to see it if you remove the driver's carpet.

Hope info helps.

Good luck.

Tks

Joe

[ptashek]

5) Once the oil line is removed you should take out the main round plug from behind the combination gauge

And make sure you don't start the engine while the oil line is detached... As silly as this may sound, I've actually done it. Luckily all the trim was already outside the car.

[Tony66_au]

First things first..........

Wrightsounds you are on the right track here.

Secondly.... Jason.

A few things I have to point out, the simple act of replacing an 8 amp fuse with a 16 amp fuse will not damage a thing.

They are simply rated to the current draw and the application.

As a safety device they prevent electrical fires caused by higher current loads than the wire can handle.

This prevents auto wire turning into Demo cord or a heater element.

Most of the calculations for fuse ratings is centered on Resistance value as this plays a crucial part in figuring out when a bit of insulated wire goes from being say a positive current feed to an interior light to a red hot firestarer buried in the guts of your car.

And the fuse rating determines how much current to allow through the circuit and when this flow becomes dangerous enough to Pop the fuse and break the flow of electrons.

White, red and Blue are what we deal with and most lighting circuits are 8 amp rated because the wire is thin and the devices are fragile low current items.

Heavier duty stuff like heater fans. cigar lighters and demisters use a heavier fuse because they use heavier duty componentry and wire that wont so easily combust under load.

So heres the problem............

Over time the resistance value of a basic car circuit will increase and a simple 5 or 6 amp light circuit fused at 8 amps will become an 8 or 10 amp circuit with an 8 amp fuse.

This happens because joints are not as clean or uncorroded as they were when new, copper goes green, solder goes dry and also oxidises and more importantly earth connections turn to crap so you need to feed more current into the circuit to get a passable result once the current has navigated all the worn out crappy joins in the wire.

The fuse is now the weakest point in the circuit and as the electron flow through the fuse is now equal or greater than 8 amps the fuses get hot as they are now a high resistance value compared to the current draw needed to make the item work properly or even a bit. (Yellow flickering headlights on old cars are a classic example).

So Mr Fuse is now stressed to the max and getting a bit hot.......

No biggie you say.

Well this is true if you have good quality fuses as supplied by Daimler Benz in Germany, But not all fuses are created equal.

A good bullet fuse is a ceramic core with a copper strip.

A cheap fuse is a Ceramic core with an alloy strip....

A Nasty arse fuse is a Plastic core with an alloy strip...............

Copper is a lower resistance which creates less heat, Ceramic is pretty much fire proof so this combination is best.

Jump straight to the plastic core alloy strip and you have a higher resistance reactive alloy (That gets very hot under load) and a plastic core that will get soft or melt when hot and you have the perfect recipe for an intermittent fault.

Add old wiring that will be of a far higher resistance than it was when new, crap fuses that melt and a reactive metal strip and its trouble on a stick.

Then add a doof doof sound system, Amp, Subs, iPlod, Phone chargers, Sat nav and all the other shit we find in cars today and the already stressed by age electrical system will often be on the verge of meltdown.

Another little known fact of auto electrics is that a bad earth (Negative side of the circuit) will increase the positive current draw to compensate effectively turning other items on the circuit into defacto earth wires, usually through other light globes or switches and a rheostat being a variable resistor will be hit fairly hard by all this which is why there are so many dud dimmers and failed dash lights.

So................

When you have the car apart fixing the issue, have a good look for a new earth to the body of the car.

Dont use self tapping screws if you can avoid it and use a nice piece of new wire when you make the earth, it will save you headaches in future.

[jbrasile]

ptashek,

I've done the same thing... and on a Diesel too!!! Luckily I only got my clothes messed up and not the seats/carpet, hehehe...

Tks,

Joe

[JasonP]

A few things I have to point out, the simple act of replacing an 8 amp fuse with a 16 amp fuse will not damage a thing.

Well, I would never do it.

Good info about the corrosion and age causing more current draw because of a higher resistance, though.