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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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Discussion Starter #1
On my 560SL the warning indicator lights went out for WW level, coolant Level and oil level. I thought I was just going to be replacing 3 bulbs but these three bulbs are actually LED's on a printed circuit board. I found the board to have a burnt out resister. I measured the resistance as installed of another working board and it was 4 Ohms. But that resistor was installed. Since this part is $500 I would like to repair it if possible. So I'm wondering what the likely hood would be of this thing working just by replacing the resister and also how can I tell what watt resistor is required.

Sorry the first picture is not clear but the large resistor in the middle is cooked and discolored. Not a hope in hell of reading the colors. The second picture is from a working board and the colors are pretty clear, However I can never tell the between yellow, orange or gold on these things.
 

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1987 300SL
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118 Posts
I believe you're correct cush
The monitor I use (photography) is colour calibrated monthly and I read from top down orange, blue, gold, gold
Auto white balance when taking the pic might skew the colours though, so what is being shown might not be accurate

Edit, just looking at the pic of Samson? in the background, the red duco is far more vivid than the orange band on the resistor, so I believe orange is correct
Blue is also correct, no question on that
And the gold bands have too much reflection to be yellow
 

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'80 450SLC Afro RHD Ikonengold
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2,925 Posts
Or ot could be more common 2.7 ohm aka red, violet. Could be either.


There is an app to read colours of resistors from an Android phone camera
Camera often skewers the colours. A picture omon outdoor sunny day is the best.

Resistors are like fuses, they burn for million reasons, but never for "no perticular reason".

To me it looks as if it is a 2W carbon film resistor
 

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1983 380SL, ivory/dk brown, 46k miles, dual roller timing chain. 1986 560SL, red/white, 190K mile.
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6,942 Posts
Where you see green, I see blue :)

Orange Blue Gold Gold = 3.6 ohms.
I think that is a 2 watt resistor. Anyone know for sure??
 

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Premium Member
1986 560SL with M120 V12 Engine, 1988 560SL Stock
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10,411 Posts
Discussion Starter #8
If I select Automotive Flame retardant as a feature the only one that comes up in 2W is this one:

Can I assume that:
If its a 2W I can install a 2W to 5W
Any composition is OK

 

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Premium Member
1986 560SL with M120 V12 Engine, 1988 560SL Stock
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10,411 Posts
Discussion Starter #9
I believe you're correct cush
The monitor I use (photography) is colour calibrated monthly and I read from top down orange, blue, gold, gold
Auto white balance when taking the pic might skew the colours though, so what is being shown might not be accurate

Edit, just looking at the pic of Samson? in the background, the red duco is far more vivid than the orange band on the resistor, so I believe orange is correct
Blue is also correct, no question on that
And the gold bands have too much reflection to be yellow
When your comparing to the color of Samson, do remember to quote nobby. "Samson is various shades of vagina pink depending on how the sun hits it"
 

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Premium Member
1986 560SL with M120 V12 Engine, 1988 560SL Stock
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10,411 Posts
Discussion Starter #10
Or ot could be more common 2.7 ohm aka red, violet. Could be either.


There is an app to read colours of resistors from an Android phone camera
Camera often skewers the colours. A picture omon outdoor sunny day is the best.

Resistors are like fuses, they burn for million reasons, but never for "no perticular reason".

To me it looks as if it is a 2W carbon film resistor
There might be a reason this happened. On several occasions while running the car with the instrument cluster loosely installed, I noticed the tach wouldn't work correctly. I though I blew the ground. But with the instrument cluster permanently installed now all works good. The real question is would that blown resistor be the total extent of the damage?
 

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1983 380SL, ivory/dk brown, 46k miles, dual roller timing chain. 1986 560SL, red/white, 190K mile.
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6,942 Posts
If I select Automotive Flame retardant as a feature the only one that comes up in 2W is this one:

Can I assume that:
If its a 2W I can install a 2W to 5W
Any composition is OK

Either 2W or 5W at 36 ohms will work.
You can also use x2 Resistors at 72 ohms each and put them in parallel with each 1W.
Two same resistors in parallel = equivalent to 1/2 the resistance of one of them.
AND, the dissipated wattage for each resistor is also halved.

You might find useful if you have difficulty finding a 2W resistor or a 5W is too big.
 

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'80 450SLC Afro RHD Ikonengold
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There might be a reason this happened. On several occasions while running the car with the instrument cluster loosely installed, I noticed the tach wouldn't work correctly. I though I blew the ground. But with the instrument cluster permanently installed now all works good. The real question is would that blown resistor be the total extent of the damage?
Without seeing it, I cannot assess.
If there is a chance something was shorting whilst it was lose, then maybe.
I do not know the function of this resistor and what purpose does it serve to answer better.
Bottom line, plug it in and see what happens. Discolouration is not a sign of a major fault and intermittent shorting can cause it to overheat.

A bit of overthinking:
When replacing the resistor, it is beneficial to observe the method of mounting and the shape of the wire terminals (i.e. bend on the wires, distance from the PCB and wire bend at the solder sides). Some serve a function and are best replaced equivalent. Bends on the wire, either side of the PCB will ensure the component stay attached to the pcb even if it gets hot enough to melt the solder. Also, I would suggest use of the same solder - so observe the sign "Pb" being crossed out or RoHS sign - unleaded solder in use.
Different resistors burn differently but in 70's metal film resistors were costly and used only where critical. 5% tolerance probably mean it was metal film anyway so product from your Digi-Key link is likely to be good, in my view.
 

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1983 380SL, ivory/dk brown, 46k miles, dual roller timing chain. 1986 560SL, red/white, 190K mile.
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6,942 Posts
Here is my analysis.
I assume that 36 ohm 2watt resistor is attached to +12 volts.
Also one end of the LED connects to that resistor and the other end to Ground.
A short event would allow I=E/R = 12/36 = 0.33 amps through the resistor.
The power dissipation on that resistor would be P=IxE = 0.33 amps x 12v = 4 watts.
That exceeds the watt rating by 100%.
 

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75, 280Sl /5speed
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1,340 Posts
While you can use a higher wattage resistor as a replacement I always assume whoever
designed the circuit had that specific wattage in mind for a reason, and I don't try to
second guess.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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10,411 Posts
Discussion Starter #16

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1983 380 SL
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3,013 Posts
The resistor didn't cook for no reason... it may have been temporary shorted when the console was out but that doesn't appear likely to me. Why don't you just measure the resistor? Often times small resistors can get really hot even though they aren't carrying much current.

There are 3 diodes on the board adjacent to the resistor. It's very easy to check diodes, with your meter set to ohms, put your meter leads across the leads of the diodes one at a time and then reverse the leads. A diode will read open or nearly open in one direction but conduct in the other direction. That's the function of a diode but diodes in DC circuits serve little purpose unless they are Zener diodes and those diodes don't look like Zener diodes to me.

The ic looks like a timer to me (like a 555) but I was unable to cross reference the numbers on it. It's function remains a mystery to me.

There is also what appears to be a transistor on the board, several capacitors, one electrolytic capacitor (makes no sense if it's a dc circuit unless the timer ic is timing the charge and discharge time of the electrolytic).

You can perform most tests with just a DVM but if the ic is bad there is no way to check it without a schematic with wave forms and a scope.

Would you happen to have a schematic?
 

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1987 560SL, 2000 Kawasaki W650
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1,048 Posts
Unfortunately this appears to be the least-fuzzy photo I took of the whole top of the board. They changed the circuitry for 1987. Mine has an 8647 IC, your working example appears to be an 8818? It’s possible they changed the IC because they couldn’t get more 8818s, which is the possible reason there’s no reference data on it.

That resistor on mine is a 3 Ohm 5% 2W or 5W. I’d go with a 5W to be safe.

A better photo of your board with the burned out resistor would help for a better comparison.

2640057


If I had that assembly out now I would replace the aluminum electrolytic capacitor at upper right on principle, as they say in the MB manuals. Over the years those dry up, I can't tell you how many of those I've replaced on old tube amps.
 

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1986 560SL with M120 V12 Engine, 1988 560SL Stock
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10,411 Posts
Discussion Starter #20
Unfortunately this appears to be the least-fuzzy photo I took of the whole top of the board. They changed the circuitry for 1987. Mine has an 8647 IC, your working example appears to be an 8818? It’s possible they changed the IC because they couldn’t get more 8818s, which is the possible reason there’s no reference data on it.

That resistor on mine is a 3 Ohm 5% 2W or 5W. I’d go with a 5W to be safe.

A better photo of your board with the burned out resistor would help for a better comparison.

View attachment 2640057

If I had that assembly out now I would replace the aluminum electrolytic capacitor at upper right on principle, as they say in the MB manuals. Over the years those dry up, I can't tell you how many of those I've replaced on old tube amps.
Nice pic That looks more like Orange, Blue, Gold, Gold
 
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