I'll be pulling my cluster soon for an odometer repair, but I imagine the design probably changed between '75 and '81.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

Ashedd, power in watts is the square of the voltage divided by the resistance.

P = VČ/R

So, if the voltage say is 10, and the resistance was say, 65 ohms - like the fuel gauge - then the wattage would be 1.5. Thus, for the wattage to be 5, then the resistance would have to be 20 ohms.

Does this makes sense?

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1975 300D

Not sure if this is what you are asking, but:
A 5 watt resistor is rated as such because that is the max amount it can safely pass. Using real-life numbers, the voltage is going to be more like 14 with the ohmic value of the resistor being 68, the current is going to be something above 2 watts. You might be able to get away with a 2.5 watt rated resistor, but you'd be bumping up against its max rating. Makes more sense to put in a 5 watt if available, doesn't it?

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-Louis

'87 300D Anthracite

LOL - +1

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

The temperature gauge responds to current, which depends on the voltage applied (between 12.5 and 14 volts) across the series combination of temperature sending unit resistance (variable, depending on temperature) and the resistance of the "broken" series resistor.

While the series resistor must be rated to dissipate the wattage (VČ/R or IČ*R, take your pick) at any current from cold to hot, this wattage rating will not really help you find its resistance.

So, we are back to the need to find the proper resistance value to get the proper replacement.

Now, if you know the resistance of the temp sending unit at gauge minimum and maximum temperatures (40C and 120C?) and amount of current it takes to drive the needle at these deflections, you can calculate the total resistance value and thus find your resistor value...

have you posted a wanted ad for the gauge? surely someone is parting one of these cars on this forum and has the part as a replacement - the whole gauge unit that is. for the 20 bucks or whatever it's going to cost for the used temp gauge you'd be done. it's fine to try fixing the gauge with a resistor but it's also a whole lot easier to swap in a good used unit and go on to frying bigger fish.

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punkinfair

how do you pull the cluster on the 115? I could probably check on mine after irene passes this upcoming week

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This post brought to you by Carl's Jr.

Everyone has added good info. I still believe the next step is to find another one to either buy or get the value off of.

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D

Okay, I had the cluster out of my '81 300TD to try to fix the odometer, today, so I lifted one leg of the resistor on the temperature gauge and measured it. It is, indeed, a 68 ohm resistor. Yours is probably the same *if* the temperature sender spec didn't change between '75 and '81. I don't know the wattage spec but judging from its physical size, 5 watts should be safe.

So, you need a 68 ohm, 5 watt resistor. I checked my junk box, and I unfortunately don't have one to give you.

If there's a Fry's in your area, they'll almost certainly have it. Radio Shack might but it's a long shot; they're mostly a cell phone store now. You could also order online from Digi-Key, but you'll end up paying $15 in shipping for a $0.50 part, if you do it that way.

You will need basic soldering skills to do the replacement.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

Thanks a lot Orv. You came to my rescue!

I will order one online. I tried sourcing these things locally, and no one carries them. At least no one who is mentioned on google.

Mark

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1975 300D

By the way, for this application, the exact type of resistor is not important. Most likely you will be getting a wire-wound one, but anything that meets the resistance and power handling specifications will be fine. It may not look exactly like what was there before.

Since the resistor affects the accuracy of the gauge I would go with 5% tolerance or better. I don't know the tolerance of the one that's in there but 5% is pretty typical. They might have used 1% but I doubt the gauge is meant to be that precise.

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

I replaced a 68 ohm resistor that was in my tach. It's still dead...

I replaced an 68 ohm resistor that was in my tach. It's still dead...

I think I measured an intact one on the same board and came up with 68. I strung two or three together to get 68 (Forget the values but I got them at RS.)

68 WTF??

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My occupational hazard bein'
my occupation's just not around...
1980 300SD
1980 300SD

Orv, the resistor for the fuel gauge is a 68 ohm with a 10% tolerance. I guess back then, that was normal.

So I will assume now that the temp resistor is 68R 5w 10% too. Although the numbers are written on it, it can be hard to see.

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1975 300D

I'd go with a junkyard one, and check to find out what other models use the same gauge and then start the search. Also, have you checked with Phil here at Peachparts to see if he has one located on the shelf . . ready and waiting for an owner? His may be a lot cheaper.

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1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002

Just happened to see your thread . . .

I recommend you use a "film" resistor since they are very stable in high temp situations. Here's link to "Mouser" . . they have everything electronic and they're are cheap!

This is 68 ohm, 0.5watt, 1% metal film resistor that costs $.13 each.
http://www.mouser.com/ProductDetail/Xicon/273-68-RC/?qs=sGAEpiMZZMtG0KNrPCHnjfSDAuUPXjOYkZ2FglWjjnc%3d

At that price, you could buy a few at different wattage, say 1W, 2W, etc.

Here's a 1W version, 1% version; price is $0.70 each:
http://www.mouser.com/ProductDetail/Vishay-Dale/CPF168R000FKB14/?qs=sGAEpiMZZMu61qfTUdNhG33BaVfMIgUsb7yqqMn1dBM%3d

Here's a 2W, 5% version; cost is $0.32 each:
http://www.mouser.com/ProductDetail/Vishay-BC-Components/PR02000206809JR500/?qs=sGAEpiMZZMu61qfTUdNhG%2f4r7Iw6CIkyvLyi6YDfBsU%3d

If this is a "current sense" resistor and it's "5W", then I doubt that it's "68 ohms"; most likely it's a "0.68 ohm". .

Here's a 0.68 ohm, 5W, axial resistor: cost is $1.20 each:
http://www.mouser.com/ProductDetail/Panasonic/ERX-5SJR68/?qs=ykEKLGTiBczSOwfH5bnJqw%3d%3d

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Regards . . . . JimF
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'94 S500 Cpe

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