Craig and Phillip, WOW! Thanks for the detailed info. SO:
1. I am certain then, that the trip is correct (will check odo too, which is also probably correct). 2. My in city MPG, then, with the '76 240D, with all the crap in the tank (bacon grease, VO, crankcase oil, gear lube, ATF, & D2) was 24 mpg. I expect it to improve with straight D2 and would like to venture out on the highway for a day trip to check highway MPG. 3. The speedo seems fixable, but since there is a replacement in the trunk (entire instr. cluster), I'll simply swap out and verify with GPS. 4. Old instrument cluster (minus bulbs) is free to whoever wants to pay shipping once I get it out of the car (moved 1 week ago, so arranging the house takes priority - or my wife will get pissed. Wife pissed at Benz is NOT good). Providing the "new one" is worse, then someone can have it and I will be playing with springs and magnets! Thanks alot! Creating new file for these e-mails - speedo repair. Christopher --- Craig McCluskey <[EMAIL PROTECTED]> wrote: > On Mon, 12 Sep 2005 15:37:36 -0700 (PDT) Christopher > McCann > <[EMAIL PROTECTED]> wrote: > > > SO, it's possible that that trip meter (and > probably odometer) are > > correct and the speedometer wrong? > > Yes, it's possible. > > > > I was being told that if the speedo was off 10%, > then the trip meter/odo > > MUST be off 10% too. > > That is wrong. The trip meter/odo MAY be off 10%, > too, but they don't have > to be. > > The speedometer and odometer work in the following > way: > > 1. The car's wheels turn on the pavement due to > friction. The number of > revolutions per mile is determined by the rolling > radius of the tires > (relatively fixed). > > 2. The car's wheels turn the driveshaft through the > differential, which > has gears, and thus a fixed ratio. > > 3. The car's driveshaft turns the speedometer (and > odometer) pick off gear > through a solid mechanical shaft, and thus a > fixed ratio. > > 4. The car's speedometer pick off gear turns the > speedometer cable via > another gear, and thus a fixed ratio. > > 5. The car's speedometer cable turns the input shaft > of the speedometer > through a (relatively) solid mechanical shaft, > and thus a fixed ratio. > > Now it gets a little tricky: > > 6. The input shaft of the speedometer does two > things: > > 6a. It turns the odometer and trip odometer through > a set of gears, and > thus a fixed ratio. > > 6b. It spins a permanent magnet near an aluminum > disk. (There is a > solid mechanical connection between the > speedometer's input shaft and > the permanent magnet.) The spinning magnet > induces a current the > aluminum disk and causes a drag force > proportional to the speed the > magnet is turning, and inversely proportional to > the spacing between > the magnet and the disk. The disk is mounted on > a rotating shaft along > with a rotary spring and the speedometer needle. > > SO, the angular position of the speedometer needle > is related to how fast > the magnet is spun by the speedometer's input shaft, > and by extension how > fast the tires are rotating. BUT, it is ALSO related > to the space between > the magnet and the aluminum disk AND where the > speedometer needle happens > to be pointing relative to the aluminum disk. > > The end result is that you can have a lot of > variance of the indicated > speed for a specific rotation rate of the wheels. > You have to juggle the > magnet/aluminum disk gap, the strength of the > spring, and the pointing of > the speedometer needle to have the speedometer be > accurate over its entire > range. > > > Craig > > _______________________________________ > For new parts see official list sponsor: > http://www.buymbparts.com/ > For used parts email [EMAIL PROTECTED] > > To Unsubscribe or change delivery options go to: > http://striplin.net/mailman/listinfo/mercedes_striplin.net > Christopher McCann, Squier Park, Kansas City, Missouri -1985 300SD, 209K miles, "Wulf" (http://www.pictureblogger.com/My-1985-Mercedes-Benz-300SD) -1976 240D, ManyK miles, "AKP-Wagen" (Alternativen Kraftstoffs Prüfenlastwagen = Alternative Fuel Test Vehicle) -1998 Toyota Sienna CE, 99K miles, "The Van" __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com
