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#1
10-20-2009, 12:38 PM
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126 Speedo reads to fast
300SDL
I just replaced the defective 140mph speedo with a 160mph speedo from a gas model and it now works but reads to fast. At an actual 40 (GPS) it reads over 50. Tire size is correct for the car. Any thoughts for the need or proceedure to recalibrate? Speed sensor problems? Maybe this has something to do with the fuel gauge being maxed regardless of fuel level or oil pressure reading unusually high? I realize the sensors are totally seperate but the speedo is electronic. Thanks in advance. Tony Thanks for any thoughts on this
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87 300SDL (Silver/red) 08 VW GTI (Sold) 89 F250 5sp turbo diesel 00 Triumph Sprint ST (4sale) 00 Polaris Xplorer 400 4x4 09 Kawi KLR 
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#2
10-20-2009, 01:06 PM
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The gas car has a different rear-end ratio, thus a different speedometer.
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 Gone to the dark side - Jeff
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#3
10-20-2009, 03:38 PM
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Since its electronic could it be as simple as a potentiometer or something?
I recall a time earlier where I was troubleshooting the speedo and I read a post indicating that a voltmeter between the brown and white wires on the plug that goes to the speedo would net me AC voltage increasing with vehicle speed. I did this and that is exactly what happened leading me to the conclusion that the speedo was defective. Maybe a variable resister inline with the white wire to gently knock voltage down until I can match the speedo with my GPS? Replace the speed sensor with one appropriate for the gear ration and speedo? What to do? Thanks Tony
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87 300SDL (Silver/red) 08 VW GTI (Sold) 89 F250 5sp turbo diesel 00 Triumph Sprint ST (4sale) 00 Polaris Xplorer 400 4x4 09 Kawi KLR Last edited by Caseyjones955; 10-20-2009 at 03:45 PM.
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#4
10-20-2009, 04:17 PM
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AC voltage on a sensor would imply to me that it's reading the pulses not the voltage.
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Gone to the dark side - Jeff
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#5
10-20-2009, 07:37 PM
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Dakota Digita SGI-5
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There is a very fine line between "hobby" and "mental illness." FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E. Bling: '87 560SEC, ported/polished, lowered Lisa: '87 560SEL, lowered
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#6
10-21-2009, 11:06 AM
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that is exactly how I will handle it, to bad I cant do any such thing for my cable operated motorcycle speedos. Thanks very much for a workable solution!
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87 300SDL (Silver/red) 08 VW GTI (Sold) 89 F250 5sp turbo diesel 00 Triumph Sprint ST (4sale) 00 Polaris Xplorer 400 4x4 09 Kawi KLR
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#7
10-21-2009, 11:11 AM
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You can order different gears to calibrate the speedo an odometer at carradio.com
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1999 w140, quit voting to old, and to old to fight, a god damned veteran
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#8
10-21-2009, 12:57 PM
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Is the scale uniformly off by 10 MPH? If so, pull the needle off the dial and stick it back on moved over a bit.
Or is the scale uniformly off by 20%? If so, you have to use a pulse stretcher like that Dakota thing. Or find and put a 140MPH gauge back in (probably easier).
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Cheers! Scott McPhee 1987 300D
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#9
10-21-2009, 06:55 PM
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its around 20%, I am nowhere near a 140mph speedo, I wish I were because that would be the simple solution. There is no Benz dealer or wrecking yard that will acknowledge the existance of MB anywhere near me. Acquiring the correct speedo would be another trip to ebay. Thinking the signal stretcher is good, then I can calibrate it to be very accurate at open road speeds.
The Dakota Digital peice is nice but at $80 I had to look for a cheaper solution, of course nothing came up. Im so burried in this car I would love to find a cheaper way to do it but I'll do what I must to get it operating correctly.
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87 300SDL (Silver/red) 08 VW GTI (Sold) 89 F250 5sp turbo diesel 00 Triumph Sprint ST (4sale) 00 Polaris Xplorer 400 4x4 09 Kawi KLR
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