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Old 07-03-2011, 07:43 AM
Join Date: May 2011
Location: Illinois
Posts: 21
94 C280 Non working Speedometer and workaround

One of my other challenges I face with my car is a non-working speedometer. I've determined it is wheel speed sensors that are causing this issue becaue ABS and traction control are knocked out as well. The car runs and drives fine so I won't be spending the $400 on all four sensors (maybe less if it is on the fronts only $275). I may take care of this come winter time when ABS and TC is always nice in the snow.

As the tachometer is still functional, I knew I could calculate speed off the RPMs the car is running. After much searching I found the gear ratio for this particular car 2.87:1. This post is particularly for the transmission series [FONT=Courier New][SIZE=3][FONT=Courier New][SIZE=3]722.424 Model W4A020.

Utilizing the gear ratio, you can calculate RPM to MPH. Here is the output of those calculations. If someone wants to test this on a working car I'd appreciate it. I took the car past a construction zone radar gun and it showed I was pretty accurate with my numbers...

Differential Gear 2.87 to 1
Transmission Gear 1.00 to 1

Vehicle MPH-------Vehicle RPM


Last edited by bbogner12; 07-03-2011 at 07:48 AM. Reason: did not import numbers from spreadsheet.
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Old 07-03-2011, 05:05 PM
Gilly's Avatar
Registered User
Join Date: Mar 2001
Location: Evansville WI
Posts: 9,616
As far as a workaround, most GPS units will give you speed.

As far as a repair, you can find out which sensor provides the wheel speed input, I know it is one of the fronts but unsure if it is right or left, but should not be super difficult to find out, then replace that one.

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