speedometer/odometer testing - PeachParts Mercedes-Benz Forum

shadetree77 · #1 03-15-2009, 04:56 PM

hello,

does anyone know how to test the speedometer/odometer when it's out of the vehicle>? i got a 86 420SEL (it has an electrical wire instead of an cable that goes to the speedo head). i know how to test the old style cable driven one (put a drill to the back of the speedo head), but i don't know how to test an electronic one. how exactly does it work? ( does it get a volt signal from the tranny?) any help will be appreciated

mbdoc · #2 03-16-2009, 08:24 AM

Increasing a/c voltage from the trans as you increase speed.

You can test the output from the trans very easy. Testing the gauge requires a frequency generator.

shadetree77 · #3 03-16-2009, 12:52 PM

thanks m.b. doc, i found the instructions to test the speedo in the manual; don't have a frequency generator though...i replaced some broken gears for the odo but it still doesn't work, resoldered connections on the board and odometer still doesn't work; i was trying to run the speedo out of the car so i don't have to keep pulling the cluster...i guess there's a little electric (electro-magnetic) motor that runs the main gear for the odometer..must be shot....guess i am off to the junkyard!!! thanks for the info though

Strife · #4 03-16-2009, 11:57 PM

Are you really, really sure that you are getting power (and ground) to it? On a 107 (which I'll bet has a similar board, anyway), there is an infamous trace on the board that blows if anyone in the past had shorted +12 to ground.

Looking at the PCB while I had a spare gauge set apart (to change the gears and to restore it), I think the frequency to voltage converter chip used is a fairly industry standard chip, which could theoretically fail - but I'd try the above first.