[Steve420SEL]

I've read all the posts I could find on speedometer bench testing, but still not quite sure if I have all the information. I replaced the odometer gears and capacitors a while back to fix that issue, and it worked okay for a while, so now I think there is a bad trace somewhere, as I could tap the dash to revive it each time it quit, but that stopped working. That old PCB was hard to solder without damaging the foil. Is there a schematic for the speedometer section? And does the square wave for testing get applied to the big pin or to the middle pin on the 6-pin connector, between the +12v and GND pins? I have a function generator to drive it on the bench. Thank you for your help!

 

[alexstorm]

I suspect a bad solder join inside the Speedo. Look to see if any connection looks milky and refresh it.

Hope that helps

 

[JamesDean]

Steve,

I usually test them by sending signal into the 6-pin connector. The bottom row is labeled. 31, G+, 15.

31= ground
G+ = square wave 12Vpp.
15= 12V.

Here's a video, of a 350SD speedo under test:

You may need to play with the duty cycle and frequency of your function generator to get it to swing full range.

 

[plinker17722]

Your talking about a 1988 car here .Are you shure its an electronic speedo .

 

[JamesDean]

Your talking about a 1988 car here .Are you shure its an electronic speedo .

The W126 and R107 are electronic speedometers.

The W123, W124, W201 are mechanical cable driven speedometers.

Here are some pictures from an 300SD:
https://www.smugmug.com/gallery/n-ZjkSXB/