I've checked the forum but don't see anything like what I'm experiencing. My speedometer pops up to about 20mph when I start the car and bounces between 20 and 40. If I whack the dash it will sometimes drop and work normally. Sometimes if corrects itself. This is an electronic speedo so I know it isn't a cable lube problem.

'84 300SD

I had a similar problem with the Temp Guage on my 84 300SD, I pulled out the instrument cluster and looked at the solder joints on a 27 year old cluster. The solder joints were all dull grey so I resoldered every connection and my problem went away. It could be that simple or not. Best of luck in your hunt for the culprit.

__________________
Gary
OBK #37

I've been chasing this bug with an oscilloscope,
for a customer, for a couple of months now.

I think, in my case, its noise in the connector between the electrical lead and
speedo. Try dis-assembling the plastic housing on the speedo end of
the cable and plugging just the terminal back in. Its a coax, so there is no
problem doing this. This worked for a while in my case. But since the
connector isn't secured properly to the speedo -- the plastic housing has
a little finger that orients it to the speedo -- the cable can work loose.

I only see the car every 4 or 6 months; so when I get to the bottom of this
I'll be sure to post it up. I'd love to find an electric speedo and drive box so
I could stuff it into my own car and rig it up properly for long term diagnosis.

Anyone have a speedo head they want to get rid of? Send me a PM.

Thanks all. I'm diving back into it tomorrow.