Electrical nightmare 1981 300sd
 

So I bought a 81 300sd with no key. Not a big deal but there was no way for me to test everything before buying the car. Got a smoking deal for it. Btw was able to make sure it ran good before I boight it. Now that I got it home I was able to take ingnition cylinder out and turn ignition on with now key. Dash lights are goin crazy. When I turn the left blinker on the fuel gauge and temp gauge move with the blinker. Also the seat belt light, glow plug and high beam indicator lights all flash with it. So my question is that is there any comin problem that would cause this issue. Like a comin connecter or comin power wire that could be it. I've tried searching through the forums to no anvil. Any help would be much appreciated. TIA

I'm not familiar with the 81 300sd, but my bid is your dash not connected to the negative or the connection is corroded.

I had similar problems with my 84 300CD. The problem was a burnt through grounding trace on the instrument mounting PC board. It was a little hard to find because the burned area was under the fuel gauge guts. I was able to repair it by installing a jumper wire around the bad spot.

+3 on a bad ground connection somewhere in the instrument cluster or from IC to the car body.

Had the same thing on my 380SL - it's such a common problem in that community, it's standard practice to add a ground wire to the cluster. Look for a bad trace on the board first, then consider adding a ground wire.

Another thing to look for is the 15 pin circular connector on the back of the cluster can get its polarity key broken off. If the connector is mated with the wrong pins matching, much mayhem will ensue.

I wouldn't have pulled the tumbler....it takes a bit but you could get a factory key for the car for under 40$ from any mercedes dealer....you just need the latest registration/title.

What did you do to pull the ignition tumbler also? Did you drop the column? Pull the ignition plug off the back of the ignition?

Finally....There is no such thing as a smoking deal aka cheap mercedes...this is just one issue of many I am sure...

14 pin round connector in connected correctly. They gauges only move when right turn signal switch is on and not the left. Also all the center console lights for the switches flash as well. Checked the ground wire for the Instrument cluster and it is ok. Also the oil pressure guage is pegged in full and only the left turn signal indicator light in on. And the tach doesn't work.

Figured out someone was in there messing with the pins. The 8 pin connector behind the speedo was all wrong. Put the pins back in the right spots and that fixed the light show but created another problem. Now the speedo is at showing 25 mph when the ignition is on but the mileage counter isn't actually moving. Double checked the pin out and its good.

The speedo and odometer are all mechanical afaik.

I thought the early 80s 126s had an electric speedometer.

Hopefully something did not get fried with the crossed pins...

I stand corrected. It is electronic!

Ground issues can still cause all sorts of gremlins in the electrical system....

Going to double check the 15 pin connector pin out and make sure that's correct. My have to send to cluster out to be repaired. Added an extra ground but no change.

The odometer is gear driven. No odo means broken or disengaged gears in the cluster.