While driving down the highway today my speedometer needle started jumping back and forth in 20 MPH intervals. As the day went on it appeared as if the speedo is starting to die, sometimes it stays at zero for a few seconds. Last time something like this happened, I cleaned the contacts for the speedo wire at the back of the transmission and it worked. That didn't help this time and I even tried cleaning the contact at the back of the speedo. I also installed my spare instrument cluster and that speedo behaved the same way. I'm guessing that the problem is the part at the back of the transmission that sends the speed signal. Here's a pic of a spare that I took off of my last transmission before I scrapped it. My old transmission and my spare part was from an '81 300SD, the current transmission is from an '83. Should I try replacing this part or is there something else to check out? Thanks

Attached Thumbnails

 

__________________
'81 300SD

Logical next move unless difficult to change. Could also be a real marginal ground as well in the system. If I did not have the part on hand I would check out the wiring first.

Just my opinion though. You did have a wiring problem earlier in the same car and same circuit. That to me makes it somewhat more suspect than the sender.

So few problems are mentioned about this particular sender have to also be kept in mind. Although you still could have a bad sender of course.

Pis you posted looks like a pressure switch to me

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks for the replies.

I installed an extra ground wire to the instrument cluster a couple of years ago when the needles would bounce in sync with the turn signal, so I'm confident that at least the cluster has a good ground connection. I'm not as confident that the part I have is for the speedo because it doesn't have the same connection as my current one which is a plug, not a small screw. The wire coming off of that part is the same shade of green as the speedo wire. What is the pressure sender for?

__________________
'81 300SD

The part pictured is the kickdown solenoid.

Thanks. Do you know if the speedo gets it's signal from an internal or external part of the transmission? Maybe I can find a replacement if it bolts to the outside of the transmission.

__________________
'81 300SD

The sensor is located at the back of the transmission and looks like this -



It's a VR sensor that reads a 4-lobe trigger wheel which is attached to the parking pawl inside the transmission tail cone which looks like this -



Since you're getting a signal to the cluster, it's extremely likely that the problem lies there...and most likely ground related.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

I already have an additional wire grounding the cluster at the moment. This problem occurred a day after I replaced my starter, so I'm now guessing that I need to clean the ground wire on the transmission. Maybe I got some oil and dirt on it when bolting the starter back on.

__________________
'81 300SD

Sounds like you have wiring and grounding problems. If you still have starter issues, you may want to resolve them first before doing more work on the starter. Oil will not cause problems with electrical contacts. I wrote in your starter thread about looking under the heat shrink at the crimps for corrosion. Either replace all the wires that are in the circuits involved or fix the ones you have.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Mercedes W126 500 SEL Kickdown Ventil 0003041790 10 Bar | eBay

Yes. I'm not familiar with his SD, just going by the 10 bar marking on the part which tells me it is actuated by pressure.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Transmission part is mechanically driven I imagine. So there are perhaps three choices of a possible defect at least.

A 300SD must be different than my 84 & 85 300D's. Mine have a cable input to the speedometer in the cluster, similar to U.S. cars thru the 1980's. I thought 300SD's used the same engine & transmission as 300D's. There must be a different connection at the transmission.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I did the same thing when I moved the diesel engine to the gas body a month or so ago. I had a ton of wiring to move so I learned to handle the wiring section of the FSM and took some youtube soldering lessons. I also bought some marine grade heat shrink. Who knows? It might work.

Glad it worked out for you.

__________________
84 300SD
85 380SE
83 528e
95 318ic

I thought that an 81 would have a speedo cable...? If so they the cable was failing and now its failed....?

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

81 - 85 SDs used an electronic speed sensor as did the 81 300TD.

__________________
'81 300SD