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

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'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

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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?

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'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.

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'81 300SD

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.

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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.

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.

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Current Stable

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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.

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

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.

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'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

The starter is working really well. I checked the ground wire at the starter bolt just to make sure that I remembered to connect it. The central locking system blew a fuse yesterday, and I just replaced that a few minutes ago. The instrument cluster fuse looks good and all of the other gauges are steady.

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'81 300SD

So my speedometer is now completely dead as well as the odometer. Before when the speedo needle was jumping around the odometer still worked. I'm pretty sure that it must be the sensor since the spare speedometer is also non functioning. I looked up speed sensor on pelican parts and got this:

1981 Mercedes-Benz 300SD Base Sedan - Sensors - Page 2

The part listed has a 2 pin male connector. The back of my speedo has a 6 pin male connection and a 1 pin male connection. Do I make a splice into two of the 6 wires that go into the back of the speedo?

Pelican parts didn't list this part for later W126s, but I've read that the same sensor is used from 1981 to 1991 models, is that true?

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'81 300SD

You want to cut up a 6 wire harness to splice in a 2 wire speed sensor? How about finding the receptacle of the 2 wire speed sensor and just plug it in?

Better yet, before replacing the speed sensor, ohm it out and compare it to the new sensor to see if it is bad or not first. I hate replacing parts by trial and error to find the problem still exists.

__________________
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