The speedometer needle on the 84SD was making huge jumps at low speed ie from 0 to 40mph when I backed out of the drive and parked on the street. It did this one other time.

Where is the connection to the trans and how difficult is it to access? What is the likely repair?

I have an 85SD for parts but many things changed slightly between 84 & 85 and the parts may not interchange.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

sometimes the alternator will do that or battery connection.Do you still have crappy aluminum fuses,or changed to copper?Sometimes its the speedo,the contacts need cleaning.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I have exactly this issue. Speedo reads 30mph while stationary in reverse.

The sender is easy to get to and pull out. It's just a little magnetic pickup on the tail section of the trans. I may pull mine for a look see today.

My voltage seems pretty stable. I can make the speedo reading go back to zero when I shift to Drive and hold the brake.

Not sure what is up but soon my transmission will be removed and replaced. I'll see if it keeps doing it.

It is odd because nothing is really at the sender while stationary. I'll get back here and post if I figure it out or if it goes away with the tranny swap.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

Page 210 Fig. 1 of the electrical diagrams shows the impulse transmitter. It's aft of the Starter lockout/Backup light Switch on the LH side of the transmission.

http://www.startekinfo.com/StarTek/outside/12253/program/ETM/126td83-85.pdf

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Sam

84 300SD 350K+ miles ( Blue Belle )