1980 300SD Nervous Speedometer?

I recently noticed that my speedometer has developed a certain nervous tendency. Where before it was rock solid, now it will twitch back and forth by about 2 to 3 miles per hour at any given speed. I guess it's
not a big deal, just a little annoying.
Anyhow, I have the instrument cluster out at the moment for other work and was wondering:
1 - Is this usually the speedometer itself, or is something else possible?
2 - Any DIY fixes I should try while I have access?
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The speedo cable core needs to be cleaned and lubed. The jerkiness is from its binding and releasing - like a coiled spring.
How many miles?
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"The speedo cable core needs to be cleaned and lubed. The jerkiness is from its binding and releasing - like a coiled spring. How many miles? "
Thanks, that's good news. Only 112,000. How do I get the cable out to clean it? What kind of lube to use?
Here's another question, though of less importance. Is there an easy way to clean the odometer numbers? For some reason, some of the widgets within the instrument cluster face have accumulated a kind of smokey coating. This was most noticeable on the clock hands, which instead of bright orange, were becoming a dull brown. The clock had stopped recently after being parked at JFK for a week in 100 deg temps. The good news is I found an article online that detailed how to fix it! Sure enough, it was a bad capacitor and now the clock is running! I cleaned the hands with some household spray on cleaner and they came out like new. The lower few digits of the odometer and the trip odometer also have this smokey kind of coating.
Only problem is, I can only get to maybe 3 digits of each wheel to clean them. Anyone know of a way to access the rest? I'm a little nervous disassembling the speedo, without knowing how it comes a part, or goes back together. I thought of hooking it up to a variable speed drill to run it up a thousand miles, though I don't know how I would connect it and worry that I would damage it. That would probably do the trick, as it's only the last few digits that are most dirty.
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My speedo was a lot worse than that - I t would make a high pitched squeal when cold and wildly jump back and forth.
I had been told to use transmission fluid, but didn't follow that advice.
What I did, was use a spray can of Teflon Lube (sold in marine stores and perhaps some hardware stores?). I did not remove the cable. There is a sort of cup at the end of the cable that attaches to the speedo. I set it in upright position and kept spraying the fluid into the cup until no more would go in. That was a few years ago - no more problems with speedo.
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To give you the "official" procedure I looked through the Maintenance Manual and the 116 Chassis Manual for speedometer cable service or replacement. Apparently it NEVER goes bad - there's no entry - anywhere. Everything but this.
Suggest you check that the cruise control transducer is on the back of the speedo rather than set into the cable in the foot well. If it's on the speedo, which it's supposed to be, then I'd drip some automatic transmission fluid or light mineral oil into the speedo tube. A product that won't get gummy as it evaporates. Many years ago Chrysler sold speedometer cable oil for this application, maybe someone still does.
Can't suggest anything about the film on the mileage digits or how to reach them - other than a speedo shop.
Since we own the same model you may be interested in the digital climate control retrofit kit that's sold by Performance Products. I installed one and it works pretty well - amplifier & servo are gone, replaced by a heater control valve and four electric vacuum valves and a digital controller. No change to the cabin controls. email me if you want more info.
Tom
'80 300SD @ 108K
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Thanks for the info. I have a set of service manuals and couldn't find anything about the speedometer cable either. Unless they talk about it in the transmission manual, which I don't have. I'll try putting some lube down the cable and see if that helps.
As for the climate control, I have one of the rebuilt servos from Performance Analysis, (George Murphy) that use an aluminum bottom section that can't crack That at least removes one big failure mode. It's been doing fine for 18 mths now and he has a guarantee to rebuild it for life for $100, so I should be in decent shape.
As for cleaning the speedo numbers, I think what I'm gonna do is put it back in temporarily without the front plastic cover. Since for some reason it's only the lower 3 digits that are really dirty, I'm gonna just drive for 1000 miles and clean em as they change! The ones I did with alcohol came out super.
I've been going nuts here the last week. Decided to fix a bunch of small things that have been accumulating for about 2 years:
Rain leak in passenger side, that was pooling water in the heater blower housing - Was coming in from windshield seal, plus fender seam
Rain leak in trunk - Was coming in from rear window seal, gas filler neck to chassis rubber seal, fuel door lock, and right rear tail light assembly
Rain leak on drivers side, with water traveling to back seat footwells - Windshield seal crack at top corner of A pillar.
It rained last night and all these passed the final test!
Engine wouldn't shut off with key in cold weather - Now here's a classic. Since it worked in warm weather, expected this to be the vacuum diaphragm gizmo on the injection pump. Instead, while fiddling around, the vacuum hose for it broke off in my hand right where it goes throught the firewall at a sharp angle. Pretty confident that was the problem, still waiting for new line I ordered online to show up, but she shuts off with minimal vac from my hand tester. BTW, I found a good article online that pointed me to a source for all the hard and soft vacuum tubing for this car.
Clock died - Fixed that after finding an article online about how it's usually a failed capacitor, which it was! Trip to radio shack and some solder and she's ticking again!
Driver Window Regulator Partially Broken so window wouldn't close straight - Waiting for a kit with new plastic bushings that hopefully will fix it
Left to go:
Antenna doesn't full retract. Gonna take that appart again. Rejuvenated it a few years ago, this time will put in new mast.
Cruise control is out: I have a spare amp I pulled from another car. Gonna try that and/or opening the original and looking for bad solder joints.
I guess a lot of people wouldn't fix a lot of this, but the car is in super shape, and I love driving it!
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I keep mine in good shape too. However, there's always something!
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Move to a warmer climate. My '87 is fine until cold weather, and it's been like that for 8-10 years. Not worth worrying about or fixing IMO.
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