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1985 500sel konig special widebody, 1979 slc, 1992 300sl
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232 Posts
Discussion Starter #1
hello guys, this is my second attempt to get help! i have a 1983 300SD, all guages work fine when car is running, except speedometer and tachometer, when i start the car the speedo climbs to 40mph and the car is parked!! tachometer is dead... please help i dont want to get a speeding ticket!!
 

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1984 300sd turbo
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18 Posts
Well you're going to have to give us some more information.
Was the Tach & speedo working before?
Did you mess with anything before they stopped working?
What happened before they stopped working?
Have you checked your fuses? With a meter? With a light?
The only thing the Tach & speedo share is ground. So behind the instrument
cluster do you have a loose disconnected ground wire?
 

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1981 300 SD, 1991 Geo Metro 4-door Automatic
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487 Posts
Tachometer is probably the tach amp, that little black plastic cylinder thingie on the left fender under the hood. There is a top cap which unscrews from the bottom half of the cylinder. In that top cap is a circuit board which can be carefully removed. Remove said circuit board, place a cigarette butt/piece of sponge/folded piece of paper in the top of the cap and replace circuit board. The object placed in the cap puts some pressure on the circuit board, and compensates for what I'm guessing is a cold solder joint on the circuit board. Replace the cap, your tach most likely will work.

The speedometer I don't have a good answer for...you might try one from a junkyard and see if that takes care of the problem.

Hope this helps...the above is how I fixed a dead tach in my '81 300SD.
 

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1986/1990 W126
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14,142 Posts
Yep i expect the two problems are unrelated. Have you replaced all the fuses in the fusebox? Often sorts out all kinds of gremlins and odd behaviour.

The cluster is super easy to remove for checking/replacement, but make sure you disconnect the battery first!
 

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1985 500sel konig special widebody, 1979 slc, 1992 300sl
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232 Posts
Discussion Starter #5
thanks so much for your reply. i bought the car a week ago this way. i did not mess with anything because the car was parked for a while and i thought it will start working after i drive it. i checked the fuse just by making sure its not blown, not with a meter or light. i'm looking for the tools to take out the cluster with the hopes of not damaging anything. where would the loose ground be?
 

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1985 500sel konig special widebody, 1979 slc, 1992 300sl
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232 Posts
Discussion Starter #6
thanks for your help, i'm hoping to fix this speedo because the car actually has 152k documented miles. i dont want to mess up the vehicle history report if i was to sell it. your suggestion is simple and i will try it this afternoon. thanks for your help!!!
 

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1986/1990 W126
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14,142 Posts
Fuses get slight corrosion on them and it makes them do weird stuff to the car. It'd be an idea to clean all the contacts in the fuse box with a Dremel or similar, and fit new fuses. The brass ones are better, available on ebay etc.
 

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1984 300sd turbo
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18 Posts
Hi,

I just got mine a month ago with similar problems :)

It is really simple to pull the instrument cluster. Just get a wire hanger,
cut a straight piece about 6-8" long, using a pair of pliers bend a 90 degree
turn about 1/2" long at the end of the straight piece.

Take that wire and slide it between the right hand side of the instrument
cluster and the dash with the bend pointing up. Slide the wire in until it
can be turned easily 90 degrees to the left behind the instrument cluster.
Pull out gently. The instrument cluster is just a friction fit in the dash
and comes out pretty easily. It surprised me the first time I did it because it
was so easy.

REMEMBER TO TAKE THE BATTERY NEGATIVE CABLE OFF FIRST!

Sorry to yell, but I'm told that you can destroy instruments if you don't.

Most of the cables on the back of the cluster are idiot proof with the
exception of the circular one on the far left side. Be carefull to mark it
before you take it off. Failing that notice that there are 2 pins missing
on the cluster and these match up to 2 socket pins, so all is not lost
if you mess up :)
 

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1985 500sel konig special widebody, 1979 slc, 1992 300sl
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232 Posts
Discussion Starter #9
i will be taking out the clock tomorrow. i checked and replace fuse, i also cleaned contacts on fuse box, the problem continues, i'm trying my best not to replace the clock :(
 

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1985 500sel konig special widebody, 1979 slc, 1992 300sl
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232 Posts
Discussion Starter #10
replaced fuse today cleaned contacts on fuse box also, the problem continues. i'm trying my best not to replace the whole cluster :(
 

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1985 500sel konig special widebody, 1979 slc, 1992 300sl
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232 Posts
Discussion Starter #11
tried putting pressure on the black circle thing on the left fender wall, same deal, tach is still dead
 

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1985 500sel konig special widebody, 1979 slc, 1992 300sl
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232 Posts
Discussion Starter #12
i replaced fuse and cleaned contacts, i also placed a test light on the contacts, it was very bright. tomorrow i will try and remove cluster and see if there's any funny business with the wire ground. i'm trying my best to not replace the cluster.
 

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1983 SPECIAL SPEEDOMETER - Just 80mph, thanks to prez Carter?

I have an '83 and its unique in that the speedo is electronically driven. Which is to say, no cable twisting from the tranny - A GOOD idea, imho, but not tamper proof (damn lawyers in the design phase).

So YES, the tach is the amp as previously covered - separate.

The speedo is a matter of electrical connections and pulses.
It fairly easy to follow the sender signal from the tranny up to the console.
Do your usual clean and reseal along the way ( i use 'pen' erasers and lithium grease to seal)
Worst case, or if you have non-standard wheel diameters, is to use an Arduino circuit to convert the signal to actual , calibrated with your phone's GPS. Life is SO easy now!!

Cheers!
 
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