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  #1  
Old 01-11-2000, 05:52 PM
Warren
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i have an 82 300 CD that the tach works intermitently, lately it hasnt worked at all.Does anyone have afix for this
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  #2  
Old 01-11-2000, 06:27 PM
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Join Date: Apr 1999
Location: Walnut,CA,USA
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Try the T.D.C sender(RPM sensor) locate at driver side under hood (001-545-59-32)I have seen a lot of senders go bad.

------------------
MB STARS Master Guild Technician,12 years MB tech
6 years independent shop owner

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  #3  
Old 01-14-2000, 11:51 PM
Bill Pelka
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You might have a blown fuse on the overvoltage protection Relay which is located behind the glove box in my 1984 300D Turbo. If the F14 fuse is loose or blown it too will cause the tac to fail. It powers a EGR Control Unit in the 1984 300D Turbo with California Emissions. On the "Federal" unit the tac is fed from an engine speed sensor that is supposed to be located between the transmission and the engine. Under the car, I think. Never seen one but that is where I read that it was on mine. According to a schematic the F12 fuse (8 amps)feeds red brake warning indicator and the clock and tac. If your clock is working then the connections from the Tac might be bad at the connector (C166). C166 is a 2 wire connector with a brown/white and a white wire.
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  #4  
Old 01-17-2000, 05:58 PM
Bill Pelka
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Today I was looking at some electrical schematics for the Tac circuit. It also shows that the TDC Detector is connected to the Diagnostics plug assembly in the engine compartment, Drivers side. It is a cylinder that screws into a connector, right above the wheel well. I have heard people say that a "plastic peanut" or a cigarette filter stuffed inside will cause the amplifier to make a better connection. This is probably why it went from failing to dead...

Good Luck and let us know how it turns out.
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  #5  
Old 01-19-2000, 01:57 PM
Wing_Nut
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Warren
I just fixed the same problem with the tach on my 82300Sd and my buddies 78300SD (we learned the trick from threads on this web-site)
On the drivers side fenderwell there is conector about 1-2 inches tall. It's cylindrical like a minutre garbage can. Unscrew this(it should have a plastic leash that keeps the top from falling once unscrewed.) and you should 6 or 7 pin connecters. Pull the connector assembly out of the cap. Place a piece of foam, a cigarette butt, anything that is pliable that will force the connectors to make better conatact when you screw it back on.
You can also go to search on this web-site and type in tachometer. That's how we fixed ours.
Best of luck
Peter Kalloday
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  #6  
Old 12-27-2000, 09:23 PM
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300 SD Tach Problem

I just recently purchased an '82 300 SD that had this intermittent tach problem. After isolating the problem to the TDC sender, and pricing the replacement part at $61.00 to $130.00, I decided to take the TDC sender apart.

Once you remove the actual unit from the cylindrical screw down cover (the part with the tether), you can take the actual sender apart. I figured; What The Hell? The sucker doesn't work, I can't break it.

So, here's what I found. The "top" of this unit has a half dollar sized circuit board under a layer of latex. After carefully scraping most of the latex off of the circuit board, it can be carefully pried up out of the cylindrical housing. The components are "potted" in latex, and are on the underside of this circuit board. I then removed as much of the latex as I could to expose the lower portion of the device, which is a plastic housing for the contact pins that plug into the socket on the inside fender/wheel well and are attached to the circuit board by seven wires. Removing the majority of the latex above the pins on the inside of this cylinder will expose the three plastic clips that hold the contacts in place. Carefully squeezing the clips with a needle nose pliers, I was able to extricate the unit from the housing.

What I saw was about $10-$15.00 worth of components including an IC, a transistor, an electrolytic capacitor, a couple of tantalum capacitors, about 8 or 10 resistors, and a few diodes. In the process of removing the latex, I managed to damage the transistor, and break one of the wires. So, with the circuit board carefully scraped, I set to work with a brass wire brush to remove the remainder of the latex so I could inspect the circuit board.

What I found was several loose "cold" solder joints as a result of hasty dip soldering and poor QC. After resoldering the bad joints, replacing the transistor I broke with a new one, and reattaching the broken wire, guess what? It works just fine.

My final analysis? The cigarette butt trick works because it mashes down on the underside of the circuit board and as a result, the loose components make contact with the bad solder joints, not because it makes a better contact at the connector. Essentially, Mercedes should recall all of these "defective" TDC senders and replace them at their cost. Instead, they are creating a huge profit for resellers (and themselves) by selling their costly replacements.
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  #7  
Old 12-28-2000, 11:30 AM
patsy
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Oddly enough, the other day I did almost the same thing as you. I pulled the cap off of the latex circuit board. I proceeded to rip all the latex off. I was not as thorough as you(what you did is amazing). I took the whole mess back out to the car, and plugged it in, I put the cap back on, but not the part that the latex mess was in. It worked for the first time in years. It has worked flawlessly since then. I think the design is horrid. I am sure that stuffing something behind would have helped, because my circuit board had been pushed back a bit. I will buy a new one when this fails, it is $59.00 here at Parts Shop. Since I don't know what 700-800 RPM's sounds like on a diesel it was nice to find out I was actually idling at 500!

[Edited by patsy on 12-28-2000 at 11:48 AM]
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  #8  
Old 12-28-2000, 01:31 PM
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Happy Holidays to all!

Ok gentlemen, I'm seeking your wisdom on this one. I have unscrewed the cap, and can NOT seem to get the unit out of the cap. I don't want to break it, but am I missing something here. I have pulled, pushed, and pried on it and can't seem to get it out of the cap.
Any advise appreciated!
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  #9  
Old 12-28-2000, 02:07 PM
patsy
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I just used a pair of channel lock pliers and grasped the inner part and pulled and turned. It came right out. Maybe I was lucky. I was prepared to break mine though, because it did not work any way.
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  #10  
Old 12-28-2000, 02:15 PM
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Removal of

First, thanks to Patsy! I was beginning to think I was the only one nutso enough to do this!

For TCCBass: Gentlemen? Patsy, are you a Gentleman?

To remove the tethered cap from the sender, you need to depress the clips that hold it on. You will see two or three of them on the sides of the unit. Look for two parallel "cuts" in the plastic barrel for each clip. depressing these gently while pulling the cap off should work. You might also get the "clips" to grab the threads of the cap and be able to unscrew it? I forget exactly how I got mine off. All I really remember was that it seemed to be impossible when I was concerned about damaging it, and easy when I wasn't...
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  #11  
Old 12-28-2000, 02:48 PM
patsy
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Gentlewoman, in a man's world so I don't pay any attention to pronouns. This is the first forum that I have felt comfortable enough with the community to post my name. I am aware that historically I should not know anything about automobiles( and I have been asked for many years what I want the welding wire for), but the Mercedes members don't seem to mind having a lady post here. I can tell you that that is not the case at other forums. Mercedes owners are a classy bunch!!! I would hope that I would never be given a simplified response, or have any advice I give be disregarded by others based on the fact that I am a "Patsy". I have not experienced that here, and I doubt I ever will. Thanks for accepting me as a member here!!!!

TCCBass, don't worry about it, because I assume everyone here is a fella too!

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  #12  
Old 12-28-2000, 04:50 PM
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Patsy,

My SINCEREST apologies for the "gentlemen" reference. Certainly I appreciate your input and should have phrased that "fellow diesel lovers"! Working for 3 fur and diamond laden bosses who don't do their own housework has me respect you tremendously for having more mechanical knowledge than a great percentage of men. Not to mention the taste to drive the prettiest turbodiesel ever (IMHO)...the 116 300SD.
Thanks for the advise. I will get out and work on it this evening since it's finally not snowing or raining.

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88 BMW E32 735i (make it go away)
95 Volvo 960 "Inga" (valve problems)
95 Nissan Maxima (the indestructible)
89 Ford Ranger XLT (ugliest truck in the South)
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  #13  
Old 12-28-2000, 07:38 PM
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Whoa! I didn't mean to unravel this thread, just to point out that some of our posters are likely to be ladies! However, I actually knew an elderly Irishman who was proudly known by the name of Patsy...

And, Patsy! I used to train people to be truck drivers, and some of my best students were ladies, and many of them went on to become owner-operators who did their own maintenance and repairs. Also, one of the local independent MB garages in my area has a lady mechanic who gets a lot of customers asking for her specifically to work on their cars.

Finally, TCCBass! Try the styrofoam packing peanut / cigarette butt (ick!) trick before you start tearing the sender apart (unless you're an electronics guy, and then go ahead!). It should work.
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  #14  
Old 12-28-2000, 09:17 PM
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warren, see the link from longston.
good luck!
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  #15  
Old 12-29-2000, 02:16 AM
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Diagnostic plug/tach amp fix

Hello all,

I had the same problem with the diagnostic connector on my '82 300td, and after taking the top cover off and removing some of the potting compound, what I found was that the pins are supposed to be retained by a backup plate that latches onto the bottom via 3 pairs of opposed locking fingers. The plastic fingers had gotten weak and weren't able to hold onto the plate anymore. I fixed it by making some little plastic wedges to go between the fingers to keep the plate latched. The fingers are less than 1/8" apart, so it doesn't take much of a wedge to fix it. Once the pins are properly retained just reassemble the unit and it should work fine. You can make a simple test to determine if this is the problem by pushing on the pins before you take the diagnostic plug apart - the pins should NOT push in at ALL, and should all be out the same distance. Hope this helps! You can see my car, which has a 4.3L Chevy V6 installed, at: http://pages.prodigy.net/rwooldridge/mercedes.htm

Richard Wooldridge
1982 300TD 4.3L
1974 280C (It's a CLASSIC!)
1977 Jag XJ6L
1977 280Z 2+2
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