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Old 12-27-2000, 08:27 PM
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(First, sorry about the double posting, I didn't know my response to an old thread would be posted as a new topic, and I wanted to have this info be immediately available to whoever might need to know about this.)

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. In other words, the majority of this unit is hollow.

What I saw was about $10-$15.00 worth of components (retail) 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 used an orange stick from a manicure kit, and a few of toothpicks), I set to work with a fine 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 now 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.

[Edited by longston on 12-27-2000 at 08:37 PM]
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Old 12-27-2000, 11:42 PM
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Hertz so good!

Gee, Mr. Bill, or should I call you TX?

OUCH! You sure jumped on my balls, didn't you? It's ok, as big as they are, they're a pretty easy target. But, as old and leathery as they are, I'm sure they can take it. Oh, and hey, thanks for the warm and heartfelt welcome to the MercedesShop family from a SENIOR member. You're sure setting a fine example...

You also assume a lot about when, or how often the TDC sender was replaced on my car (and other cars). I really don't know if it was the original one, or when it was last replaced, but it seems to me that an awful lot of people have/are replacing them (every 300SD I've looked at recently has a bad one).

Are you serious? What are you, a member of the Daimler Mafia, or some kind of pro-MB Corp. Nazi? I just wanted to know what was actually in this TDC sender puppy, and thought that someone else might find what I discovered to be interesting.

Should it be recalled? Hell YES! Anything that is shoddily manufactured, or that has a high failure rate should be recalled out of pride in the quality of the product a company manufactures, and the reputation for quality that the company aspires to. It's not all about safety, it's about quality. We all know that Mercedes has always been a leader and an innovator in the automotive industry.

Besides, even if I am off my ass about the recall part, aren't you the least bit curious wny so many of these units are defective? You might be if you owned one. And, I don't know about your finances, but there ain't no money tree growin' in my backyard. I need to save money where and when I can. I thought that was what this forum was all about. And if I want to take the time to disassemble this part, then that's my business. I really don't need your approval.

Did you take the time to read your reply, and to consider whether or not I might find it offensive or rude? Man, I can take the heat, but the real problem with a response like yours is that it has a negative effect on some timid people's willingness to ask a "stupid" question, or to share something that they might otherwise not.

Hell, I only posted what I considered to be a pertinent piece of information that might be of some interest to the membership at large, but you just had to try to discredit my efforts, didn't you?

Wow, what a guy. I really hope you enjoyed your negative posting, because you actually succeeded in taking all the fun out of this for me (NOT!).

Sorry to waste your precious time, me boy-o. Try to be a little more supportive, I need this forum as a source of reliable and honest information and assistance. If I want someone to try to humiliate me, or if I want to fight with someone, there's always the rest of the internet to use for idiotic flames.

Get a sense of humor. Oh, and Happy Holidays...Mr. Bill.

[Edited by longston on 12-28-2000 at 12:23 AM]
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Old 12-28-2000, 08:15 PM
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now now, can't we all just get along. I personally appreciate the remark. Because I am not very handy I wouldn't go to all that trouble but if it was a recall defect, i would be first in line.
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Old 12-29-2000, 12:06 AM
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Well, thank you, diesel brother Bill! Y'know, I always end up buttin' heads with guys who I have a lot in common with, or at least ones that I'd have a beer or two with under other circumstances. You sound like one of those kind of guys.

I did my homework, and found your website. As a former truck driver, and as someone who owned his own trucking company, I can see why I would lock horns with a dispatcher at a trucking company in Houston! (Heh, heh, heh...)

We who live by the busted balls also die by the same process. My response to you was intended to get some good natured licks in at you as well (big surprise, eh?). I obviously know that the difference between a Senior member and a Junior one is a matter of a few mere months. But, hey, I'm ruthless, what can I say?

Perhaps the term recall was a bit strong, or maybe you missed the humor in my suggestion for a recall of a minor part. I was only half serious. The serious part was objecting to MB overcharging for a part that costs them pennies to produce in quantity. The rest was strictly, as the G-Man sez, Bravo Sierra.

Bill, old man, you sound like an excellent source of information on the subject of 300SD's, and I would welcome the opportunity as a new owner of one of these cars, to pick your brain.

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Old 12-29-2000, 09:48 AM
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Old 12-29-2000, 10:03 AM
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I need a clarification on this problem. Is it the round connection on the left fender well that you had the problem with. I think this is where a tester can be hooked up. Is this correct.
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Old 12-29-2000, 10:47 AM
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  #8  
Old 12-29-2000, 11:43 AM
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Back to the problem now that the emotional crisis has passed....

Hello there,

I had trouble with my '82 300TD's diagnostic plug, and it was of a different nature than Mr. Longston described... here's what I found.
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
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  #9  
Old 12-29-2000, 01:24 PM
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OK, OK, I get the point. Next time I'll take it outside, or at least "get a room".

Reply to mreid: Yes, I am told by my local MB dealer that this a diagnostic connector that they can use to download information on a car. But in this instance, we are dealing with the device that is usually plugged into that connector. That device is a sender that reads information from a sensor in the engine that indicates when a car's # 1 piston is at Top Dead Center, or TDC. This device reads how many times per minute the sensor has reached TDC, and transmits that information to your tachometer where it is displayed as RPM.

Reply to shoe: It's "where's" :p

Richard has addressed the other weak part of this assembly. The interior IS hollow, and the only support for the connector pins are the three pairs of "clips", or "opposed locking fingers" as Richard calls them. However, there is one other weak area. So, when testing to see if the plate that holds these connector pins is loose, don't push very hard, or you are likely to dislodge one or more of them from the plate that they are fitted into that is held down by the three pairs of "clips", or "opposed locking fingers" (whew!). These connector pins are simply glued into that plate, and not very well. Also, don't bother with the center, as that is just the latex potting compound, and pushing on that won't do you much good.
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