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  #1  
Old 06-21-2018, 11:50 AM
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yes, another w123 tach thread

OK, this isn't about fixing the tach with a cig butt trick.

I have 2 problems with my tach. I'm not so much interested in #1, but I'm mentioning it because it certainly may be relevant.

1) It's works intermittently - about 50% time, it just works when I start up the car to when I shut down. Occasionally, it will stop working on a drive, and other times it doesn't' work at the start of driving, but it just kicks on randomly at some point. Sometimes, it just doesn't work period. There is absolutely ZERO touching of the tach amp unit at all during any of these on/off cases. I've replaced the original amp with a known working one (at least it was when I replaced it a year ago), and I've cleaned and inspected the pickup/sender (though haven't thoroughly tested the cable itself).

2) When the tach is working (about 50% of the time) it doesn't really registered ANY rpms under 1000. So if I'm idling, the needle is just at rest. I have to have my toe on the pedal a little bit, and it immediately bounces up to 1k or above. When I'm driving around in various gears, and watching it during shifts, it seems to be reacting normally.

Anybody have any clue why the tach just seems to ignore anything below 1k? Is that threshold related to some specific component in the tach amp?

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1982 300D Turbo
120k
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  #2  
Old 06-21-2018, 01:20 PM
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Hi,

This was an awesome post on the tach amp.

How the Tach Amp works

I can only take a wild guess at what is wrong. I was thinking the threshold was a static value but apparently it isn’t. Capacitor C4 in the amp is some kind of a peak detector circuit that adjusts the threshold based on the frequency of the pulses coming in. You’ll apparently get less energy on C4 at lower rpms because the pickup fires less often. Maybe C4 isn’t working correctly and it is bleeding off power too fast at low rpms. Also there may be a short across C4 causing a higher bleed rate than the 1.2Mohm resistor puts across it.

Tantalum capacitors are usually pretty durable but maybe something has gone wrong after 30 years.

The other possibility is the tach. The schematic suggests you can hook up a signal generator to the tach and sweep the frequency to see if it works. Alternatively you can use an oscilloscope to check the signal between your tach amp and tach to see what’s going on.

Or you can try swapping parts if you don’t have the instruments. Here’s a thought, people here, and me, have had good luck resoldering the solder bumps in the tach amp. Maybe you have a weak connection in series with C4 causing the peak voltage to read low.

Good luck.
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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
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  #3  
Old 06-22-2018, 10:33 AM
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thanks. I figured this could be the case.

When i was investing my original tach amp, I had removed all the silicon from the top (i guess the bottom) of the board to inspect all the solder points. I couldn't see any that were broken, even though it clearly had the problem that was temporarily solved by the cig butt trick. Since I found another tach amp for 40 bucks, I just moved on. Maybe i should take another look. I'm a little wary of scrapping off all that silicon on the newer tach amp since I'm not really in the electrical union when it comes to trouble shooting circuits.
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  #4  
Old 06-22-2018, 01:39 PM
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Why even mess with the silicone sealant? I'd say the odds are decent that you will mess up the tach amp that way, make a good amp into a bad one.
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'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
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  #5  
Old 06-22-2018, 01:46 PM
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Lot's can happen with a 40 year old tach. Components (especially capacitors) can degrade, solder joints fail (bad tach amp soldering job from the factory), potted component gets hot and burn out (power input resistor).

From your description, it could be too large a gap between the round tooth on the harmonic balancer and the magnetic pickup. Try making that gap smaller (make sure they don't collide), which will increase the output voltage of the pickup a little and see if that fixes it.
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  #6  
Old 06-22-2018, 01:49 PM
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Quote:
Originally Posted by Maxbumpo View Post
Why even mess with the silicone sealant? I'd say the odds are decent that you will mess up the tach amp that way, make a good amp into a bad one.
Removing the silicone is the first step in fixing a bad tach amp. No access to components otherwise. You won't mess it up if you are careful. I have fixed a bunch of them. Removing the silicone makes the tach amp run cooler. Heat is a problem!
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  #7  
Old 06-22-2018, 03:07 PM
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yeah I was going to say, no way to check or repair the solder points if you don't remove the silicon first. is there a solvent that works well on that stuff? on my old one i just carefully scraped away with xacto but it was very, very tedious to get the board cleaned.
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  #8  
Old 06-24-2018, 05:18 PM
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I never replaced the glue after I picked it off. I think I used a combination of tweezers and a toothpick. Then I just resoldered every joint. Reheated and added a bit of solder to each point. That solved it.

It has been working well for years.

I like the theory of the pickup being moved far away from the coil. That makes sense.
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82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
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  #9  
Old 06-24-2018, 06:21 PM
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I did he scrape off the silicone and re solder with mine as well. I seem to remember I used hot glue instead of silicone.

Worked great for as long as I had the car.

Picking off the silicone is very tedious, best done in front of a movie, with a beverage, and just spend the time doing it.
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1997 E290 Turbo Diesel Wagon -traded for above
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  #10  
Old 06-24-2018, 07:31 PM
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Funolas gap theory sounds interesting. Especially since my tip had shown signs of damage/contact at one point earlier in its life. I had cleaned off the ragged bits and resealed with thin coat of epoxy. However, should add behavior has not changed since doing that.

wonder if there is anything i could safely bend a little to get closer. These are not easy to access without removing some accessory. Otherwise is alternative ro afix a new metal shim or something on end?
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  #11  
Old 06-24-2018, 08:18 PM
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Move the tooth under the mag pickup, measure gap with feeler gauge. You can decrease the gap by epoxying dots of sheet metal (must be magnetic) from a tin can onto the tooth.
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  #12  
Old 06-24-2018, 08:32 PM
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So I should mention that any old “known good” tach amp is suspect. I recall checking mine under a microscope and finding at least half a dozen cracked solder joints before I repaired it. I think they all suffer from this disease and must be redone with quality solder at some point or else they’re 30 year old questionable circuitry. The fact it’s intermittent is just begging for a resolder job.

While you’re at it if there are any connectors between the pickup and amp they should be inspected.

Good luck.

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82 300SD 300k miles
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