w123 tach - what is under that cap?

Sorry to address this tach issue again - I have a tach that usually reads 1000 RPM no matter what the engine speed. It will vary sometimes, especially if I let off the accelerator.
I found the unit on the fender that has the amplifier. But, I don't think my car has the circuit board people have spoken of. I unscrew the cap and the part that is still attached to the fender has a number of female electrical contacts. Under the cap is nothing at all. Where is this amplifier unit? Could my tach work at all without it? I am confused...
Thanks
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Yours is missing apparanly.
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You need a "tach amplifier" - costs about $50, including a new cap. Sold online by M-B parts vendors.
Then you'll KNOW the engine rpms - idles at 700 maximum 4,500 and transmission shifts in between.
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He NEEDS to say what year he has. 84 and 85 does not use the cap. The rpm sensor is mounted on the rear adapter plate and the rpm is read off the flywheel.

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I wondered how it could have any reading, my old car reads zero without the cap!
Karl knows, always.
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Sorry... it is a 1984
so, I'm looking in the wrong place?
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szcz wrote:

oh, yeah, by the way - there is a small (thing?) mounted on the front of the engine with a sensor that is adjacent - I am pretty sure this is the tach sensor. that has a wire that goes to the cap like thing and then into the firewall. Is that consistent for an '84?
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Wrong place. The sensor is down on the drivers side of the rear of the engine. Look up at the top of the fire wall. Middle, near the alarm switch. You will see a 2 wire black flat plastic connector. That is it. The 2 wires to the right go to the sensor. The 2 wires to the right go the large EGR control unit behind the right front kick panel.
Have you had to jump a dead battery lately?
Remove the lower dash cover below the glove box. Remove the right kick panel. Next to the large black ECU is a silver relay with a red lid on it. Lift up the lid and pull out the 10 amp fuse. Is it blown? If yes, replace it. The tach should work now.

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Wrong place. The sensor is down on the drivers side of the rear of the engine. Look up at the top of the fire wall. Middle, near the alarm switch. You will see a 2 wire black flat plastic connector. That is it. The 2 wires to the right go to the sensor. The 2 wires to the leftt go the large EGR control unit behind the right front kick panel.
Have you had to jump a dead battery lately?
Remove the lower dash cover below the glove box. Remove the right kick panel. Next to the large black ECU is a silver relay with a red lid on it. Lift up the lid and pull out the 10 amp fuse. Is it blown? If yes, replace it. The tach should work now.

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Not sure which model you have, but on many the amplifier/electronics is contained and sealed in the screw on top of that diagnostic connector. The electronics only consists of a few components and it's just sealed up is some sort of plastic or epoxy in the test cap, with nothing visible That was exactly what was wrong when my tach went whacko on my 116 300SD. I replaced it with one from the stealership and voila, tach fixed.
PS: I think you guys talking about this made it go again. Three days ago, it started acting erratically again, just like last time. The original lasted over 20 years, the new one maybe 5, so I guess the Germans still can't make electronics. Which reminds me of an old episode of All in the Family, where Archie was arguing with the meathead. He told him the only mistake Nixon made with Watergate was using Germans for anything to do with bugging and electronics, because everyone knows the Japanese are much better
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Karl wrote:

I also have a non-working tach. I have an '85 300d and I have checked the fuse in the relay per your instructions (an "American" 10 amp fuse by the way) and it is fine. I checked it with an ohm meter just to be sure.
I've also removed the pickup in the bellhousing (again, driver's side, rear of engine behind the oil filter) and cleaned it.
Still no tach. Is there anything else I can check?
Thanks in advance for any thoughts! RSC
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Go to the 2 pin plug on the middle of the firewall, above the valve cover. Disconnect it. Get a multi-meter and set it on AC volts. Connect it to the plug end that has the wires going towards the drivers side. Start the engine. The sensor is a AC generator. Read the voltage at idle. It should be around 4V AC. Increasing engine speed should increase volts. Zero volts? Either a bad rpm sensor or a bad flywheel. A flywheel from MB is 'order from Germany' and over $400!!! And yes, they do go bad. I just replaced one 2 weeks ago for this exact problem.

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Karl wrote:

I'll check it as soon as I get home.
Thanks!
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Karl wrote:

I disconnected the plug and hooked my multi-meter up to the two connections on the plug going to the driver's side. I set the multi-meter to AC volts and started the car. At idle, the meter reads about 4.5 volts. As I give the engine throttle with the linkage the voltage increases at a pretty constant rate. I did not rev the engine too high, but the voltage increased to 10-11 volts. It would appear that it is working.
Again, I checked the fuse in the relay behind the kick panel on the passenger's side and it is OK also.
Thoughts or ideas?
Thanks for the help. I've been tinkering with this for quite a while and picking up ideas where I can and so far your help has been the most accurate and valuable!
RSC
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problem.
Shucks before I would spend $400 for a flywheel I would replace the magnet stuck it the flywheel. Sounds like that would be a quickie job for a machine shop.
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Roger Shoaf

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