It takes a while for the filament to heat the cathode in the vacuum tubes. Or don't most of U remember vacuum tubes? LOL

BTW: Mine tach starts working immediately because I replaced the vacuum tubes with tranistors.

P E H

I would rather make one out of vacuum tubes if I could figure out how. Much more durable and parts are easy to find and replace!

The tach amp that I repaired does not register immediately (as it is supposed to) after engine start. I played around with the pulse R/C time constant by lowering the 470K ohm resistor in order to make the pulse width wider. It seems to help as it is working better now. The tach needle registers about 1 sec after engine starts, but it does not do it everytime, sometimes it takes longer. I would like to take some measurements on a 123 with fully a normally working tach and see what the differences are

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

good luck with that! HA!

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

The tach amp that I repaired more than 2 months ago for a local Mercedes owner has been testing in my car w/o any problems other than tach needle sometimes not registering until I rev the engine to 1500 rpm. I have since found that if I just tap the dash above the tach, it will register immediately (w/o revving engine). That led me to believe that it may be due to a "sticky tach" in my car.

I finally gave the repaired tach back to the owner and the tach registered immediately in his 300 D upon engine start (w/o revving engine). This reinforces my theory that I have a sticky tach. I will take my tach out at some point and see if I can "unstick" it.

Anyway this bring to a conclusion my tach amp repair attempt. My next project will be to add a tach to my 240 D.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Sorry but I am not the tech whiz you guys are from reading the posts here. BUt my tach works intermittently, not working 90% of the time or more and it seems as though its functioning directly corresponds with the AC working properly.

Can any of you help me to fix my tach/ac problem before I melt?

Thanks in advance for any help.

Hey, if you've got a working schematic, how much $$ to make a functional circuit board that I could use? I want to have something I can soldier into the wiring harness and get rid of that stupid plug under hood altogether..... That way the only "plug in" connection in the system would be inside the dash......

Or, if you're feeling real generous, a schematic of your working design...

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-Matt

EPA Section 609 Certified MVAC Technician
-----------------
Oil Burner Kartel Member #10

Ahh the smell of Diesel Fuel, it's like coffee in the morning!

My Car:

1982 300SD Turbo Diesel (231,500 miles!) RIP

1984 300SD Turbo Diesel Custom (235,500 mi on driveline.) - On Road!!

www.icsrepair.com

The schematic is not complete yet. I need another tach amp to get some component values off of so if you have a non working one you can send me, pls pm me.

Moving the tach amp underdash is not a bad idea. Try to maintain the shielded coaxial wire all the way. IMO, it is the heat stress that causes the tach amp to fail. It is hot underhood in summertime bumper to bumper traffic.

If you need a working tach amp, check Ebay, Last I looked they were going for around $60.

and

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I have no experience with the tach amp and tach system in an 87. Is it the same setup as in the W123's? Maybe others can chime in?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The Mercedes '83 300D schematic shows one wire per connector on the tach-amp receptacle (diagnostic plug), but I have a receptacle with two wires on one of the connectors. Any ideas to where the extra wire goes?

Stop by the house.

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83 SD

84 CD

You are a bit far from CT, but if I am in the area I'll contact you.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

That does not sound right. Can you post the schematic you are talking about?

There has to be the following # of connections at the tach amp receptacle in the 83 300D:

power- 2 wires

Mag pickup- 2 wires

output to tach- 2 wires

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked