My 82 300SD has this shall we say oddity...

the tach revs up at idle, and drops to 0 when the throttle is pressed...

I made a video and am uploading it as we speak... it's a little long.

I'm sure it's just the tach picking up an odd ground from somewhere, what do you all think?

__________________
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!

here's the video...

Cluster

__________________
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!

Does it have this odd behavior "only" when the turn signal blinker is blinking???? I think you named the link above correctly.

__________________
daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more

My 82 300td acted like this. I won some used parts off eBay. It had a tach driver in it and I tried it. That fixed my problem. If you have one to try it might be worth a shot. Hope you get it fixed.

no, it's not only doing it with the blinker on...
it's all the time. pretty much anytime the engine is on, the tach climbs. if I have the headlights on, it won't drop when the accelerator is pressed...

__________________
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!

I had one that did that....and it was 20 years ago so it was a pretty new car at the time. The fellow I bought it from said to remove the knurled diagnostic port cap and spray electrical cleaner into it. I believe it would work for a while to do that.

Other than that I would suspect a ground issue.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Ground between the cluster instrument segments bad?

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Tach amp....

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Alternator rectifier?

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

vstech,

Did you figure out your tach issue yet? I would say ground issue first. Mine cuts in and out but the reading is accurate. My issue seems to be loose connectors in the tach amp mount. Try cleaning all your connectors at the amp and behind the cluster. Also, check the alternator output to see if it is too high and how it changes when you bring the RPMs up. Volts should increase a little off idle.

__________________
1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

nahh. the tach's not worked in the car since I got it in 2000... I just posted it up because it started doing strange things...
I'll be changing the alt, as it's not handling the load with lights, wipers and heat blower running...
I've got a nice 115A unit sitting on the bench, and a cool 150A unit too...

Mmmm upgrades...

__________________
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!

Still, check your grounds and connections. I have seen bad alternators do strange things in cars and trucks before. I know I have a bad ground somewhere in my wiper system. If I have the CCU on, sometimes the wipers will come on when I start the car. I have to fiddle with them for few seconds to get them to quit. It hasn't happened in a month but since I mentioned it...

__________________
1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

My '83 300SD did that exact thing after it had a couple hundred thousand miles on it. I bought a new screw on cap thingy at the MB house, and all was better, never another problem again. The item is located under the hood on top of the left fender well. I don't even recall what the name of the item is, but it was about $70.00 or so. Do you have a few 300SD parts cars about you can swap black caps with?