odd tach behavior
 

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?:confused:

here's the video...

Cluster

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

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...

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.

Ground between the cluster instrument segments bad?

Tach amp....

Alternator rectifier?

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.

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...

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...

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?