Who can help here with RPM guage, anyone?

[rbort]

OK, I got this rpm guage for my 190E, mine didn't have it.
I put it into the instrument cluster and hooked it up to what I thought is right. It has three connections:

+
TD
Ground

I hooked up + to a live 12volt when ignition is on (coil + side).
I hooked up TD to the - terminal of the coil.
Ground is hooked up with the instrument cluster itself.

I start the engine, while starting the RPM guage hovers around 300 rpms (seems normal) but when the engine starts it jumps to 2k rpms. It bounces around there but if I rev the engine once it gets to 2k rpms and faster it works fine (like on the highway).

My problem is that it doesn't indicate correct rpms below 2000 rpms. My first thought is that the rpm guage is bad. So I went back to the junk yard and picked up another one but the second one behaves exactly the same as the first one.

Now for the big question: How do you wire this thing to work properly? Does it need a capacitor or resistor or something to make it work, or did I wire it in the wrong place?

Can anyone please help?!!

Thanks,

-=>Raja.

[stevebfl]

I would suggest that you try hooking to the real TD signal.

The engine speed signal TD is a square wave pulsed output created by the ignition controller from the engine input (crank sensor or inductive pickup in the distributor). Many tachs including all of MBs will not recognise primary voltage (what you have hooked to) as engine speed.

Most aftermarket tachs will accept coil primary but I presume yours must be looking for a real TD signal.

[rbort]

I did some reading and apparently I need a tach sending unit that is bolted to the left fendor. Can someone post a picture of it so I know what I am looking for when I go to the junk yard again?

I also understand I need a reference sensor for the engine, but that my car may already have it. If you can cost a picture of that too it will help to see if I have it.

Lastly, what about wiring this sending unit to the tach? How should I do it?

Thanks all!

-=>Raja.

[rbort]

Thanks to Steve, he led me in the right direction as I was thinking I needed a tach sending unit and a reference sensor. Apparently that is for diesel cars and I didn't pick that up when reading the forums. Since I didn't need those, I found this green wire coming from the distrubuter and going to a part on the left fendor that it plugs into like an rca jack. From there there was a connector going to the coil, and some wires went to the coil, and another to another part on the left fendor. I decided to try to hook to this other part (it has a round screw on cap with a connector with 9 pins - maybe for diagnostics?) where that wires goes, and VOILA, the tach works perfectly!! Yeeeha!!

Next problem was my idle speed being too high. I read alot on this forum and people were talking about an idle valve. I looked in performance products and I can't find this guy in my car. I have a 1983 european Mercedes so it may be different than the 84 US version the catalog is talking about. What I did find was a screw that is accessable from the center of the valve cover and right below the air cleaner housing. I turned that screw 3 turns and my fast idle dropped from 1250 rpms to about 750 rpms. I set the idle so the oil pressure guage was at 2.1 or so (used to be pegged at 3) and with headlights on it drops to 2. I seem to remember years ago, when the car was hot in the summer at idle the oil pressure was at 1.5. Since it is winter and the oil is colder hence more pressure, I set it to 2.1 and with 750 rpms as I would expect to be right.

Can someone comment on if what I did was right or wrong? I mean I know you should not be turning screws and should be replacing bad parts like the idle valve or something else, but its possible that some mechanic turned this screw in the past and make the idle faster. Its been fast like this for as long as I can remember. Just happy to see it as you guys say now (750 rpms) for a 190e.

Thanks to all!!

-=>Raja.