After reading the modification (wire jumpers) in this thread just now, it seems the tach amp is not in the EGR computer but in the cluster itself?

That would be correct. The EGR controller appears to be just a simple pass thru for the tach signal, probably done for wire routing purposes rather than anything else. (Wish I had known when I swaped in the '85 to the 107 as I spent a lot of time finding a mounting location and tracing wires and testing)

I thought we determined it was a sneaky way for Mercedes to discourage removal of the EGR computer? Pull the module and you lose tach (that is, until you know which wire to jumper).

I took apart the EGR computer from my 85 to have a look. Here are some pics. When I get a chance I will try to buzz out the tach connections and see where they go in the box. I don't see the tach chip that is in the round tach amp or the 83 240D EGR computer (uses the same chip). Unless Mercedes changed the design of the tach amp in the 85 model year? That's a lot of circuitry for a stinkin EGR computer!

http://i31.photobucket.com/albums/c3...117_133211.jpg

http://i31.photobucket.com/albums/c3...117_134809.jpg

http://i31.photobucket.com/albums/c3...117_134818.jpg

Any of you guys do this on an '85 SD? I ask is that my car has ABS and the '86+ all have issues with ABS not being functional when you remove the OVP.

I traced where the pins 14, 16 20, 25 go on the PCBs and marked them in these photos. I also tried the jumpers and verified the tach works with one or two jummpers in the pics. (note the pic with the white jumpers above do not agree with one of the pins 16 in description vs 18 in the pic).

andrewjtx, I am curious how you came up with the idea to make these jumpers? I think the EGR computer needs the RPM signal for EGR control. So the EGR computer is not just a pass through for this signal. I am going to do one more thing, which is look at pin 25 with a scope when the EGR computer is installed.

http://i31.photobucket.com/albums/c3...s/image_01.png

http://i31.photobucket.com/albums/c3...s/image_03.png

http://i31.photobucket.com/albums/c3...118_150837.jpg

http://i31.photobucket.com/albums/c3...118_150748.jpg

That could be true, however as a practical matter, most people have the EGR stuff disconnected, which makes the EGR controller at that point redundant. So it's function becomes a pass through for tach.

There are probably a bunch of those kinds of little quirks floating around. As an example, I found when trying to get the kick down working on my transmission after the diesel swap, that the kickdown gets its voltage off the fuel pump relay, which needs a signal from the ignition to activate. Diesels don't have ignition so the kickdown never worked. Jumpering the internals of a gutted fuel pump relay to send voltage to the kickdown fixed the problem.

I was trying to undersyand how it originally functions as designed by Mercedes. Does the 85 300D use a different cluster than 83 and prior?

Yes it does. Well, I guess technically it uses the same cluster, but has a different tach module.

The one on the left is out of an SD, which would presumably be the same one as pre-85 123s and the one on the right is the one out of an '85 Cali 123.

http://www.peachparts.com/shopforum/...ct-380-101.jpg

The electronics have to be completely different as the non-85s take their tach signal off the front pulley that provides 1 pulse per revolution, whereas the '85s take their tach signal off the flywheel teeth with 50-some-odd pulses per revolution.

The connectors are also different even though both are a two wire setup.

http://www.peachparts.com/shopforum/...ct-380-102.jpg

Good info! Can you post the chip p/n's on the tach's pictured?

Do you have a link to a wiring diagram of the 85 tach system including the diagnostic connector? I checked out the diagnostic connector on my 85 and it has the same pinout as the connector of the 83 tach amp, which is: power, ground, magnetic pickup (69 ohms). The only thing missing is an output pin to the cluster. I plugged in a round tach amp into the diagnostic connector and brought the output pin out with a wire and was able to measure a nice square wave pulse signal with a period of 84 milli sec. which is 11.9 hz which is 714 RPM, which means it is picking up a 1 per rev signal.

Is there a mag pickup on the harmonic balancer on the 85 or is there only one mag pickup on the flywheel?

The tach wiring diagram is attached.... which is to say, the EGR wiring. :)

Can't give you chip info as the tach pictures is installed in the car.

The '85 does indeed have all the "stuff" for a pre-85 tach system, except the tach amp and the wiring to the cluster. It's there for diagnostic purposes as far as I can tell, presumably allowing techs to use the same tools with that engine as the earlier ones.

How about the diagram for the diagnostic connector and the instrument cluster?

I seem to be missing the section of the electrical that deals with the gauges. I'll look further.

LMK if you find anything on the diagnostic port.

Did some measurements on the diagnostic port and the EGR computer connector. The diagnostic port has 12 V on it as well as a 68 ohm RPM sensor that is on the block picking up a tang on the harmonic balancer. What is this RPM sensor used for on the 85 300D? I know it is not for the tach since I measured 1.65 K ohms at pins 16,20 of the EGR computer connector which is the RPM sensor at the bell housing. Also pins 16,20 of the EGR computer connector showed 4.15 V AC at idle. Scope trace shows a very strong 1,666 Hz sine wave.

Does anyone know how many teeth that bell housing sensor is picking up? Is it around 142?

I think I have come up with an explanation why using jumpers to bypass the EGR computer works on the 85 model year 300D.

This is what pin 25 (output to tach) of the EGR computer looks like. It is a 3.8 v
1666 hz square wave AC signal.
http://i31.photobucket.com/albums/c3...s/image_12.png



This is what pins 16, 20 (transmission RPM sensor) looks like. It is a 10 v, 1666 hz sine wave AC signal. It has much higher output compared to the RPM sensor s on 83 and prior model years (around 6 to 7 mv). Due to the high voltage signal from the RPM sensor, it is strong enough to drive the tach directly.
http://i31.photobucket.com/albums/c3...s/image_13.png