Quick Fix: 1985 w123 Tach & A/C after EGR Failure
 

I've been pouring over all the threads on the 84 Cali and 85 Federal models with EGR and their propensity for water damage of the harness plug into the EGR control module. No one ever seems to offer anything other than repairing the harness and replacing the module.

While I did go and find a pristine EGR module, clean plug and large chunk of harness at the PnP yesterday, I wondered, "Why the hell am I even putting this module back in? All I need is my tach and A/C back. All of the EGR system has been removed!"

So I spliced the the white (RPM sensor signal wire) and grn/bk (to tach) wires from the harness together and guess what? SUCCESS!

I made sure to cover the power lead from the OVP so it doesn't short out on anything. But if you're running sans EGR, there doesn't appear to be any reason to run the EGR module.

YMMV

-Andrew

Nice! Do you have a pic and p/n of the egr module?

No pic right now. Just imagine all the pins are shiny...

P/N: 002 545 6032

Found this one...

http://www.peachparts.com/shopforum/...1&d=1374682769

Wow, that's pretty interesting. So what you are in essence saying is that the EGR controller is just serving as a "pass through" of the signal. And since the VR sensor is grounded outside the EGR controller only the one wire needs to be spliced. In thinking about that it makes sense in that the other major VR sensor on vehicles that have it is the speedometer, and that connects directly to the speedometer module in the cluster.

All along I assumed that the EGR controller did some kind of signal processing such that the Tach module in the cluster was just basically a voltmeter.

I'm not sure I want to rip out my module as it's working just fine, but I'd really like to be able to trace the circuit inside the EGR controller and verify that it really is just a pass through.

And the final speculative question would be, why would the German engineers route the tach through the EGR controller in the first place. I don't think it simplifies any wiring harness issues or solves any problem.

Anyway, bottom line, great find that can really simplify our cars and save big bucks on replacing modules that really don't need it.

The signal goes through the EGR controller to keep people from removing the controller to disable the EGR. This was probably mandated by our Federal Government. They have been doing things like this for many years.

Paul

Crap, there I go again looking for a logical explanation when the explanation is a political one. I guess it's kind of like the regulation that says you can't vent that horrible, dangerous, environmentally harmful R134a refrigerant (it has to be "recovered") from an A/C system, but if you buy cans of computer duster, you can vent that same 134a all day long.

Thanks for bringing me back to reality

It also looks like you can remove the OVP relay, trimming plug, rack position sensor, EGR vacuum amplifier, coolant temperature sensor and recirc air safety valve to clean things up even more.

I was wondering if it's the same p/n as the EGR computer in the 83 240D p/n 002 545 29 32, which it is not. I think they must do similar functions, which uses the tach signal to electrically control the EGR valve.

http://i31.photobucket.com/albums/c397/funola/EGRC.jpg

Don't think so. In 85 300D's the EGR controller needs the tach signal to control the (elec.) EGR valve, so the tach is driven off the EGR controller instead of a separate tach amp like in 84 and prior 300D's (which has a vacuum controlled EGR valve).

Hmmm. the '85 still had a vacuum actuated EGR valve. Now, the vacuum is sent to it via the vac amplifier that is electrically controlled by the EGR module...

The EGR module is a consumer of the RPM sensor signal along with water temp and rack position. It combines this data to know when to active the EGR for best NOx reduction.

It doesn't amplify the signal. Passing the signal through without the module gives me accurate tach and the compressor cuts out when expected. My '84 did't have a tach amplifier either. I'm not sure the actual wiring differences that changed from 83-84, but the 85 just routed the same rear rpm sensor to through the EGR module that the 84 used.

I was actually wondering about that...which would make engineering sense, not just political sense.

Next time I'm out at the junk yard I'm going to pull an EGR controller and see if I can verify that there is an internal "Y" with the signal being shared by the EGR and the tach - the tach portion being just a pass through and no signal processing involved, as andrewjtx has apparently figured out!

I've looked at the PCBs on my old module. If you can figure that out, all the power to you... ;)

Full-blown dummy here.
Where are these wires, the white and br/gn? how is the splicing done? Am I just looking at the wires that are connected to the OVP?
I want to do this for my 85 and yep, it sounds easy and probably is I just don't know where to look. OVP is out, I know where the EGR thing is behind the little door.

Bwaa!!! I just did this and it worked. thank you guys!

Ok easy way. 5mins max. No cutting or splicing required.

Go to radio shack and pick up a "jumper wire kit" and some electrical tape.

Open 'secret' door, passenger side footwell, and remove EGR control unit. (flathead useful for pulling aside plastic retainer tab)

Look at the diagram below.

(80) Control unit

On the wiring harness (female) side, with your jumper wires, connect...

20 to 25 (RPM sensor-->to tach)

16 to 14 (RPM sensor-->ground)



http://www.peachparts.com/shopforum/...ontrolunit.jpg

Test it out.
Wrap up harness with electrical tape and chuck the EGR control unit!
No cutting, no splicing!

Working tach!
Good to go!!!

Wanted to confirm the above as working.

In my case, my AC and such was all functional, just could never get a response from the tach. I finally got around to doing this. Popped my door and popped the plug from the EGR computer, expecting to find nastiness like others have reported. My pins and harness are pristine, like new. I figured I'd disect the computer just to look since I will not be replacing it. Again, like new.

As such, I have no idea why my EGR system and everything else worked fine, but the tach wouldnt.

I grabbed some scrap 22ga speaker wire and shorted pins 20 over to 25 and 16 over to 14. Instant signal. Its reading low... 500rpm idle, 3100rpm no load governed. Not sure if the reading discrepancy lies within the jump "around" the EGR computer, or in the junkyard replacement sensor I swapped in a month ago to try and fix the problem.

Either way, here's some pics for those of you that are diagram challenged. ;)

http://i.imgur.com/lXzPw7Jl.jpg
http://i.imgur.com/EjDdR0vl.jpg

And my final product once I trimmed down the excess. I tucked it into the opening around the edge. Theres no reason for the wires to come loose from their pins let alone fall out with the middles being jammed into the gap.

I would guess, ideally, you'd put a zip tie or something around the plug to be sure nothing moves. Whatever. I fixed my tach for $0. I slid the EGR computer back into its "port" and laid the plug on top. Once I replaced the kick panel, the plug cant move anywhere.

http://i.imgur.com/5myoRgyl.jpg

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

The raw variable reluctor signal should look something like the top trace in this image

http://www.molla.org/DIY-CDI/DIY-CDI...ger_Fig05s.jpg

The bottom is a switching signal from a transistor.

It appears that based on your scope traces, that the EGR controller is indeed running the raw signal through some circuitry to convert to a square wave. I'm wondering if the higher signal voltage would be detrimental to the tach over time.

And, I wonder if the appearance of your VR trace as a pseudo-sine wave is due to the closeness of the teeth on the flywhee (trigger wheel), as compared to the 4-tooth trigger wheel normally associated with a transmission for speed signals. (as shown in the following image on the right)

http://www.peachparts.com/shopforum/...edo-drive4.jpg

Interesting stuff....

Is the trace below what you captured on your 85 300D engine?

[QUOTE=mach4;3274321]The raw variable reluctor signal should look something like the top trace in this image

http://www.molla.org/DIY-CDI/DIY-CDI...ger_Fig05s.jpg

The bottom is a switching signal from a transistor.

The square wave is driving the tach. I soldered a wire onto pin 25 of the EGR computer internally, drilled a hole and brought it out of the box and captured that trace while the engine was idling with the tach registering.

That is actually a very pure sine wave from the VR sensor. Not sure why you call it a psuedo sine wave? What you see on right portion of the trace is shifting frequency, i.e the idle is not perfectly stable but varying, that is what you are seeing on right portion of the trace with muti waves. The engine was cold when the measurement was made. After it is fully warmed up, idle may smooth out and the trace may be more stable- but I won't know for sure till I try it.

What is that 4 wheel trigger wheel from? Don't tell me it is what the VR sensor is picking up in the 85 300D. :confused:

I called it a pseudo-sine wave because a VR signal should look like this. (although this is running with reverse polarity from "normal" as it shows a rising edge) There is normally a distinct rising voltage, then a near instantaneous drop to negative. This is what makes it so good for timing as you get a very distinct point where the voltage changes.

http://www.import-car.com/Content/Si...0000019330.gif

As the teeth get closer together (as in the MB flywheel) the falling edge gets less distinct as the following image shows, so much so that if the polarity is reversed, it's difficult to tell what is the rising edge and falling edge. (Note the trace is from a timing wheel, like perhaps an EDIS, where the anomaly is from a missing tooth that marks the beginning of each revolution) Someone could easily mistake the trace as a sine wave, when it is in reality not.

http://img32.picoodle.com/img/img32/...rm_37b2dec.gif

The 4-tooth trigger wheel image is from a 722.3x transmission. It's used as a trigger for the speedometer VR sensor.

I don't own a scope, so everything I've shown is from the internet.

I don't know why you think a VR signal should look like your first pic. Where did you get that pic from? The flywheel, with it's evenly spaced and uniform profile (rising and falling) teeth, should produce a sine wave output from the VR sensor like what I captured.

In your second pic, I have no trouble telling a rising edge from a falling edge.

Depends on the tach circuit. If no one has reported any damage with the mod by now, it is probably ok.

.... So I spliced the the white (RPM sensor signal wire) and grn/bk (to tach) wires from the harness together and guess what? SUCCESS!

I made sure to cover the power lead from the OVP so it doesn't short out on anything. But if you're running sans EGR, there doesn't appear to be any reason to run the EGR module.

YMMV

-Andrew[/QUOTE]
I like this and would like to try it since my tach unexpectedly quit working. The 10amp fuse at the OVP is good but the OVP rattles when shaken. If I were to open the harness going to the EGR black box and splice the two wires you mentioned and presumably the tach works again is the OVP of any use anymore?

It Works
 

This fix works.
Nice job, thanks.
:)

'85 300 TurboD
'78 300D
'70 220D

Just did this to my 85' 300TD, and my tach is now working! The pins from the EGR module were all green and crusty, and I have removed my EGR valve, so one more thing checked off the list of things to do before putting the car back on the road.

Save all EGR related parts that you remove in case some day you may have to re-install them to pass emission. Your car may not require emission test today but that could change with tightening of the noose by the EPA.

Sorry to revive an old tread but I'm in the process of trying to get the EGR controller on my om617 swap working correctly. There isn't a lot of information out about this subject and this was one of the only threads I was able to find that actually discussed the function of the EGR control besides just trying to bypass the thing entirely. Im looking for information on mainly how changing the tooth count on the ring gear might effect the function of the EGR system.

Wow, I just read this entire thread and it is mind boggling.

This reminds me why I love my '81 300SD and its simplicity. I will probably never get rid of it as my almost daily driver.

Good luck to you computer savvy guys; you are admired.

Changing the tooth count will change the perceived RPM seen by the EGR system. If you have a situation where it is critical that it function as designed at various engine RPM, it needs to be assembled as designed with the correct tooth count. There's not really any other way around it.

Thank you Diseasel300 I figured it would change the percieved Rpm I was hoping it wouldn't change it enough to raise eyebrows upon inspection. Kind of at a loss for how to make the system function properly because I am unable to run the factory flywheel in my conversion.

New to me 85 fed 300D turbo had intermittent tach issues. Found this thread and decided to pull the EGR controller and try the jumper wire trick.

I know my EGR valve is bad, assumed failed closed and finally blocked off as of today. I plugged all the EGR vacuum lines last month to see if I could get it running better. I felt it ran worse but never reconnected the lines. Figured I didn't have much to lose by pulling the EGR controller.

The EGR controller and connector had tons of green corrosion. I regret not taking pictures at this point. Cleaned up the connector, installed the jumper wires and got my tach working great, however now it won't shift out of 1st gear, which shocks me.

I cleaned up the EGR controller pins and put it back in. I lost my tach again but it shifts exactly as it did before. I'm not grasping why it refuses to shift without the EGR controller installed.

Since then, I bought 2 EXACT same EGR controllers off e-bay with NO corrosion at all, and it still DOESN'T shift out of first gear, but the tach works...

Of course, still running off the original EGR controller so I can drive the car but have no tach.

Any thoughts?

I just did this splice fix and it worked perfectly on my 85. Posting in case someone else has my symptoms.....
My tach worked through about half of the throttle range. Where it operated it was fine, but through about half the pedal travel it would drop to idle speed and a loud clicking would come from the rack position sensor on the injection pump.
The clicking was loud enough to be heard in the car and loud enough to make folks on the sidewalk look as I drove by.
The EGR connector was full of water from a new windshield leak I now know I have and must have been shorting across to the rack position sensor.

To the previous poster, I am not sure the transmission has much electrical control beyond the kickdown switch behind the throttle pedal. I would suspect something going on in that circuit.
I think the kickdown switch circuit uses signal from the tach so if it is stuck closed or otherwise malfunctioning, it would cause the transmission to hold first gear like that because it thinks you are holding the throttle to the floor. Without tach signal its not working at all and shifting normally.

The leak into your EGR controller is not from the windshield! It is from the opening(s) of one (or both) of the 2 wiring harness's outer shield in the engine bay. One is under the battery tray, the other is a connector middle of the firewall.

Per this diagram, the EGR controller has no connections to the transmission or kickdown switch. Maybe there are other diagrams that does?
http://www.peachparts.com/shopforum/...ilure-egr2.jpg

Good find. Swapping a 83 motor into my 85 3000sd and I belive troubles ahead!

The engine should not have anything to do with it.
Swap the adapter plate off the 85 engine, and bolt it
onto the 83 engine. Then plug in the sensor wire to
the adapter plate.

Iam assuming the SD is the same as the 85 300D, as
far as I know it is the same.


Charlie

Yeah well I discovered the intermediate plate difference after bolting up my flywheel clutch and trans to the 83 motor. DOH!!! I assumed the front sensor ran the tach on all of them. I just don't know if the tach will still work without the IP sensor wire hooked up and all that emissions bul Zip taking up room on my floor. Being that I wasted a day swapping that plate and trying get unsuccessfully to get the hex plug out of the water jacket for the second temp sensor I sure hope it works. My first turbo manual swap afyer years of wanying to, living without a tach would really annoy me.