PeachParts Mercedes-Benz Forum - Quick Fix: 1985 w123 Tach & A/C after EGR Failure

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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.

Quote:

Originally Posted by NTXw123300D (Post 3796543)

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?

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.

Having read this entire thread, I still have a question. (My tach has stopped working, after working intermittently for a year or so.)

A: First post said to splice white (20) to grn/blk (25). (and plug back in??)

B: Later post says to jumper 20&25 as well as 16 &14 (and presumably tape up and leave unplugged)

I have no EGR, A/C doesn't work, 85 Turbo. What is the difference and which is preferable.

Or could this be OVP or it's fuse? I read that fuse is up under passenger dash. Is there a relay there as well. If I jumper the wire to tach, are the fuse and OVP required? (I obviously need to read up on this stuff :( )

I did like post 21 by funola. I removed the EGR Box since all the vac parts were suspect and thus removed. My problem was the OVP Relay (not it's fuse atop), which is only used for the EGR Box. Those are expensive to replace, so I just tossed it and the box. For those readers jumping in here, if your tach stops working in a 1985, you will lose AC since the Klima relay requires that signal. It routes thru the EGR Box. I think 1984 is the same re fixing the dash tach, though doesn't have Klima for AC.

Quote:

Originally Posted by RavenTBK (Post 3237451)

Wanted to confirm the above as working.

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.

Old thread, but there should have been a correction to this. I think Funolo mentioned it later in thread. Maybe it doesn't matter but the wiring pictures show 20-25 correctly as per the text, but the other jumper in pictures goes from 14 to 18 instead of 14 to 16 as per the text.

Seeing 20-25 worked in post 1, perhaps that jumper is all that was needed? I tried that but it did not work. So I added the 14-16 jumper and the tach came to life!

Not sure what pin 18 to the “trimmer plug” does. From the two wiring diagrams one lead from the rpm sensor goes to 16 which should logically connect to something…otherwise you have effectively disconnected your magnetic pickup. Doesn’t make sense, but hey if it works on your car, go for it.

The above photos sure look like 14-18.

Quote:

Originally Posted by ykobayashi (Post 4287760)

The above photos sure look like 14-18.

That is exactly what I was saying.

The pictures don't match the text of the posts and are likely wrong.

20-25 and 14-16 is what I used. And this worked.

Graham