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?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

This fix works.
Nice job, thanks.


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

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!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

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).

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

.... 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?

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.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

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?

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?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

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.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

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.