I don't think there is a setup 2).

No need to get your hands dirty, the bellhousing pickup goes up to a disconnect by the middle of the firewall which then goes to a bundle to a grommet (behind the battery) into the cabin to the EGR controller. If you unplug that disconnect you should measure 1.6 K ohms on the pickup side if the pickup is good and the wiring is intact.

__________________
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

But...I swear guys, there is no EGR controller on mine. It's not there. The wiring harness is not there, the plug's not there, the controller isn't there. It's not like it has been removed by a previous owner (unless he was extremely thorough), but it looks like the car never came with it.

Same for the tach amp, which on mine is just a diagnostic plug, not even wired for a tach signal.

So confused right now...

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

OK.

Your car is a Federal or Canadian version. You have a speed sensor on the rear of the engine that has brown and white wires going to a 2 pin connector C155. It is located to the left of the brake booster (right if looking from front). You can unplug it and check the sensor coil resistance. Also check to ground for infinite resistance.

From C155, wires go to C154 on the back of the cluster. Wire colours are now brown/white (pin 1) and white (pin 2). Pin 3 is the power feed to the tach that comes from fuse 12 (8 amp) and is black/red.

If your sensor checks OK and you have 12 volt power and ground to the tach, then your tach may be at fault. Ground for the tach is connected internally in the cluster and should be checked. Among the things that can fail in the tach is a calibration pot that can open and dried out electrolytic capacitors.

When pulling the cluster remove the plastic clip holding the speedo cable (engine compartment) to allow the cable enough slack.

Very interesting. Do you have a diagram showing this kind of setup (tach sensor direct to cluster with no tach amp in between, with connectors C155 / C154? Do you have a car with such a setup? What year, market and VIN?

You said "Your car is a Federal or Canadian version" Is the Canadian version same as the Federal version but different than the California version?

I have an 85 300D Federal version and it is nothing like what you described. I do have an OVP, my bell housing tach pickup goes to the EGR controller before going to the cluster.

__________________
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

Courtesy of joshuajeeper. Little hard to read, but maybe he can scan a better version.




.

Attached Thumbnails

 

__________________
1983 123.133 California
- GreaseCar Veg System

@Rick76
Thank you so much! That is EXTREMELY helpful! Really appreciate you taking the time to write all of that down. I will go trace some wires this weekend!

@Funola
So, my research combined with what Rick76 is saying means the following:

1) All W123's up to '84 use the tach amp
2) Early Federal 84's use pick-up directly to cluster, but all California 84's have signal through the EGR controller with the OVP under the passenger kick panel as do most/all late 84's
3) 85's are the same as late '84's, and use EGR controller, both Federal and Californian

My car is early '84, so apparently it's the weird setup directly into the cluster (although I haven't checked or traced any wires yet), but what Rick76 is saying matches perfectly with what I've found so far.

@DeliveryValve
Thanks bud!

You guys rock.

__________________
"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

I do believe ALL Federal 1984 w123/w126 617.95x's do not have an EGR computer. Only the Cali's got them.

.

__________________
1983 123.133 California
- GreaseCar Veg System

My 85 300D has the regular air cleaner like the 83 300D and I believe that makes it Federal? California models have the oddball expensive trash can air filter by the head light?

My 85 definitely have an EGR computer. I've had to work on mine 3 separate times already (due to water damage in the connector) to get the AC and tach to work. I think the 3rd time finally fixed it. It's a kluge and a PITA to trouble shoot due to lack of good documentation. Be glad you don't have such a system.

As for the late 84 with tach pickup direct to cluster, this is the first time I have heard of it. I think it would be useful to be able to define it by VIN # range rather than "late 84". I went to look for the diagram you posted on Startek online electrical and that diagram was missing. Where did joshuajeeper get it from?

__________________
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

The Vin decoder will show whether or not you have a Federal or Cali version. My 1983 is a Cali, but I couldn't differentiate what different compared to the Federal.

What you are describing, your '85 is a Federal vehicle. But the EGR computer setup is the same as in the '85 Cali.

The Federal 1984 never had an EGR computer. But the '84 Cali did. The tach setup in all 1984 uses the bellhousing pickup sensor instead of the front pulley sensor and tach amp.

Is joshuajeeper is or was a Mercedes dealer tech. He might have the actual service manual on this. I don't know, you may have to ask him.


.

__________________
1983 123.133 California
- GreaseCar Veg System

The fact that the diagram shows two "paths" to the tach module from the pickup is pretty strong evidence that the EGR is just a "pass thru" connection and has no role in signal processing or conditioning. This is further evidence that bypassing the EGR for the tach signal as has been done many times is a good solution not just an "it works" solution. I always suspected this, but never opened the EGR to trace the connections.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Thanks for the info. Learned something new about the 84 tach today. Maybe it is because it is the simplest system having only 2 parts, sensor and tach in the cluster that very little goes wrong with them thus the reason never hear about them?

__________________
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 can assure you the tach pickup to my 85 EGR controller is not a pass through. It comes in the EGR controller as a sine wave and goes out as a square wave to the cluster. I did ask andrewjtx (question was unanswered) in this thread Quick Fix: 1985 w123 Tach & A/C after EGR Failure how he ever came up with the idea of by passing the EGR controller. I think someone must have messed with an 84 tach and saw that the bellhousing sensor went directly to the cluster and decided to bypass it on models with an EGR controller and found that it worked. It'd be interesting to see what the resistance of the 84 tach pickup is. I am betting it is the same as an 85 at around 1.6K ohms (1600 ohms).

__________________
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 did some more research and the MY84 was as I described.
MY85 Federal and Canadian were given the California version with EGR controller and OVP relay.

Be nice. I like the 1985 CA frame-mounted air filter so much that I put one in my 1984 car (no more cracked air-filter mounts). Not expensive if you stack 2 cheaper air filters in the can. I posted the Wix PN (for Toyota MR2).

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I put this thread in the condensed sticky thread...

Great info!

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!