I have an 81 300TD that didn't come with a tachometer. I also have an 85 300D donor car (cali model). The tach on the donor car is the 3-pin style which I believe is the type that did not use a tach amplifier but rather ran off of a transmission bell housing sensor routed through an ECU.

A PO of the 81 300TD swapped the engine and transmission at some point, I don't have any info on the swap, but there is a two prong wire coming off of the transmission that isn't connected anywhere and is just hanging out in the engine bay.

Assuming this wire is the rpm sensor attached to the transmission, doe anyone know if it is possible that I could make the donor tachometer work in the '81 300TD? Would I need to pull the EGR from the donor car to make this happen?

I think it would be easier to install a 2 prong tach.

Your '81 should have a diagnostic port on the driver's side finder well that can be converted to a tach amp base.

If you look under the cover of the port, there are holes with numbers associated with them. #2 is ground and #6 is power and leads to the firewall. You will need to add in a wire for #3. Get the wire and plug for the back of the tachometer (instrument cluster) from a donor vehicle. The green-black wire from the tachometer plug goes to the #3 prong on the tach amp and the red-black one goes to switched power that you will need to locate. Then hope it works!



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1983 123.133 California
- GreaseCar Veg System

Funola did a write up on installing the egr Computer for the tach...

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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!

DIY tach for 240D w EGR- easy!

__________________
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!

I've read through funola's diy and that may work, although he demonstrates adding signal to an older 2-pin tach while I have the newer 3-pin tach available to me from my donor parts car.

Searching around it seems that some people have bypassed the EGR altogether and have found that the EGR is really just a pass-through and isn't doing any sort of signal conditioning or amplification. I'm not 100% sure if I've understood that correctly though. Ideally I would like to wire the signal from the bell housing unit directly to the 3-pin tachometer.

What I have is this two pronged wire (one brown and one white) hanging out in the engine which leads down to a unit attached to the transmission bell housing:





The tach I have is 3-pin type marked with W, G and +

From this thread: '84 W123, no tach amp?

This quote makes me think that I should be able to connect the two wires coming out of the sensor directly to the 2 of the pins on the tach, and then provide 12v to the tach on the 3rd pin.

Has anyone ever tried this specific setup/hack, i.e. wiring sensor directly to tach? Or rather, does anyone know which of the sensor wires should go to the which of the pins on the tach?

If your cluster is from an 85, then the tach in it should be able to be driven directly from the sensor on the transmission bell housing. It is not polarity sensitive. Just run 2 wires (twisted pair, or shielded cable).

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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

Since it is a 3-pin tachometer should I assume that the bottom pin marked with a '+' symbol would be for the 12v power?

Does anyone know what the 'G' pin (middle pin) and 'W' pin (top pin) stand for? Do you think I should be able to run wires from the sensor to these two pins?

From nomenclature conventions used by Mercedes on 83 and prior 300D tachometer, I'd say the "+" is terminal for +12V, the "W" is terminal for the sensor. The "G" would be the return for the sensor. I do not have a diagram for an 85 tach. It would be best to verify by finding such a diagram. Disclaimer: use above info accordingly.

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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

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

After reading through that thread several times and staring at the wiring diagrams associate with adding jumper wires to the EGR connector I think:

1. The white wire from the RPM sensor needs to go to the top 'W' pin on the Tach

2. The brown wire on the RPM sensor needs to be grounded

3. The bottom '+' pin on the tach needs 12v power


I'm still unsure about the middle 'G' pin on the tachometer. I need to look the tach connector from the donor car and see if it is even used. The 'G' indication makes me think it is possibly a ground from the tachometer, but I'm not sure yet.

The middle pin on the tach marked 'G' doesn't appear to be used, i.e. in the donor '85 vehicle there are only two wires running to the tach, a green one to the top 'W' pin and a red/black wire to the bottom '+' pin.

I wired the tach up as described above:

1. The white wire from the RPM sensor to the top 'W' pin on the tach
2. The brown wire on the RPM sensor to ground
3. 12v power to the '+' pin on the tach

For ground I used one of the grounding clusters behind the gauge cluster in the dash.

For power I temporarily connected to the #12 fuse clip in the fuse box.

I turned the engine over and didn't get response from the tach, so back to the drawing board for now.

3. The bottom '+' pin on the tach needs 12v power

If you have the 85 donor vehicle with a complete harness, you should be able to buzz it out and figure out how the tach is wired.

__________________
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'm least proficient at troubleshooting electrical problems, that's why I like diesel w123s 🙂

Possibly a dumb question, but why does the rpm sensor have two wires? I believe the white wire to be the signal wire, but if the sensor isn't powered what is the brown wire for?

A sensor must have 2 wires, one for the signal, one for the return. Unless the sensor is grounded by its mount, then it could have one wire. This type of sensor is a winding of many turns of wires around an iron core and does not need to be powered. A powered sensor usually has 3 wires.

Do you have a ohmmeter? Buzz out the harness connector "+" whether it goes to +12V (fuse) or not.

__________________
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

Is the rest of the cluster working? I see a connector on pic of your cluster marked +12V. Is that hooked up? Is the clock working?

__________________
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