I assumed the spade marked 12v was for the clock, but no I haven't added power to that yet. This is turning into a good opportunity to learn got how to start using volt meter and ohm meter

Did you have the circular connector hooked up to the cluster?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Would having the circular connection plugged in make a difference? If the vehicle didn't originally have the tachometer I don't see where that plug would make a difference, i.e. I don't think there are any connection to the tach/clock portion of the gauge cluster.

I have the original gauge cluster in the car with everything plugged in. The new/donor tachometer is on the seat next to me. As described before I have everything wired as follows:

1. The white wire from the RPM sensor to the top 'W' pin on the tach (for testing i'm just using a long wire coming from the engine through the driver window)

2. The brown wire on the RPM sensor to ground (this is going from the tach to grounding cluster behind the gauge cluster)

3. 12v power to the '+' pin on the tach (another longish wire attached to the #12 fuse holder coming through the drivers window to the tach)

I'll try a few more things tomorrow. I'm not sure it would make a difference but I will run power to the 12v clock spade on the combo tach/clock.

I made a few more attempts yesterday but didn't get a response from the tach. Unfortunately I don't know whether the tach was functional when I took it from the donor car or not. The clock portion of the gauge is definitely not working, i.e. when 12v is applied the clock doesn't start turning (I left it plugged in for a while to see if the hands would move).

All that to say as a next step I will need to figure out how to test whether the RPM sensor is working and whether the tach is working.

You should post pics of how you have the tach hooked up. Iirc you have it taken out of the cluster, laying on the seat. If that is true you need to ground the case of the tach for signal and power return.

To test the sensor, check the resistamce. It should be around 1900 ohms (1.9k).

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Yep, that was it, I needed to ground the tach itself. I went ahead and just put the tach into the cluster and put everything back together. Jumped up to ~750 RPM as soon as I turned it over.

Thanks for your helpful suggestions Funola

My final question is regarding power for the tach. Currently I have the power temporarily connected to fuse #12 directly in the fuse block, but I'll need to find a long term connection.

The easiest connection would be tapping into the 12v line that goes to clock, but that would mean power would be going to tach when the engine is off, not sure if that is problematic or not?

Wiring it to the clock will work but add additional drain to the battery. It would be interesting to try it since it may allow you to see what rpm your engine cranks at when starting. If you do not want to put additinal drain on the battery, I have a pic in my 240D egr tach diy DIY tach for 240D w EGR- easy! which shows where to solder a wire onto a rivet for switched +12. If you do not have a soldering iron, you can buzz out that rivet to find the circular connector pin it goes to and splice into the harness for that pin.

Edit: I'm not sure if your cluster has that rivet. If not, poke around with a voltmeter to find a point with switched +12V.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I went ahead and tapped into the cigarette lighter power. Put it all back together and seems to be working great. Thanks again for all the input.

How did you ground the tach?

Typically the RPM sensor provides an AC signal, which varies in frequency with the rotation speed of the engine because it is sensing and responding to either metal target or a magnet passing by. Your sensor can probably be tested with a digital multi-meter set to measure AC voltage, and with the meter leads connected to the two wires from the sensor. I don't know how the '85 sensor works exactly (magnet or metal target) but either way you got lucky and it works. I suspect that the sensor is pointed at the starter ring gear, and as each tooth passes by, it generates a signal.

The Tachometer uses that signal by measuring the frequency, doing some math to determine the number of signals that correspond to one rotation of the engine, and then moving the needle appropriately.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Turns out the tach is grounded by simply adding it to the gauge cluster. When I had the tach out of the cluster and simply sitting on my dashboard it wouldn't work, but once I installed it in the gauge cluster it started working.

The other necessary ground is the brown wire coming off of the rpm sensor attached to the bell housing. I grounded this to one of the grounding points behind the gauge cluster. When you have the gauge cluster out there are a few spots under the dash where multiple wires are grounded. I simply added a connected to one of those locations.

He didn't get lucky. He is running the stock configuration of a rare 84 300D minus the harness. There is a forum member from Alaska with an 84 300D with this factory stock configuration, i.e. sensor on the flywheel, running directly to a tach that is unique to 85 (and some 84 MY's).

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Actually, the car is titled as an '81 300TD. A PO swapped the engine at some point. I don't think the car originally was a turbo as the hatch says just 'Diesel' not 'TurboDiesel'. Also the SLS has been deleted and the engine doesn't have a block off plate where the SLS pump would have been, i.e. it isn't an engine from a 300TD.

I bought the car from someone who claims her 'ex-boyfriend' did the work, so unfortunately I don't have a complete history on the car.

I suspect the engine is from a later model 300D that had the flywheel RPM sensor.

In your donor 85, is the RPM sensor grounded at the bell housing? I don't think so. Did you cut into the sensor cable and patched a ground at the bell housing?

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The donor '85 had the same sensor at the flywheel as my '81 with the same two-prong plug at the end. In the '85 it plugged into another set of wires at the firewall leading to the passenger side of the vehicle. I suspect in the '85 it led to the ECU thing behind the front passenger kick-panel.

In the '81 I grounded the sensor to a grounding cluster behind the gauge cluster. I probably could have grounded it in the engine bay.

Your 81 had an 85 or 84 transmission put in, those are the only 2 model years with a RPM sensor in the bell housing. Prior to 84, the RPM sensor is located at the front of the engine near the harmonic balancer. You should not ground the sensor at the bell housing which is poor practice.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked