I have a 1984 300D W123. I have been looking for a replacement combination gauge for the instrument cluster for it and the printed circuit board they are mounted to. (You know, the fuel, oil pressure, and temp gauge on the left side of the instrument cluster) My car is a Euro version non-turbo (with A/T and dealer added A/C). So, no tachometer. Just the big oversized clock all by itself on the right. Anywho, the big ol' round primary multi-pin plug on the back of my instrument cluster has 15 holes and there are 15 pins on the printed circuit board. (Also, there are 16 wires leading into the plug) Most of the used instrument clusters I have located on eBay, etc. have 14 pins and there is a gap where there would be a 15th pin. Anyone know why mine has 15 and most others have 14? And what the differences in the circuits and components are for 15 pin versus 14? Also, there is a square black relay on the back of my instrument cluster on the printed circuit board right next to the dimmer rheostat and below the oil pressure gauge fitting. Many of the used instrument clusters do not seem to have this relay. What is it?

That's not a relay. It's a buzzer to tell you the headlights are on.

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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

There have been a number of threads about clusters. This one, andin particular first and last post might be of some help:

http://www.peachparts.com/shopforum/diesel-discussion/346954-instrument-cluster-repair.html

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

Forgot about that thread!

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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

Ahhhhhhh so that's what the little black box is on the printed circuit board? So, that's where the buzzer is that warns us we left the headlights on? Thank you!

Yes. Thanks. I had already read through that thread several times. You'll notice it doesn't really address why the difference of sometimes 14 pins and sometimes 15 pins and whether or not the circuit boards are interchangeable with no adverse effects

From other posts, someone with an 83 and 85 300D has a cluster with only 14 pins (missing pin #9). Strange that your cluster is an 84 and has 15 pins. The cluster where I labeled everything (about 3 to 4 years ago) has 15 pins but I do not know what year /model W123 it came from. At the time I did not even know there were 14 and 15 pin clusters. I'll have to look see how many pins my 85 cluster and plug has next time I have it apart.

Can you take a look at cluster harness plug how many sockets it has and whether #9 is missing or not?

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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

First of all, remember, and it may or may not make a difference, my 300D is a Euro model. All 15 sockets in the instrument cluster harness plug are present and each has a copper female connector inside. There are actually 16 wires going into the harness plug. 2 are doubled up to one of the plug's sockets. They are all original wires. Not add ons. All 15 pins on the PCB are present. Also, the 15 pins on the back of my PCB are located low and to the right as you look at the back of the PCB. Not high and above the oil pressure line connection like many I have seen. I will post photos after I take some.