Tach not working.....wiring is suspect

[Morrison]

Car: 1987 325iC

Problem:

Okay, so I've been trying to get the tach to work, along with the mpg gauge just below it. None of this worked since I got the car. The 13 button obc seems to work, althought it only displays 5.9 avg mpg, which seems kinda low. It was just a little around town driving so maybe that's right.

Steps taken so far to correct it:

Took out instrument cluster to inspect SI board. Discovered the newer style lithium board, but went ahead and replaced the batteries with two externally mounted battery boxes, both of which contain 2 AAA lithium batteries. This was needed to equal the 3 volts of each of the lithium cells. The SI board seems fairly normal, as far as resetting the lights is concerned. One odd behavior though is that the lights never "count down." All four green leds stayed lit for the past 8 months. I then went to the junk yard this past weekend and picked up a cluster. Installed it and couldn't get anything on the SI section to light up, still no tach or mpg gauge. Inspected the SI board to find the standard twin AA NiCad board with the acid leaked out. Soldered in a new remote mounted battery holder and reinstalled. SI section now works, although I don't know if it will "count down." Still no tach.

[Morrison]

This got me thinking it may be a wiring issue. I dropped down the glove box to inspect the wiring by the Motronic unit and found a few suprises.
First off, I found the following wires unplugged or cut as described.

3 pin white plug with yel/wht, Gr/Red, and Blk on one side, which goes to White, Green, and Blk/Blu on the other side, and then those wires are just cut and hanging.

There is also a single black connector with Blk/Purple on one side and Brown/Red on the other. This is also cut and hanging.

There is also a single black connector with a blu/yel wire, which I believe to be one of two wires that connects on the other end to the speed output of my cluster.

There is a white spade type connector with a red/green wire and another white spade type connector with a brown/blue wire.

There is also a solid yellow wire which come from the back of the obc to a white connector which is unplugged.

There is also a green wire with black stripe which kind of splits off and goes to a back single pin connector and a white single pin connector.

The only wires coming from the Montronic harness itself terminate in two black connectors. On of them is a 2 pin, with single red/blue wire. The other is a 4 pin with black/grey and purple/grey wires.

[Morrison]

My working hypothesis so far is that this car had an engine swap at some point in its past and not all the wiring got hooked up properly. (Aside from the tach, mpg gauge, and brake fluid level sensor, the car starts, idles, and runs like a champ) From the searching I've done it seems that the problem may lie in the C104 plug which it appears that an '87 should have. However there is no C104 plug dangling from the Motronic harness in the car.....it's just one big group of wires that go to the engine compartment, aside from the two black connectors.

Thoughts?

[Morrison]

Sorry for such a long post, but I answered my own question today. For whatever reason, the car had a different harness from the ecm to the c101 plug for who knows what reason. I ended up cutting open the main harness right by the ecm and splicing into the wires there, for what would have went to the c104 plug. Tach and mpg gauge work perfectly now.