Regarding one of the schematics I'm looking at 




First question is on the Oil Warning lamp (H) - clearly the Oil Pressure sense is a (-) input - yet the schematic above shows what should be the high side of that lamp connected to 'ground' 
Mistake? Surely should be connected to the Green/Black (+) ? 

Next are the M (choke) and N (overtemp) lamps - I presume there should be a connection where the connection between two lamps crosses the green/black trace? 
(i.e. a connection 'dot' at that intersection) 
And therefor those inputs on 12 & 10 are also  (-) triggers. 
And contrary to my earlier perception, it looks like the 'charge' signal is also (-) as is 'lamp module' 

Edit - I see by looking at other diagrams that indeed that one I posted above is in error and should be as I deduced. Would not surprise me if it came from Haynes, I find issues with their schematics frequently! 

I see a number of users with the 2853 Acewell 
The only available (-) inputs warning lamps for that gauge are the Oil and Neutral - the only other 'spare' one (besides the turns and the hi beam) is the 'hazard' warning, but the logic for that lamp requires a (+) 
Edit - this is yet another documentation error, this time from Acewell - the Hazard Lamp is in fact a '-' input also.  
obviously the Neutral is required: are users combining all of those other warning lamps (Oil, Overtemp, Charge, choke, lamp monitor) into that Oil  indicator? 
I see on the BEP2.0 schematic that they have multiplexed these signals via diodes into a common connection (as well as providing independent outputs) but again, that will require a (-) input on the instrument 
If you wanted to use the 'Hazard' as an alarm, it would required a transistor (or relay) to switch the logic to (+) be able to utilize that. 
The TGPI has no provision for this multiplexing so you would have to do this diode-multiplexing externally (in which case you could add the transistor if you wanted to use the (+) input of the hazard alarm light)  

So it appears with the 2853 you are stuck with having one warning lamp (oil) for all those functions - so I guess if you have a problem you just panic, stop then try to figure out which one it is!   
Incidentally what function ARE 2853 users using that 'hazard' input to display? 

I think for the application I'm assisting with, looks like what might be best is to use the in-instrument oil indicator as intended solely for that purpose and have additional (external) LEDs for Overtemp and Charging* - don't think the others worth bothering with, or individuals could add additional LEDs as they choose. 
The 'hazard' indicator will have another purpose (in conjunction with a Key-Less system) 

* as an alternative to using the OEM charging signal, I personally use this little device - very nice little product, compact and inexpensive 
http://www.sparkbright.co.uk/sparkbright-eclipse-battery-voltage-monitor.php

http://www.sparkbright.co.uk/sparkbright-monsoon-battery-voltage-monitor.php (non adjustable display intensity - only marginally cheaper) 

I prefer the 5mm LED size myself - see the notes about the control pcb for that one vs the bigger sizes. But the smaller ones are just nicer for my taste.

That of course only addresses the display - you must install a 50 ohm 3W resistor between Ignition Power (Green/Black) and the Blue Wire from the Alternator field coil - this is necessary to ensure the field coil is adequately charged at alternator start-up.

Assuming I have the Logic of the alarms correct .......... 

Edit - The previous post is incorrect  I found two separate diagrams for the 2853, one showing that the 'hazard' lamp was a '+' input and another showing a '-'
It is in fact a '-' as it turns out. 
So you can actually just diode multiplex those other alarms (as you choose) via diodes directly to the Hazard input WITHOUT having to use the Inversion Switch as diagrammed below. For other types of gauges where you might come across similar I'll leave it in there

Here's how you can multiplex the alarms to drive a positive input (e.g. the hazard warning light on a 2853 Acewell) 
The oil pressure in this instance is shown as it own (-) input to the Oil indicator
I've shown a relay version for those who may be uncomfortable with electronic components. 

When the Transistor gate (or the Relay 85) sees a (-) through ANY of the warning circuits, it will light a (+) input LED. 

Of course if you want to simply send ALL the signals to the 'Oil' LED, you don't need transistor or relay, just connect the common Anodes to the LED (-) (adding in the Oil to the matrix as well via another diode) 

Note on the Fuel Level options (this ONLY applies to Tanks with Float Level Sensors - NOT for earlier Thermistor types) :
The AceWell 2853 very conveniently has both + & - inputs; the Level Sensor on the BMW has 'inverted' resistance (higher resistance at low level) vs most Japanese bikes (higher resistance at high level); 
@rbm wrote an excellent article on (physically) inverting the level sensor in K100 forum - however with the 2853 you simply have to select the 100* range for the fuel level; this will then display the fuel level correctly without any need to do the physical inversion of the sensor. 
(*The range options are 100, 250, 510, Off, -100, -250 -510) 
You do however need to swap the Fuel Level Wires:
The White wire originally coming from the Pump Connector is the 'Low Level' (Reserve) signal; this is the wire going to the Instrument Panel; this signal is only 'Low' or 'Not-Low'
The actual 'Fuel Level' from the float resistor, is via the Yellow wire - this goes to the auxiliary instrument connector which is located under the tank.
So the simplest method is to just swap the yellow and the white wires at the pump and now the 'white' wire going to the Instrument connector becomes your actual Fuel Level, no longer a low-level signal only. 
You can of course run a new wire from the Yellow at its auxiliary connector, but since you already the white at the instrument connector, it's much simpler to just swap the yellow & white at the pump connector. 
If you wish to also have a separate warning light function with an auxiliary LED, you can use the Yellow wire on the auxiliary connector for that function as that yellow has now become 'low-level' signal. 

If you wish to correct the fuel level signal to be 100 ohms on empty tank (BMW sensor is 130 ohms) then simply connect a 470 ohm resistor between the Signal and Ground (i.e. in parallel with the sensor) 470 in parallel with 130 is 470*130/(470 + 130) = 101 ohms. 
It won't be exactly linear but will be better at the empty end of the scale which is more important.

For Tach, the best result was to simply connect the OEM Tach Signal Wire (from Coil #1 drive) directly into the 2853 via a 1Mohm resistor.
This was extremely stable on this device, no conditioning necessary. 
It has good noise rejection as-delivered. 
The setting on the instrument was SP 1r1P - one pulse every revolution (this will be the case whether a K75 or K100)