Since I got the important bits from junkyards, there was of course always the issue of them not being fully functional, so I considred this as being a gamble I was willing to make.
I opted for the later model amplifier/ECU with the coding resistor as I more less hoped it would be customizable in some manner, thus enabeling me to make fit my car. Of course I based this on absolutely nothing except the fact that the same ECU was installed in W140 300SD and W124's. W140 only have a speed signal derived from its ABS tone rings, (which are in the order of 40-50 pulses per wheel revolution), and the W124, which in some models uses a hall sensor mounted in the speedometer cluster (getting its signal from the still mechanical speedometer cable). I assumed this frequency to be considerably less.
In order to be able to find this out I needed a test rig. I hooked everything up to a signal generator in order to try things out.
A note on the speedometer sensor is that at least in standalone mode it needs a pullup resistor in order to be able to provide a signal. This MAY be present inside the amplifier. I just left it on there to make life easier for me when I did the installation.
I found out that one of the resistors DO infact control minimum engagement speed! the other ones obviously control things as well, but so far I have not been able to figure out what.
I was able to source some extra coding plugs from
(he seems to be known as JamesDean on this forum, and provided valuable input even though this isn't the most ordinary project), So I will eventually produce a permanent one.
Attached is first a picture of the amplifier with test wiring. Also visible is the modified coding plug with potentiometers (for experimenting purposes)
Secont is an overall picture of the full test setup. just to be fully sure the speedometer sensor worked with the system (signal levels can be a problem some times) I hooked it up to a power drill. Everything worked fine!
There is also a picture of the speedometer sensor, which I had to machine slightly. originally it was for M18x1.5 threads, but on fintails they obviously use M16x1.0
I machined some plastic and mad botha an insert thread for the female side, and a new one for the male side. Do NOT attemt to thread in the black plastic, it really doesnt respont well to it...