What a mess! Soldering wires if done properly should not affect readings however soldering is not performing well under mechanically stresed connections.
If I was repairing that loom I'd make intermediate plug/crimp joint in the wires but thats me, even RAAF teaches its technicians that "soldering is for PCBs and crimping for cables and I had same training for my trade from the vehicle manufacturers....
As with the chip your best bet if you really want to know is to expose its internals aka check the PCB with all the electronic hardware. Like this one ==>
Essentially repackaged DTE chip the grand daddy of them all.
Anyway, yours seems awfully like the DPU system or Diesel Power Unlimited
I found the link to to it on Russian E-bay (DPU chip)